The Carbon Trust
Marine Energy
Challenge
Oscillating Water Column
Wave Energy Converter
Evaluation Report
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 The Carbon Trust 2005
Contents
Executive Summary ............................................................................................................................................. 1
1
2
Scope of Work & Report Structure......................................................................................................... 7
1.1
Background to the OWC Evaluation ..................................................................................... 7
1.2
First Stage Evaluation........................................................................................................... 7
1.2.1
State-of-the-art Review ......................................................................................................... 7
1.2.2
Market Assessment .............................................................................................................. 8
1.2.3
Report Structure ................................................................................................................... 8
1.3
Second Stage Evaluation...................................................................................................... 8
1.3.1
Scope for improvement......................................................................................................... 8
1.3.2
Future Visions....................................................................................................................... 9
1.3.3
Shoreline against near-shore device installation................................................................... 9
1.3.4
Sensitivities and prioritisation................................................................................................ 9
1.3.5
Report Structure ................................................................................................................... 9
1.4
Third Stage Evaluation.......................................................................................................... 9
1.4.1
Recommendations for development work............................................................................. 9
1.4.2
Report Structure ................................................................................................................... 9
1.5
Developer input and peer review ........................................................................................ 10
Glossary of Terms ............................................................................................................................... 11
2.1
Oscillating Water Column Terminology............................................................................... 11
2.2
Terminology ........................................................................................................................ 11
State-of-the-art Review & Market Assessment .................................................................................................. 13
3
Existing Shoreline OWCs .................................................................................................................... 14
3.1
LIMPET 500 [3,5,6,27,56] ................................................................................................... 14
3.1.1
Concept .............................................................................................................................. 14
3.1.2
Geometry ............................................................................................................................ 14
3.1.3
Design Parameters (Assumptions and working principles) ................................................ 15
3.1.4
Project Experience.............................................................................................................. 15
3.2
Pico [4,26,45,46,67,68] ....................................................................................................... 18
3.2.1
Concept .............................................................................................................................. 18
3.2.2
Geometry ............................................................................................................................ 18
3.2.3
Design Parameters (Assumptions and working principles) ................................................. 19
3.2.4
Project Experience.............................................................................................................. 19
3.3
OSPREY [10]...................................................................................................................... 22
3.3.1
Concept .............................................................................................................................. 22
4
3.3.2
Geometry (OSPREY 1)....................................................................................................... 22
3.3.3
Design Parameters (Assumptions and working principles) ................................................. 24
3.3.4
Project Experience & Reported Conclusions ...................................................................... 24
3.3.5
Performance ....................................................................................................................... 25
3.4
Port Kembla, NSW, Australia [47,70,71] ............................................................................. 26
3.4.1
Concept .............................................................................................................................. 26
3.4.2
Geometry ............................................................................................................................ 26
3.4.3
Design Parameters (Assumptions and working principles) ................................................. 26
3.4.4
Project Experience.............................................................................................................. 27
3.4.5
Performance ....................................................................................................................... 28
3.5
Vizhinjam OWC, Trivandrum, India [11,48]......................................................................... 29
3.5.1
Concept .............................................................................................................................. 29
3.5.2
Geometry ............................................................................................................................ 30
3.5.3
Design Parameters (Assumptions and working principles) ................................................. 31
3.5.4
Costs................................................................................................................................... 32
3.5.5
Performance ....................................................................................................................... 32
3.6
Sakata, Japan..................................................................................................................... 34
3.6.1
Concept .............................................................................................................................. 34
3.6.2
Geometry ............................................................................................................................ 34
3.6.3
Design Parameters (Assumptions and working principles) ................................................. 36
3.6.4
Performance ....................................................................................................................... 36
3.7
Other OWC installations ..................................................................................................... 36
3.8
Recent Developments......................................................................................................... 37
3.9
Risk Matrix .......................................................................................................................... 38
3.10
Key Performance Indicators................................................................................................ 41
3.10.1
Structural Quantities v Output............................................................................................. 41
3.10.2
Wave Energy at Collector v Output..................................................................................... 41
Summary of OWC technology (State-of-the-Art) ................................................................................. 42
4.1
Resource Assessment [2,51] .............................................................................................. 42
4.2
Modelling and Performance Prediction [9,13,29,30,32,33,34,40,41] .................................. 42
4.3
Structural Form ................................................................................................................... 43
4.4
Turbine Design and Optimisation [77,78,79,82,84,85,87 & as shown below] ..................... 44
4.4.1
Summary of the Current State-of-the-Art ............................................................................ 44
4.5
Controls [83, 86] ................................................................................................................. 45
4.5.1
Treating the water Column itself as a tuned oscillator ........................................................ 45
4.5.2
Modifying the Characteristics of the Turbine-Generator...................................................... 45
4.5.3
Machine Technology........................................................................................................... 45
5
6
4.5.4
Control Systems ................................................................................................................. 46
4.6
Power Conversion Efficiency .............................................................................................. 46
4.7
Maintenance and Reliability ................................................................................................ 46
Market Size.......................................................................................................................................... 47
5.1
Assessment Method ........................................................................................................... 47
5.2
UK Wave Energy Resource ................................................................................................ 47
5.2.1
Distribution.......................................................................................................................... 47
5.2.2
Variation / range ................................................................................................................. 48
5.2.3
Ranking method.................................................................................................................. 50
5.3
Sea Bed Profile................................................................................................................... 52
5.3.1
Method of review ................................................................................................................ 52
5.3.2
Ranking............................................................................................................................... 54
5.4
Tidal Range ........................................................................................................................ 56
5.4.1
Ranking............................................................................................................................... 57
5.5
Grid Accessibility ................................................................................................................ 59
5.5.1
Ranking............................................................................................................................... 59
5.5.2
Results................................................................................................................................ 59
5.6
Local Construction Factor ................................................................................................... 62
5.6.1
Shoreline Construction........................................................................................................ 62
5.6.2
Near-shore Construction..................................................................................................... 62
5.7
Relative Economics of Near-shore versus Shoreline Construction..................................... 62
5.8
Ranking Matrix, Combination and Weighting ...................................................................... 63
5.9
Potential Resource ............................................................................................................. 67
5.9.1
Shoreline............................................................................................................................. 67
5.9.2
Near-shore.......................................................................................................................... 68
5.10
Existing Structures .............................................................................................................. 68
5.11
Conclusions ........................................................................................................................ 69
Generic Base Case Elements.............................................................................................................. 71
6.1
Existing device similarities .................................................................................................. 71
6.2
Existing device differences ................................................................................................. 71
6.3
Focus Areas for Stage 2 Evaluation ................................................................................... 72
6.3.1
Generic design.................................................................................................................... 72
6.3.2
Plant optimisation ............................................................................................................... 72
Scope for Improvement & Future Vision ............................................................................................................ 73
7
Choice of device type and structure..................................................................................................... 74
7.1
Shoreline OWC Devices ..................................................................................................... 74
7.1.1
Resource Assessment ........................................................................................................ 74
8
7.1.2
Site specific design and preparation ................................................................................... 75
7.1.3
Construction techniques ..................................................................................................... 76
7.1.4
Weather and programme risk ............................................................................................. 78
7.1.5
Contractual risks ................................................................................................................. 78
7.2
Near-Shore OWC Devices.................................................................................................. 79
7.2.1
Advantages......................................................................................................................... 79
7.2.2
Disadvantages .................................................................................................................... 80
7.3
Breakwater OWC devices................................................................................................... 80
7.4
Conclusions ........................................................................................................................ 81
Scope for Improvement ....................................................................................................................... 82
8.1
Site–specific tailoring of designs......................................................................................... 82
8.1.1
Chamber geometry ............................................................................................................. 82
8.1.2
Chamber lip ........................................................................................................................ 83
8.1.3
Device Capture Width ......................................................................................................... 83
8.1.4
Anti-slosh Devices .............................................................................................................. 84
8.1.5
Air Plenum Configuration .................................................................................................... 84
8.1.6
Collector Width ................................................................................................................... 85
8.1.7
Tidal range.......................................................................................................................... 85
8.2
Turbine choice and specification......................................................................................... 85
8.2.1
Air flow rate matching ......................................................................................................... 85
8.2.2
Blade and vane configuration ............................................................................................. 86
8.2.3
Orientation of axis ............................................................................................................... 87
8.3
Device construction methods.............................................................................................. 87
8.3.1
Balance of Plant Materials .................................................................................................. 87
8.3.2
Mass production / manufacture........................................................................................... 88
8.4
Conversion efficiency.......................................................................................................... 89
8.4.1
Understanding Pneumatic Capture ..................................................................................... 89
8.4.2
Wave to pneumatic efficiency ............................................................................................. 89
8.4.3
Pneumatic to mechanical.................................................................................................... 92
8.4.4
Mechanical to electrical....................................................................................................... 93
8.4.5
Total device efficiency......................................................................................................... 93
8.5
Survivability......................................................................................................................... 95
8.5.1
Wave loading ...................................................................................................................... 95
8.5.2
Return Period...................................................................................................................... 95
8.5.3
Balance of Plant.................................................................................................................. 96
8.6
Reliability, operation and maintenance ............................................................................... 96
8.7
Prediction and modelling..................................................................................................... 97
8.8
9
10
Control of export power....................................................................................................... 97
Future Visions...................................................................................................................................... 99
9.1
Location .............................................................................................................................. 99
9.2
Optimum water depth.......................................................................................................... 99
9.2.1
Energy vs. Water Depth...................................................................................................... 99
9.2.2
Wave Loading versus Water Depth .................................................................................. 100
9.2.3
OWC cost versus water depth .......................................................................................... 101
9.2.4
Sea-bed Conditions .......................................................................................................... 102
9.2.5
Overall Economics of Power Capture against OWC CAPEX............................................ 103
9.3
Structure ........................................................................................................................... 103
9.3.1
Design description ............................................................................................................ 103
9.3.2
Foundation........................................................................................................................ 104
9.3.3
Device concept ................................................................................................................. 105
9.3.4
Scour Protection ............................................................................................................... 105
9.3.5
Construction...................................................................................................................... 105
9.3.6
Installation......................................................................................................................... 110
9.4
Turbine.............................................................................................................................. 110
9.4.1
Turbine sizing ................................................................................................................... 110
9.4.2
Blade profile...................................................................................................................... 111
9.5
Generator.......................................................................................................................... 111
9.5.1
Specification ..................................................................................................................... 111
9.6
Costing.............................................................................................................................. 111
9.6.1
Caisson............................................................................................................................. 111
9.6.2
Turbine.............................................................................................................................. 112
9.6.3
Generator.......................................................................................................................... 112
9.6.4
Grid connection................................................................................................................. 113
9.6.5
Operation and Maintenance.............................................................................................. 119
9.7
Costing Summary ............................................................................................................. 120
9.8
Risk................................................................................................................................... 121
9.8.1
Risk Schedule................................................................................................................... 121
Economics ......................................................................................................................................... 125
10.1
Economic models ............................................................................................................. 125
10.1.1
Common assumptions ...................................................................................................... 125
10.2
Sensitivity assessment...................................................................................................... 127
10.2.1
Upper bound / Lower bound sensitivity assessment ......................................................... 127
10.3
Improvement priorities / targets ........................................................................................ 130
10.3.1
Priorities............................................................................................................................ 130
10.3.2
Targets.............................................................................................................................. 130
10.4
Learning Rates ................................................................................................................. 131
10.4.1
Shoreline Targets ............................................................................................................. 132
10.4.2
Near-shore Targets........................................................................................................... 132
10.4.3
Port Kembla ...................................................................................................................... 133
Research & Development ................................................................................................................................ 134
11
12
Areas for Research and Development............................................................................................... 135
11.1
Wave Climate ................................................................................................................... 135
11.2
Wave Collector/Capture.................................................................................................... 135
11.3
Construction and Installation............................................................................................. 135
11.4
Turbine.............................................................................................................................. 136
11.5
Generator and Power Take Off......................................................................................... 137
11.6
Controls ............................................................................................................................ 137
11.7
Grid Connection................................................................................................................ 137
11.8
Power System Dynamics .................................................................................................. 137
11.9
Business Development and Partnerships ......................................................................... 138
Conclusions ....................................................................................................................................... 139
APPENDICES
Appendix A
References
Appendix B
OWC Construction Programmes
Appendix C
OWC Cost Estimates
Appendix D
Economic Model
Appendix E
Discount Rate Sensitivity
Appendix F
Improvement Variables Discount Rate Sensitivity
Appendix G
Peer Review Responses
Submission Reference 115214-00
Description Final Issue with minor corrections
Prepared by
Name Ian Webb, Chris Seaman,
Gordon Jackson
Reviewed by
Approved by
Adrian Fox
Gordon Jackson
Initials
Date
4 February 2005
The Carbon Trust
Marine Energy Challenge
Oscillating Water Column Wave Energy Converter Evaluation Report
Executive Summary
Oscillating Water Column (OWC) wave energy conversion devices comprise a chamber in which air is
compressed and rarefied by wave action. The energy transmitted to the air is converted to useful
power by passing it through a turbine. Several prototype scale OWCs have been constructed and
operated with varying degrees of success over the last 30 years. There are examples of shoreline,
near-shore and breakwater devices in a number of countries. The device with the greatest operational
history and evaluation work in the public domain is the shoreline LIMPET device on the island of Islay,
Western Scotland.
This report considers the current state-of-the-art in OWC deployment as a basis for establishing the
future potential and economics of this technology.
State-of-the-art Review and Market Assessment
An assessment of the state-of-the-art in the design, construction and operation of near-shore and
shoreline OWC wave energy devices was first conducted. The state-of-the-art review, together with an
assessment of the potential range of applicability of OWC technology around the UK coastline, was
used to identify the most promising conditions in which to develop OWCs.
All presently operational OWCs (December 2004) have been constructed in concrete and have relied
on its weight (with ballast when deployed near-shore) to ensure foundation stability. The concrete
devices have had fairly regular geometry and have not sought to focus the wave energy. One
unsuccessful OWC and a new facility being constructed in Australia have chosen to construct in steel
and have aimed to minimise the structure size to both reduce quantities and survival wave loading
whilst aiming to capture the maximum possible wave energy through incorporating guides that are
wider than the air collector chamber.
All the structures have been bespoke designs for particular locations. The selected geometric form of
the collector chamber is considered to depend on the wave climate in each location. The greater
prevalence of larger breaking or near-breaking waves at the shallow North East Atlantic shoreline sites
requires a sloping chamber for greatest efficiency. Deeper water devices in less severe metocean
conditions can have near vertical chambers.
The construction experience of the shoreline devices has been poor with extended construction
schedules and difficulties in providing adequate temporary works to allow safe year-round working.
This has not been the case with near-shore and breakwater devices that have largely been
constructed off-site.
A number of prototype OWCs have had too great an installed power capacity compared to the actual
wave resource encountered. In some cases, significant modification has been undertaken, or is
planned, to match the power train to the actual resource encountered. In one typical example
operational average wave-to-wire efficiencies have been consistently below 10% despite initial
estimates of greater than 30% efficiency. These difficulties highlight the need to match the power train
and OWC so that these experiences are not replicated on future devices.
The UK coastline has been evaluated with respect to wave resource, sea-bed slope, tidal range, water
depth and proximity of suitable grid connection points. A matrix method was adopted to rank regions
of the coastline. The most promising areas for OWCs are in Northern and Western Scotland, the Outer
Hebrides, the Shetland and Orkney Islands and the north coast of SW England. A potential for 141km
frontage of near-shore OWCs and 8km of shoreline OWCs was identified although the assessment of
the shoreline potential was fairly cursory within the confines of this study. The near-shore OWCs have
the potential to supply power to around 1.8m households (7.8TWh/annum).
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The Carbon Trust
Marine Energy Challenge
Oscillating Water Column Wave Energy Converter Evaluation Report
Scope for Improvement & Future Vision
The existing operational experience has been critically appraised with a view to identifying the scope
for improvement in OWCs. This included an examination of the potential and the constraints in
developing a larger market for shoreline devices and a brief review of the alternative shoreline OWC
technology of tunnelling. This led to the conclusion that, while significant improvements could be
made to shoreline devices, there are substantial barriers to developing a resource that could be
considered significant in terms of national power consumption. Shoreline devices are accordingly
considered most appropriate for serving smaller isolated communities such as on Islay.
Attention was then focused on the improvements that might be made on near-shore devices by
incorporating the lessons learnt from previous projects and by assessing, through analysis and related
experience, the effectiveness of a range of improvements in design, fabrication and installation of
OWCs. Such improvements would reap greater reward in view of the larger near-shore resource that
can be realised.
The key elements of improved economics are reduced structural quantities and survival loading
combined with the maximum possible wave energy capture.
Generic Future Vision – a near-shore concrete caisson OWC
A generic near-shore OWC was developed to take advantage of the following factors:
•
A greater available resource, with a more flexible choice of locations.
•
The ability to use a generic design rather than a site-specific design for each device.
•
The possibility for mass production
•
Little or no below-water working is required
•
The structure is built in a controlled environment and as such is less susceptible to adverse
weather conditions and programme delay.
•
Ability to tune the structure for a particular wave climate by varying the device depth and / or
orientation.
Concrete was chosen as the material of construction as it appeared marginally more cost-effective
than steel for UK application in a brief screening exercise. It should not be concluded from this that
steel should be discounted when examining future OWC potential.
The OWC would be fabricated offsite, towed and installed offshore. The device was configured for the
more prevalent rocky or sandy areas of the western UK where it was sited in a fairly energetic location
with an average energy flux of 34kW/m passing the 20m depth contour (a depth being promoted for
consistency of reporting between nearshore devices). A water depth of 9m was selected, following an
optimisation exercise that indicated that depths of 9 - 11m would yield the most economic fixed
structure design in relation to the wave power that could be collected. The OWC was sited where the
tidal amplitude was no greater than 2m, as greater tidal amplitudes had been shown to be detrimental
to the economics.
The generic near-shore OWC was assessed to perform as follows:
•
42% of the average wave flux at 20m contour over the maximum width of the OWC was
convertible to pneumatic power in the collector (or 59% at the collector)
•
65% of the pneumatic power was convertible to mechanical power (average)
•
91% of the mechanical power was convertible to electrical power.
This gives an overall wave-to-wire efficiency of 24.8% (or 34.8% using energy fluxes at the collector).
Developers have agreed that such an efficiency is achievable on prototype devices to be developed in
the near term.
Arup
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The Carbon Trust
Marine Energy Challenge
Oscillating Water Column Wave Energy Converter Evaluation Report
The suggested geometric form is shown below:
Power Production Cost
Cost estimates were prepared for a shoreline device, one near-shore unit 82.5m wide, and 10 such
units. The expected benefits of applying mass production techniques and the learning curve of
repeatability were then applied to the 10 near-shore units to arrive at a capital cost for economic
modelling. All modelling was based on UK costs and productivities. Lower construction costs can be
obtained in some parts of the European Union (and elsewhere in the world) and should be considered
in the future scope for improvement.
Device
Generic
Shoreline
1 x Near-shore
10 x Nearshore
10 x Nearshore (with
productivity
benefits)
Total frontal width (m)
20
82.5
825
825
Rated power (kW)
500
2,250
22,500
22,500
1,480
6,100
61,000
61,000
£1,762,605
£4,884,376
£40,088,497
£33,278,908
£310,000
£1,795,000
£17,250,000
£12,750,000
£2,072,605
£6,679,376
£57,338,497
£46,028,908
85 %
73 %
70 %
72 %
£4,145,210
£2,968,612
£2,548,378
£2,045,729
£103,630
£80,962
£69,601
£55,793
Annual power capture
(no outage)
(MWhr/year)
Structure cost (incl.
temp. works /
installation)
Balance-of-plant and
grid connection cost
Total Cost
Structure cost as %
total cost
Cost / MW rated
capacity
Cost / m collector
Power Production
Cost (10% discount
rate) p/kWh
17.5
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The Carbon Trust
Marine Energy Challenge
Oscillating Water Column Wave Energy Converter Evaluation Report
Economic models were established for shoreline and near-shore devices to investigate the likely
p/kWh power output cost, and to test the sensitivity of this cost to the main design elements of an
OWC development.
Improvements with the biggest impact on the power production cost, requiring the least effort to
implement, were then identified.
Sensitivity analysis showed that increased capture efficiency had the biggest impact on the
development’s economic viability. The wave resource at the development site was the next most
critical factor.
The effect of development size was also investigated in relation to power output cost. It was
concluded that, for smaller developments up to 2 MW, shoreline OWCs are preferable. Near-shore
devices are less viable for small-scale development due to the dry dock and installation vessel
mobilization costs (but can be made more viable if construction facility costs can be minimised). The
facility and vessel mobilisation costs, when shared on a multi-device development, have a reduced
impact on the final cost and continue to reduce with development size.
Grid connection costs are site-specific, however, significant capacity was found to be available at a
cost of around £120,000/MW. As grid connection is a mature technology, improvements were not
explored in this area.
Viability of OWCs
In order to illustrate the future viability of this technology in the medium term, a power production cost
target was chosen for each device type, and the viability of the improvements required to reach the
target assessed.
Due to the more limited market resource and the lower CAPEX and power production cost for smaller
developments (less than 2 MW); shoreline OWCs would be more appropriate for remote power supply
to small communities. This could be as an alternative, or supplement to, small diesel generator power
production. As such a target power production cost of 8 to 10 p/kWh is considered reasonable.
Our current assessment of this technology suggests a power production cost of 17.5 p/kWh for a
500kW shoreline OWC (with a 10% discount rate). Reaching the 8 p/kWh target would require a
combination of capture efficiency being increased by 10% to 52%, turbine efficiency being increased
by 2.5% to 67.5%, generator efficiency being increased by 3% to 94%, and civil costs being reduced
by 50%. Such a reduction in civil costs would have to be achieved through a change in concept –
perhaps to the ‘roof only’ LIMPET proposed by Wavegen.
Near-shore development is better suited for power supply above 2 MW, with greater economies as the
project increases to the 20 MW scale. This type of development should be compared with the current
offshore wind technology, as such targeting a 5 p/kWh power production cost.
Our current assessment of this technology suggests a power production cost of 9.6 p/kWh (with a
10% discount rate) for a 22.5MW near-shore demonstration project. Reaching the 5 p/kWhr target
would require a combination of capture efficiency being increased by 13% to 55%, turbine efficiency
being increased by 2.5% to 67.5%, generator efficiency being increased by 3% to 94%, balance of
plant costs decreasing by 40% to £300/kW, civil costs reducing by 20% and the cost contingency
reducing from 10% to zero. The reduction in civil costs may be achieved by importing from a lowercost country of construction.
Achieving the Targets
An engineering assessment approach has been taken in this report whereby the potential for
refinement and development of sub-systems and components has been evaluated. An alternative
empirical approach, based on learning rates – i.e. the rate at which cost decreases with each doubling
of cumulative production - has been considered to assess how achievable these improvements are.
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The Carbon Trust
Marine Energy Challenge
Oscillating Water Column Wave Energy Converter Evaluation Report
Learning rates in renewable energy technologies have been assessed to be 10-30%. The rate of
progress achieved on Arup dry-built offshore concrete platforms was examined, as this form of
construction is directly comparable to the future vision for the civil construction of a near-shore OWC.
A learning rate of 7.3% was found, which reflects the fact that marine concrete design and
construction is a relatively mature field.
Since the relatively low rate of learning foreseen in the large civil works cost element will dominate
over the improvements in wave-to-wire efficiency and the balance of plant, a learning rate of 13% was
suggested as being applicable to OWC technology (compared to 18% for onshore wind).
For shoreline OWCs, if the learning rate of 13% is applied, the cost of power will decrease from
th
17.5p/kWh to 8p/kWh by the time the 50 unit has been constructed. This would amount to 12.5% of
the total market foreseen in the UK for shoreline OWCs, or a capital expenditure approaching £80m. It
is considered unlikely that the necessary financial support to allow this quantity of units to be
constructed will be readily secured by developers without assistance from government. The capital
cost is approaching twice the level of funding currently being made available through the Dti for all wet
renewable power demonstration projects. This suggests that efforts are best directed at researching
designs that may achieve substantial cost savings through a ‘concept shift’ such as Wavegen’s roofonly LIMPET.
For near-shore OWCs, 5p/kWh would be achieved when 250 number 82.5m long units had been
installed at a rated capacity of 562MW and a capital cost of approximately £1bn. These would
constitute 15% of the total market foreseen for near-shore OWCs around the UK. By comparison, this
is the size of five of the UK round 1 offshore windfarms for which some capital grant funding was
available. It is considered unlikely that this volume of construction would secure the level of financial
support to make it attractive to investors, especially considering that developments on this scale would
merely permit near-shore OWCs to reach the level of current power cost performance of offshore
wind. Instead, some further stimulus to test whether economics can be improved should be the first
step.
The target improvements, whilst considered achievable, only take OWCs to the stage of a first
medium-scale demonstration project. A greater range of improvements must be sought or a greater
rate of learning achieved to be sure that the industry can grow to the level where better economics
would be achieved. Increasing the learning rate to 18% would mean that 5p/kWh was achieved by the
th
production of the 100 near-shore unit and 2p/kWh if the full UK resource was developed.
Deployment of OWCs in other countries would, of course, mean that 2p/kWh was reached before the
whole UK resource had been exploited.
Research & Development Priorities
The research & development priorities identified to help deliver the improvements needed are:
•
To make wave climate data available for representative near-shore sites
•
To conduct parametric exercises using computational fluid dynamics (CFD) tools matched by
wave tank testing to distil out the fundamental principles of good collector design in order to
deliver the improved capture efficiency essential to improved economics
•
To conduct a further Marine Energy Challenge type exercise to brainstorm and prove construction
and installation methods that can show savings in the dominant civil works costs
•
Conduct research coordinated with breakwater and coastal engineers into the nature of impact
loading and structural response
•
Explore the potential for adopting reduced load and material factors in design recognizing the
consequences of failure of an OWC
•
Developing a renewable energy development contract that would aim to place risk with those best
able to carry it
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•
Comprehensive benchmarking of fixed-bladed turbine, guide vane, throttle valve and control
systems using CFD tools validated against large-scale prototype devices followed by investigation
into variable pitch blade control
•
Development of performance standards for defining the performance characteristics of OWC
installations to allow like-for-like comparisons
•
Assessment of the scale of sub-synchronous resonance in power systems to establish the nature
of this issue
•
Encouragement of the creation of strategic partnerships between technology developers and
potential volume manufacturers
•
Investment in the use of LIMPET as a proper test-bed for OWC technology
Conclusions
If OWCs are to have a long-term future, rate of progress must be improved. The Marine Energy
Challenge was devised as a way of pushing wet renewable technology forward and encouraging
faster learning. Hopefully, this report for the Challenge demonstrates that near-shore OWCs can,
within a reasonable timescale, reach a point where they can work within the current UK framework for
renewable power generation without needing additional government support. The improvements
reported here in performance and cost to reach this position do seem achievable. However, the OWC
industry will not deliver these improvements without a more coherent development approach and
appropriate financial assistance. The best use of any future funding would be to stimulate the rate of
learning that will deliver a similar progress rate to that of successful renewable energy technologies
such as onshore wind. It is believed that attention to the identified research and development
priorities will show whether the potential can be realised.
Shoreline OWC technology will never achieve as low a power price as near-shore OWCs.
Nonetheless, it can still play a role in serving isolated communities where there is power price
premium. Efforts commensurate with the scale of resource should continue, to help the shoreline
OWCs reduce costs and improve performance, so that they can be a viable niche power provider.
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Scope of Work & Report Structure
1.1
Background to the OWC Evaluation
The Marine Energy Challenge aimed to inform the Carbon Trust (CT) on the scope and potential for
marine energy technology. One important area that the CT wished to understand was shoreline
Oscillating Water Columns (OWCs). The CT wanted to form a view on the status and prospects for
such devices.
Shoreline OWCs are the most widely deployed wave energy technology, with several large-scale
prototype devices installed in the Azores (Portugal), China, India, Japan, Sweden and the UK. These
designs are broadly similar and comprise:
•
A shoreline superstructure, incorporating a wave ‘collector’ (with an opening to sea and
containing a water column).
•
A turbine-generator power take-off system.
•
A flow control system
The design and construction of OWCs depends significantly on their intended location, and many
designs must be tailored to site-specific conditions. The market size for a particular device
configuration is highly sensitive to the adaptability of that design to local site environments (and costs),
including the depth of water at entry to the collector and method of superstructure construction.
The term shoreline refers to OWCs that are constructed on land at the shore or in cliffs. The
evaluation work was extended to incorporate bottom-founded OWCs denoted as near-shore devices
in this evaluation. Floating OWCs were being separately evaluated by the Carbon Trust.
1.2
First Stage Evaluation
1.2.1
State-of-the-art Review
Numerous studies have been made of OWC designs. In fact, the device category may be the most
reported of all wave energy systems. A literature review was required containing a concise summary
of:
•
The main characteristics of existing devices, in terms of their conceptual designs, geometric
forms, main components and performance were to be identified. This was to illustrate the extent
of development to date and demonstrate the state-of-the-art. Where key design decisions had
been justified in the literature (e.g. with regard to selection of materials), these were to be noted,
together with important design assumptions and working principles.
•
The prevailing technical uncertainties, risks, and associated opportunities for either cost-reduction
or performance improvement, from the perspectives of various authors were to be summarised.
The second part of the literature review was to enable CT to form a view on the potential markets for
shoreline OWCs. The design, performance and cost of OWCs are sensitive to the situations in which
they are sited; e.g. against a cliff, or as part of a breakwater; in shallow or deep water. The ability of an
OWC design to be installed in different locations affects its scope for replication and consequently the
size of its potential market.
A number of applications was to be identified for shoreline OWCs in terms of the primary siting
requirements e.g. water depth, wave climate, proximity to electrical demand centres or infrastructure,
etc. These were to be in the applications for which the largest markets were thought to exist. The
contractor was required to estimate the overall market size, through an assessment of resource and
primary constraints to exploiting the resources. Detailed market analysis was not required.
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This market review served two purposes:
•
A first indication to the CT of the potential markets for OWCs.
•
A ‘base-case’ for use in the scope for improvement assessment discussed below. This was to
define the applications for which most OWCs should be designed.
1.2.2
Market Assessment
The market size for a particular OWC device configuration is highly sensitive to the adaptability of that
design to local site environments, including the depth of water at entry to the collector and method of
superstructure construction.
The UK coastline has been evaluated with respect to wave resource, sea-bed slope, tidal range, water
depth and proximity of population centres. A matrix method was adopted to rank regions of the
coastline. The most promising areas for OWC developments were identified.
This phase of the study assessed the potential scope for improvement measured against the existing
device technology. It went on to consider the most promising near-shore areas to develop a base
case OWC with this design evaluated on the basis of cost of energy and market size. This included
an estimate of both the capital cost and the performance of the design.
1.2.3
Report Structure
The results of this first stage evaluation are included as chapters 3 to 6 following the divider entitled
‘State-of-the-art Review and Market Assessment’.
1.3
Second Stage Evaluation
1.3.1
Scope for improvement
With respect to the state-of-art of this technology identified in the first stage, this second stage of the
evaluation identified the potential areas for improvement to shoreline OWC designs, in terms of both
installed costs and expected performance.
The study considered:
•
Site –specific tailoring of designs
•
Turbine choice and specification
•
Incorporation of devices into existing structures
•
Conversion efficiency
•
Mass production / manufacture
•
Materials selection
•
Survivability
•
Reliability, operation and maintenance
•
Economic design life
•
Prediction and modelling
•
Resource assessment
•
Control of export power
•
Device construction methods
For each area identified, the potential to lower the cost of energy was to be estimated with reasoned
assumptions. ‘Knock-on’ effects to other design attributes were also to be stated, so as to define the
relationships between various design parameters.
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1.3.2
Future Visions
Based on the list of areas offering scope for improvement, a future vision of this type of device was to
be developed and an economic model established. This was to be used to explore the potential
improvements in terms of cost of energy and market size by the combination of several design
changes. These were to be:
•
The current state-of-the-art
•
An optimistic case
•
A base case (being the most likely)
•
A pessimistic case
The scenarios were to be defined by –
•
The types of improvements assumed for each
•
The overall effects on cost of energy
•
The assumed relative contribution of each area to the overall effect
These scenarios were to be compared on the basis of cost of energy and market size. These were to
include, where appropriate, estimates of both the capital cost and the performance of the designs.
Where there was a trade-off between cost of energy and market size (estimated in stage 1) this was to
be explained.
1.3.3
Shoreline against near-shore device installation
The stage 1 evaluation concluded that a near-shore device offered a greater potential resource than a
shoreline device. As such the focus of the stage 2 evaluation was to be directed towards this
technology. However, this report continued to study the shoreline OWC concept as a base line
comparison to near-shore OWCs.
1.3.4
Sensitivities and prioritisation
Using the scenarios, the sensitivity of cost of energy to different design changes was to be explored.
A well-reasoned, technically-rigorous summary was required of where the greatest improvements
were most likely, and consequently efforts for cost reduction/performance improvement should be
focussed (Stage 3). This was a key deliverable of the work.
1.3.5
Report Structure
The results of this second stage evaluation are included as chapters 7 to 10 following the divider
entitled ‘Scope for Improvement & Future Vision’.
1.4
Third Stage Evaluation
1.4.1
Recommendations for development work
For each area identified as a priority for improvement and therefore further work, well-considered
proposals for reducing cost/improving performance were to be made and justified. Estimates of costof-energy benefits and market size were to made with reference to the development scenarios
previously outlined in stage 1. Amongst the recommendations, some elements of technology transfer
from other industry sectors were expected, and suggestions were to be given on how to motivate this.
1.4.2
Report Structure
The results of this third stage evaluation are included as chapters 11and 12 following the divider
entitled ‘Research and Development’.
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Developer input and peer review
The views of people and organisations involved in the design, construction and operation of OWCs
were to be sought since they had not been directly involved in the Marine Energy Challenge. These
views, where accepted, are incorporated throughout the report and in the final conclusions. The
comments of the peer reviewers and the responses to their views are recorded in Appendix G.
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Glossary of Terms
2.1
Oscillating Water Column Terminology
AIR CHAMBER OR
PLENUM
COLLECTOR
OR
/RAREFIED
LIP
Figure 2.1
2.2
An OWC (Source: The Green Trust)
Terminology
Capture width – The maximum linear dimension of the collector chamber, or collector chamber plus
guide walls where present, perpendicular to the approaching wave front
Capture efficiency – The proportion of wave energy captured in the compression and rarefaction of
the air in the collector out of the total incident wave energy flux (commonly taken at the 20m contour)
crossing a line equal to the capture width
Pneumatic efficiency – Sometimes used as an alternative to capture efficiency
Mechanical efficiency – the proportion of pneumatic power that is converted to mechanical power in
the turbine
Turbine efficiency – Sometimes used as an alternative to mechanical efficiency
Electrical efficiency – the proportion of power that is converted from mechanical power to electrical
power in the generator
Wave-to-wire efficiency – the product of capture efficiency, mechanical efficiency and electrical
efficiency
Availability – the proportion of the year that the OWC is available to generate power ie 8760
hours/year less planned maintenance
Capacity factor – Describes how well the generator capacity is utilised. It is the actual electrical
energy yield as a percentage of the maximum theoretical or ideal yield.
Shoreline – An OWC is classified shoreline if connected to the shore above mean high water. It also
includes OWCs that use tunnelling technology
Nearshore – A device is classified as nearshore if it is not connected to the shore. The OWC is fixed
to the sea-bed for the purposes of this review. Access will normally be by boat.
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Breakwater – a structure that aims to provide sheltered conditions in its lee
Caisson – a monolithic structure usually in steel or concrete and usually divided into compartments,
deployed in the marine environment
Power train – the combination of a turbine, any gearbox or flywheel and the generator
Balance of plant – the power conversion elements of an OWC that are not civil works, namely the
turbine, generator and controls
Market – the quantity of power that can credibly be produced using the OWC technology, recognizing
limitations in grid capacity, acceptability of the technology and existence of suitable conditions for its
deployment
Cost of energy – the calculated cost at which the net present value for the project become zero ie the
cost beyond which a profit can be returned from the investment
Equity discount rate – the rate at which costs are depreciated in future years to reflect the time value
of money
Out-turn costs – the entire capital costs of developing an OWC
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State-of-the-art Review & Market Assessment
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Existing Shoreline OWCs
This section provides a summary of the existing more recent OWC projects. The LIMPET and Pico
projects are shoreline mounted systems, the OSPREY and Port Kembla are examples of near shore
OWCs, with the Trivandrum and Sakata projects examples of breakwater devices. These projects
have been key to contributing to the current understanding of OWC technology, and in many cases
serve either as on going test beds or are used for data collection. While there are many other OWCs
throughout the world including many powering navigational buoys, these are considered the most
important in defining the current state-of-the-art.
3.1
LIMPET 500 [3,5,6,27,56]
3.1.1
Concept
The LIMPET 500 (Land Installed Marine Pneumatic Energy Transformer – 500kW) is an OWC built
into the shoreline near Portnahaven on the island of Islay. Construction began at the end of 1998,
with the civil structural works substantially complete by the end of August 2000. The plant was
commissioned at the end of 2000. It comprises an insitu concrete collector, with the generation unit
installed immediately behind the rear collector wall. Self- rectifying Wells turbines mounted directly on
the generator shaft have been used, enclosed by a small turbine hall. The process air exits the
turbine hall via an acoustic attenuator, installed behind the turbine hall.
Foam from a spent breaker cascades down the face of the
LIMPET. (Wavegen)
The LIMPET structure was built in-situ into a rock cliff. This was done by excavating from the cliff top
and leaving a temporary rock bund on the seaward side of the construction site. The collector
chamber was then constructed and plant installed prior to removal of the bund. This left the device
slightly set back from the original shoreline.
3.1.2
Geometry
The device consists of 3 chambers 6 x 6m built at an angle of 40 degrees to the horizontal.
/RAREFIED
3D View of LIMPET Chamber (The Green Trust)
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The inclined column has been shown to offer an easier path for water ingress and egress resulting in
less turbulence and lower energy loss. This is particularly true at the shoreline where shallow water
effects are increasing the surge motions relative to heave. (The improved performance of an inclined
water column in comparison to a vertical column was established through tank testing).
The width of the device was split into 3 chambers to reduce the risk of transverse wave excitation
reducing the energy capture performance.
3.1.3
Design Parameters (Assumptions and working principles)
The following table summarizes the principal design characteristics of this project.
Geometry
Wave / Sea Characteristics
Total Width
No. of Chambers + Width
Chamber Angle
Lip depth below MWL
Total Height above MWL
21.0m
3 x 6.0m
40o to horiz.
2.3m
12.8m
Depth at lip
6.0m
Design Wave Height, Hs max
4.4m
Design Wave Period
13.4s
Return period
(n/a waves depth limited)
Max design wave pressure
600kN/m2
Turbine
Generator
Turbine Dia.
2.6m
Nominal operating speed
1050rpm
No. of Turbines
2
Arrangement
In line, contra rotating
Type
F3GTS 400 G8G
Power at Generator Terminals
2 x 250kW
Duty Type
continuous inverter driven
Rotor Type
Wound rotor
(but used with rotors shorted)
Rated Voltage
400V
Rated Speed
1016rpm
Max test speed
1500rpm
Inertia
Generator 11.5kgm2
2
Load 1300kgm
Blade Form
Number of blades
Blade Chord
Hub to tip ratio
NACA 12
7
320mm
0.62
A wave climate of approximately 20kW/m was initially estimated during the concept stage to exist at
the collector. Model tests during the design period reduced the estimated resource to 17.9kW/m at the
OWC lip. Values measured in service are only 12kW/m at the lip.
3.1.4
3.1.4.1
Project Experience
Turbines
LIMPET uses two back-to-back, horizontal axis, contra-rotating Wells turbines. This configuration was
believed, based on model test results, to give better performance than the equivalent single stage
machine, due to the nozzle effect of the first stage on the second stage. However, in practice, the
cyclical motion does not allow the system to stabilise and energy recovery is minimal.
Two inlet valves are used in series for isolation (including emergency shut down) and control, although
it is indicated that only one of these would probably be sufficient when technology is proven through
operational experience.
An actuated and counterweighted butterfly valve is used as the primary means of flow modulation and
also serves as an emergency stop valve. Modulation of the valve gives flow throttling, thereby
smoothing the velocity profile over time and improving performance of the turbine. Problems have
been encountered with ensuring free movement of the valve spindle and with flow induced vibration,
although both of these are claimed to have been overcome.
An array of radial vanes is also provided for isolation purposes downstream from the butterfly valve.
The arrangement of the vanes, and indeed of the air inlet structure, appears to be similar to the nose
cone and inlet guide vanes (IGVs) on gas turbine compressors. Modulation of the radial vanes to
achieve control is not currently possible due to lack of instrumentation, but steps are being taken to
address this. Actuator power needed for radial vanes may be lower than for the butterfly valve due to
lower mass and inertia.
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A general conclusion made by the project report [5] was that to achieve high turbine efficiency will
require development of variable pitch machines with an appropriate control strategy. It may be
possible to combine of the stop valve radial vanes with inlet vanes for control of incidence angle onto
the moving blades.
3.1.4.2
Control and Electrical Aspects
The project has produced a number of useful lessons; among these, are not only the obvious
recognition of the need for protection of the turbine/generator from over-speed events, but a useful list
of the measures taken to ensure this under a wide set of circumstances.
The literature [5] refers to the ‘layered’ measures taken in the automatic control and protection
software provided, which ensures that the plant comes to no harm. These layers consist of a series of
progressively more positive actions taken by the protection systems (mechanical and electrical),
ending up with full turbine/generator trip, and isolation of the plant from both the grid and from the
power source (the OWC).
Protection measures for the electronic control systems (PLCs) themselves are listed, and include the
provision of an Uninterruptible Power Supply (UPS), and the designing-in at an early stage of fail-safe
measures for the main electro-mechanical protection and isolating systems. Thus a butterfly valve
used to control the air inlet to the turbine is operated by a solenoid valve. If power is lost, the solenoid
is de-energised and the valve closes to isolate the turbine on the inlet side. Similarly, the variable
vane control actuator is pneumatically operated. If the actuation signal is lost, then the piston cylinder
inlet is opened to atmosphere and the reservoir dumps actuation air to the closing side of the piston.
In both cases, loss of the main controller power system will result in the fail-safe shutdown of the
installation
The literature [5] also gives extensive information on the instrumentation systems installed to provide
an internal health-check (condition monitoring) of the rotating and actuator parts, and to provide
information for the protection and automatic control systems themselves.
The original induction machine drive design used resistors to dissipate rotor energy and provide
variable speed operation. This would, as with the 75kW prototype, give rise to poor efficiencies.
Replacement with an industrial standard drive system should have given much better electrical
efficiencies than those reported. If the figure of 32% electrical efficiency is correct than it is probably
due to a combination of rotor resistance losses, over rating of the machine and operation in low sea
states.
The lessons gained from LIMPET are valuable, and provide insight into some of the problems
encountered in an industrial size OWC generator, and the solutions adopted in overcoming them.
There is an active programme at LIMPET designed to address the issues raised.
3.1.4.3
Problems
The following problems have been reported has having a major detrimental effect on the overall
project programme, cost and performance.
•
Feasibility study – a lack of detailed site data prior to the final device location being chosen
led to a lower than predicted wave energy resource due to sea bed friction. In addition the
benefits of ‘harbour wall focus’ do not occur in shallow water. In such shallow water a parallel
gully was found to be detrimental to the overall performance of the collector.
•
Civil construction planning – a lack of flexibility in resource allocation and programme for
activities to be tuned around weather windows.
•
Civil construction method – insufficient protection of the work area by the temporary bund.
•
Civil construction method – Removal of temporary bund led to a scatter of rock fragments
and water chamber blockage.
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•
Operational - Deluge of the turbine hall from overtopping, later improved through installation
of a marine standard doorway, although one wooden door is still present on the turbine room.
•
Efficiency - The acoustic attenuation was found to reduce the efficiency of the turbine. This
was due to an unbalanced airflow through the turbine during the inlet stroke, achieving only
75% of the outflow.
•
Efficiency – At low generator speed the percentage parasitic losses are very high.
3.1.4.4
Successes
The following have been reported as particular successes on this project.
•
Low maintenance costs.
•
Remote control by modem
•
Grid connection
•
Pre-cast elements for turbine hall
•
Use as a test-bed has allowed operational experience to be matched to engineering models
3.1.4.5
Performance
A summary of the device estimated and operational efficiencies is listed below.
Type of energy
conversion
Structure / device
Wave to pneumatic
(measured at lip)
Oscillating water
column
Pneumatic to mechanical
Wells turbine
Mechanical to electrical
Generator
Overall efficiency
Estimated
Efficiency%
Measured
Efficiency%
80
64
60 (average)
40 (average)
100 *
32
48 (average)
8
* Note that Wavegen now accept that this figure was unrealistically high and should have been 90%.
While the original estimate of wave resource was 20kW/m the actual measured resource at the OWC
lip has only been 12kW/m. This equates to an initial estimate of 202kW output against a measured
output of only 21kW.
A crew works on LIMPET's turbine (Wavegen)
3.1.4.6
Reported Conclusions
The project participants have reported a number of findings that could have an impact on any future
OWC design. These include:
•
The need to optimise the device design for a particular water depth.
•
Measurement of applied loads proved to be less than extreme used in design by a third, with
wave slam and peak pressure not co-existent.
•
Internal wave slam has been less than predicted.
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3.2
Pico [4,26,45,46,67,68]
3.2.1
Concept
The Pico project is a 400kW OWC built into a small natural gully near Porto Cachorro on the island of
Pico in the Azores. Construction of the civil structural works began in 1996 and was completed in the
summer of 1998. The plant was first commissioned at the end of 1999, however flood damage to the
electrical and control equipment led to a further year delay. The plant comprises an insitu concrete
collector, with the back wall stepped and the generation unit installed immediately behind the upper
constriction of the collector wall. The plant is equipped with a horizontal-axis Wells turbine-generator
set enclosed by a small turbine hall.
Front view of Pico OWC (Maretec)
3.2.2
Geometry
Elevation of the chamber design for the PICO plant (IST 1996)
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Rear view of Pico OWC (Maretec)
3.2.3
Design Parameters (Assumptions and working principles)
The following table summarizes the principal design characteristics of this project.
Geometry
Wave / Sea Characteristics
Total Width
No. of Chambers + Width
Chamber Angle
Lip depth below MWL
Total Height above MWL
12.0m
1 x 12.0m
60o to horiz.
2.5m
15.0m
Depth
8m at lip, 8 – 10m approach
Design Wave Height
7.5m
Design Wave Period
12.0s
Return period
Max design wave load
700kN/m2
Turbine
Generator
Turbine Dia.
2.3m
Nominal operating speed
750 – 1500rpm
No. of Turbines
1
Arrangement
Horiz. axis
Blade Form
3D symmetrical NACA15
at root and NACA12 at tip
Number of blades
8
Blade Chord
375mm
Hub to tip ratio
0.59
Type
Induction, Wound Rotor, Kramer
Power at Generator Terminals,
400kW
Duty Type
50Hz, 8 Pole
Rotor Type
Wound, slip rings
Rated Voltage
400V
Rated Speed
Max test speed
Inertia
1400 RPM
1500 RPM
600kgm2
From offshore measurement with wave-rider buoys, and close shore measurement using ultrasonic
probes a wave resource of 13.4 kW/m at the OWC lip was determined.
3.2.4
3.2.4.1
Project Experience
Problems
The following problems have been reported has having a major detrimental effect on the overall
project programme, cost and performance
•
A new access road was required to the construction site.
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•
The temporary cofferdam, erected to protect the construction works was badly damaged
several times.
•
Underwater work was particularly diificult.
•
Flooding of the control room both during installation and commissioning caused a major
programme extension (The control room has subsequently been moved).
•
Little infrastructure on the island
•
Limited choice of contractor
3.2.4.2
Performance
The rated power of the Pico plant is 400 kW, which is estimated to deliver around 0.5 GWh per year.
A summary of the device estimated and operational efficiencies is listed below.
Type of energy
conversion
Structure / device
Wave to pneumatic
Oscillating water
column
(measured at lip)
Pneumatic to
mechanical
Horizontal axis Wells
turbine
Mechanical to electrical
Wound rotor, induction
Generator
Overall efficiency
Estimated Efficiency
Measured Efficiency
%
%
NA
NA
75 (peak)
NA
NA
NA
35 (average)
NA
The plant is equipped with a single horizontal axis Wells turbine with fixed pitch blades and guide
vanes. The plant is designed to have a controlled bypass relief valve located on the roof of the
structure, although it is believed not to have been installed to date. (Reference report was dated
December 2000 [26]). The bypass relief valve would be expected to enhance the performance under
heavy sea conditions by limiting the air velocity through the turbine and thus preventing stall. However,
it is important that the power consumed in operating the valve does not outweigh the benefits gained
from increased power capture in the turbine.
The assumed turbine peak efficiency of 75% is that obtained in a steady flow. The time-averaged
efficiency in a bi-directional flow will clearly be less than this at around 50%.
A second horizontal axis machine with variable blade pitch, designed under a JOULE project, is
planned for installation alongside the fixed pitch machine.
In general horizontal axis machines should be easier for maintenance than vertical axis machines. The
thrust bearing loading will be reduced (not required to support the machine weight) but the thrust
bearing will need to be designed for loading in both directions to allow for bi-directional air flow.
A second JOULE project which started in 1999 sought to optimise OWC control and blade geometry.
3.2.4.3
Reported Conclusions
The project participants have reported a number of findings that could have an impact on any future
OWC design. These include:
•
The use contractors experienced in working in marine environments is crucial to the success
of the civil element of the works
•
Underwater work should be minimised.
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•
Two air valves were installed in series with the turbine to enhance safety. One adequately
designed valve would allow a cost reduction.
•
A by-pass valve control of the turbine flow rates is a relatively inexpensive way of increasing
efficiency of a conventional fixed pitch Wells turbine.
•
Variable speed generator enables the turbine to respond efficiently to a to a wide range of
sea states, and also allows a temporary storage of excess available energy (by fly wheel
effect) with a short term smoothing effect on the electrical power supplied to the grid. The
wound rotor induction machine with Kramer variable speed drive allows variable speed
operation.
•
A study of the performance of various construction materials is on going.
The plant developer reported ‘Considerably more than half of the total cost of the Pico plant concerned
the civil construction. Recent studies carried out in Portugal have shown that the cost of the concrete
structure of a commercial OWC of similar size (and in a less remote site) can be reduced by a factor of
more than three.’
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3.3
OSPREY [10]
3.3.1
Concept
The OSPREY (Ocean Swell Powered Renewable EnergY) is a near-shore device designed to operate
in a water depth up to 15m and up to 1km offshore. OSPREY 1 was the first variation of this 2 MW
concept. It was constructed in 1995 to be installed 100m off the coast at Dounreay in the North of
Scotland, and comprised a rectangular steel collector chamber with trapezoidal steel ballast tanks
fixed either side for gravity anchoring of the device. Mounted on top of the collector chamber was the
power module containing the turbine- generators (mounted vertically) and the control equipment.
However during the installation of the 750 tonne structure, a large wave smashed open the ballast
tanks and the device had to be abandoned.
The OSPREY (Universitat Leipzig including subsequent images)
Wavegen has re-designed this device, known as the OSPREY 2000, as a composite steel and
concrete unit. This led to the announcement in 1998 that Wavegen had gained backing from the Irish
Government (through its Alternative Energy Requirement III). The tender would have resulted in a 15year power purchase agreement, however funding was not forthcoming and the project is not
considered likely to proceed.
The following sections comment on the design and technology associated with the OSPREY 1 project.
3.3.2
Geometry (OSPREY 1)
The collector chamber is 20m wide and is fabricated using a double skinned, composite construction,
comprising panels each containing a layer of concrete sandwiched between two sheets of steel.
Each ballast tank is a maximum of 20m high and 44m long, manufactured with stiffened 10mm thick
plate. The bottom of the tanks was covered with a layer of concrete to provide trim and additional
stiffness during tow out.
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3.3.3
Design Parameters (Assumptions and working principles)
The following table summarizes the principal design characteristics of this project.
Geometry
Total Collector Width
No. of Chambers + Width
Chamber Angle
Lip depth below MWL
Total Height above MWL
(to collector roof)
Turbine
Wave / Sea Characteristics
20.0m
1 x 20.0m
Curved
3.8m
5.24m
Turbine Dia.
3m
Nominal operating speed
800-1400rpm
No. of Turbines
4
Arrangement
Back-to-back pairs
Blade Form
NACA12
Number of blades
9/11
(different to minimise effects of blade passing)
Blade Chord
444/363mm
Hub to tip ratio
0.62
Depth
14.5m
Design Wave Height, Hs
8.6m
Design Wave Period, tz
10.9sec
Return period (n/a – depth limited waves)
Max design wave load
Generator
Type
Induction / Cage
Power at Generator Terminals
4 x 500kW
Duty Type
continuous inverter driven
Rotor Type
Cage
Rated Voltage
600V AC
Rated Speed
1000 rpm
Max test speed
Inertia
1500rpm
1500kgm2 per rotor
A design wave climate of approximately 30kW/m was estimated to exist at the OWC lip. The climate at
the test site was 12kW/m but the extreme conditions were typical of more energetic sites.
3.3.4
Project Experience & Reported Conclusions
No installed experience and performance monitoring exists for this project as, after it was towed to its
station, it was destroyed by the tail end of hurricane Felix before it could be secured to the sea-bed.
An extreme wave smashed open the structure's ballast tanks. Although OSPREY's builders had time
to rescue valuable equipment, the sea eventually tore the device apart.
The project participants have reported a number of findings that could have an impact on any future
OWC design. These include:
•
Part of the problem was the inability to ballast the device quickly enough to beat the weather,
before 7,000 tonnes of sand-ballast had been pumped in. Any future device would need to
look at the weather exposure risk incurred between installing and ballasting the structure.
•
Towing the structure to the site with neutral ballasting. This would stabilise the structure
during tow out and could assist during the placement process.
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•
3.3.5
Cellular construction such that localised damage would only lead to flooding a small volume
of the structure.
Performance
A summary of the device’s estimated average efficiencies is listed below.
Type of energy
conversion
Structure / device
Wave to pneumatic
Oscillating water
column
(measured at lip)
Estimated Efficiency
Measured Efficiency
%
%
115 *
NA
Pneumatic to
mechanical
Wells turbine
70
NA
Mechanical to electrical
Slip-ring induction
generator
80
NA
65 (average)
NA
Overall efficiency
* The wave-to-pneumatic efficiency exceeds one because the effective capture width of the device is
greater than the collector width. The mechanical and electrical efficiencies are acknowledged to be
high.
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3.4
Port Kembla, NSW, Australia [47,70,71]
3.4.1
Concept
This 300kW wave energy demonstration device is a near-shore OWC to be installed at Port Kembla,
part of the Illawarra Region, approximately 100km south of Sydney, Australia. It will be installed 200m
off Port Kembla Harbour breakwater with taut mooring lines and legs extending to the seabed. The
structure is currently being fabricated to the design produced by Energetech and JPKenny Ltd.
An alternative bi-directional turbine to the Wells turbine, the Denniss-Auld turbine will be installed in
the plant, which is planned to be commissioned by the end of 2004.
(Source: Energetech)
The device is intended as a demonstrator project and has been designed for a three-year deployment
period. Energetech has plans for future devices to follow this demonstrator.
3.4.2
Geometry
The device is approximately 36m long and 35m wide, and uses a ‘parabolic wave focuser’ to focus all
the energy of a plane surface gravity wave crest to a single area. As the wave converges, the crest
height grows to a maximum in the focus area. At this focal point the collector chamber is positioned. It
extends to a depth below the minimum possible wave trough such that the oscillatory wave motion
causes a forward and backward airflow through the chamber.
To obtain the greatest capture efficiency, the device should be aligned so that the wave crests
propagate parallel to the axis of symmetry of the parabolic focuser. The greater the angle between the
axis of symmetry and the propagation direction, the lower the level of wave energy entering the device
(a cosӨ term).
3.4.3
Design Parameters (Assumptions and working principles)
The following table summarizes the principal design characteristics of this project.
Geometry
Wave / Sea Characteristics
Total Width
36.0m
No. of Chambers + Width
1 x 10m
Chamber Angle
vertical rear, approx 45 o front
Lip depth below MWL
3.0m
Total Height above MWL
12m
Depth
Design Wave Height
Design Wave Period
Return period
Max design wave load
Turbine
Turbine Dia.
Nominal operating speed
No. of Turbines
Arrangement
Blade Form
Number of blades
(variable pitch)
Blade Chord
Hub to tip ratio
Generator
Type
Squirrel cage/induction
Power at Generator Terminals
200/300kW
Duty Type
50Hz, 12-pole
Rotor Type
Cage
Rated Voltage
415 V
Rated Speed
500RPM
1.6m
500rpm
1
horizontal axis
21 blade
0.75
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Max test speed
Inertia
8-16m
7.0m
9 to 12s
100 year
750RPM
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A 9 kW/m wave has been assumed at the chamber mouth. The sea bottom should be reasonably flat,
so it does not disturb the wave direction, and deep enough so that when wave section enters the
parabola, the crests do not steepen and break as they grow. Energetech believes that the relatively
shallow slope of the sea-bed will ensure that the breaking waves are of the spilling type which would
reduce loads considerably compared to plunging breakers. Nevertheless, the structure has been
designed against a breaking wave.
A choppy scattered incoming wave will scatter some energy away from the focus. The energy loss
arising from the above-mentioned conditions can be minimized by choosing the appropriate focal
length of the parabola, so that the waves do not have the time or space to vary greatly.
3.4.4
Project Experience
This project is still in the development stage, and as such no installed project experience is available.
Based on the main characteristics of this device, an assessment of the inherent project risks is
included in section 3.9. Particular project specific challenges that must be overcome during the
execution phase include:
•
Providing sufficient strength to the parabolic reflector during extreme wave loading.
•
The complexity of the steelwork detailing and its resultant behaviour in fatigue
•
Keeping the anchors taut
•
Providing access for maintenance past the taut-moored anchoring system
•
Any requirement to adjust the legs in service
Problems may occur during the commissioning of both the turbine and control equipment due to the
advanced and untested nature of this plant. The use of this technology is, however, one of the
principal drivers of this project.
Progress Photos in October/November 2004 (www.energetech.com.au)
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3.4.5
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Performance
Energetech’s Port Kembla Wave Energy plant is expected to produce almost 1 GWh of electricity per
year.
A summary of the device’s estimated efficiencies is listed below.
Type of energy
conversion
Structure / device
Wave to pneumatic
Oscillating water
column
Pneumatic to
mechanical
Denniss-Auld turbine
Mechanical to electrical
Not known
Estimated Efficiency
Measured Efficiency
%
%
67
NA
54 average
NA
80 peak
Overall efficiency
90
NA
32.4 (average)
NA
The turbine has been tested by Lloyd’s Register and showed lower than expected losses from
windage and bearing friction, averaging around 3kW at a normal running speed of 500rpm.
Noise testing has also been conducted, with decibel readings taken at a number of locations one
metre from the turbine. The result was an average reading of 73dB.
3.4.5.1
The Denniss-Auld Turbine
The Energetech turbine is a variable pitch machine that uses a slower rotational speed with higher
torque. This they believe improves efficiency and reliability and reduces the need for maintenance.
The turbine uses a sensor system with a pressure transducer. This measures the pressure exerted on
the ocean floor by each wave as it approaches the capture chamber, or as it enters the chamber. The
transducer then sends a voltage signal proportional to the pressure, thus identifying the height,
duration and shape of each wave. The system is calibrated to ignore small-scale “noise” from
activating it.
The signal from the transducer is sent to a Programmable Logic Controller (PLC), which adjusts
various parameters to optimise turbine efficiency that are selected based on particular conditions and
energy content of each site.
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3.5
Vizhinjam OWC, Trivandrum, India [11,48]
3.5.1
Concept
A 150 kW pilot OWC was built onto the breakwater of the Vizhinjam Fisheries Harbour, near
Trivandrum in India in 1991. Located directly in front of a rubble-mound breakwater at Vizhinjam
harbour a cellular reinforced concrete caisson providing the geometric structure for the OWC was
floated into place. The installation comprises a 3,000 tonne concrete caisson topped by a steel tower.
The turbine is connected directly to the local grid
The structure was not integrated into the breakwater as no actual reconstruction or new build
breakwater projects were available at the time.
The hydrodynamic efficiency was enhanced by continuing the chamber side-walls towards the sea.
This (has been claimed) helps to smooth the different wave frequencies to an optimum range at the
collector chamber.
The following pictures show the OWC caisson situated in front of the harbour breakwater of
Vizhinjam.
Vizhinjam OWC caisson in front of rubble mound breakwater structure [Graw, 1999]
Caisson construction took place locally despite the fact that no heavy plant, wharves or docks were
available. A small construction dock was excavated on the beach inside the harbour and kept free of
water by means of pumps. The 3 m high base plate was then cast in this pit. The excavation was then
flooded, the sand between the pit and the harbour basin removed and the base plate towed out into
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the harbour. The side-walls of the caisson were then cast using sliding formwork (slipform), with a
steel gate placed across the seaward opening to prevented the ingress of water. The progress of the
work was organised in such a way, that the caisson floated horizontally throughout the entire process.
Upon completion of the casting, the caisson (now with a draft of 9.9 m) was towed to its final location,
lowered onto the prepared stone bed by flooding and subsequently filled with 3,000 tonnes of sand
ballast.
Vizhinjam OWC under construction
3.5.2
Geometry
The concrete caisson structure is 23.2m x 17.0m x 15.3m high. The chamber entrance is 10m wide by
6m high. The top of the CWC chamber is a double cubic curved shell in concrete 10 x 7.75 m at the
bottom, reducing to 2.0 m circle at the top and 3.0 m high to support the power module. Other
principal dimensions are shown on the following diagrams.
(Source: niot, India)
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(Source: niot, India)
3.5.3
Design Parameters (Assumptions and working principles)
The following table summarizes the principal design characteristics of this project.
Geometry
Total Width
No. of Chambers + Width
Chamber Angle
Lip depth below MWL
Total Height above MWL
Wave / Sea Characteristics
14.0m
1 x 10.0m
vertical
1.1m
14.5m
Depth
Design Wave Height
Design Wave Period
Return period
Max design wave load
Turbine
Generator
Turbine Dia.
Nominal operating speed
No. of Turbines
Arrangement
Blade Form
Number of blades
Blade Chord
Hub to tip ratio
Type
Power at Generator Terminals
Duty Type
Rotor Type
Rated Voltage
Rated Speed
Max test speed
Inertia
2
NACA 0021
8
380mm
0.6
10.2m
7m
8 to 12s
1000kN/m2
1:10 model of the OWC (TU Berlin)
A sea-bed slope of 1:50 exists in front of the caisson, with a wave resource of 20kW/m average
monsoon input wave. 5 to 10 kW/m is typically available outside monsoon periods.
During caisson design the wave forces were estimated by treating the caisson as a vertical wall
obstruction to the waves. The highest probably non-breaking lateral wave force was 12,000 kN. The
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highest probable breaking wave was estimated to be 7 m. The peak design force intensity used was
2
1000kN/m .
3.5.4
Costs
The costs for erection of the OWC were reported to be approximately £400,000 (1990 prices?), with
the electricity cost claimed to be 3.5 p/kWh, if the additional costs for the harbour construction are
taken into account. It has not been possible to verify these figures and they should not be relied
upon for future power cost prediction.
3.5.5
Performance
A summary of the device estimated and operational efficiencies is listed below.
Type of energy
conversion
Structure / device
Estimated Efficiency
Measured Efficiency
%
%
Wave to pneumatic
Oscillating water
column
50
Pneumatic to
mechanical
Wells turbine
25
Mechanical to electrical
Slip-ring induction
generator
50
Overall efficiency
6.3
The OWC has functioned successfully, providing data on the in-service performance of the Wells'
turbine and the mechanical and electrical plant. After the initial tests, the plant has been used to test
different types of generating equipment. This information has been used to produce new designs for a
breakwater comprising 10 OWC units with a total capacity of 1.1 MW.
Plant operational performance revealed several areas where improvements could be made. The
power module, designed for a peak power of 150 kWe, had large no-load losses and windage losses
amounting to almost 15 kW. When the instantaneous wave power was low even the no-load losses
could not be met by the turbine. Hence, the system would motor, drawing power from the grid. This
resulted in poor long-term efficiency of the wave power plant. Also, the squirrel cage induction
generator had a limited variation in speed up to eight per cent of synchronous speed. The vertical axis
assembly of the power module posed operational and maintenance problems. Based on the
experience, several improvements were incorporated in a new module installed in April 1996:
•
A tapered chord design was chosen for the turbine blades
•
two horizontal axis thrust opposing turbine rotors were coupled to an electrical generator on a
common shaft, each having peak installed capacity of 55 kW to allow one module to be run during
non-monsoon times and both during the peak season
•
A slip-ring induction electrical generator replaced the earlier squirrel cage machine
•
Better instrumentation and data acquisition system installed
•
A current controller was installed on the stator of the electrical machine in order to reduce the
‘peak to average ratio’ of electrical power exported to the grid.
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3.5.5.1
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Performance Analysis of the Second power module
Efficiencies as function of wave power-Second module (niot, India)
The efficiencies of the various element of the power take-off, as a function of incident wave power,
measured during its operation from April 1996 to July 1996 are shown above. A drop in efficiency for
increasing incident energy is evident.
As the output was most limited by the turbine, efforts were directed towards increasing the absolute
value of the efficiency and the dynamic range over which the efficiency remains high. Wells turbines
are claimed to have efficiencies of around 40% under oscillating flow conditions. Such efficiencies
were being obtained at an average pneumatic input of only about 20 kW or only 3 kW/m average wave
power. The dynamic range of the turbine was increased by increasing the external resistance
connected across the rotor in conjunction with a controller. The resistance value was chosen so that
the plant behaves as a high slip machine, thus enabling the turbine to freely ride the wave. When the
speed exceeded a preset upper limit, the external resistance was reduced. Some optimisation of
resistance values along with the speed settings resulted in improved overall efficiency, particularly with
higher incident wave power where stalling had previously occurred, as shown below.
Effect of increase of rotor resistance on plant efficiency (niot, India)
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3.6
Sakata, Japan
3.6.1
Concept
This project consists of a five chamber OWC, built as part of the harbour wall at Sakata Port in Japan.
The device became operational in 1989 but, after a test programme, only three air chambers were
used for energy production. A turbo-generator module of 60 kW has been installed and is being used
as a power generator unit for demonstration and monitoring purposes. This will possibly be replaced
by a larger turbine (130 kW) sometime in the future.
The project was developed by the Port and Harbour Research Institute, a division of the Ministry of
Transport.
Wave power generation system used in Sakata Port, Yamagata
The Japanese Sakata OWC was implemented as part of the port infrastructure, integrated into the
ports second northern breakwater as one of the reinforced concrete breakwater modules.
The construction method for this caisson was similar to the Vizhinjam OWC project with the bottom
section of the caisson cast first in a dry-dock. The completed base was then floated, before being
completed to its final height. The caisson was then towed to its required location, lowered onto the
prepared sea-bed and protected against scouring around the foot of the structure by placing rock or
tetrapods.
Sakata port is an industrial harbour and was able to provide all the necessary technical equipment for
the implementation of this project.
3.6.2
Geometry
The principal dimensions of each caisson were 20.0 m X 24.5 m X 27.0 m high, with an operating
water depth of approximately 18 m. Each caisson had 5 openings on its front surface facing the
predominant wave direction. Corresponding with the frontal openings, partition walls divided the
internal space into 5 chambers.
The front wall of the OWC caisson was inclined to 45°, designed to reduce the horizontal force and
causes a vertical force downwards, stabilising the caisson.
The following picture shows the OWC integrated into the northern breakwater during the construction
phase.
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Construction phase of Sakata caisson breakwater OWC [Graw, 1999]
Sakata breakwater caisson in heavy sea [Graw, 1999]
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3.6.3
Design Parameters (Assumptions and working principles)
The following table summarizes the principal design characteristics of this project.
Geometry
Wave / Sea Characteristics
Total Width
20.0m
No. of Chambers + Width
5 x 3m
Chamber Angle Wave chamber – vert, air 45 o
Lip depth below MWL
3.0m
Total Height above MWL
12.5m
Depth
Design Wave Height
Design Wave Period
Return period
Max design wave load
Turbine
Generator
Turbine Dia.
Nominal operating speed
No. of Turbines
Arrangement
Blade Form
Number of blades
Blade Chord
Hub to tip ratio
1.337m
16
18.0m
15.3m
14.5s
Type
Power at Generator Terminals
Duty Type
Rotor Type
Rated Voltage
Rated Speed
Max test speed
Inertia
60kW
200V AC
.
3.6.4
Performance
Information regarding the performance of this plant has not been located.
3.7
Other OWC installations
The following table is a summary of other shoreline OWC devices.
Location
Type
Rated
Output
Width
Water Depth
Operation
Period
Shanwei,
Guangdong,
China
Coastal OWC
100kW
20m
?
Since 2001?
Dawanshan,
China
Coastal OWC
3kW
4m
10m
Since 1990
Isle of Islay,
Scotland
Coastal OWC,
(Islay 1)
75kW
17m
3m
1988 – 1999
Kujukuri, Japan
OWC with
pressure
storage
30kW
10 x 2m dia.
2m
Since 1987
Niigata, Japan
Breakwater
OWC
40kW
13m
6.5m
1986 – 1988
Toftestallen,
Norway
Coastline OWC
500kW
10m
70m
1985 – 1988
Sanze, Japan
Coastal OWC
40kW
17m
3m
1983 – 1984
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3.8
Marine Energy Challenge
Oscillating Water Column Wave Energy Converter Evaluation Report
Recent Developments
Another form of shoreline device is the bored tube/cavern. This report has considered this technology
briefly in the stage 2 evaluation based on the plans (SeWave – Wavegen) for this technology to be
deployed on the Færoe Isles.
Wavegen is re-focussing its attention on the OWC market and has developed new designs for nearshore deployment that it hopes to implement as a demonstrator project in the coming years. A smaller
OWC power take-off system is being developed by Wavegen for deployment on breakwater projects
worldwide.
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3.9
Risk Matrix
The following table summarises an assessment of the project risks encountered, or possibly to be
encountered, for each device. The degree to which the identified risks affected the project outcome, or
may affect projects under development, was assessed to be high, medium or low based on our
observations. A comment on the risk mitigation for future projects is included in the comment column.
Shore
line
Feasibility
Energy resource
assessment
H
L
M
L
L
L
Risk reduced if initial
investigation thorough
Permitting
L
L
M
M
L
L
Risk increased if large
development, particularly if
considering shoreline device.
Costing
inaccuracy
M
M
H
M
L
L
Future designs can get
contractor buy-in before
committing to project
Schedule
inaccuracy
H
H
M
M
M
L
Promoter to employ
contractors with adequate
standing that will be capable of
the necessary planning
Sea bed
conditions
H
L
L
L
L
L
Risk reduced if initial
investigation thorough
Bathymetry
H
L
L
L
L
L
Risk reduced if initial
investigation thorough
Correct channel
geometry
M
L
na
na
na
na
Risk reduced if initial
investigation thorough
Rock integrity
M
M
na
M
na
na
Risk reduced if initial
investigation thorough
Concept freeze
M
M
H
H
M
L
Do not commit to final
investment until sufficient
engineering completed
Material
selection
M
M
H
M
L
L
Steel structures have a shorter
maintenance-free life than
concrete offshore
Scour /
undermining
L
L
H
H
M
M
More prevalent the closer
inshore the device is placed
Turbine
efficiency
M
M
M
H
M
M
Port Kembla is adopting new
technology
Control systems
L
L
M
M
M
M
Significant development has
taken place in this field
Site
Investigation
Design
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Sakata
India
Breakwater
Port Kembla
Nearshore
OSPREY
Pico
Risk
Description
LIMPET
Stage
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M
H
M
M
M
Onshore
Access to site
L
L
H
L
M
L
Construction
Component /
material delivery
L
M
L
L
L
L
Contractor skills
/ equipment
M
M
H
M
M
L
Productivity
Gain
H
H
H
M
M
M
All previous designs have been
bespoke with no opportunity for
productivity gain through
repetition.
Late delivery of
components
M
M
H
H
H
M
Port Kembla schedule
considered tight
Errors in fit up
M
M
H
H
H
M
Temporary
works
H
H
L
L
M
L
Will remain difficult for all
shoreline devices
Workforce
safety
H
H
L
L
M
L
Will remain difficult for all
shoreline devices
Weather down
time
H
H
L
L
L
L
Will remain difficult for all
shoreline devices
Adverse sea
state
na
na
H
M
M
M
Devices should be made as
sea-state tolerant as possible
Ballast
placement
na
na
H
na
M
M
Devices should be stable
without ballast in moderate
sea-states
Bed preparation
na
na
H
M
M
M
Difficult in exposed sites
Skirt penetration
na
na
na
na
na
na
Commissioning
na
na
H
M
M
L
Shore
connection
na
na
H
M
L
L
Setting taut
mooring
na
na
na
H
na
na
Tug / barge
availability
na
na
M
M
M
L
(solid/water)
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Sakata
M
India
Breakwater
Port Kembla
Peak wave load
est.
Offshore
Installation
Nearshore
OSPREY
Shore
line
Pico
Risk
Description
LIMPET
Stage
Understanding of impacting
wave pressures has improved
over the period of development
of these devices
Short taut moorings are difficult
to install and maintain taut.
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Operation
and
Maintenance
na
H
H
M
L
Access
L
L
H
M
M
L
Number of
moving parts
M
M
M
M
M
M
Any moving parts must be
accessible and replaceable
Davits / cranes /
runway beams
M
L
H
H
M
L
Maintenance provision must be
built-in from the outset
Marine fouling
L
L
H
L
L
L
Port Kembla is a temporary
structure so any fouling should
be less significant
Fatigue
L
L
M
L
L
L
Complex connections and
multiple members undesirable
on long-life devices
Chamber
blockage
M
M
L
L
L
L
Only an issue on shoreline
devices where rock removal
may be incomplete.
Integrity
inspection
L
L
H
M
M
M
Deluge of plant
rooms
H
H
M
M
M
M
Devices should be detailed to
prevent this in future
Cathodic
protection
inspection
na
na
M
M
na
na
Not required on concrete
structures unless embedded
steel present below water-line
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Sakata
na
India
Prior experience
Arup
Breakwater
Port Kembla
Nearshore
OSPREY
Shore
line
Pico
Risk
Description
LIMPET
Stage
Experienced installation teams
not yet established for work of
this nature.
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3.10
Key Performance Indicators
3.10.1
Structural Quantities v Output
Device
Schedule
Conceptual
Development
/ Design
Construction
and
Commissioning
Principal
Structural
Quantity
Estimated
Delivered
Cost
(UK 2004
Prices)
CAPEX
expressed in
p/kWh
amortised over
25 years
(no discounting)
LIMPET
1992 – 1998
Pico
1993 – 1995
OSPREY
1994
Nov 1998 – end
2000
1386m³
(concrete)
1995
£2.2m
5.0 estimated
(£1.75m
reported in
1999
money)
48 actual
1400m³
£2.1m
4.2 estimated
750te (steel)
£5.9m
2.2 estimated
7000te (ballast)
Port
Kembla
2003
Jan – Dec 2004
485te (steel)
£2.6m
2.6 estimated
Vizhinjam
1987 – 1989
Dec 1990 – Oct
1991
1200m³
£2.0m
198 actual
£4.7m
Not known
(concrete)
3000te (ballast)
Sakata
3.10.2
OWC
Device
LIMPET
Complete 1989
2900m³
Wave Energy at Collector v Output
Wave Energy
(kW/m) at
Collector
Device
frontage
20 estimated
21
m
12 measured
Wave
energy
x
frontage
Annual Average
Output
Overall Average
Wave-to-Wire
Efficiency
420
202kW estimated
48% Estimated
252
21kW measured
8% Measured
Pico
13.4 measured
12
161
57kW estimated
35% Estimated
OSPREY
29.9 estimated
20 chamber
up to 44 with
wing walls
598 to
1200kW estimated
91% Estimated
based on wing
walls
1316
Port
Kembla
10
35
350
114kW estimated
33% Estimated
Vizhinjam
5 to 10, 20
average
monsoon
10
100
average
6kW measured
6% Measured
Sakata
20
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4
Marine Energy Challenge
Oscillating Water Column Wave Energy Converter Evaluation Report
Summary of OWC technology (State-of-the-Art)
4.1
Resource Assessment [2,51]
Northwest Europe is situated in the mid-latitudes, which are affected by the west-to-east passage of
Atlantic depressions. The winds associated with these depressions create large waves over long fetch
lengths travelling across the Atlantic Ocean into the Western coast of Europe.
The offshore wave climate around the Western coast of the UK is therefore very energetic. The most
exposed regions of the Western coastline have the most vigorous climate, in particular, the islands off
the Western and Northern coasts of Scotland. The more sheltered East coast of the UK has, in
general, low wave energy resource.
When considering the development of a shoreline or near-shore device, the energy losses from
offshore locations into near-shore and shoreline locations have to be determined. The main energy
loss mechanisms are related to refraction and shoaling, seabed friction and wave breaking.
The prediction of wave energy at offshore locations can be carried out without too much difficulty by
utilising metocean hindcast models available from the UK Met Office and other commercial services.
The hindcast models cover a variety of areas at differing levels of resolution. The hindcast model for
UK waters has approximately a 12km grid. With the increasing sophistication of computer technology,
the Met Office hindcast models have been continually improving over several years and are
benchmarked against data from wave rider buoys.
A large database of satellite wave and wind data is held by various companies (e.g. ARGOSS, [57])
allowing access to retrieve joint probability distributions, histograms or plot short time-series.
Parameters including wind speed and direction, significant wave height, mean period and zerocrossing period and mean direction are available as well as statistics for wind-sea and swell waves.
Such data is available at much lower cost than commercial hindcasting services.
The Department of Trade and Industry (DTI) has commissioned a consortium led by ABP Marine
Environmental Research Ltd (ABPmer) to produce the UK Marine Renewable Energy Atlas [81]. The
purpose of the Atlas is to spatially map wave, tidal and offshore wind resource potential within the
limits of the UK Continental Shelf (UKCS). Its use for OWC evaluation is limited as the coarseness of
modelling make predictions within 12km of the shoreline less reliable. The DTI plan to use the Atlas to
assist decisions on future rounds of licensing for large-scale deployment of marine renewable
technologies. This work was undertaken as part of the DTI-led Strategic Environmental Assessment
(SEA) combined programme covering Oil & Gas and Marine Renewable agendas.
The challenge in bringing predictive data inshore is to reliably account for the seabed profile, shielding,
shoaling and wave breaking at every frequency for each wave energy spectrum around the coastline
to provide design data at the prospective OWC locations. A possible solution to this problem, which
reduces the required computational effort, is to calculate the inshore energy for target development
sites that are broadly representative of larger areas of the coast.
4.2
Modelling and Performance Prediction [9,13,29,30,32,33,34,40,41]
A number of technical papers have been written over the past twenty years on the modelling and
performance prediction of Oscillating Water Column (OWC) structures.
During the design process, a scale model test of OWC structures is carried out in a wave tank that
accurately reproduces the wave climate at the site. During these tests, the maximum calculated
design loadings are verified and the performance of the device is assessed.
The cost of tank testing for these structures is relatively high and thus several methods of
mathematical modelling are used to minimise model testing time.
Some examples of the design methods used are summarised below:
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The calculation of the device resonant frequency can be carried out initially by simplified mathematical
formulae that include the geometrical size of the opening, excitation forces, damping and stiffness.
Some of the available formulae are device specific such as those integrated into a breakwater. Other
formulae are more general based on calculations of semi-submerged ducts representing the OWC.
When the resonant frequency and wave amplitude of the device is known, further approximations of
the available power take-off can be made based upon the change in damping.
More sophisticated methods, such as boundary element methods, have been used that more
accurately predict the diffraction around and inside the device. Transfer functions can be derived from
the surface velocity within the diffraction model that can relate to the air pressure inside the chamber.
A more accurate prediction of the power available can then be made based on the site spectra
encountered throughout the year. A further sub model based upon the performance of the turbine and
other electrical equipment can be made that will further refine the model to give a full wave-to-wire
prediction.
One of the shortcomings of the boundary element method is that it is based upon linear diffraction
theory and therefore cannot account for higher order viscous effects. This is particularly important in
the design of the lip of the inlet to the chamber.
A method used for design of the lip into the chamber is Particle Image Velocimetry (PIV) [30]. This
method illuminates the water particles by use of very thin laser light and a combination of lenses. The
losses around the lip can be quantified using this method. It has been found through using this method
that the size, shape and inclination of the lips are of importance.
Computational Fluid Dynamics (CFD) software has been used in some applications of wave energy
devices. There do not appear, however, to have been any models of the entire OWC device. CFD
software has the capability to model the waves, viscous effects, air pressure movement and turbine
rotation. A model of this size would involve considerable computational effort and the costs would
possibly become similar to tank testing. This could, however, become a more favourable option in the
future as the software and computer technology improves.
The theoretical maximum conversion efficiency of the OWC (wave to pneumatic) at any given period
(frequency) of the incoming wave is thus determined purely by its geometry, the direction of the wave,
and an optimum value of the ‘damping’ in the chamber. The damping is determined by the turbine and
generator characteristics. This optimum value of damping is frequency dependent. Thus, the
conversion efficiency of the OWC under operating conditions is governed by the load
characteristics. Device developers have claimed average capture efficiencies of 50 to 80%. These
values are examined more closely in subsequent sections of this report to assess whether there is
potential to reach such capture efficiencies.
4.3
Structural Form
The main requirements of the structure of an OWC is to create a chamber of air suitable for
compression and rarefaction by wave action, and provide a platform and protection to the turbine /
generator equipment. This structure must maintain a fixed location while exposed to an extreme and
varied loading regime.
The geometric properties of the OWC chambers are fairly similar from device to device. This includes
a lip, inclined chamber and main air void leading to the turbine collector duct.
The lip is set to a depth below mean water level to limit the occurrence of inlet broaching. This is
when the water level falls below the level of the entry lip and a direct air passage is opened between
the working chamber and the atmosphere (ending the loss of pressure and hence capture efficiency).
The wave height at which this broaching occurs is a function of the lip penetration at still water, the
state of the tide and the dynamic characteristics of the water column. The LIMPET design
incorporates a restriction at the lip that restricts the outflow from the column during the down stroke
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hence allowing the turbine to operate with an extreme low water of two to three metres below the entry
lip. A curved entry lip profile helps to reduce turbulent losses at the entry.
An inclined water column has two main advantages. Firstly it offers an easier path for water ingress
and egress resulting in less turbulence and lower energy loss. Secondly the plane area of water rising
in the column is increased. This permits the primary water column resonance (a function of the water
plane area to entry area ratio) to be tuned to the predominant period of the incoming waves. The
water column should be sufficiently long that water does not rise high enough to flood the rear wall,
which could either over pressurise the turbine collector ductwork or rear wall.
The main air chamber should allow for control of the peak turbine flow pressures.
In addition, the Port Kembla project includes wing walls to focus the incoming wave energy. The true
benefits of wave guides have not been fully demonstrated and would benefit from future attention.
4.4
Turbine Design and Optimisation [77,78,79,82,84,85,87 & as
shown below]
4.4.1
Summary of the Current State-of-the-Art
Turbine technology for use with oscillating water columns (OWCs) relies primarily on the extraction of
useful work from the induced, bi-directional flow of air above the water column as it rises and falls. The
majority of OWC turbine studies, both theoretical and experimental, have been to investigate the
performance characteristics of variants of the Wells turbine. Work on impulse turbines has also been
investigated but to a much lesser extent.
Wells turbines with various blade and guide vane geometries, with and without bypass valves have
been investigated and performance curves have been generated over a range of representative flows.
The work has generally sought to identify the optimum configuration to overcome the inherent
weaknesses of the Wells turbine in a variable flow environment, particularly at low or high stall air
velocity when the turbine may actually consume power to maintain rotation.
Performance is considered in terms of turbine efficiency as a function of flow rate and / or aggregated
net power. The performance is significantly influenced by the blade and vane configuration and
whether or not a bypass valve is used.
Machines with fixed blade pitch angle generally have a narrow efficiency curve (peak efficiency over a
narrow flow range) whereas machines with variable blade pitch angle maintain efficiency over a wider
flow range, albeit at a lower absolute level. Aggregate net power is generally improved by using a
bypass valve to vent excess air at higher air flow rates.
One of the studies [38] describes a theoretical study of a complex system for control of vane and
blade pitch angle in a modified wells turbine, including a predictive model for wave patterns. The study
applies the model to the Pico plant and concludes that the performance gains appear relatively small.
It may be questionable whether such a complex control system is of real benefit on a commercial
scale machine where it could result in poor reliability and high maintenance requirements. The
concept of such a control system, including wave prediction, is expected to be incorporated in the Port
Kembla machine currently under construction [47].
By comparison a relatively simple control option involves the use of an automatic blow off valve where
the flow rate is capped. The valve may be actuated or a simple non-return valve, the latter having the
advantage of no parasitic power requirement but at the expense of not being under positive control.
Alternatively a throttling device may be used in series with the turbine, such as at Pico. The civil
structure for an installation with blow off valves would most likely need to be custom designed for the
specific location, but this potentially offers the option of a range of standard, geometrically similar
turbines with narrow efficiency curves to operate at varying levels of output power.
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4.5
4.5.1
Marine Energy Challenge
Oscillating Water Column Wave Energy Converter Evaluation Report
Controls [83, 86]
Treating the water Column itself as a tuned oscillator
Various attempts have been made to ‘tune’ the water column to the peak energy frequency of the
incoming waves under varying sea conditions. These can involve actively controlling the physical
profile of the column, in order to maintain the resonant frequency at the desired value. Alternatively,
active control of baffles or walls using sensors and actuators, or opening up/closing off smaller side
chambers to the main water column chamber, have been considered.
Allied to this, active control of the resonance wave itself within the column is important, so that its
amplitude neither decays nor increases uncontrollably. Too far in one direction would reduce the
energy content of the water; too far the other way might physically destroy the OWC installation.
Research efforts in this area have taken place to actively control the ‘resistance’, ‘damping’ or ‘inertia’
of the ‘terminations’ within the water column. In this case, ‘terminations’ usually refer to the physical
properties of the escape route for the compressed air at the top of the water column. This can refer to
the turbine characteristic itself, and how this can be modified and controlled, or to other devices placed
in series or in parallel with the turbine to modify the dynamic characteristics of the air/water interface.
In many cases, these ‘other devices’ consist of an actively controlled valve or damper which affects
the amount of compressed air flow to, or around, the turbine. Physically, this active tuning of these
parasitical routes can be seen as varying the physical and dynamic properties of the OWC, and if
done in a sympathetic way, achieve the desired aim of the researcher.
Much of the research carried out in this area has been to compare alternative strategies for control,
with a view to optimising the energy abstracted from the incoming waves. The work has been carried
out for simple wave spectra, and for time-varying wave profiles.
Much of this work has been done by computer simulation, some by using small-scale physical models
in the laboratory, and some on small-scale trials on functioning OWCs. These have been done under
particular sets of wave conditions and considerably more research is required in this area.
4.5.2
Modifying the Characteristics of the Turbine-Generator
In general, researchers have settled on the use of a Wells-type (rectifying) turbine for converting
kinetic and potential energy in the OWC to rotational energy suitable for driving an attached electrical
generator.
The characteristics of the Wells turbine show that the torque/air pressure relationship peaks at a
certain value of OWC air pressure. Above this value, the turbine output torque falls and the turbine
can enter a ‘stall’ phase, where energy conversion efficiency falls off rapidly. It is therefore important
that the incoming wave energy profile is controlled so that the unwelcome stall condition is not
reached.
A number of research projects consider the benefits and drawbacks of varying the geometry of the
Wells turbine blade (variable pitch system), and of running the turbine as a fixed or variable speed
device, but as yet no conclusions could be drawn as further research work needs to be done.
Consideration has been given to the possible sources of power for driving any regulating surfaces
(control valves, dampers, blade vanes etc) of the control philosophy adopted. This work includes
consideration of the electrical power inputs required by the electrical rectifier/inverter (converter)
system required to maintain the electrical power from the turbine generator train at a constant
frequency, given that the turbine/generator may run at a different, and time varying frequency itself.
4.5.3
Machine Technology
Current projects use both synchronous and induction generators, generally with electronic control of
speed. The review of existing projects and associated studies makes little mention of doubly-fed
induction generators (DFIGs), since they largely predate the more recent popularity of such machines
used in the wind power industry, however these modern drive systems are very similar to the well
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established Kramer drive systems which have been used in OWC applications. It should be noted that
whether the design is considered as a single system optimisation problem or a series of smaller subsystems to be optimised, each sub-system’s design and performance is dependent on the others.
4.5.4
Control Systems
The control of the machine itself is largely a feature of the machine design, rather than the prime
mover. In many of the prototype OWCs a traditional slip energy recovery or Kramer drive, has been
used. Modern DFIG drives are essentially the same concept, but with increasingly more complex
control strategies.
4.6
Power Conversion Efficiency
It is clear from the wealth of papers and project reports published on the subject of the OWC and its
associated air turbines that overall efficiency is still somewhat of an unknown quantity. Developers
have embarked upon prototype installations with high expectations, hoping to achieve wave-to-wire
efficiencies of 70-80%, only to find after months or years of operation that the best that can be
achieved is more like half this figure.
As described in the research papers on the subject of the OWC, the matching of the wave frequencies
to that of the resonant frequency of the water column is key to the efficiency of energy conversion.
Interactions within the chamber can also have a considerable impact. Efficiency figures for the OWC
independent of the turbine were not available from the papers reviewed. So far it doesn’t appear that
much of the state of the art R&D from the universities laboratories has been successfully implemented
in the prototype installations. Implementation of the best automatic control schemes could also have
considerable impact on the efficiencies currently being achieved in prototypes.
As described in the survey of the turbine design papers, there are currently a number of different
designs of air turbine being used with the OWC. Peak efficiencies of 70-80% for the turbine are
theoretically possible but only at specific air velocities. Practical prototype installations are giving much
lower average efficiencies due to the variation in air velocity, due to the variation in wave conditions.
Electrical induction machines normally operate at around full load with typical efficiencies of around
98%+, however some prototype OWC installations report efficiencies of as low as 50% and similar
stories apply to inverter efficiencies. Such reports do not seem to be credible, average electrical
efficiencies of 90%+ should be easily achievable even with a variable load. Correct sizing of the
equipment and the use of modern commercial designs should resolve these issues.
4.7
Maintenance and Reliability
The OWC and its associated air turbine either Wells, Impulse or variations of these types form the
OWC based WEC. This WEC system has been under development for some thirty or so years in
several countries. Despite many years of R&D the technology is still very much at the early stages of
development.
A number of prototype projects have been successfully operated around the world, however these
have so far been largely run very much as R&D projects managed by academics with limited funding.
The technologies and designs are many and varied, characteristics that are unlikely to be taken into
the commercial exploitation stage. In contrast to this, many small-scale navigational buoy OWCs have
operated successfully over many years and proven to be low-maintenance facilities.
The current prototype projects have suffered from maintenance problems and plant failures however
this should not be an indication that the commercial installations will be the same. Clearly the marine
environment creates its own problems however these issues are all resolvable, given the time and
money to design and develop the technology. Most of the issues have been faced before in the
offshore and marine industries so it should be largely a case of transferring existing technologies.
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5
Marine Energy Challenge
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Market Size
5.1
Assessment Method
The ‘market’ in this report is taken to be the amount of power generation that can realistically be
generated using OWC technology. An assessment method has been developed to evaluate the
market size. As part of this exercise areas around the UK were ranked in terms of their potential for
construction and operation of a shoreline or near-shore OWC. The proposed method will not match
that produced by other authors [73] and the market size should always be considered as an indicative
figure given the judgements made by various authors in arriving at market size estimates.
The factors considered in the ranking exercise were:
•
Wave Energy resource
•
The average sea-bed slope
•
Tidal range
•
Grid capacity
•
Cost of local construction or ease of transportation of units built off-site
•
Overall capital cost of shoreline and near-shore devices
The rankings were used to prepare an evaluation matrix that would show both the best areas to
develop and the potential resource from each location. A range of 0 to 4 being worst to best
respectively was used for each ranked variable.
Factors were derived for each ranked variable to allow different importance to be ascribed to each
aspect of the overall cost and suitability of OWC deployment. These importance factors are
necessarily somewhat subjective since they could only be otherwise established by extensive
technical and economic modelling. Since the main aim was to rank areas and then assess resource, it
was considered to be a reasonable method.
5.2
UK Wave Energy Resource
5.2.1
Distribution
Figure 5.1 shows the available offshore wave energy resource for UK waters.
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Figure 5.1
UK Offshore Wave Energy Resource (kW/m) (source: Dti Marine Atlas)
It can be seen from Figure 5.1 that the wave energy along the Western coast of the UK is high. This is
due to the long fetch of the Atlantic. The wave energy around the West coast of the UK is perhaps the
highest in the world. It can also be seen, however, that the available power levels are significantly less
for the East coast of the UK.
5.2.2
Variation / range
Figure 5.2 shows an estimate of the available near-shore wave energy around the coastline compiled
from [2] and other data. The energy levels reported are predominantly in the West coast of the UK
where the resource is highest.
Near-shore values appear to be somewhat higher than the Atlas of UK Marine Renewable Energy
Resources [81] would suggest.
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Figure 5.2
Near-shore Average Wave Energy Resource (at 20m contour in kW/m)
38
46 32
28
42
19
31
35
37
26
40
13
24
9
7
33
44
48 20 22 40 36
22 25
9
21
34
23
1
36
1
21
4
43
3
17
2
15
40
26
31
8
15
2
3
9
7
11
13
29 15 14
39
29 22
26
14
35
35
32
34
24
49
28
6
6
6.5
12
10
5
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5.2.3
Ranking method
It can be seen from Figure 5.2, that the near-shore wave energy available around the coast varies
from 0-50 kW/m. The ranking of each area is as follows:
Power <5kW/h
0
Power 5-10kW/h
1
Power 10-15kW/h
2
Power 15-25kW/h
3
Power >25kW/h
4
A colour plot of the wave energy ranking is shown in Figure 5.3. A weighting factor of 2 was applied to
the wave energy ranking.
The contour plot is to be used to rank large areas of coastline as part of the overall ranking process
and not as a definitive assessment of the wave energy at any specific location.
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Figure 5.3
Marine Energy Challenge
Oscillating Water Column Wave Energy Converter Evaluation Report
Wave Energy Ranking at 20m Contour Colour Plot
United Kingdom Wave Energy
Ranking
Energy <5 kW/m
Energy 5-10 kW/m
Energy 10-15 kW/m
Energy 15-25 kW/m
Energy >25 kW/m
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5.3
Sea Bed Profile
5.3.1
Method of review
The seabed profile has a significant effect on the available energy at inshore locations. A shallow
sloping seabed is more likely to create more energy loss mechanisms such as seabed friction and
wave breaking than a steep sloped seabed.
An approximation has been made into the seabed slope around the UK and is shown in Figure 5.4.
The assessment of the seabed slope has been made using a combination of previously published
literature and admiralty charts. Although the assessment is relatively simplistic, it serves as a
reasonable indicator of seabed slope ranking for the purposes of this study.
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Figure 5.4
Seabed Slope
United Kingdom Seabed Slope
Approximation
Slope 1:96
Slope 1:107
Slope 1:36
Slope 1:120
Slope 1:267
Slope 1:150
Slope 1:0.025
Slope 1:0.05
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5.3.2
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Ranking
As discussed in section 5.2.1, a steeper seabed slope results less energy losses between the offshore
and near-shore / shoreline locations. The ranking of the seabed slope is therefore:
Slope 1:500+
0
Slope 1:100-500
1
Slope 1:50-100
2
Slope 1:1-50
3
Slope 1:0-1
4
Figure 5.5 shows a contour plot of the seabed slope ranking. A weighting factor of 0.5 has been used
to allow for the assessed importance of sea-bed slope.
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Figure 5.5
Seabed Slope Ranking
United Kingdom Seabed Slope
Ranking
Slope 1:500+
Slope 1:100-500
Slope 1:50-100
Slope 1:1-50
Slope 1:0-1
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5.4
Marine Energy Challenge
Oscillating Water Column Wave Energy Converter Evaluation Report
Tidal Range
The tidal range around the UK varies considerably from almost zero to over 14m, one of the highest
tidal ranges anywhere in the world.
Figure 5.6 shows the spring tidal contours around the UK.
Figure 5.6
Note;
Spring Tidal Amplitude around UK (Source: HSE)
Spring tidal amplitude is half the spring tidal range
Contours are in metres
To obtain the approximate highest and lowest astronomical tides (HAT and LAT respectively) from
Figure 5.6, the conversion factors shown in Table 5.1 should be used in conjunction with the following
formulae.
+
T = Level of HAT relative to MSL
Spring tidal amplitude
T- = Level of LAT relative to MSL
Spring tidal Amplitude
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Table 5.1 HAT / LAT Conversion Factors
5.4.1
Ranking
Figure 5.6 can be used for approximate ranking of areas around the coastline. Further more specific
data would be required for the actual chosen sites as tidal range can vary significantly over small
localised areas.
In general, the tuning of oscillating water columns is more complicated when the device is situated in
an area with a large tidal range. This is due to the changing natural frequency of the chamber and the
requirement to alter the damping of the system.
The ranking of the area with respect to tide is therefore:
Range >6m
0
Range 4-6m
1
Range 2-4m
2
Range 1-2m
3
Range 0-1m
4
Figure 5.7 shows a colour plot of the tide range ranking around the coastline.
A weighting factor was devised for the effects of tidal range on overall cost. This was based on the
assessment that capital cost would be proportional to the maximum design water depth squared. An
average water depth of 12m was assumed. Thus the factor became:
2
2
Depth /(Depth + tidal amplitude)
Smoothing the numbers resulted in the following:
Tidal Factor
Rank
0.3
0
0.5
1
0.7
2
0.85
3
1.0
4
The tidal factor is used as a multiplier on the overall weighting.
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Figure 5.7
Figure 5.7
Tide Range Ranking
Tide Range Ranking
Tide Range Ranking
Range >6m
Range 4-6m
Range 2-4m
Range 1-2m
Range 0-1m
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5.5
Marine Energy Challenge
Oscillating Water Column Wave Energy Converter Evaluation Report
Grid Accessibility
Although the smallest wave power installations can expect to connect at 33 kV or even 11 kV, the vast
majority of commercial-scale projects will require a connection at or close to a 132 kV substation.
New 132 kV substations are possible, but add considerably to the price, so the distance to the nearest
existing substation has been taken as an indicator of the accessibility of the grid from any location.
5.5.1
Ranking
A rank was assigned to each location based on distance as follows:
Distance
Rank
Up to 10 km:
4
11 – 30 km:
3
31 – 50 km:
2
51 km or more:
1
A point was deducted if the connection was insecure; that is, if it relied on a single 132 kV circuit, or if
the 132 kV system was known to be already utilised to capacity with existing generators.
Similarly a point was deducted if the connection required a substantial sea crossing, since this is
considerably more expensive than a land connection. This applied to islands such as Arran or Mull,
which currently have no 132 kV infrastructure.
The Isle of Man has a 90 kV interconnector to the mainland. The terminal of this interconnector, near
Douglas, was taken as the nearest 132 kV substation for the purpose of the study.
5.5.2
Results
Figure 5.8 shows the approximate distances for various stretches of coastline. In the case of islands,
no distinction has been made between exposed and sheltered coasts (i.e. facing the sea or the
mainland), but a distance from the approximate centre of the island, or an average around the whole
coast, has been used. Figure 5.9 shows the rankings on a map of the coast.
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Figure 5.8 Connection distance and ranking
Typical /
Average
Distance (km)
Secure Connection
Sea Crossing
Required
Ranking:
8
Yes
No
4
10
Yes
No
4
10
Yes
No
4
10
Yes
No
4
10
Yes
No
4
12
Yes
No
3
15
Yes
No
3
15
Yes
No
3
15
15
Yes
No
No
No
3
2
18
Yes
No
3
20
Yes
No
3
20
Yes
No
3
20
20
Yes
No
No
No
3
2
25
25
25
Yes
No
No
No
No
No
3
2
2
30
35
No
No
No
Yes
2
0
Isle of Lewis and
North Harris
North Uist
North of Scotland
Islay, Jura
Colonsay
Benbecula
South Uist
Orkney Islands
Mull
Tiree
Coll
40
45
50
50
50
50
50
60
60
100
100
No
No
No
No
No
No
No
Yes
No
Yes
Yes
No
Yes
No
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
1
0
1
0
0
0
0
0
0
0
0
Shetland Islands
240
Yes
Yes
0
Area
Stranraer Lancaster
Hastings Felixstowe
Weymouth Portsmouth
Portsmouth Hastings
Inner Bristol
Channel
Lancaster Birkenhead
North East of
Scotland
South Cornwall /
Devon
Outer Bristol
Channel
Kintyre
North Cornwall /
Devon
Felixstowe Cromer
St Davids Head Outer Bristol
Channel
North / West
Wales
Isle of Skye
Bridlington Montrose
Cromer-Bridlington
Isle of Man
Cardigan - St
Davids Head
The Small Isles
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Figure 5.9
Coastal Grid Accessibility Ranking
Great Britain
Grid Accessibility Ranking
Rank 0: Insecure/Submarine Connection > 50 km
Rank 1:Secure Connection > 50 km (or insecure/submarine 31–50 km)
Rank 2: Secure Connection 31–50 km (or insecure/submarine 11–30 km)
Rank 3: Secure Connection 11–30 km (or insecure/submarine < 10 km)
Rank 4: Secure Connection < 10 km
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5.6
Local Construction Factor
5.6.1
Shoreline Construction
A local construction factor has been ascribed to each location to reflect the ease of supplying labour,
plant and materials to the site of the OWC construction. Relative construction cost factors were
selected based on concrete gravity substructure cost estimates previously prepared for NE England,
Nigg Bay in Ross-shire and Loch Kishorn in the Western Highlands.
The factor reflects the fact that construction in remote regions is more costly due to transport
premiums on materials and plant and the additional costs of acquiring labour. The cost factor is
multiplied by the site weighting.
Cost Factor
Rank
0.7
0
0.8
1
0.9
2
0.95
3
1.0
4
5.6.2
Near-shore Construction
A construction cost factor was selected based on the distance a unit might be transported from an offsite construction facility to the final installation site. The factor has been based on an assumption that
locations that have previously been used for the construction of steel or concrete gravity platforms
would be candidate locations for building OWCs.
Once again, the cost factor is multiplied by the weighting.
Cost Factor
Rank
0.8
0
0.85
1
0.9
2
0.95
3
1.0
4
5.7
Relative Economics of Near-shore versus Shoreline Construction
The cost of an offshore concrete gravity substructure constructed in a developed country is often
considered to comprise 1/3 labour, 1/3 plant and 1/3 materials.
At this stage of assessment, it has been assumed that shoreline and near-shore OWCs might require
the same amount of materials per kW of power captured. Onshore OWCs have had extended
construction schedules thus far so a relative labour productivity of 1.5:1 between shoreline and nearshore construction (off-site) is considered reasonable. Plant demands for temporary works and actual
construction are assessed to be in the ratio 1.25:1 between shoreline and near-shore. This results in a
relative weighting of:
(1.5/3 + 1.25/3 + 1/3) : (1/3 + 1/3 + 1/3)
or
1.25:1
in the overall efficiency of shoreline compared to near-shore construction. Near-shore weightings of
1.25 are thus applied.
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5.8
Marine Energy Challenge
Oscillating Water Column Wave Energy Converter Evaluation Report
Ranking Matrix, Combination and Weighting
The aim of the ranking assessment in Section 6 of this report has been to gain a broad outline of the
areas around the UK coastline where the conditions for installing a wave energy device would be most
favourable. The assessment has aimed at general large areas of coast where the focus of further
refined study should be undertaken.
The totals in the ranking table are derived as below:
(Seabed Rank *0.5 + Wave Rank *2 + Grid Accessibility Rank *0.5) * Tide Factor * Construction
Location Factor * Near-shore or Shoreline Factor
A summary of the ranking matrix is shown in Figure 5.10 for near-shore, Figure 5.11 for shoreline and
Figure 5.10 for the combined rankings. The latter figure shows that the highest ranked shoreline
th
location lies in 12 place. This shows a clear conclusion that near-shore OWC development should be
considered ahead of shoreline when aiming to maximise the resource that can be developed.
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Figure 5.10
Marine Energy Challenge
Oscillating Water Column Wave Energy Converter Evaluation Report
Near-shore Ranking Matrix
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Figure 5.11
Marine Energy Challenge
Oscillating Water Column Wave Energy Converter Evaluation Report
Shoreline Ranking Matrix
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Figure 5.12
Marine Energy Challenge
Oscillating Water Column Wave Energy Converter Evaluation Report
Composite Ranking Matrix
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5.9
Potential Resource
5.9.1
Shoreline
An assessment has been made of the suitability of the UK coastline for shoreline OWC deployment.
The coastline of the UK was examined on Admiralty charts to assess areas where water depths of at
least 10m were present close to shore. These areas were then examined in relation to topography to
assess which areas of coastline could be considered for construction. Generally, coastline with cliffs or
scarps was targeted and a height limit of around 20m considered the limit of practical construction.
Actual access was not considered in the selection process and the actual water depths shoreline have
not been verified.
A percentage of the available coastline was assessed for all regions of the UK in this way. It was
further considered that considerations such as measured water depths, environmental suitability, loss
of amenity, ease of maintenance and so forth might reduce the prospective sites to 1/10 of the
possible maximum. Furthermore, only the top twelve ranked locations were considered that had
shoreline possibilities. This resulted in 8.8km of coastline as being the maximum that could be
considered for development of shoreline devices.
If the average shoreline wave energy resource were 15kW/m at the lip, and an overall wave-to-wire
efficiency of 20% could be realised, this would yield 230GWh of power per annum, or enough power
for 58,000 households. This is somewhat lower than has previously been forecast by ETSU who
reported that 400GWh might be realisable [ 73 ].
5.9.1.1
Shoreline Resource Example
An example is given for Islay, adjacent to the LIMPET site:
Cliffs < 20m high
Fig 5.13 Ordnance Survey Map of Islay near LIMPET site (shown as crimson spot)
This shows that, of the 100km available coastline on Islay and the adjacent Isle of Jura, only around
7km has access to the sea in cliff areas that are less than about 20m high. This area lies near the
LIMPET site as shown in Figure 5.13 above.
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Deepwater Near-shore
Fig 5.14 Part of Admiralty Chart 2168 covering Islay © Crown Copyright
The Admiralty Chart shows areas where acceptable, deep water appears to exist at the shoreline. The
area around The Oa has cliffs too high and the area on the SE side of the Rhinns of Islay does not
face the wave resource.
Combining siting factors with the presence of deep water at the shore might leave around 700m of the
Islay coastline, or 1/10 of that practicable, as being suitable for shoreline OWC deployment.
5.9.2
Near-shore
A similar process has been followed for near-shore devices. Here the proportion of coastline with
reasonable opportunity to deploy near-shore OWCs was assessed. Steeply shelving areas at the base
of cliffs were avoided. Then an assessment of direct conflicts with OWC deployment was made
including port access, conurbations or conflicting amenity use and the coastline proportion accordingly
reduced. It was then further considered that 20% of the available coastline might be a practical upper
limit for deployment before environmental effects such as coastline shielding or current blockage
became too significant. This reflects the greater degree of flexibility afforded by a larger choice of
development locations. The land use, construction access and coastal geometry constraints
experienced on shoreline devices are not present.
Furthermore, only the top twelve ranked locations that had near-shore potential were included in the
total. This resulted in 141km of coastline as having potential for development of near-shore devices.
If the average near-shore wave energy resource were 25kW/m (at the 20m contour), and an overall
wave-to-wire efficiency of 25% could be realised, this would yield 7.8 TWh of power per annum, or
enough power for 1.8m households. This is somewhat higher than has previously been forecast by
ETSU who reported that 2.1TWh might be realisable [ 73 ].
5.10
Existing Structures
The integration of wave energy devices has been carried out successfully in a few locations around
the world. A review of the major ports around the UK in areas of significant wave energy resource was
carried out. It was found that there were very few breakwater locations that were in the normal OWC
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operating depth and tidal range. It may be possible that a device could be fitted at a small number of
locations although the supply of a significant proportion of grid energy through integration into existing
structures is unlikely.
5.11
Conclusions
A sizeable market has been identified for OWC deployment based on near-shore application. All the
prospective sites, with the exception of North Cornwall and the North of Scotland, are remote from
both population centres and grid infrastructure. However, the potential market is of sufficient size for
commercial scale developments to be considered. Such development might help to overcome the
current grid access difficulties faced by the more remote locations.
The market for shoreline devices is much more limited and is more likely to be realised as a power
supply to remote communities, as is the case at Islay.
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5
4
4
3
4
3
3
3
3
4
1
3
1
1
1
3
3
33 3
3
4
4
2
4
3
3
4
3
1
0
3
0
0
1
0
0
2
2
1
1
4
0
2
3
2
1
2
0
Seabed Slope Ranking
0
2
2
2
2
2
1
0
2
2
0
0
2
1
2
0
1
1
3
3
4 1 2
4
4
1
3
3 33
3
3
3
2
2
4
1
1
0
4
1
1
1
Wave Energy Ranking
Tidal Range Ranking
0
1
2
3
4
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6
Marine Energy Challenge
Oscillating Water Column Wave Energy Converter Evaluation Report
Generic Base Case Elements
6.1
Existing device similarities
Most of the existing OWCs have used a structural form bespoke to the specific location of the project.
Concrete has been the material of choice for the completed projects. Generally, smaller designers and
contractors have been involved in the project realisation and the overall skill required in the project
execution has been relatively low. This may have been the best approach for these small projects as
the overheads associated with major contractors could probably not have been borne by the
developer.
Previous OWC projects have demonstrated that the site preparation and civil construction cost is
approximately 70% of the completed project, and can contribute approximately the same percentage
to the on site programme. Clearly, there is a desire to develop a structural system that would be
suitable for a wide range of locations, that could be mass-produced and that would demand less of the
total project cost and programme resource. This can be more readily achieved near-shore than
shoreline.
All devices with collector chambers above a certain size have sub-divisions in the chamber to reduce
the risk of transverse wave excitation reducing the energy capture performance.
All devices, apart from the Port Kembla device, use the established Wells’ turbine technology. Most
have found that the generator originally specified has been too large thus reducing the power-train
efficiency under the lower than expected wave climate. Most have, or plan to, re-configure the power
train to improve efficiency. This will have the largest effect on the power losses being experienced at
low load. Most devices were not well configured to deal with the wide fluctuations of power output to
be expected between calm and storm conditions.
While much research effort has been expended upon the design and efficiencies of the mechanical
and control equipment, there appears to have been less of a focus on the marine and civil aspects of
OWC facilities.
It has also been the wish of many developers to incorporate the structural part of the device into a
breakwater or other structure (e.g. wind turbine foundation) in order to offset a portion of the project
CAPEX. While valid for countries that are developing their infrastructure (e.g. India), the opportunity in
the UK for incorporation into breakwater new build is very limited. The only real potential would be to
add a device in front of an existing breakwater, as at Trivandrum. Such a device would have more in
keeping with a near-shore device, however.
The OWC technology can readily provide a foundation for an offshore wind turbine thereby improving
the energy capture density and economics of any development. However, OWC technology is not
mature enough for major incorporation in the current licence rounds of UK offshore wind development.
6.2
Existing device differences
Of the operational devices, the major difference has been in the inclination of the OWC chamber. An
inclined column offers an easier path for water ingress and egress resulting in less turbulence and
lower energy loss. This is particularly true at the shallow water shoreline devices where the horizontal
component of wave motion begins to dominate compared to the vertical motion. This is most
pronounced at Islay.
The OSPREY and Port Kembla devices have chosen steel and are relatively light-weight construction.
Their lack of weight has been compensated for by ballast and tethering respectively. Both have kept
the structural quantities to a minimum by utilising wave focusing devices.
Steel solutions have most potential where stability can be readily assured, such as through piling.
Concrete operates best as a gravity structure.
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The estimated, or achieved, wave-to-wire efficiencies vary widely. Measured efficiencies have been
less than 10% whereas estimated efficiencies exceed 30%.
6.3
Focus Areas for Stage 2 Evaluation
6.3.1
Generic design
The market assessment has identified that the greatest potential lies with near-shore devices. The
highest ranked locations all have relatively low tidal range and substantial incident wave power so
these factors should be targeted when seeking scope for improvement.
The task in the second stage was to identify the most promising design configuration. There was a
need to optimise the design for water depth, distance from shore, wave height and structure loading.
This would lead to the selection of a generic design for which improvements could be sought. Inherent
in this development would be a choice of location: shoreline or near-shore.
The second stage work placed less emphasis on shoreline devices and integration opportunities
because of their more limited applicability and repeatability.
6.3.2
Plant optimisation
The following aspects arising from the state-of-the-art review of the mechanical plant were to be
assessed during the second stage evaluation:
•
evaluation of asymmetric inward and outward air flow and associated optimisation of turbine
geometry;
•
development of a range of standard, geometrically similar turbines of high efficiency for use
in associated custom built civil structures incorporating one or more bypass valves;
•
wider investigation of alternative turbine types, for example the impulse turbine.
Improvements were to be assessed against the operational performance of existing plant and other
examples of plant in comparable service conditions. The issues examined were to include:
•
Reliability
•
Robustness of design
•
Materials issues - hostile environment
•
Performance under real conditions
•
Maintenance (including access, frequency)
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Scope for Improvement & Future Vision
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Choice of device type and structure
There are three classes of fixed OWC device; shoreline, near shore and breakwater. While many of
the elements, and as such sources of improvement, are the same for each (particularly in relation to
the balance of plant), there are significant differences in relation to the structural form and potential
market size for these devices. The initial decision in relation to the scope for improvement of this
technology is the choice of generic OWC device.
7.1
Shoreline OWC Devices
The majority of UK and European operational experience of fixed OWC plant is with ‘shoreline’
devices, i.e. with the structure built into a coastal cliff or gully. An efficiently designed, well planned
and constructed shoreline device should be capable of producing power at a competitive price for
power from a renewable source. However, actual projects have reported considerable cost and
programme overruns relating to the civil construction element of the projects.
The main challenges faced by the developer include:
•
Resource assessment
•
Site specific design
•
Site preparation and rock removal
•
Site access and material supply
•
Specialised construction skills and equipment required
•
Temporary works and risk of water ingress
•
Weather dependant construction, prediction and planning
Experience to date, and possible improvements to eliminate or reduce the risk of occurrence of such
problems are discussed in more detail in this section.
7.1.1
7.1.1.1
Resource Assessment
Challenges
Accurate resource assessment has been difficult to achieve close to shore due to the greater
departure from linear wave theory leading to less certain numerical modelling, and a number of factors
such as bed friction losses and local shoaling effects. These can lead to an inefficient design with the
device pneumatic and turbine performance not matching the chosen generator. In these
circumstances the machine losses incur a higher percentage reduction to the actual output than
originally estimated.
7.1.1.2
Improvements
At present, inshore data in the public domain is available from an early study carried this out using
forward and back tracking wave ray models that considered the refraction and shoaling and used the
Bretschneider and Reid spectrum to model seabed friction [2].
The forward and back tracking ray method is the most commonly available method for calculating
inshore conditions. More recently, though, a more sophisticated spectral transformation model known
as SWAN has been developed that can be set up to predict inshore conditions to a 100m grid. The
model can be set up to account for depth induced breaking, seabed friction, tidal range and wind
induced wave rise inshore. The SWAN method involves substantial computational effort and accurate
grid bathymetry.
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Further advances in wave prediction have been made involving the use of satellites that can measure
the instantaneous wave heights. The most recent advances in this field involve the use of microwave
radar on the satellites that can predict wave energy spectra at specific points.
It is unlikely, however, that all the latest technological advances in wave prediction will result in
significantly different wave energy levels from earlier studies such that it would affect the wave energy
device location focus. However, at a project level the accurate prediction of wave resource without
extensive wave-rider buoy deployment remains difficult. The problems are exacerbated the closer the
OWC is to the shoreline as the influence of local bathymetry and coastline is magnified.
Large area bathymetric surveys of shallow seas are also now available using satellite imagery to map
the depth. Radar images routinely acquired by satellites since 1991 have allowed production of charts
using only a fraction of the sounding data normally needed. This saves money when a large area
needs to be charted in shelf and coastal seas where depths are less than about 30 m.
Whilst these improvements in data mapping can aid initial assessments, additional site specific studies
would still be required. This could include tank testing of the near-shore bathymetry in conjunction with
the OWC. However, this can be costly and as much of the initial scheme design as possible should be
completed with the satellite imagery techniques.
Developers should have an appreciation of importance of the survey data required at the project
conception, and include sufficient budget and programme allowance from the outset.
7.1.2
7.1.2.1
Site specific design and preparation
Challenges
Due to the nature of the shore line, each cove, gully, and cliff face location will be slightly different from
location to location. This will include:
•
Rock type and formation affecting strength, method of removal and OWC structural
geometry.
•
Local bathymetry, affecting the modification of the incoming wave resource.
•
Access road / infrastructure requirements differ from site to site.
•
Proposed development site land use, cost, and planning requirements may vary.
The energy output of an installation is proportional to the length of coastine used by the installation.
As fewer large sites are available, and a greater number of small sites are available, the specific
market for OWCs is restricted by the OWC site. A multiple location development does not gain the
benefit of economies of scale due to site specific re-design and resource analysis required, multilocation construction costs, plus the additional grid connection costs.
7.1.2.2
Costs
As such it becomes very difficult to produce a generic design suitable for many locations. Therefore
each location will attract a site-specific design cost. This cost would include:
•
Survey costs, approx. £75k per device location
•
Re-calculation of wave resource losses, including tank testing costs, approx. £60k per device
location
•
Re-design of civil structure for adaptation to site-specific geometry and rock interface, approx.
£40k per device location
•
Design and establishment of site-specific access, approx. £100k per device location
•
Planning approval
•
Additional costs for multi-location working. (E.g. plant required at each location.)
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7.1.2.3
Improvements
The possible improvements will depend upon the size of the proposed development.
•
Only develop large continuous coastal frontages
•
Limit the site-specific variables such that each location is prepared (excavated and dredged)
to be near identical
7.1.3
Construction techniques
Shoreline devices all require tailoring of the local water line / coast interface and the immediate seabed approach; however several different shoreline concepts exist with differing associated
construction methods.
1.
In-situ construction behind a rock bund or cofferdam (e.g. LIMPET).
2.
A roof-only OWC using pre-cast elements fixed to a rail bolted into an excavated / profiled rock
face.
3.
An OWC tunnelled into a cliff face.
7.1.3.1
Challenges
Each construction method has particular challenges plus the generic problems associated with
exposed or remote coastal construction sites.
In situ construction
The decision to construct a device in an excavation behind a rock bund has several impacts on the
device situation. Firstly it pushes the collector back from the original coastline. This arrangement has
been shown to be detrimental in shallow water. Secondly, it requires a specific coastal bluff / cliff
height to provide sufficient protection to the construction site. This limits the number of suitable site
locations.
Final bund rock removal has proven to be less straightforward than anticipated, with loose rock
scattered in front of the collector and deposited by wave action into the chamber itself that had to be
cleared in a separate work programme.
Protection of the construction site with temporary works would also be problematic as by the very
nature of the project the wave loading would be fairly extreme. As with the permanent works, the
temporary structure design would be site-specific offering limited repeat construction savings. It would
be costly to provide a wide frontage and difficult to install, particularly if a more traditional sheet pile
coffer dam was used. Final removal and other access problems would also increase the cost of this
type of construction.
Roof-only OWC
A roof-only OWC concept has been devised by Wavegen that utilizes the natural cliff face as the
bottom element of the chamber with pre-cast elements for the chamber roof fixed to a rail bolted to the
excavated rock face.
A roof-only OWC concept requires extensive in-situ rock shaping with specific requirements in relation
to access, geometry and geological properties. Given these requirements, the potential resource that
can be realised from shoreline OWCs will most likely be unaffected by this development.
The majority of construction would be required to be completed in favourable weather windows, and
as such programme contingency and risk would need to be considered. In addition, construction and
finishing below the water line would attract a cost premium.
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Cliff face tunnel OWC
Some of the difficulties associated with exposed working at the shoreline can be avoided by using
tunnelling techniques to form the OWC collector and plant space within a cliff. The OWC collector exits
below the water line as the tunnel is broken through to the sea. The tunnel OWC is less visually
intrusive so may prove more acceptable to the public.
This method is resource constrained in a similar manner to shoreline devices, needing deep
competent rock and an exit to deep water direct from the cliff. Such conditions may be relatively
common on parts of the Shetland and Orkney Isles, but are less common elsewhere.
A project on the Færoe Isles is currently at the development stage. Wavegen has formed a joint
venture with local utility company to undertake this project.
It is beyond the scope of this study to evaluate the general economic applicability of the tunnelling
approach. This could be done and would need to consider typical geologies and available specialist
tunnelling equipment. Wavegen report that power costs in the range 9-15p/kWh are expected in the
Færoe Isles. This would appear to be somewhat better than the economics of shoreline demonstration
projects.
Generic
The majority of the preferable OWC development sites are in fairly remote and exposed sites. For in
situ constructed projects this presents a number of problems:
•
Labour and equipment may need to be specially mobilised due to lack of indigenous resources
•
Guaranteed material supply may involve establishing a dedicated batching plant
•
Summer working period only
7.1.3.2
Costs
The nature of the in situ work in exposed or remote locations either attracts a premium on to standard
working methods, or requires specialist activities (such as below-water working) or requires extensive
additional temporary works:
•
Temporary works cost and design
•
Specialist equipment and skilled contractors working at a premium compared with less remote
sites
•
Labour and accommodation premium
7.1.3.3
Improvements
The lessons learnt by the developer on LIMPET have identified several areas for improvement:
•
The use of pre-cast elements for the chamber roof reduced in-situ construction time, reducing the
programme and temporary works and hence the associated weather risk.
•
The extension of the rock bund provided additional protection to the work site. A more robust and
higher extension would have meant construction activities could have continued on a greater
number of days and would have limited the amount of re-work and repair required to the formwork.
•
Flexible working methods, resource allocation and programme would have allowed construction
to match weather windows but possibly with an associated higher labour cost.
A near shore device prefabricated in a dry dock would again be a significant improvement mitigating
many of the problems associated with an in-situ construction technique.
•
Limited or low cost site preparation required.
•
No requirement for local land take or access during construction
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•
Temporary works costs are shared over several devices
•
Limited underwater working that is non programme critical
•
Locally available skilled construction labour and equipment at lower cost.
•
Year round working
7.1.4
Weather and programme risk
The nature of shoreline sites means that they are exposed to storm and extreme wave conditions
particularly in the winter months. This means that the construction programme is best tailored to
match the summer months. Appendix B shows the programme for the LIMPET project [56] plus a
generic programme if all the improvements outlined in chapter 6 were implemented. It can be seen
that it would be difficult to complete all activities within the summer working window even for the
improved schedule, automatically extending the overall programme by four months.
7.1.4.1
Challenges
•
Constraining the on site construction to be completed within the summer construction window.
•
Planning construction activities to be flexible such that storms and unpredictable weather can
be accommodated.
7.1.4.2
Costs
•
Storm damage rework and repair impacts the planned programme. Contingency and
flexibility to mitigate such problems has an overall project cost. Additional cost of standby
equipment, a flexible but inefficient programme and extended working hours could add
approximately 10 to 15% onto standard construction rates
•
Programme extension, demobilisation and re-mobilisation in the spring also would have a
cost impact. See the programme and cost model included in Appendix B.
•
Costs for meteorological and metocean forecasting need to be included in the project cost
build up.
7.1.4.3
Improvements
•
Construction could be broken into short-duration tasks completed within weather windows,
with the non-weather-dependant activities saved for storm periods.
•
It should be noted that while a near shore device would be sensitive to weather conditions
during installation, all other activities could be completed within the shelter of the dry dock
making the construction programme more flexible than shoreline in situ construction.
7.1.5
Contractual risks
The development of OWC projects is still in its early stages. A pool of experienced developers,
designers and contractors is not available. Considerable risks are thus present throughout the project
development stage. Proper engineering and planning can mitigate some of these risks, but it is likely
that new entrants will not understand all the risks to which they will be subjected.
It is a common requirement of financial institutions that they require contracts to be placed on a lump
sum design and build basis on the assumption, commonly misplaced, that this gives greater certainty
in project financing. However, given the immature market, if risk is passed from the developer to the
contractor this will typically result in the contractor either underestimating the scope of the works, or
the contractor making extensive provision within his pricing to cover the unknown. The former can lead
developers to believe that developments will be more economically attractive; the later can lead to
overly costly OWCs.
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An example of the differences that might arise from the two approaches was given by Wavegen, who
stated that a turnkey design and build estimate for an OWC was twice the sum of individual
subcontract prices. Part of the difference relates to main contractor overheads and management, but
the remainder probably can be apportioned to risk. Costings presented for future visions in later
sections of this report have been based on risk levels that have been mitigated as far as is currently
considered practicable given the current state of OWC development.
7.2
Near-Shore OWC Devices
The UK experience of near shore structures to date has not been favourable. The OSPREY (Ocean
Swell Powered Renewable Energy) was designed to operate in a water depth up to 15m and up to
1km offshore. It was constructed in 1995 to be installed 100m off the coast at Dounreay in the North
of Scotland. During the installation of the 750 tonne structure, a large wave smashed open the ballast
tanks and the device had to be abandoned.
This experience however should not prejudice any future use of near shore structures.
The Port Kembla project plans to demonstrate that viable near-shore devices can be deployed. The
team hope that the successful deployment of this device will generate substantial learning that will
allow future designs to be much more economic. The team has opted for offsite construction, utilising
a low-cost fabrication location, Indonesia. This is an important factor in the economics of the device.
The construction methods, technology and skills required to successfully install near-shore structures
are readily available and have been successfully proven in developing countries, as witnessed by the
Trivandrum OWC, India.
Hence the ability to construct offsite and a critical appraisal of the best materials for a particular site
should be carried forward into the future vision for OWCs.
7.2.1
Advantages
Many of the inherent limitations of ‘shoreline’ OWC devices are eased by placing them in a ‘nearshore’ location. These include:
7.2.1.1
Generic design / Mass production
The use of a dry dock or fabrication yard as a facility for the construction of the near-shore devices
negates any requirement for in-situ temporary works. This provides a more secure safer working
environment, offering increased levels of productivity. While the use of the dock or fabrication site itself
could prove costly, once spread across a multiple device development the overall cost becomes more
acceptable. This is demonstrated in the cost estimates to support the future vision included in
Appendix C.
A modular approach to the construction of the devices could allow mass manufacturing techniques to
be employed. This would lead to cost reduction through increased productivity and programme
reduction. Greater levels of quality would also be achieved.
A modular approach could also lead to a flexible device size better matched to site-specific resource
or balance of plant.
7.2.1.2
Controlled construction environment
•
Little or no underwater working is required
•
The structure is built in controlled environment and as such is less susceptible to adverse
weather conditions which may delay the programme
•
Locally available plant, material, labour and skills
•
Little or no site preparation required
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•
7.2.1.3
Sufficient number must be constructed to justify dry dock cost.
Water depth optimisation / sea bed losses
A near shore device chamber approach depth can be optimised to minimise cost simply by moving the
structure further towards or away from the shore. Shoreline sites have little opportunity to adjust the
inlet / lip depth other than through careful choice of development site or through excavation of the seabed approach.
Wave sea-bed friction affects the performance of the device, with shoreline devices typically being
most prone to wave breaking prior to the collector and greater bed friction losses.
7.2.2
7.2.2.1
Disadvantages
Installation
•
Equipment availability / cost
•
Installation weather windows
•
Cable connection underwater.
7.2.2.2
•
7.2.2.3
Maintenance and operation
Access is more difficult increasing maintenance costs, although the basic scope of
maintenance for both at shore and near shore devices is expected to be similar.
Offshore environment
The lateral load resistance of the structure must rely on the self weight / ballast. A sloped front wall
can be beneficial in this regard as this encourages a downward-acting wave loading component.
7.2.2.4
Visual impact
All OWCs involve a change of land use and some change to the visual amenity of a particular location.
It could be argued that near-shore OWCs, sited less than 5km offshore will be more visually intrusive
that shoreline devices, particularly if developed on a large scale. A near-shore structure will have an
impact on benthic processes and change the pelagic environment.
It seems likely that any near-shore device development will have to expect the level of interest
expressed in offshore wind turbine development and it is hoped that a greater public realisation that
renewable energy developments are beneficial in the round will eventually ease the development
process.
7.3
Breakwater OWC devices
The integration of wave energy devices has been carried out successfully in a few locations around
the world. A review of the major ports around the UK found that there were very few breakwater
locations that were in the normal OWC operating depth and tidal range. It may be possible that a
device could be fitted at a small number of locations although the supply of a significant proportion of
grid energy through integration into existing structures is unlikely.
While this would obviously reduce the relative CAPEX apportioned to the OWC development, this
method of OWC improvement is not considered further in a UK context. However, there is
undoubtedly potential to incorporate OWCs in breakwaters being considered in exposed locations for
port development around the world. Baja California is one such area where substantial breakwater
structures are planned for LNG import facilities.
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7.4
Conclusions
The inherent constraints on shoreline OWCs outlined above limit the scope for improvement and scale
of the resource that potentially can be realised. Near-shore devices can avoid some of the shoreline
constraints. This includes:
•
A greater available resource, with a more flexible choice of locations.
•
The ability to use a generic design rather than a site-specific design for each device.
•
The possibility for mass production
•
Little or no underwater working is required
•
The structure is built in a controlled environment and as such is less susceptible to adverse
weather conditions and programme delay.
•
Greater ability to tune the structure for a particular wave climate by varying the device depth
and / or orientation.
Nearshore Cost Split
Shoreline OWC Cost Split
3%
12%
19%
3%
Temp Works / Site Prep
10%
6%
Temp Works / Site Prep
Structure
Structure
Turbine / Generator
Turbine / Generator
Grid Connection
24%
Elec Connection
57%
Installation Vessel Costs
66%
Shoreline v Near-shore Power Production Cost
24.00
22.00
20.00
p/kWhr
18.00
16.00
14.00
12.00
10.00
8.00
6.00
4.00
0
5
10
15
20
25
Development Size MW
Shoreline Trend
Nearshore Trend
This relationship is discussed in detail in section 10.
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Scope for Improvement
The technical review of OWC demonstration devices has highlighted aspects of the technology and
means of delivery of projects that show scope for improvement. The potential and technical proposals
for improvement are discussed in this section. They are intended to be as generic as possible
although LIMPET experience is frequently referenced.
The scope for improvement is related to the energy flux at the 20m contour in this chapter.
8.1
Site–specific tailoring of designs
8.1.1
Chamber geometry
An OWC formed from a circular tube submerged in water can be considered as the simplest
embodiment of this type of device. The frequency (in rad/sec) of oscillations, neglecting end effects, is
[40]:
ω = √ gL
L
Fig 8.1
Submerged Tube Resonance
The submergence depth, L, is related to the mass (and added mass) of the moving water column and
determines the natural frequency of oscillation. For practical submergence depths of around 5m, the
natural period of oscillation is approximately 5 seconds. Adding a horizontal section to the tube will
increase the effective mass thus increasing the natural period. The use of a constraint at the lip and
adopting an inclined tube such that the water-plane area in the tube increases and the area at the
outlet decreases are both beneficial in increasing the natural period of oscillation.
The best geometric form of the air chamber also appears to be dependant on the wave climate in each
location. The greater prevalence of larger breaking or near-breaking waves at the shallow shore-line
sites, where the energy is more concentrated into horizontal motion, suggests that a sloping chamber
will result in greater capture efficiency since horizontal movement can be converted more readily into
vertical movement. Selecting an inclined water column in shallow shoreline conditions also offers an
easier path for water ingress and egress resulting in less turbulence and lower energy loss. A
downside of having the sloping face is that water surface pitching (i.e. front to back slosh can be
induced) rather than heave with consequent loss of captured power.
Deeper water devices in less severe metocean conditions may exhibit acceptable performance with
near-vertical chambers.
In order for the maximum benefit of resonant response to be utilised both a crest and trough need to
develop at the chamber. However, it has been observed that a trough has not been forming in the
LIMPET device and this has been attributed to the water running down the inclined outer face
inhibiting trough development in front of the wall. A possible design refinement would be profile the
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front face of the chamber to shed the receding wave flow to one side or to explore whether a lip on the
base of the sloping face would stop this backwash affecting the developing trough.
The natural period of oscillation of practical OWC devices lies below the practical range of periods
containing the most significant wave energy. The table below, for conditions in 8m water depth
offshore Islay, shows that the most frequent energetic sea-state has a mean period of 10.5sec, well in
excess of the resonant period of 5 seconds. Wave amplification away from this resonant period will
thus be reduced. For example, the response of a system with a natural period of 5 seconds to a
forcing function with 10.5 second period would only be 1.3 times ambient, in the absence of damping.
Hence the OWC might be better described as an amplifier of forced oscillations.
Hs vs te
2.8
3.9
5.0
220
0.5
1.5
6.2
1,352
5,680
2.5
7.3
3,877
30,303
2,551
8.4
8,163
76,448
33,094
3.5
4.5
5.5
9.5
8,367
90,992
110,398
17,083
10.6
6,628
87,854
138,590
88,577
9,762
11.8
4,454
52,340
117,548
169,764
50,112
12.9
2,145
29,531
78,876
123,677
92,001
15,270
14.0
15.1
16.2
1,281
14,991
41,642
62,783
83,027
15,504
519
5,835
16,208
31,767
42,010
15,689
131
3,534
6,545
6,414
10,603
17.4
6.5
7.5
Wave Energy Flux in W/m times percentage frequency of occurrence
For UK applications with energetic shallow water conditions, a 45° chamber angle appears the most
appropriate. Such a choice may incur a small constructability penalty but the need to capture the most
power will over-ride such considerations.
8.1.2
Chamber lip
The lip depth should be selected to limit the occurrence of inlet broaching. Broaching is undesirable as
it causes loss of power take-off and sudden pressure changes in the collector. Broaching occurs when
the water level falls below the level of the entry lip and a direct air passage is opened between the
working chamber and the atmosphere. The wave height at which this broaching occurs is a function
of the lip penetration at still water, the state of the tide and the dynamic characteristics of the water
column. The LIMPET design incorporates a restriction at the lip that constrains the outflow from the
column during the down stroke hence allowing the turbine to operate with an extreme low water of two
to three meters below the entry lip. A curved entry lip profile helps to reduce turbulent losses at the
entry.
Given that the selection of a lower lip, whilst increasing the natural period of oscillation of the OWC,
will also be cutting out some of the incoming wave energy and reflecting it back, the highest lip level
should be sought. The depth of wave troughs below still water level in the steep waves expected
near-shore is around 1⁄3 the wave height. Further, the maximum wave height encountered at the OWC
will typically be depth-limited at around 0.78 times depth. It is thus suggested that the lip is set at a
minimum of 25% water depth below Mean Low Water Springs to minimise the likelihood of chamber
broaching.
8.1.3
Device Capture Width
The capture width associated with an OWC has often been assumed to be the width of the collector.
Incidences of capture efficiency (pneumatic power as a percentage of energy flux) being greater than
1 have been reported which is unhelpful in terms of understanding the capacity and efficiency of a
particular device.
The reason for apparently high capture efficiencies is usually some form of focusing device used in
conjunction with the collector. Both the OSPREY and Port Kembla devices incorporate such features.
Subsea guide walls or structures are more likely to be adopted on steel substructures where there is a
desire to minimise the area exposed to the most severe wave loading yet still capture power over the
entire width of the structure.
An individual OWC will have an influence on the wave climate beyond its collector width. It will thus
have a higher apparent capture factor than an infinite length of devices. The quantification of the
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extent of influence is less tractable than on devices such as point absorbers as energy can be
reflected, re-radiated and lost in viscous effects in a more complex pattern. It is hoped that arrays of
OWCs will be deployed near-shore thus making the consideration of capture width less relevant.
However, for the present, it is suggested that the maximum width of the device is used to define the
potential energy capture, to minimise the potential for pneumatic power capture factors being reported
that exceed one, as was the case with OSPREY.
The effectiveness of guide vanes has not yet been proven in practice, although Port Kembla will help
in this regard. It is recommended that research and development activities are directed to evaluating
how effective the guide vanes are.
8.1.4
Anti-slosh Devices
The chamber must be suitably sub-divided where the formation of standing waves within the chamber,
that would store energy but produce no usable power, is expected. The wave sloshing period is well
known and should generally be kept below 5 sec. This would imply that the chamber should be
subdivided into chambers where the maximum plan dimension of the chamber exceeded about 10m.
The actual choice of chamber width is thus a balance between avoiding sloshing and having element
spans that are suitable for the loading to which they are subjected.
8.1.5
Air Plenum Configuration
The geometry and volume of the plenum should be selected in combination with the turbine to
optimise power capture. A smaller air plenum will be subject to higher pressure and greater flow both
of which must be considered in relation to the turbine characteristics.
The air plenum may be subjected to high pressures if the flow exceeds the turbine capacity and the
control valve is closed. The maximum crest height (assuming 2⁄3 crest and 1⁄3 trough) has been
assessed for the LIMPET geometry.
Hs vs te
2.8
3.9
0.5
5.0
6.2
7.3
8.4
9.5
10.6
11.8
12.9
14.0
15.1
16.2
17.4
0.21
0.24
0.73
0.26
0.79
1.31
0.28
0.83
1.38
0.28
0.85
1.42
1.99
0.29
0.87
1.45
2.04
2.62
0.29
0.88
1.47
2.06
2.65
0.30
0.89
1.49
2.08
2.68
3.27
0.30
0.90
1.50
2.09
2.69
3.29
0.30
0.90
1.50
2.10
2.70
3.30
0.30
0.90
1.50
2.11
2.71
0.30
0.90
1.5
2.5
3.5
4.5
5.5
6.5
Estimated Crest Height in Collector (m)
The maximum predicted crest is +3.3m, which would be a level of +5.9m with respect to local datum at
the time of MHWS for the nearby Orsay Island. The lowest point on the turbine inlet is +8.54m with
respect to local datum and a bench is provided at +4.94m. Thus there seems to be scope for lowering
the overall height of the chamber.
The plenum was originally designed for a considerable internal pressure. The corresponding locked in
pressure associated with the above crest level has been assessed to be 0.34barg as shown in the
table below. This value is relatively small compared to the external design hydrostatic head.
Hs vs te
0.5
1.5
2.5
2.8
3.9
5.0
0.02
6.2
0.02
0.06
7.3
0.02
0.06
0.11
8.4
0.02
0.07
0.12
3.5
9.5
0.02
0.07
0.12
0.18
4.5
5.5
10.6
0.02
0.07
0.13
0.19
0.25
11.8
0.02
0.07
0.13
0.19
0.26
12.9
0.02
0.07
0.13
0.19
0.26
0.34
14.0
0.02
0.07
0.13
0.19
0.26
0.34
15.1
0.02
0.07
0.13
0.19
0.26
0.34
16.2
0.02
0.07
0.13
0.19
0.26
6.5
Estimated Plenum Peak Pressure in bar gauge
It should thus be possible to effect savings in the collector chamber design compared to previous
more conservative assumptions. Such savings will be carried forward to the future vision generic
design.
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The air plenum could also be actively adjusted using baffles and valves to better match the required
turbine damping requirements. This has to be viewed further in relation to the power required to
control the valves and the additional instrumentation and control system that would be needed.
8.1.6
Collector Width
The maximum frontal width that can be selected for one air chamber depends on the degree to which
long wave fronts can be expected to reach the device at the same instant. A collector that was linked
to a crest in one location and a trough in another would have a reduced capture efficiency. It would be
possible to have a wide collector facing the waves with only one turbine set but a significant amount of
valves and controls would be needed to ensure useful power was not lost. It seems preferable to limit
the collector width to the continuous crest length that can be reasonably expected. A maximum
collector width of 40m is suggested, which would automatically limit the size of turbine that needed to
be developed for OWCs. For an incident wave power at the 20m contour of 34kW/m and a pneumatic
capture of 40-50%, the maximum rated power for a turbine set would be around 1300kW based on the
rated power being around three times the average power captured.
More accurate determination would require the degree to which shoaling effects encouraged offshore
spread seas to become more long-crested to be assessed. Further, angular offset that would be
expected between the line of advancing wave fronts and the contour of constant water depth on which
it was designed to site the OWCs would need to be determined.
8.1.7
Tidal range
It has been shown in the stage 1 evaluation that the capital cost of an OWC could be considered
proportional to the maximum design water depth squared since the device has to become
proportionally wider or heavier to resist wave loading whilst also getting taller.
Taking a typical water depth of 10m, the sensitivity of device CAPEX to tidal amplitude could be
assessed to be proportional to:
(Depth + tidal amplitude)
2
If an average UK tidal amplitude of 2m is assumed compared to a location without tidal influence, a
cost increase of 44% might occur. Hence it is important to concentrate on sites with the smallest tides.
It is for this reason that the vast majority of the development sites associated with the future vision all
have a tidal amplitude less than 2m.
8.2
Turbine choice and specification
8.2.1
Air flow rate matching
For the limited number of projects developed to date, mismatching of the collector and turbine appears
to have been a critical factor in the poor performance that has been achieved. Although there has
been much theoretical analysis, primarily in the area of turbine damping, there has seemingly been
little effort directed towards the creation of accurate, generic predictive tools for matching collector and
turbine characteristics. Much of the work appears to have been undertaken on an empirical basis and
where theoretical models have been developed their validation has been rather limited, typically
against a very limited range of experimental data. Consequently the design process and matching
between collector and turbine is in many cases likely to be sub-optimal.
Modern CFD techniques should be capable of analysing the problem. A comprehensive dynamic
model incorporating the collector, variable speed turbine, exhaust system (including any noise
abatement) and cyclic, reversing flow would undoubtedly require substantial computational capacity
and although feasible may not be practicable in all cases. Nonetheless, if the limit is solely one of
computational capacity then it should be possible to create a number of more manageable sub-models
with matching boundary conditions. Development of these techniques would enable a better
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understanding of the interaction between collector and turbine characteristics and should lead to
improved designs with better performance and output.
Overall performance will be improved if turbine efficiency can be maintained over a wide range of sea
states. This implies either that the collector structure and intake duct must be designed to deliver
steady flow to the turbine over a wide range of sea states (e.g. by turbine inlet flow modulation using
bypass and/or inlet throttling valves) or that the turbine must be capable of efficient operation over a
broad range of inlet flows. The latter option would seem preferable as there is less risk of flow
mismatch due to incorrect design of the collector structure.
At least one developer has observed that the performance of Wells turbines is better at low wave
energy input levels with low air flow rates whereas an impulse design is potentially more suited to
higher wave energy input applications. It may therefore be appropriate to select the basic type of
machine based on local wave energy available. This approach would be similar to that for water
turbines where the basic design (Pelton, Francis or Propeller) is determined by the available input
head and flow.
The goal should be to develop a range of standard turbines with known characteristics such that
collector structures can be designed to deliver the required air pressure and flow conditions. In this
respect a parallel can be drawn with thermal power generation where it is usual for a steam boiler to
be custom built against a recognised code to deliver the required steam conditions to a steam turbine
selected from a standard range.
For near shore applications where the collector structure is not constrained by local topographical
considerations, it should be possible to develop a range of matched collectors and turbines. Such an
approach would be expected to deliver significant cost savings in terms of both reducing the amount of
site-specific design activity and enabling savings due to standardisation of the manufacturing process.
8.2.2
Blade and vane configuration
The performance of the turbine will be substantially determined by the blade and vane configuration
and each design will have a specific characteristic for efficiency against inlet flow. The characteristic
may be a single curve that includes speed variation or a family of curves at constant speeds. The
shape of the performance characteristic will be determined by the generic blade type. For example, an
impulse machine will in general have a broader characteristic curve than a Wells machine, primarily
due to it being less susceptible to stall at higher air flow rates.
Much of the development work to date has considered variations on the Wells turbine in order to
broaden the performance characteristic. This may be due to the simpler blade design with fewer
blades and hence lower cost in a research and development environment. However, in its simplest
form the Wells turbine suffers from inherently poor performance and a narrow characteristic due to its
low torque development and susceptibility to stall at off-optimum air flows. Variants investigated to
improve performance include variable and fixed guide vanes, variable pitch blades, bypass valves and
inlet flow throttling. Whilst such schemes can be shown to improve the theoretical performance, there
is a danger that for those involving additional moving components the advantages conferred by
simplicity of the blade design could be outweighed by an over-complex control system (with inherent
reliability and maintenance risk), increased maintenance burden and increased parasitic losses. An
impulse design appears equally valid and greater evaluation of its potential would be justified.
Notwithstanding the above, the use of guide vanes is in general likely to improve performance and in
the case of impulse design machines is essential. Fixed guide vanes offer the simplest solution with
minimal maintenance and would be expected to give higher reliability due to the simpler control
system. However, the reversing nature of the air flow and variable turbine speed makes optimisation
of the design somewhat difficult; exit vanes that become inlet vanes on flow reversal generally reduce
performance as the optimum aerofoil geometry for an exit vane differs substantially from that for an
inlet vane under reverse flow conditions.
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Valve arrangements to rectify the flow could be possible. This would obviate the need for exit vanes
but a complex control system may still be required, reliability may reduce and parasitic losses would
increase if the valves required powered actuation.
Partial arc admission has also been proposed, for example the air flow path could be divided into
quadrants with two diametrically opposed quadrants being optimised for forward flow and two for
reverse flow. However, the practical implementation of this would need some development as simple
blanking would not be possible in a reversing flow stream.
For machines with variable pitch blades it is likely that a pitch change would be required during each
forward and reverse flow cycle; this is the philosophy for the Dennis Auld turbine under construction
for the Port Kembla project where real time wave measurement is incorporated in the control software
to maintain optimum performance. The success of this approach has yet to be demonstrated and the
increased complexity may not be cost effective and could introduce poor reliability.
Both Wells and impulse turbines for OWC applications have traditionally used symmetric blade profiles
on the basis that forward and reverse flow will be the same. Consequently much of the theoretical and
experimental work to date has been based on uni-directional flow, often under steady state conditions.
However, there is evidence (Pico, LIMPET) that forward and reverse flow rates are not the same and
thus there is a case for the investigation of asymmetric blade profiles optimised for each direction of
flow. For configurations in which guide vanes are used it follows that these can be of different profile
on each side of the rotating blades.
The use of asymmetric blade profiles also opens the possibility of introducing blade twist, lean and
section changes with radius to better match the incidence angles to the radial distribution of flow within
the annular duct.
The contra-rotating design has not proved successful at LIMPET and simpler monoplane and bi-plane
designs appear to be favoured for the future. The disappointing performance of contra-rotating
designs is believed to be due to having too much damping for the OWC. However, it should be
possible to design multiple stage turbines to provide similar damping to single stage turbines by
changes such as increased turbine diameter and reduced blade area. In the final analysis multiple
stage machines may only offer modest performance improvements that are not sufficient to justify the
additional costs.
8.2.3
Orientation of axis
The most common arrangement to date is for horizontal axis machines in a horizontal duct. This
reduces the thrust bearing loading (no self weight to be carried) and allows horizontal split casings for
maintenance access. For shoreline installations the risk of water ingress to the turbine under storm
conditions is also reduced as the atmospheric vent can be directed away from the wave source (at
LIMPET waves have been observed to break over the cliff top structure).
An alternative configuration is for a vertical axis machine in a vertical duct. However, this arrangement
has the potential disadvantage of turbine deluge during wave overtopping unless the atmospheric vent
is positioned substantially above the sea level.
8.3
Device construction methods
8.3.1
Balance of Plant Materials
Construction materials must be capable of withstanding operation in a hostile environment with
respect to corrosion, dynamic stressing and possible impact loading in the event of water carry over
from the wave collector.
To date, blades generally have been manufactured from a metallic material, the most common being
aluminium (LIMPET) or stainless steel (Port Kembla) or both (Pico).
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Aluminium offers some advantage in terms of being resistant to corrosion but may suffer long term
limitations in terms of fatigue under cyclic loading. Fatigue cracking under cyclic loading due to blade
vibration under stall conditions in a Wells turbine is potentially a significant operational and
maintenance issue.
At LIMPET, aluminium blades have been used in conjunction with steel rotor discs. The use of
dissimilar metals can present a natural tendency for corrosion due to galvanic cells being created,
particularly in a moist atmosphere. Blades removed from the LIMPET machine are reported to have
suffered particular corrosion pitting and possible cracking in the region of the blade to disc interface. It
was suggested that the blades would not be re-useable after just 6,000 hours running in 30,000 hours
of service.
Stainless steel is likely to be more fatigue resistant and due to its greater density will give a greater
contribution to rotor inertia (see below). However, the blade weight would lead to greater parasitic
losses for applications involving variable blade pitch due to greater inertia about the blade radial axis.
Blades manufactured from composite materials (e.g. resin impregnated carbon fibre) would be
expected to have excellent corrosion resistance and would have low pitch inertia for variable pitch
applications. However, composite materials may be constrained by strength limitations perhaps
making them more suitable for lower speed, lower diameter impulse type machines. In addition a
composite blade aerofoil is likely to be less resistant to water droplet erosion in a high moisture
environment and will therefore require a protective metallic coating. The application of the protective
coating may present manufacturing difficulties in terms of ensuring adequate adhesion to the
composite substrate. Nonetheless such blades are being proposed for a variable pitch machine
intended for installation at the Pico site in 2005.
The choice of material for rotating components is likely to have an impact on rotor inertia. This can be
significant in terms of response to load rejection (potential over-speed) and power quality issues. A
high inertia rotor is preferable and will reduce the need for a flywheel with a potential performance
benefit due to reduced windage losses.
Casings are generally fabricated structures manufactured from coated steel. Hot dip galvanising and
aluminium flame sprayed coating have been used to enhance corrosion resistance. Cathodic
protection may be appropriate in some circumstances. Casing technology has synergies with that for
duct structures for thermal plant and if correctly maintained these structures should be capable of at
least 25 years operational life and perhaps longer. Corrosion is likely to be the main life limiting issue
and high integrity corrosion protection or use of non-corroding materials such as plastic or fibre glass
will be required.
From a production viewpoint, metallic materials are most likely to offer the potential for mass
production, these being readily suited to existing fabrication and automated CNC machining methods.
8.3.2
Mass production / manufacture
Mass production would be feasible for a range of standardised turbines and a potential volume
manufacturer has indicated a maximum volume of around 200 units per week to be a realistic
production target within five years, a rate that is likely to far outstrip demand.
There is general consensus that the market is likely to be very large; one developer has suggested a
total worldwide market potential (for their own products alone) of around 12,000 grid connected
machines and 2,500 machines for stand alone remote generation. Current effort is concentrated on
demonstration plants to prove the technology prior to development of mass production sources.
Costs for mass manufacture would initially be high but would be expected to fall over a three to five
year period. Cost for a one off development machine at LIMPET was around £900/kW. This would be
expected to reduce substantially for a standardised design, perhaps to around £300/kW. Such figures
should be possible by the time 1000 units have been produced if previous energy technology learning
rates are experienced [76].
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It should be noted that for large coastal installations the cost of the turbine and balance of plant could
represent 5% or less of the total project cost. Unit costs for the turbine may not therefore be critical in
determining the viability of such projects. However, for near shore devices where a steel collector
structure is used the cost of the turbine and balance of plant is expected to represent a more
significant proportion (perhaps 20% to 30%) of total costs due to the lower cost of construction and
opportunities for standardisation of the collector structure.
8.4
Conversion efficiency
8.4.1
Understanding Pneumatic Capture
Pneumatic efficiencies of OWCs have generally been assessed using model testing. Although the
principles of wave capture are well-known, a ready means of developing and evaluating new collector
performance does not seem to be available. A simple spreadsheet based system was thus developed
to gain better confidence in the understanding whether developers were reporting capture efficiencies
in a manner that would allow general capture efficiency conclusions to be drawn. It also allowed some
basic testing to sea how capture efficiencies might be improved through the change of basic
parameters such as collector size and turbine damping.
8.4.2
Wave to pneumatic efficiency
An OWC sited in a wave flume could be configured for a particular monochromatic wave to have
100% power capture for the correct value of damping associated with the turbine. In reality, sea-states
comprise waves of a range of frequencies and this makes continuous turbine matching more difficult.
An energy model (developed for LIMPET) is typical of OWCs:
Fig 8.2
Energy model developed for LIMPET [6] (source: Wavegen)
The only part of the energy model that produces useful power is the heave of the water column
caused by incoming waves. The geometry of the collector should thus be selected to maximise the
energy in heave motion whilst minimising the losses and energy contained in outgoing waves.
The theoretical maximum conversion efficiency of the OWC (wave to pneumatic) at any given period
(frequency) of the incoming wave is thus determined by its geometry, the direction of the wave, and an
optimum value of the ‘damping’ in the chamber. The damping is determined by the turbine and
generator characteristics.
8.4.2.1
Resonant Frequency
The resonant frequency of OWCs can be established by numerical modelling or through model
testing. Resonant frequency determination has been examined using a linear diffraction analysis code,
AQWA, using LIMPET as a reference case.
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Response Amplitude Operator (RAO)
in Heave (m/m)
Dynamic Amplification on LIMPET model (7m water depth case)
14
12
10
LIMPET in Isolation
8
LIMPET with channel
geometry modelled
6
4
2
0
0
5
10
15
20
25
30
Period (s)
Fig 8.3
Resonant frequency determination using Linear Diffraction Modelling
Two analyses were carried out using 1m amplitude waves: one with the local gully geometry modelled
and one with the OWC in isolation. The resonant amplitude peak without the gully in the collector
chamber is around 4 seconds, perhaps illustrating the importance of the added mass of water in the
gully in increasing the resonant period. With the gully present, a resonant peak is observed around 5
seconds (the model would not calculate for lower periods than 5.5seconds) and secondary peaks are
observed that possibly relate to the harbour resonance effect (reported to be around 10 seconds
period).
Hence, although diffraction analysis is relatively quick to perform and gives a reasonable indication of
natural period, it is expected that a CFD model that included the air chamber and turbine
characteristics as well would be the more appropriate analysis platform. It is recommended that
research be directed to such CFD modelling techniques to assess their effectiveness.
8.4.2.2
Numerical Modelling
The spreadsheet model was based on the geometry and performance characteristics of the LIMPET
device. The model assumed a resonant period for LIMPET of 5 seconds [6]. Standard relationships for
the response of a single degree of freedom (SDOF) system (i.e. heave in the collector) to a forcing
frequency were used to determine the wave amplitude expected at different forcing frequencies.
Spectral techniques were used to establish the resonant response of the collector for each sea-state
the collector would experience in one year. A critical damping value for the collector was estimated by
calculating the mass of water in the collector (plus some added mass) and its water-plane stiffness. A
critical damping value of 3500kNs/m was estimated. The damping value due to viscous losses
caused by friction on the walls and the 40° bend was evaluated but this was much lower than the
expected turbine damping and was thus ignored.
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The work done by the turbine was calculated for the standard SDOF relationship for the working done
during resonant sinusoidal motion:
π P a = π c ω a2
where:
P
=
external force
c
=
turbine damping
ω
ω
=
natural frequency
a
=
amplitude of motion
P is determined from the potential energy of the oscillating water column.
The percentage of critical damping of the water column was set to be that given by the turbine
damping coefficient and the turbine damping coefficient was adjusted until the work done by the
external force matched the work done in the turbine.
Hs vs te
2.8
3.9
0.5
1.5
5.0
6.2
7.3
8.4
9.5
10.6
11.8
12.9
14.0
15.1
16.2
17.4
83.2%
74.3%
74.3%
66.6%
66.6%
66.6%
60.0%
60.0%
60.0%
54.4%
54.4%
54.4%
54.4%
49.6%
49.6%
49.6%
49.6%
49.6%
45.5%
45.5%
45.5%
45.5%
45.5%
42.0%
42.0%
42.0%
42.0%
42.0%
42.0%
38.9%
38.9%
38.9%
38.9%
38.9%
38.9%
36.2%
36.2%
36.2%
36.2%
36.2%
36.2%
33.9%
33.9%
33.9%
33.9%
33.9%
31.9%
31.9%
2.5
3.5
4.5
5.5
6.5
Capture factors for individual sea-states
The capture factors (lip power to pneumatic) produced by the spreadsheet were comparable to the
values reported by Wavegen for monochromatic waves. A typical value of 40% was reported by
Wavegen for 10 second waves. This gave confidence that the spreadsheet was a useful tool for
assessing the scope for improvement in capture efficiency.
The total capture efficiency obtained was 33% with respect to the 20m contour value and 47% with
respect to the chamber lip.
QUB Wide Tank - LIMPET 3D Optimum Damping Trials - August 2003
7m Water Depth
400
Sea31 - Pi
Sea29 - Pi
Sea24 - Pi
Sea21 - Pi
350
67.3kW/m
39.6kW/m
21.2kW/m
5.26kW/m
- 0.5%
- 1.5%
- 7.1%
- 9.5%
Pneumatic power [kW]
300
250
200
150
100
2800kNs/m
50
0
[Ns/m5] 0
[kNs/m] 0
17340
Fig 8.4
100
2890
200
5780
300
8670
Damping coefficient
400
11560
500
14450
600
LIMPET turbine damping optimisation tests [60]
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This compares with peak capture values reported for LIMPET [60] of 28.5 to 40% based on the 20m
energy flux values. These are estimated to be 43 to 60% with respect to lip energy fluxes.
An average turbine damping value for all sea-states of 2800kNs/m was assessed. This is close to the
optimum values that Wavegen reports, again raising confidence in the spreadsheet method.
8.4.2.3
Scope for Improvement
If the LIMPET chamber is increased in area (200m² rather than 159m²) until the capture factors
become one at the lower sensible energy ranges of the sea-states, the capture factor can be
increased to 59% at the lip and 42% with respect to the 20m contour. This increase will only be real if
this amount of energy is actually transmitted across the lip. This aspect will still need to be assessed
through CFD or model testing.
Further refinement of the chamber and possible variable damping arrangements might increase the
capture factor to 65 or 70% at the lip or conceivably 80% as some developers claim [5]. This
compares with 64% reported, by Wavegen [5], to be currently achievable for an ideal LIMPET. The
scope for improvement in both the understanding and assessment seems to be substantial and further
research effort in this area will probably yield the greatest dividends in better OWC performance.
8.4.3
Pneumatic to mechanical
Wavegen reported the following findings following investigation of the performance of the turbine
installed at the LIMPET device:
The efficiency of the Wells turbine depends on the turbine flow coefficient; with a clear stall point at
high flow coefficients resulting in a rapid reduction in efficiency.
To maximise turbine torque the amount of turbine flow needs to be limited to avoid an excess flow
coefficient and thus low turbine efficiency. For a Wells turbine the flow rate is proportional to the
pressure drop across the turbine and thus this condition is equivalent to limiting the pressure in the
plenum chamber. Thus, a perfect blow-off valve would operate so that the chamber pressure was
never higher than a critical value defined by the turbine characteristics.
The matching of the wave frequencies to that of the resonant frequency of the water column can
involve the active control of baffles or walls using sensors and actuators, or by opening up/closing off
smaller side chambers to the main water column chamber etc.
Research efforts in this area have taken place to actively control the ‘resistance’, ‘damping’ or ‘inertia’
of the ‘terminations’ within the water column. In this case, ‘terminations’ usually refer to the physical
properties of the escape route for the compressed air at the top of the water column. This can refer to
the turbine characteristic itself, and how this can be modified and controlled, or to other devices placed
in series or in parallel with the turbine to modify the dynamic characteristics of the air/water interface.
In many cases, these ‘other devices’ consist of an actively controlled valve or damping which affects
the amount of compressed air flow to, or around, the turbine. Physically, this active tuning of these
parasitical routes can be seen as varying the physical and dynamical properties of the OWC, and if
done in a sympathetic way, achieve the desired aim of providing optimum damping for the OWC.
The characteristics of the Wells turbine show that the torque/air pressure relationship peaks at a
certain value of OWC air pressure. Above this value, the turbine output torque falls and the turbine
can enter a ‘stall’ phase, where energy conversion efficiency falls off rapidly. It is therefore important
that the incoming wave energy profile or collector geometry is controlled so that the unwelcome stall
condition is not reached.
For an optimised design an instantaneous peak turbine efficiency of between 60% and 70% is
probably a realistic expectation for a Wells or an impulse turbine with flow in either direction. With
appropriate design development it should be possible to extend the breadth of the performance
characteristic such that greater than 50% efficiency is achieved over a defined range of flows.
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Mechanical to electrical
Experience with wind turbines has shown that directly connected synchronous machines do not cope
well with fluctuating power sources, especially in the range of frequencies observed in oscillating water
columns, but rather reflect these with undesirable sub-synchronous oscillations on the system. For a
directly connected machine, an induction machine is preferable, since the slip characteristic tends to
damp such oscillations. Generators for wind turbines usually operate at a much higher level of slip
than commercial induction motors, the increased losses being a small price to pay for the vital
damping action.
The review of existing projects and associated studies makes little mention of doubly-fed induction
generators (DFIGs), since they largely predate the more recent popularity of such machines used in
the wind power industry, however these modern drive systems are very similar to the well established
Kramer drive systems which have been used in OWC applications. It is hoped that experience gained
with wind power will prove useful in marine energy.
The control systems employed are largely a feature of the machine design, rather than the prime
mover. In many of the prototype OWCs a traditional slip energy recovery or Kramer drive, has been
used. The modern DFIG drives which are the current state of the art in the wind industry are
essentially the same concept, but with increasing more complex control strategies. In wind power
these schemes are generally cheaper than the other popular method, decoupling the machine
completely from the grid with an electronic converter. In wave power, however, if the machine is
frequently cycled between full speed and stationary, the converter for the DFIG may be no cheaper
than the in-line converter used with other kinds of machine.
The intermittency of the power source is something that OWCs have in common with WTGs, however
with current designs of OWC the low frequency pulsating nature presents a more onerous power
system issue. However as has been found with large wind farms, large arrays of OWCs will help to
smooth out this variation. Other existing power system techniques such as use of SVC and STATCOM
may also be needed to improve the quality of the supply. The use of existing technology and what has
been developed for the wind industry is likely to be used.
The control of output is more involved because it depends on the nature of the power source and the
nature of the electrical system. Historically, network operators have required a strong enough
connection for a small generator that minimal control is required; where this has been impossible, they
have tended to require extensive remote control of the machine from the network control room. There
are a number of innovative control strategies currently being developed and considered as part of the
OFGEM remit to connect more distributed generation. The developments fall largely into two
categories, enhanced and novel generator control systems, and co-ordinated network based control.
Again these are generic issues with the connection of generation into weak power systems that were
designed only for passive loads. It is likely that any solutions developed for the connection of
distributed generation will transfer into the wave power market.
In contrast to the turbine the electrical plant will make use of existing state of the art industrial and
wind industry technology. The range of mechanical to electrical efficiencies is expected to be 90-95%.
This range will depend to a certain degree on the type of technology used such as cage machine
direct drive or DFIG. However the most significant variations will result from the design choices made
in the range of device operation in varying sea states.
8.4.5
Total device efficiency
Measured performance data from LIMPET indicate that instantaneous peak turbine efficiencies of
around 70% for forward flow and 30% for reverse flow have been achieved. However, this was over a
relatively narrow energy input range and the challenge will be to extend this performance over a wider
range of sea conditions. Average performance for the LIMPET machine over the full energy spectrum
is substantially lower with overall wave to wire efficiency of less than 10%. This is understood to be
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largely due to poor original wave climate data and the consequential mismatch between the collector
and turbine.
The reverse flow efficiency of the LIMPET turbine is significantly influenced by the retrofitted acoustic
attenuator which causes poor circumferential distribution of the air flow. This is not expected to be an
issue for a machine in which the acoustic attenuator is included in the original design. Nonetheless,
differences between forward and reverse flow turbine efficiencies may persist due to differences in
boundary conditions in each part of the cycle. It is therefore considered more appropriate that an
overall efficiency should be stated that takes account of this variation.
The Dennis Auld turbine for Port Kembla has a predicted efficiency based on numerical modelling and
wave tank tests in excess of 80% peak. Overall turbine efficiency is expected to average around 54%.
No operational performance data are available for the Pico project due to a series of operational
problems not directly related to the turbine technology. Nonetheless from limited data the developer
(IST) is confident that the turbine efficiency will achieve that predicted in the design process. The
maximum attainable overall efficiency for the Wells turbine (forward and reverse flow) was thought to
be around 60% compared to around 50% currently.
When comparing overall efficiency values it is important to determine the basis on which efficiency is
being quoted, in particular the reference point for wave energy. Each developer currently appears to
be free to quote efficiency on any convenient basis, potentially making like-for-like comparison of
designs difficult. It is therefore recommended that a common standard should be developed for the
definition of the performance of wave energy devices (in line with current practice for fossil fired plant).
Some progress is being made on his issue at the EMEC facility on Orkney [80].
In the case of the LIMPET site the generator and turbine were over rated for the wave resource
available. This was due to mistakes made in assessing the resource. The question has been asked as
to whether the turbine and generator should be rated for the average or peak wave energy. The best
analogy here is with wind farm wind resource calculations. Once a set of annual wave resource data is
available for a given site then yield calculations can be carried out of different turbine and generator
sizes. The lowest p/kW will depend on the capital cost and wave distribution and cut in and out speeds
of the turbine. It seems likely that the turbine and generator will be rated above the average available
resource but well short of the peak storm conditions.
Until resource calculation tools and devices have reached greater maturity, it is difficult to determine
the best rating as many developers have found. Typically wind turbines have an electrical rating based
on around twice the average wind speed, however this typically corresponds to around four times the
power. Energetech the developer of the Port Kembla project have chosen to rate their turbine and
generator at three times the average available power.
A rating of three times the average power has been selected for the generic base case design. It is
suggested that two turbines be adopted per collector with the second generator being activated when
higher sea-states are expected. On this basis, only 2.6% of the available resource might be lost or not
fully-utilised as shown below.
Hs vs te
0.5
2.8
3.9
Not operating
1.5
2.5
5.0
733
6.2
902
8,114
One 375kW turbine
3.5
4.5
7.3
1,020
9,183
25,508
8.4
1,103
9,928
27,579
9.5
1,162
10,459
29,052
56,942
Two 375kW turbines
5.5
6.5
10.6
1,205
10,846
30,128
59,051
97,615
11.8
1,237
11,136
30,934
60,630
100,225
12.9
1,262
11,358
31,550
61,839
102,223
152,704
14.0
1,281
11,532
32,032
62,783
103,784
155,036
Flow to be throttled for Two 375kW turbines
15.1
1,297
11,669
32,415
63,534
105,025
156,889
16.2
1,309
11,781
32,724
64,140
106,027
17.4
1,319
11,872
% Resource
2.1%
52.0%
43.4%
2.6%
7.5
Turbine selection for range of energy fluxes at 8m depth site
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8.5
Survivability
8.5.1
Wave loading
The design of near-shore caisson structures requires a detailed understanding of the wave loading to
which they are subjected. Wave loading can be determined from diffraction theory for unusual
geometries or from breakwater theory for reasonably uniform-fronted caisson or shoreline OWCs.
The study of impacting loads on caisson structures has been the subject of recent research through
the PROVERBS [61] programme. Caisson structures and their sub-elements have previously been
designed for high impact pressures treated as quasi-static loading. However, the nature of the
impacting load events is that they typically act over less than 100msec. An approach that considers
the natural period of the element being designed in relation to the impact duration allows a much
smaller quasi-static force to be designed for. This approach should be researched further as it will
have an important bearing on future OWCs designs.
It is preferable to site an OWC in an environment where impacting waves are less prevalent.
PROVERBS indicates that if the proportion of impacting events is less than 2% then impacting
conditions can be ignored. Figure 8.5 below shows that these conditions are present above about 11m
water depth taking offshore Islay as a reference case.
45.00%
40.00%
Waves Breaking or Impacting
35.00%
30.00%
25.00%
Breaking Waves
Impacting Waves
20.00%
15.00%
10.00%
5.00%
2%
0.00%
6
7
8
9
10
11
12
13
14
Depth (m)
Fig 8.5
Percentage of Breaking and Impacting Waves in Sea-State
The adoption of a sloping face to the OWC also limits the potential for a plunging breaker impacting
the OWC. Tank testing by Wavegen seemed to confirm this for LIMPET [6]. If plunging breakers are
not present this allows the front face of the chamber to be designed for normal reflecting wave
pressures such as those given by Goda [62]. This should be taken forward in future designs as it
reduces the capital cost of the structure.
8.5.2
Return Period
In practical near-shore water depths, the design wave height is limited by the water depth. A maximum
wave height of 0.78 x water depth is conventionally used [63]. The concept of a return period for
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design thus has little meaning as the depth will govern for waves of the minimum return period
appropriate for a design life of typically 25 years.
8.5.3
Balance of Plant
Both LIMPET and Pico have experienced problems with flooding of the plant room during wave
overtopping. At Pico this is being addressed by proposed relocation of all electrical and control
equipment except the generator to a separate installation around 100m inland. Where electrical
equipment is located within a plant room in the main structure it will need to be enclosed to at least
IP56. Some of this protection may be provided by the structure of the module. Water tight
construction techniques should be applied to the construction of the plant room including water tight
doors and access penetrations.
The turbine should ideally be designed to withstand 100% deluge, particularly if used in a vertical axis
configuration. The Port Kembla machine (horizontal axis) is understood to have been designed on this
basis. Where such capability is not part of the turbine design the layout of the installation should be
such as to minimise the risk of major water ingress.
Location of the turbine rotor body and the generator in a nacelle structure will give additional
protection and limit the risk of water ingress.
8.6
Reliability, operation and maintenance
Operational reliability of the LIMPET machine is reported to be high. However, the availability is rather
low (65%) due to frequent loss of grid connection. The problem is thought to be more related to poor
design than any inherent weakness in the local grid system. The Pico plant is located on a small, and
potentially less stable, island grid system and no such similar problems are reported albeit operation
has been somewhat limited for other reasons.
The control system at LIMPET is such that remote manual intervention is required for a re-start
following a plant trip under fault conditions. In general this is a sound philosophy, as it will protect the
plant from further damage in the event of a genuine fault. However, the current arrangement includes
re-starts following loss of grid connection, even if the loss is only of short duration. The noncommercial basis of operation has meant that depending on the time of the grid loss, the plant can be
shut down for several hours or days. The provision of auto restart following loss of grid should enable
an availability of greater than 90% to be achieved.
For unmanned operation the reliability of the control system must be high. The plant must be capable
of automatic start up and shut down according to wave climate. The capability for remote interrogation
of the control system for condition monitoring and investigation and re-start after trip is essential.
Standard remote monitoring systems similar in principle to that for the LIMPET installation are likely to
be readily available in the market place.
Maintenance requirements must be low for an unmanned site and the plant must be capable of
sustained operation without manual intervention for long periods. Given the relatively simple nature of
the OWC plant, maintenance should not prove too onerous in comparison to floating devices and
devices that have hydraulic power conversion systems. Planned routine maintenance at 8,000 hour
(1 year) intervals should be the minimum expectation as for conventional thermal plant. Annual
maintenance would be expected to encompass inspection of the turbine and generator bearings,
inspection of the turbine blades (fixed and moving) and blade actuator mechanisms (variable pitch
machines) and checks on the control and electrical systems. Typical routine operational maintenance
would include greasing of inlet valve bearings and actuator linkages, and draining of compressed air
receivers. Both of these activities should be achievable by automatic means.
In time, the requirements for maintenance and reliability can hopefully become as low as those for
hydro-electric plant.
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For shoreline OWCs the main maintenance consideration is the likely remote location of the plant and
the potential for associated transportation difficulties if major work is to be undertaken such as removal
of plant to works for maintenance or repair. However, provided adequate facilities are incorporated in
the design (e.g. local lifting and workshop facilities) on-site routine maintenance activities should be
expected to proceed as with any other electro-mechanical installation of similar size.
For near shore devices there is the additional consideration of access to the structure and
maintenance in a potentially more hostile environment. One approach to alleviate this issue might be
to develop modular turbines that could be removed for maintenance on shore in a controlled workshop
environment. Use of a rotating spare would minimise the downtime for maintenance to that for
exchange of the module. The same principle could also be applied to shoreline OWCs.
8.7
Prediction and modelling
It has been illustrated earlier that the calculation of the device resonant frequency can be carried out
initially by simplified mathematical formulae that include the geometrical size of the opening, excitation
forces, damping and stiffness.
More sophisticated methods such as boundary element methods have been used that more
accurately predicts the diffraction around and inside the device. Transfer functions can be derived
from the surface velocity within the diffraction model that can relate to the air pressure inside the
chamber. A more accurate prediction of the power available can then be made based on the site
spectra encountered throughout the year. A further sub model based upon the performance of the
turbine and other electrical equipment can be made that will further refine the model to give a full
wave-to-wire prediction.
One of the shortcomings of the boundary element method is that it is based upon linear diffraction
theory and therefore cannot account for higher order viscous effects. This is particularly important in
the design of the lip of the inlet to the chamber.
A method used for design of the lip into the chamber is Particle Image Velocimetry (PIV). This method
illuminates the water particles by use of very thin laser light and a combination of lenses. The losses
around the lip can be quantified using this method. It has been found through using this method that
the size, shape and inclination of the lips are of importance.
Computational Fluid Dynamics (CFD) software has been used in some applications of wave energy
devices. CFD software has the capability to model the waves, viscous effects, air pressure movement
and turbine rotation. A model of this size would involve enormous computational effort and the costs
would possibly become similar to tank testing. This could, however, become a more favourable option
in the future as the software and computer technology improves. It should be noted that such CFD
models have been created for the Port Kembla project currently under construction in Australia and to
a partial extent for the Pico project.
8.8
Control of export power
The primary source of short-term variations in mechanical power will be the waves themselves –
generating a high level of output as the water rises and falls, and much less output in between these
states. The frequency of this variation will vary according to the wave conditions, and there will be a
range of frequencies characteristic of any particular site.
This is by contrast with wind power (from which the control system is likely to be adapted), which
experiences random variations (gusts), and a sub-synchronous variation at the blade frequency,
directly related to the mechanical speed. The OWC controller must cope with sub-synchronous power
variation at a range of frequencies; it will probably be necessary to analyse the system for subsynchronous resonance and program the controller for maximum damping at the resonant
frequencies. On the other hand random variation is likely to be much less, and the controller will
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probably not need to accommodate power spikes of several times the continuous rating, as are
observed from wind turbines.
It is likely that initial commercialisation of OWC will utilise fully rated converter drives and cage
induction generators rather than the DFIG generators that have become the standard on large WTG.
Power quality issues may lead to the adoption of DFIG in the long term once a significant small
generator market is established.
Steady state control of export power will be carried out by operating vanes in the air path, but transient
control, using the energy stored in the converter electronics, can be given the required characteristic in
software.
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9
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Future Visions
A generic design incorporating the improvements highlighted in the previous sections has been
developed as the future vision of OWC technology. The generic design is a near-shore OWC device.
The exact nature of the generic device, its geometric form and materials of construction are merely
one future vision. The aspects considered in this section could be applied to a number of OWC
devices and the most successful will emerge once a number of these fundamental design exercises
have been worked through. The designs that are ultimately successful will most likely be the ones that
can deliver the highest learning rate.
9.1
Location
An assessment method was used to rank areas around the UK in terms of their potential for
construction and operation of the generic OWC [Section 5]. The top twelve UK near-shore areas
identified were:
1.
Shetland Islands
2.
Islay, Jura
3.
Orkney Islands
4.
Tiree
5.
The Small Isles
6.
Colonsay
7.
Isle of Lewis and North Harris
8.
Benbecula
9.
North Cornwall / Devon
10. North of Scotland
11. North Uist
12. South Uist
In developing a test case future vision, Islay has been chosen as a development location. Although
not the highest ranked it does allow some of the learning from the existing LIMPET device to be used.
9.2
Optimum water depth
There is a need to optimise the design for water depth, distance from shore, wave height and structure
loading. Such work will identify a preferred depth range in which to site a fixed near-shore device for
the greatest overall capture efficiency and economy.
9.2.1
Energy vs. Water Depth
When considering the development of a shoreline or near-shore device, the energy losses from
offshore locations into near-shore and shoreline locations have to be determined. The main energy
loss mechanisms are related to refraction and shoaling, seabed friction and wave breaking.
The seabed profile has a significant effect on the available energy at inshore locations. A shallow
sloping seabed is more likely to create more energy loss mechanisms such as seabed friction and
wave breaking than a steep sloped seabed.
It is recommended that reporting of wave capture efficiency is based on the energy flux at the 20m
contour. Studies on the near-shore wave climate have typically been carried out by adjusting offshore
data for near-shore conditions. Data on the reduction in wave energy with depth has been less widely
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published so a typical calculation was performed using hindcast data corrected for shallow water
effects provided for offshore Islay by Argoss [57]. This gives an energy versus depth relationship as
shown in Figure 9.1.
Energy Flux
28.0
24.0
kW/m
20.0
Pierson-Moskowitz
JONSWAP
16.0
12.0
8.0
4
8
12
16
20
Depth m
Fig 9.1
Energy Flux versus Water Depth
Figure 9.1 shows that the penalty in reduced energy flux only becomes marked with water depths
below 9m. The choice of a Pierson-Moskowitz spectrum [58] gives a 2% greater resource value than
the peakier JONSWAP spectrum developed for fetch-limited conditions.
It may be more appropriate for shallow water spectra, such as the TMA spectrum, to be used for
energy flux determination as these include the energy that is shifted from the peak due to wave
breaking [59]. This could have a bearing of capture efficiency.
9.2.2
Wave Loading versus Water Depth
The wave loading on a caisson structure can be established by using Goda’s formula in reflecting
conditions. In impacting conditions, the approach of Allsop and Vicinanza [64] can be used together
with an assessment of the lateral and rocking natural periods of the caisson structure. Using a typical
design, the variation in wave loading with water depth was assessed as shown in Fig 9.2.
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1400
Horixontal Load (kN/m run)
1200
1000
800
Goda
Impacting
600
Design Load
400
200
0
6
7
8
9
10
11
12
13
14
Water Depth (m)
Fig 9.2
Gravity Structure Wave Loading versus Water Depth
This shows that depths of 9 to 10m are promising in terms of minimising wave load whilst maximising
available wave energy.
9.2.3
OWC cost versus water depth
The results of the wave load determination can be used to perform outline sizing of structures capable
of resisting the loading in sliding and overturning.
Designs for four types of structure were considered:
•
Concrete gravity substructure on sand (or gravel/rock) foundation with sand ballast
•
Concrete gravity substructure on stiff clay foundation with sand ballast
•
Steel gravity substructure on sand foundation with sand ballast
•
Piled steel structure with steel cylindrical collector structure with subsea wave guides
The capital costs were normalised against the minimum cost concrete substructure on the sand
foundation. The results are shown in Fig 9.3.
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2
1.8
Relative cost (min sand cost = 1.0)
1.6
1.4
Relative cost concrete caisson
medium-dense sand
Relative cost concrete caisson stiff
clay
Relative cost steel piles
1.2
1
0.8
Relative cost steel caisson
0.6
0.4
0.2
0
6
7
8
9
10
11
12
13
14
Depth (m)
Fig 9.3
Assessment of CAPEX versus water depth
The structure size and cost in Fig 9.3 follow a similar trend to the wave loading water depth
relationship. Although structures in the minimum water depth considered appear cheapest, costs
exhibit a second minimum between 8 and 10m. The most cost effective solution is the concrete
caisson structure on stiff clay soil. However, a survey of surface sediments around the target areas for
near-shore OWC development showed a greater prevalence of sand or rock at the surface. Sand/rock
foundations have thus been considered as the future vision.
Steel caisson structures of similar geometry to the concrete caisson (i.e. relatively continuous
structure with a collector width similar to the device width) appear less favourable because of the cost
of steel fabrication compared to concrete construction. However, a more favourable cost for the steel
caisson might be obtained by reducing the collector width at the water line and using subsea guide
walls, as with OSPREY and Port Kembla. This might reduce capture efficiency, however, thereby
negating some of the cost saving arising from reduced wave loading.
Based on this assessment, a concrete caisson OWC appears most favourable for near-shore
installation.
This evaluation also shows that the rate of increase of cost with water depth is just less than
proportional to water depth squared for depths exceeding 9m, as has been used in the tidal amplitude
impact assessment (Section 5).
9.2.4
Sea-bed Conditions
The ideal choice for the gravity caisson would be a level sand or clay foundation. However, the most
likely foundation material in the better resource areas is rock that could well have an uneven even if
covered with a layer of sand or gravel. The caisson structures cannot tolerate excessive out-of-plane
deformations so the sea-bed has to be prepared in some way to best suit the OWCs. An OWC
founded on uneven sediments can adopt underbase grouting together with the means to break the
structure free at the time of decommissioning. Rock anchors could be considered where the OWC is
founded directly on rock, although this does increase the offshore work content. Siting on rock is
beneficial in that solid ballast can be reduced. In economic terms, the reduced cost of ballasting is
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considered to equate to the cost of either under-base grouting or rock anchoring in the costing
exercises presented in later sections.
9.2.5
Overall Economics of Power Capture against OWC CAPEX
If the curve of available power versus water depth is combined with the average OWC capital cost, a
further trend curve can be prepared as shown in Fig 9.4. This clearly shows that it is advantageous to
site the device in around 10m water depth.
1.4
Relative capital cost of power production
1.3
1.2
1.1
1
0.9
0.8
6
8
10
12
14
Depth (m)
Fig 9.4
Relative capital cost of delivering power versus water depth
9.3
Structure
9.3.1
Design description
Having established the ideal coastal area and optimum depth for the proposed near-shore device, the
wave loading regime and other design drivers can be assessed.
The device principal dimensions were established for the installed depth and wave climate to ensure
that the wave trough does not cause chamber broaching, that the amplified wave within the chamber
does not cause turbine deluge, and that the device is not significantly over topped. From these
constraints a caisson was developed that would have an installation draft less than 8m.
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OWC Finite Element Analysis Stresses
A reference design was developed for conditions offshore Islay in 9m water depth to chart datum and
tidal amplitude of 2m.
Tow out conditions were analyzed to establish internal cell pressures and assess device torsion. The
ballasting requirements both for device stability at float out and to resist horizontal loading, sliding, and
overturning once installed was calculated, as well as checking the peak base contact pressure.
The chamber front wall wave loading was also analyzed to check the structural integrity of the caisson.
OWC Analysis Deflections
9.3.2
Foundation
The foundation design has been based on medium-dense sand conditions. Such foundations can be
made stable in sliding and overturning through the addition of solid ballast material. The device has
been configured to allow sand ballasting and the ballast chamber has been configured so that the
ballast is best placed to resist overturning.
Experience has shown that, where sandy sea-beds are found, they are normally acceptably flat for
directly placing the OWC on the sea-bed so that under-base grouting is not required.
The foundation design of caisson structures could follow the established Det norske Veritas (DnV)
method used for oil and gas installations [65] or the International Navigation Association (PIANC)
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approach used for caisson breakwaters [66]. The PIANC approach accepts a greater probability of
foundation failure than is inherent in DnV and will thus lead to lower solid ballast requirements for
gravity foundations. The PIANC design guidelines recognize that near-shore wave protection facilities
are usually multiple structures and the failure of one unit can perhaps be tolerated. There is a lower
threat to personnel safety on an unmanned caisson compared to a manned offshore oil and gas
installation. Both these considerations would apply to near-shore caisson OWCs so it would seem
appropriate to consider the PIANC approach for design. It would also be a useful exercise to consider
overall safety and reliability levels that ought to be achieved for these unmanned devices. This work is
currently being undertaken by DnV for the Carbon Trust.
9.3.3
Device concept
This is illustrated below:
9.3.4
Scour Protection
Scour protection is required, particularly to the front of the support piers and in front of the collector
chamber, to prevent foundation undermining. This would be a two-layer graded rock blanket. A steel
skirt is provided to allow the structure to be founded below any looser sand at the surface and to
inhibit undermining between device emplacement and the placing of scour protection.
Similar arguments to the risk-based approach to foundation design can be advanced for the design of
scour protection. Complete loss of scour protection would lead to undermining of foundations on sand
but a design that permits some loss of material in larger design events followed by inspection and
maintenance is probably a more cost-effective approach than designing the scour protection to
withstand the worst conceivable conditions without damage.
9.3.5
Construction
Construction of near-shore OWCs can take place away from the offshore site making weatherdowntime considerations less relevant. There is no requirement for the design and construction of a
temporary cofferdam and bund/cofferdam overtopping leading to possible formwork damage does not
have to be considered. The adoption of a construction site dedicated to OWC production allows much
greater potential for productivity gain leading to lower capital cost.
The initial stage of construction would be the preparation of the dry dock.
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A suitable dry dock for multiple device construction would be Hunterston Marine Yard, located near
Clydeport 30 miles south west of Glasgow on the west coast of Scotland. The dry dock facility covers
an area of 90 acres and was a specialist in the construction and decommissioning of offshore
structures.
Hunterston’s main features are:
Dock Floor Length
230 m
Dock Floor Width
150 m
Width (Earth Bund Entrance)
150 m
Maximum Depth (In Dock)
14.5 m
Maximum Depth (At Sill)
11 m
The yard benefits from good road communications, easily accessible from Glasgow and has rail
facilities available from Clydeport’s adjacent Hunterston Coal Terminal site.
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Once the dock is prepared, and the base slabs constructed slipforming of the buoyancy tank piers can
take place.
The OWC chamber cell modules would be constructed horizontally, for ease of construction.
The chamber modules are then rotated into position
The balance of plant would be fabricated off site and installed as a complete containerised module into
the buoyancy pier. It is envisaged that there would be three generators per device. This ensures that
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wave fronts that are not totally parallel to the device do not cause an out of phase wave pneumatic
contribution to a single turbine.
Due to the requirement for installation in relatively shallow water, and to reduce the size of the
buoyancy piers (thus reducing wave loading), the chamber volume is required to provide buoyancy
during transportation. This would be achieved by placing buoyancy bags in the chamber modules.
The Dunlin A Platform utilised this technology during float-out.
Example : Concrete Pipe Positioning Floats
With the device construction complete the dock is flooded….
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…… and the bund removed.
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Installation
OWC installation should follow normal working practice for the installation of offshore oil & gas
platforms. A transportation sea-state can be selected based on the duration of the tow. Typically, a 10year seasonal storm should be considered for tow and installation durations that exceed 3 days.
The marine transportation and installation will usually be insured and the insurer would normally
appoint a Marine Warranty Surveyor (MWS) to evaluate the transportation and installation plans. The
MWS would record approval to proceed with particular stages in the marine operations, once satisfied
that the operations met normal industry standards and that metocean conditions were favourable.
OWCs should be designed to satisfy normal intact and stability criteria for oil and gas installations [69].
The scheduling of OWC construction and installation should aim to complete all offshore installation
work between typically mid-April to late September. A maximum installation sea-state of 1.5 to 2.0m
Hs is the practical limit for personnel transfer to the OWC to operate the flooding system for set down.
The design of caisson devices should recognize that swell conditions are likely to be present at the
time of installation, even if the total sea-state is less than 1.5m Hs. Swell can cause substantial heave
motions on caisson devices and the design should ideally be configured to suppress heave excitation
to allow a straightforward set-down procedure.
On sand sites, the installation should not proceed unless the scour protection vessel is on standby to
place the initial protection material. Failure to place scour protection within around 24 hours after OWC
emplacement could lead to the foundation being undermined.
9.4
Turbine
9.4.1
Turbine sizing
Development machines to date have been typically of the order 500kW either as a single unit or as
two 250 kW modules in series.
Economies of scale are likely to be achieved for larger units. There is, in theory, no limit to the size of
unit that could be manufactured. However, the practical limit for a device will be the wave energy
available to the collector structure over a reasonable length of capture and the energy losses
associated with wave interactions inside the collector structure. Such considerations are likely to limit
the maximum energy that can be obtained such that the maximum unit size is never likely to exceed
2MW.
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Optimum performance may best be achieved by designing unit capacity for commonly occurring sea
states rather than extreme storm conditions. The performance would then be optimised for a greater
time period and the parasitic losses (particularly on the generator) would represent a smaller
proportion of average output. To permit continued operation during storm conditions wave collectors
would need to be designed for partial close off to moderate the pneumatic power input to the turbine,
or additional turbines brought on line.
9.4.2
Blade profile
It is clear that a broad variety of blade and vane configurations is possible. Future effort should be
directed to establishing the optimum configuration and, to support this, a structured development
process is required. In the first instance the options should be evaluated by the use of CFD
techniques, leading on to scale model tests in a wind tunnel or purpose designed test rig before final
development of a prototype for field trials. All generic blade types should be considered.
At all stages the aim should be to achieve acceptable turbine performance over as broad a range of
air flows (sea states) as possible, thereby reducing the sensitivity to collector design. From a reliability
point of view the design should aim to be as simple as possible with the minimum number of moving
parts and without an overly complex control system. Limiting the number of moving parts will also
deliver benefits in terms of a reduction in parasitic losses and reduced maintenance burden.
The benefits of this approach are expected to be a reduction in cost of turbine manufacture
(standardisation, fewer components, simpler control system), improved performance (estimated up to
a factor of five compared to current prototypes) and improved reliability.
9.5
Generator
9.5.1
Specification
The generator will be a doubly-fed induction generator (DFIG), i.e. a wound-rotor induction machine
with an ac-dc-ac converter in the rotor circuit. The rating of the generator stator will be 750 kW
electrical for the near-shore device; the rating of the rotor circuit will need to be determined following
detailed modelling of the turbine and water column. To a first approximation, the rotor power is given
by:
Rotor Power = Stator Power ×
Mechanical Speed − Synchronous Speed
Synchronous Speed
The rating of the rotor circuit is therefore determined by the range of speeds over which the turbine is
to operate.
For simplicity of design, both the stator terminals and the grid-side converter terminals will be
designed to connect at 400 V.
9.6
Costing
9.6.1
Caisson
A generic shoreline cost was produced from a quantity take off of the LIMPET as-built structure. This
was based on a modified design-then-build approach where the design development risk is taken by
the operator and the constructor and installation risk is taken by suitably experienced contractors in
their respective fields. A novation arrangement, whereby the OWC designer completes the detailed
design for the civil contractor, would allow constructability input to be brought into the design, helping
to reduce construction cost.
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From the scheme near-shore design a cost was produced for the production of a single device. This
cost was then modified for the construction of a 10 device production run such that certain economies
of scale could be applied, then again with productivity discounts applied to certain construction
activities.
Economies of scale cost reductions include•
Preparation / modification of dry dock
•
Re-use of pre-fabricated formwork
•
Repeat design
•
Offshore SI
•
Tug / ballast vessel mobilisation
•
Flotation bag re-use.
Increased productivity discount rates were applied to –
•
Pre-fabricated rebar panels
•
Slipform preparation / re-use
•
Installation procedure
•
Plant and equipment usage.
9.6.2
Turbine
Manufacturing experience to date is limited to a small number of development machines and costs of
production are consequently high. In addition the cost breakdown between the major constituent parts
of the machine (turbine, generator, control system) is not readily available from the developers. While
costs for mass production would be expected to fall substantially compared to those for small volumes
of prototype machines, it is difficult, if not impossible, to give an accurate prediction of future
manufacturing costs based on current experience.
The commercial viability of OWC technology will be determined by the total costs for initial
construction (and hence debt financing) and ongoing maintenance in comparison to the expected
income stream over the life of the plant. For an end user, the breakdown of construction costs
between the constituent parts is usually less important than the overall unit cost and in evaluating the
commercial viability of a project it is standard practice to consider the capital cost of plant in terms of
cost per kW installed. Costs to date for an OWC turbine, generator and balance of plant range from
around £900/kW (excluding civil work) at LIMPET to around £2500/kW at Port Kembla (including
design development). The Port Kembla team foresee this cost reducing to £625/kW by the time 1000
units have been constructed. The Port Kembla costs have been queried, as the device does not seem
likely to become economic if correct. Instead, the LIMPET costs have been used as the starting point
for future improvements.
9.6.3
Generator
The generators proposed for the reference design are the doubly-fed induction machines now almost
universal in the wind industry, and the costs are based on figures provided by wind turbine
manufacturers. The figures cannot be used directly because the requirements for the generator are
not the same as for a wind turbine:
•
In the wind industry it is now standard, due to the large distances between turbines, to
include a step-up transformer and high voltage switch with each generator. For OWC
devices, it may be more economical to cable between generators at low voltage, and share a
transformer between two or three generators. The step-up transformer and HV switch are
therefore not included here but in the grid connection costs.
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•
Even off shore, wind turbine plant is elevated well above the salt spray, and protection
against moisture ingress is often somewhat rudimentary. By contrast, wave power devices
are potentially fully exposed to the waves, and a higher degree of protection is required.
•
The nature of the power fluctuations for wind power (deriving from gusts, and from the blades
passing the tower)) differs from that of the fluctuations due to waves. Some custom tuning of
the controller software may be required to suit the wave climate of individual sites.
While the wind industry is now concentrating on large machines, typically above 600kW, the reference
OWC design uses 2 x 375kW machines (the few wind turbine manufacturers still offering 375kW
machines use conventional induction machines). A choice will have to be made between the use of
cage induction generators with a fully rated converter drive and the DFIG currently favoured by the
wind industry. It is likely that initial commercialisation will use conventional cage machines. The DFIG
may become favoured in the long term once there is a significant sub 1MW market for DFIG. For the
750kW reference design considered, the choice of cage induction generator or DFIG will not
significantly affect the BOP costs.
The cost of a 250kW generator, together with associated plant and a suitable enclosure, has been
estimated at £100,000. This gives a cost for the generator of £400,000 per megawatt.
If the cost of the enclosure, and the cost of installing the generator in it, are accounted elsewhere in
the model, a figure of £250,000 per megawatt may be achievable for the generator and auxiliaries
alone. If larger generators are used, such as are now standard in the wind power industry, the costs
may be reduced even further.
9.6.4
Grid connection
The purpose of these studies is to establish the approximate cost of connecting OWC generating
modules to the grid in the regions identified as most suitable in phase 1 of the project. The figures
should be treated with some caution; in particular, the following caveats apply:
•
No actual sites have been selected at this stage, and the costs given are indicative for each
region. The costs for actual sites will depend strongly on the site chosen.
•
Where a major reinforcement, such as a 132 kV cable, is indicated, the cost of the
reinforcement is largely independent of the capacity installed. In such cases, the permegawatt figures quoted refer to installed capacities close to the limit stated. Lower
capacities, unable to achieve the same economy of scale, will have higher costs per
megawatt.
•
The costs quoted assume that development is in small clusters of no more than a few
megawatts each, and that each cluster requires switchgear, step-up transformer(s), and
overhead line to connect to the system.
The costs are quoted in most cases for 11/33 kV and 132 kV. 11 and 32 kV are treated together
because the cheaper plant at 11 kV is balanced by the smaller cluster size that can be
accommodated.
9.6.4.1
Orkney and Shetland
Orkney and Shetland are two groups of islands off the north coast of Scotland. The Orkney islands
are separated from the mainland by a strait, the Pentland Firth, which at its narrowest is little more
than 10 km wide. This is crossed by two 33 kV cables, with a capacity of 20 MW each, connecting the
Orkneys to the mainland.
Demand on the Orkney islands varies from a summer minimum of 6 MW to a winter maximum of
31 MW [52]. The supply is therefore adequate, but not secure; security is provided by a 16.5 MW
diesel power station at Kirkwall, and there is a gas turbine of unknown capacity, apparently associated
with the oil terminal on Flotta [53].
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The capacity for generation on Orkney is 26 MW if a firm connection is required, or 46 MW if an
interruptible connection is used (i.e. up to 20 MW would be tripped on the loss of a cable to the
mainland). Above this level, load following would be required, or an additional cable to the mainland.
The price of an additional 33 kV cable has been estimated at £15 million. In any case, additional
voltage support would be required.
The Shetland islands are approximately 213km, by a likely cable route offshore, and have at present
no connection to the mainland. The maximum demand is believed to be around 50 MW, so minimum
demand is unlikely to exceed 10 MW. At present all the electricity in the Shetlands is supplied by a
67 MW diesel power station at Lerwick; a replacement power station, or possibly a DC link to the
mainland, are being considered [54].
11/33 kV Connection
The islands provide two variants of the connection problem: on the western part of Mainland Orkney
and the northern two-thirds of Mainland Shetland, the power lines run 20 km or more from the coast.
On most of the smaller islands, the southern peninsula of Mainland Shetland, and the east coast of
Mainland Orkney, power lines run within 10 km of the coast.
Substations are plentiful, and a reasonable density of wave power generators might be connected by
perhaps 10 km of overhead line per 5 MW of generation. Each cluster of around 5 MW would also
require switchgear and a step-up transformer.
Smaller clusters might be able to connect at 11 kV. This would allow the use of cheaper switchgear
and transformers; but, apart from the smallest clusters, an 11 kV connection would require a longer
overhead line (in order to connect at a suitable substation), and the price per megawatt remains much
the same.
As noted above, the maximum (cumulative) size for such schemes is around 40 MW on Orkney plus
10 MW on Shetland. The price (based on Central Networks data) is around £100,000 per MW.
132 kV Connection
A 132 kV connection on Orkney or Shetland would require a new cable. The most likely route is from
the supergrid substation at Dounreay to a new substation at Stromness (for Orkney) or Scalloway
(Shetland). The cost of this cable would dominate the cost of the project: it has been estimated at
£40 million for Orkney, or perhaps £150 million for Shetland, dependent on capacity. Connecting
generators to the cable substation would cost only slightly more than connecting to the existing 33 kV
system, since the latter could transmit much of the power required.
The capacity of such a cable would be between 100 MW and 200 MW, and would not be secure. A
reasonable estimate of the cost of connecting generation would therefore be £1.5 million per
megawatt on Shetland, or £400,000 per megawatt on Orkney, provided the full capacity of the cable is
utilised.
Orkney
11/33 kV, up to 40 MW
£100,000 per MW
132 kV, 150 MW
£400,000 per MW
Shetland
11/33 kV, up to 10 MW
£100,000 per MW
132 kV, 150 MW
£1,500,000 per MW
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Islay, Jura, Colonsay
This group of islands, at what might loosely be described as the Southern end of the Inner Hebrides,
connects to the mainland by a 33 kV cable across Jura Sound to the Mull of Kintyre. The 33 kV power
line runs along the East coast of Jura and across the centre of Islay; in most parts of the island it is
around 10 km from the presumed preferred wave energy sites on the West coast.
The transmission system on Kintyre, from a generation point of view, is full. With all the connected
hydro and wind power operating at full output, the 132 kV line from Inveraray to Loch Sloy is not
secure: an outage of one circuit of this line could leave the other slightly overloaded.
Any generation, therefore, connecting in these islands, would need a control system for load following,
an intertrip, or a system reinforcement.
Minimum demand on the islands is small – around 2 MW – so the limit on generation derives from the
booster transformer at Tayvallich, which is rated at 15 MVA.
11/33 kV Connection
Islay has two 33/11 kV substations rated at 8 MVA, but the 11 kV system on Jura is extremely weak,
with 1 MVA being the highest transformer rating seen. On Islay, then, there is the possibility of
connecting a small installation to the 11 kV system, while on Jura, all the installations will be
connected at 33 kV. As with Orkney and Shetland, 11 kV connections are likely to be possible only for
small installations, and 33 kV is likely to be required for larger projects. The economies of scale
balance the increased costs of 33 kV plant, and the cost is about £120,000 per megawatt at either
voltage.
The link to the mainland limits the capacity to around 15 MW. Larger capacities could probably be
achieved by reinforcing this link with a second cable. Best use of existing assets would be achieved
by routing the cable from Port Ellen to West Park Fergus, on the Mull of Kintyre. This would be
expensive – around £20 million – but cheaper options are possible if a lower capacity is acceptable.
This gives a total cost of around £500,000 per MW.
132 kV Connection
A 132 kV connection is required if large numbers of generators are to connect. This would involve a
new cable to connect the islands; 132 kV infrastructure on the islands themselves; and a
reinforcement of the existing 132 kV system between Kintyre and the rest of Scotland.
The cost of this reinforcement is approximately the same whether the existing lines are re-strung
between Loch Sloy and the tee point for the new cable, or a new line is constructed from Port Ann to
Dunoon. It is largely independent of the amount of generation to be connected, so the 132 kV
connection is only worth considering if development proceeds on a very large scale
9.6.4.3
Colonsay and the Small Isles
The Small Isles are a group of islets off the East coast of Jura. The 33 kV system is relatively close,
but water must be crossed. The total cost may be assumed to be the same as for Jura itself.
Colonsay is a larger island about 15 km off the West coast of Jura, and a similar distance off the North
coast of Islay. There is at present no 33 kV connection; since it would connect via the Jura 33 kV
system, generation installed on Colonsay would use up the capacity on Jura. It is unlikely to be
beneficial to locate generation here rather than on Jura itself.
Islay / Jura / Small Isles
11/33 kV, up to 15 MW
£120,000 per MW
33 kV, up to 40 MW
£500,000 per MW
132 kV, 100 MW
£550,000 per MW
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9.6.4.4
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North Devon and Cornwall
This coastline runs diagonally south-west to north-east, with a few small bays along most of its length
and one large bay at Barnstaple. Some 30 km of coastline in the Bristol Channel also belong to
Devon, but this has less wave energy and a greater tidal range than the rest of the coast, and
accordingly has not been studied. Rather, the preliminary studies showed that part of the southern
Cornish coast, from Land's End to Lizard Point, had similar conditions to the north coast and this is
included.
South-west England is much more densely populated than Scotland, and this coast is well-served with
33 kV and 132 kV infrastructure apart from a short stretch near Bodmin, where the moor reaches
almost to the coast, and another stretch north of the Tamar [55]. Unlike most of the Scottish locations,
this coast has several 132 / 33 kV substations, and the capacity of the system is quite high provided
the generators are not all connected in one neighbourhood. It is therefore unlikely that a 132 kV
connection would be required, and only the 33 kV option has been studied.
33 kV Connection
There are few places on this coast where the 33 kV lines are more than 8 km away, and there are long
stretches where the nearest 33 kV line is within 5 km. On the other hand, the high population density
means that some of the connection to the 33 kV system may require underground cable.
Assuming as elsewhere a 5 MW block to connect at 33 kV, with a 5 km connection which is 50%
overhead and 50% underground, the price is comparable with a typical Scottish site, at around
£600,000, or £120,000 per megawatt. If the entire connection is constructed as overhead line, this
falls to around £400,000 or £80,000 per megawatt.
A careful selection of sites could utilise seven or eight bulk supply points: Camborne, Fraddon, Hayle,
Rame and Truro in west Cornwall; St Tudy and Pyworthy either side of Bodmin Moor; and East
Yelland if development extends into Barnstaple Bay. Excluding East Yelland, the total load on these
sites is about 460 MW at winter peak, so it may be expected to be around 90 MW at the summer
minimum. One may therefore assume that perhaps 100 MW could be accommodated with minimal
system reinforcement, and maybe 300–400 MW with suitable uprating of 33 kV circuits, and voltage
control plant.
For a development of up to 400 MW, it is likely that most of the individual installations would be larger
than the 5 MW assumed elsewhere in this study. The consequent saving on switchgear means that
the price is kept to around £50 million, or about £125,000 per megawatt.
Cornwall and Devon North Coast
9.6.4.5
33 kV, up to 100 MW
£80,000 per MW –
£120,000 per MW
depending on cable
requirement
33 kV, up to 400 MW
£125,000 per MW
Tiree, Coll and Mull
The islands of Mull, Tiree and Coll belong to the Inner Hebrides, between Jura to the South and Skye
to the North.
The isle of Mull derives its supply from two 33 kV cables connecting it to the mainland, of which only
one is switched in at any time. There is a small generating station on the island, at Tobermory.
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The isles of Coll and Tiree derive their supply from a small diesel plant on Tiree. As a backup there is
an 11 kV cable that can import power from Mull.
These islands, like Islay and Jura, are connected to the Kintyre 132 kV system. As noted above, this
system already contains as much generation as it can securely accommodate; intertrips are necessary
even for small projects, and reinforcements for more extensive developments
11/33 kV Connection
The 33 kV line runs around the Northern side of Mull. There are few places where it approaches the
western (exposed) coast, and a long peninsula (the Ross of Mull) that appears to have little or no
electrical infrastructure. As an average, installations may require around 20 km of overhead line to
connect to the network. The capacity, limited by the cable to the mainland, is around 10 MW.
It is not known where the 11 kV line runs on Coll and Tiree, but it is unlikely that any installation will be
further than 10 km from it. The capacity on these islands is very small, though – around 2 MW.
Connecting within the existing capacity costs in the region of £130,000 per megawatt. To increase the
capacity would require an additional 33 kV submarine cable; a capacity of around 20 MW could be
achieved at a cost of about £300,000 per megawatt.
132 kV Connection
This involves reinforcing the line from Oban via Inveraray to Loch Sloy, and building a new line and
cable to Mull. Once again the cost of this is largely independent of the capacity, and it only becomes
competitive on a per megawatt basis above about 100 MW.
Mull / Coll / Tiree
9.6.4.6
Coll/Tiree, 11 kV, up to 1 MW
£130,000 per MW
Mull, 11/33 kV, up to 10 MW
£130,000 per MW
Mull, 33 kV, up to 25 MW
£400,000 per MW
Mull, 132 kV, 100 MW
£400,000 per MW
Lewis and North Harris
The Isle of Lewis (of which North Harris is a part) lies at the Northern end of the Outer Hebrides. It
connects to the mainland via the Isle of Skye, with a 33 kV cable across the Little Minch, the strait that
separates the islands. There is 132 kV infrastructure on the island, and an interconnector at this
voltage appears to be planned, but is not shown on the system diagrams for 2010.
The existing inter-connector has a capacity of around 20 MW, which is not secure. In addition there is
around 9 MW of minimum demand. The total capacity is thus in the region of 30 MW.
11/33 kV connection
Lewis is a large island, and the 33 kV system does not go very close to the coast. By the same token,
there appears to be a reasonable 11 kV system reaching the scattered settlements, with a number of
33/11 kV transformers rated between 2 and 8 MVA.
Assuming a cluster of about 5 MW capacity could connect to the 33 kV system with about 20 km of
overhead line, and a cluster of about 2 MW could connect to the 11 kV system with 15 km of overhead
line, the cost of connecting generation on the island works out around £130,000 per megawatt.
132 kV Connection
The capacity of the system could be markedly increased by adding a 132 kV cable from Harris Grid
Substation to Ardmore. This would connect with the 132 kV system on Skye, and while this is only a
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single circuit, it could provide 100 MW of capacity if backed by suitable intertrips. This capacity would
apply to all the generating plant on Skye and in the Outer Hebrides, and further reinforcement would
be necessary to accommodate (say) 100 MW on Lewis, 20 MW on Uist and 20 MW on Skye.
The cost of the 35 km cable has been estimated at £30 million, adding some £300,000 per MW to the
cost of connecting the generation.
Isle of Lewis
9.6.4.7
11/33 kV, up to 30 MW
£130,000 per MW
132 kV, 110 MW
£400,000 per MW
Uist
Uist, for the purposes of this study, comprises a string of islands in the Outer Hebrides, of which the
major members are North Uist, Benbecula and South Uist. A 33 kV system covers the three islands,
running (for once) close to the West coast, although much of North Uist is not served by 33 kV.
Like the Isle of Lewis, Uist is connected to the mainland via a 33 kV cable to Ardmore; there is also a
12 MW diesel power station at Loch Carnan. The cable can carry about 20 MW, and the islands have
a minimum load of about 2 MW, giving a total capacity of 22 MW. As with most of the other island
systems, this capacity is not secure.
11/33 kV Connection
The topography of the coast is quite different on the East and West coasts of the islands, and wave
power development is likely to see a bias to one or the other. (Gently shelving beaches, as on the
West coast, tend to reduce the energy, while cliffs and deep water, as on the East coast, tends to
make installation expensive.)
On the West coast, the 33 kV power lines are within 5 km of the coast, and readily accessible. This
leads to the cheapest connection costs observed [for any of the Scottish sites], around £80,000 per
megawatt
On the East coast, and most of North Uist, the power lines are much further away, and the cost of
connection is greater. A small amount of generation (perhaps 2 MW on the whole of North Uist) could
be connected at 11 kV, for a similar cost to the West coast schemes; developments over most of the
islands would require longer 33 kV connection, leading to a cost of around £120,000 per megawatt.
132 kV Connection
Unlike Lewis, Uist possesses no 132 kV infrastructure; and the crossing to Skye is slightly further than
that from Harris, leading to a more expensive scheme. The capacity would be around 100 MW, but
this would have to be shared with Lewis and Skye.
Allowing about £40 million for the cable, the cost of connection comes to around £600,000 per
megawatt.
Uist
North Uist, 11 kV, up to 1 MW
£90,000 per MW
N Uist and East Coast, 33 kV,
up to 20 MW
£120,000 per MW
West Coast of S Uist /
Benbecula, up to 20 MW
£80,000 per MW
132 kV, 100 MW
£600,000 per MW
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9.6.5
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Operation and Maintenance
To minimise operating costs, future commercial installations will need to be designed for fully remote,
unmanned operation. It follows that the control system must be robust and have a high level of
reliability. Remote monitoring and diagnostic capability will be an essential requirement.
In much the same manner as wind turbines have a permissible range of wind speeds for which
operation is possible, it is likely that OWC installations will have a permissible range of sea states. The
control system must therefore be capable of implementing fully automatic start up and shut down
determined by measured sea state. The optimum method for defining and measuring sea states will
need to be developed.
Maintenance requirements for the turbine and electrical equipment are expected to be generally low. A
target of at least 8,000 operating hours between maintenance inspections should be possible. Remote
monitoring should eliminate the need for frequent on-site visual inspection of the plant. An availability
of 95% or more should be attainable for mature technology.
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9.7
Costing Summary
Device
Generic
Shoreline
1 x Near-shore
10 x Nearshore
10 x Nearshore (with
productivity
benefits)
Total frontal width (m)
20
82.5
825
825
Rated power (kW)
500
2,250
22,500
22,500
1,480
6,100
61,000
61,000
£1,762,605
£4,884,376
£40,088,497
£33,278,908
£310,000
£1,795,000
£17,250,000
£12,750,000
£2,072,605
£6,679,376
£57,338,497
£46,028,908
85 %
73 %
70 %
72 %
£4,145,210
£2,968,612
£2,548,378
£2,045,729
£103,630
£80,962
£69,601
£55,793
Annual power capture
(no outage)
(MWhr/year)
Structure cost (incl.
temp. works /
installation)
Balance-of-plant and
grid connection cost
Total Cost
Structure cost as %
total cost
Cost / MW rated
capacity
Cost / m collector
Power Production
Cost (10% discount
rate)
17.5
9.6
p/kWh
Nearshore Cost Split
Shoreline OWC Cost Split
12%
3%
19%
3%
Temp Works / Site Prep
10%
6%
Temp Works / Site Prep
Structure
Structure
Turbine / Generator
Elec Connection
Turbine / Generator
Grid Connection
24%
57%
Installation Vessel Costs
66%
A detailed cost build up for the construction of the OWC structure has been included in Appendix C.
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9.8
Risk
The potential risks in terms of safety, cost overrun, programme etc, that the future vision development may encounter have been considered, divided into the
project stage when such problems may be encountered. The risks considered are the same as those in the stage 1 report, allowing direct comparison with
existing devices. A method of risk reduction has then been applied and the residual risk classed as low medium or high. Low risks would thus require no
further action and medium risks would require a contingency sum assigned. The remaining high risks help to highlight the focus of further R&D of this
technology.
9.8.1
Risk Schedule
Stage
Aspect
Comment
Residual Risk
H, M, or L
Feasibility
Site
Investigation
Design
Energy resource
assessment
While this has been a problem for a number of the existing demonstration devices, an adequately funded
project would be able to obtain sufficient data to accurately assess the available wave resource.
L
Costing
inaccuracy
Detailed cost plans and scheme design should be developed to a level suitable to obtain sub-contractor
prices and buy-in prior to committing to project. A contract strategy should be set that considers risk
management / risk ownership. However, a cost contingency must be included in the build up for
inaccuracy due to the pioneering nature of the development.
M
Schedule
inaccuracy
Device construction in a dry dock facility mitigates the impact of adverse weather. Construction by
slipforming and other known technology should increase the accuracy of the programme estimate
providing the promoter employs contractors with adequate experience.
L
Sea bed
conditions
Risk reduced if initial investigation is thorough with sufficient cost allowance. The percentage cost
contribution of the SI is reduced for a large-scale development.
L
Bathymetry
As above
L
Rock integrity
As above
L
Concept freeze
Do not commit to final investment until sufficient engineering completed
L
Scour /
undermining
More prevalent the closer inshore the device is placed, which pushes the development into the deepest
water that is practicable. Sea bed preparation must be included in the cost build up.
L
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Stage
Aspect
Comment
Residual Risk
H, M, or L
Onshore
Construction
Offshore
Installation
Chamber
efficiency
Biggest impact on the project economy, but the most difficult to mitigate / estimate through model test and
calculation.
H
Turbine efficiency
Considerable design development needed before full potential can be realised. CFD modelling of the
turbine and OWC is a complex undertaking.
H
Control systems
The greatest areas for development are around OWC tuning, and turbine blade pitch control.
M
Peak wave load
est.
Understanding of impacting wave pressures has improved over the period of development of these
devices
L
Access to site
Dry dock access is very good.
L
Component /
material delivery
As above
L
Contractor skills /
equipment
Locally available skills
L
Productivity
Productivity gain through repetition. A covered construction yard could be contemplated.
L
Late delivery of
components
Programme flexibility and concrete modular construction allows activities to continue.
L
Errors in fit up
As above
L
Temporary works
Dry dock construction minimises the reliance on the competence of the temporary works.
L
Workforce safety
Controlled environment of the dry dock is a safer working environment.
L
Weather down
time
Dry dock is less susceptible to weather down time.
L
Adverse sea
state
Devices should be made as sea-state tolerant as possible, however programme contingency should be
allowed for.
M
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Stage
Aspect
Comment
Residual Risk
H, M, or L
Operation
and
Maintenance
Ballast placement
(solid/water)
Devices should be stable on the sea-bed without ballast in moderate sea-states. This is well understood
from the offshore industry.
L
Bed preparation
Difficult in exposed sites. Sites should be avoided that would require bed preparation.
M
Skirt penetration
Suitable device location sites should be chosen such that with adequate SI the correct skirt can be
designed.
L
Commissioning
Offshore activity attracts a degree of programme uncertainty.
M
Shore connection
Offshore activity attracts a degree of programme uncertainty.
M
Tug / barge
availability
Large-scale development would help to secure sufficient tug operator commitment.
L
Prior experience
Lack of exact experience, however similar skills to other offshore activities. Programme contingency
should be considered.
M
Access
Offshore activity attracts a degree of programme/ availability uncertainty. Maintenance / availability
assessment should be included in income estimate.
M
Number of
moving parts
Any moving parts must be accessible and replaceable. Modular approach to balance of plant.
L
Davits / cranes /
runway beams
Maintenance provision must be built-in from the outset
L
Marine fouling
M
Fatigue
Concrete structures are less fatigue sensitive. BOP cost and design should mitigate such risks.
L
Chamber
blockage
Only an issue on shoreline devices
L
Integrity
inspection
Offshore activity attracts a degree of programme uncertainty. Concrete structures have an excellent
maintenance free record in seawater if well constructed from good quality materials.
M
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Stage
Aspect
Comment
Residual Risk
H, M, or L
Deluge of plant
rooms
Modular BOP container design to be watertight.
L
It can be seen that the highest residual risk with the greatest impact on any future development is an inaccurate estimate of the chamber wave to pneumatic
efficiency. The residual risk in the future vision has been reflected by applying a 10% contingency to estimated costs in the economic modelling.
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10
Marine Energy Challenge
Oscillating Water Column Wave Energy Converter Evaluation Report
Economics
Section 7 assessed the relative merits of shoreline versus near-shore OWCs. Section 8 outlines the
potential and technical proposals that could improve the performance of OWCs, with section 9
describing a future vision of a device that has incorporated these improvements.
In order to evaluate the effectiveness of these improvements economic models of a shoreline and a
near-shore device have been created. The method of evaluation, results and conclusions are
summarised in the following sections.
10.1
Economic models
The economic models produced uses the project Net Present Value to test the project commercial
viability to varying CAPEX, power production cost, wave power, and capture efficiency. It is based on
UK costing and power pricing regimes.
Other countries have different construction cost, power cost and ways of encouraging renewable
power generation. This would make direct use of these financial models outside the UK somewhat
misleading. However, as an initial indicator, the total required power sales production cost to achieve
economic viability can be examined.
10.1.1
Common assumptions
The assumptions listed below represent our current best estimate of the OWC design parameters. The
effects of upper bound and lower bound variables have been tested by the economic model and are
reported in the sensitivities section.
A wave power at 20m water depth of 34kW/m has been assumed to be present in any future
development site. Other assumptions relating to size, location, input energy etc. are set out below.
10.1.1.1
Assumed yearly averaged efficiencies
Type of energy
conversion
Structure / device
Assumed Efficiency
Wave to pneumatic (The
energy coming in to the
advice is taken to be the
maximum width of the
OWC facing the waves
multiplied by the energy
flux passing the 20m
contour)
Oscillating water
column
42.0*
Pneumatic to
mechanical
Wells turbine
65.0
Mechanical to electrical
Generator
91.0
%
Overall efficiency
24.8
* Shoreline efficiency would be reduced due to sea bed friction in shallow water, however
with no specific site chosen actual sea bed loss has not been calculated and a capture
efficiency equal to near-shore has been used for direct comparison purposes. Also, the cliff
effect tends to increase efficiency compared to an isolated near-shore OWC.
In addition to the above, a 2% transmission loss has been assumed.
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10.1.1.2
Assumed Operating, Maintenance and Insurance costs
•
A decommissioning cost at 10% of CAPEX has been assumed (inflated).
•
A Crown Estate rent of 2% of energy revenue
•
Contractors all risk insurance is assumed at 2% of initial CAPEX.
•
Cost overrun insurance is set at 3% of the first year revenue.
•
An operational insurance of 0.8% of the CAPEX has been assumed.
•
Business interruption insurance of 2% of energy revenue
•
Maintenance cost of 1.5% of BOP
The total operating cost equates to £16.60/MWh generated. This figure is in line with experience from
offshore wind farm operations and similar to that suggested by other authors [72].
10.1.1.3
General Financial assumptions
•
A 10% contingency on initial CAPEX has been included
•
2.5% rate of inflation
•
Renewable Obligation Certificate value assumed to inflate over life of project.
•
Climate change levy remains constant
•
Capital grant financing has not been included in model
•
Tax liability has not been included
•
The equity discount rate has been plotted against varying CAPEX, power production cost,
wave power, and capture efficiency to examine the project Net Present Value as a test of the
project’s commercial viability.
10.1.1.4
•
Shoreline assumptions
Each device has a 500kW generator using a 20m collector frontage.
10.1.1.5
Near-shore assumptions
•
Each 750kW generator has a 27.5m collector frontage made up of 20m open plus 7.5m
collector focusing structure. 3 such units / generators per device
•
A development field of 10 devices. This allows certain economies of scale to be applied,
reducing the balance of plant cost to £500/kW, and increasing the OWC structure
construction productivity.
•
A modular system for the incorporation of the BOP into the device structure assumes
interchangeable units. This combined with the simple nature of the BOP means that only 3%
of the total operating hours have been assumed to be lost to maintenance activities. It should
be noted that wave availability is accounted for in pneumatic capture efficiency figure.
•
An initial Third Party insurance cost of £100,000, inflated over the project period.
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10.2
Sensitivity assessment
The main variables that influence the outturn power production cost are the development size, wave
resource, device efficiency (capture, turbine and generator efficiency), and the initial capital
expenditure including civil, Balance of Plant, and grid connection costs.
Section 9.6 has highlighted the cost reduction possible as the scale of the development increases.
Shoreline v Near-shore Power Production Cost
24.00
22.00
20.00
p/kWhr
18.00
16.00
14.00
12.00
10.00
8.00
6.00
4.00
0
5
10
15
20
25
Development Size MW
Shoreline Trend
Nearshore Trend
The above figure shows that below 2 MW shoreline technology is more cost effective. The rate of the
cost reduction with increased development size plateaus for shoreline devices at approximately 5 MW.
This is because a single construction location would not be available for such a large wave capture
frontage, the development site would become more disparate and economies of scale are lost.
Near-shore devices are less viable for small-scale development due to the dry dock and installation
vessel mobilization costs. These costs reduce quite quickly with development size as the mobilization
costs are shared, leading to the fairly pronounced change in curvature above. However, cost savings
would also plateau as a development size reaches 30 MW as a second dry dock facility would be
required and multiple vessels needed to support multi-location activities. This would assume a
reasonably compact construction programme of approximately 18 months.
While the possible cost improvement by development scale should be recognized, the development
size will largely be governed by the developer’s financial commitment and project ambition.
10.2.1
Upper bound / Lower bound sensitivity assessment
Each of the other variables has a range of possible values, a worst-case scenario, a current best
estimate, and an upper bound value that may be achieved with further development.
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To assess the sensitivity of the project to each variable, the improvement scale of these factors has
been ranked, with the best estimate ranked zero with a +/- 5 limit. Each step change from zero has an
increasing degree of difficulty of achievement.
Wave resource
A wave resource of 34 kW/m has been chosen as the zero rank. This is the highest wave resource
that could be expected at the 20 m contour while still offering a degree of flexibility of project site
development areas. 51 kW/m should be considered the absolute maximum that could be achieved for
a single development, offering little site flexibility with the high probability of suffering other detrimental
factors to the project in the limited locations available. A lower range of 17 kW/m was chosen to test
the viability of a wider range of site locations.
Capture efficiency
A yearly capture efficiency of 42% of the 20m contour resource (59% at lip) over a capture width taken
as the maximum width of the OWC represents the current best estimate. Developers have estimated
that 80% capture of the energy at the OWC lip should be possible. Allowing for some loss in energy
from the 20m contour, a highest expected capture efficiency of 67% is suggested. A low value of 17%
would represent poorer performance than any existing OWCs have reported.
Turbine efficiency
For an optimised design an instantaneous peak turbine efficiency of between 60% and 70% is
probably a realistic expectation for a Wells or an impulse turbine with flow in either direction. With
appropriate design development it should be possible to extend the breadth of the performance
characteristic such that greater than 50% efficiency is achieved over a defined range of flows.
Generator efficiency
The probable range of mechanical to electrical efficiencies is expected to be 90-95%, with 91% the
current best estimate of likely efficiency. This range will depend to a certain degree on the type of
technology used such as cage machine direct drive or DFIG. The maximum of 96% corresponds to
the manufacturer's upper limit. The lower limit of 86% is based on the likely result of a greater range of
rotor speed and increased rotor resistance to enhance damping.
Out-turn Civil Costs
+/- 20% would be the typical range that civil costs could vary compared to the level of cost estimate
produced for this report. Increased accuracy could be obtained through engaging project participants
across the range of subcontractors involved in such a project following the production of a more
detailed tender design.
Greater cost changes could only be expected if a step change in design could be achieved for
shoreline construction (perhaps through development of the ‘roof only’ LIMPET) or by considering
overseas construction for near-shore devices.
Balance of Plant Costs
The upper bound reduction to £300/kW corresponds to manufacturers’ expectations of mass
production of near-shore devices. The lower bound of £700/kW corresponds to the use of state-of-theart available technology on a smaller scale limited production basis.
Grid Connection
The potential for improvement in grid connection cost is not related to OWC performance so has not
been considered as a variable in the sensitivity exercise.
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Lower Bound
Best
Est.
Upper bound
Rank
-5
-4
-3
-2
-1
0
1
2
3
4
5
Wave resource kW/m
17
20.4
23.8
27.2
30.6
34
37.4
40.8
44.2
47.6
51
Capture efficiency %
17
22
27
32
37
42
47
52
57
62
67
Turbine efficiency %
50
53
56
59
62
65
66
67
68
69
70
Generator efficiency
86
87
88
89
90
91
92
93
94
95
96
Outturn civil costs %
20
16
12
8
4
0
-4
-8
-12
-16
-20
BOP cost £/kW
700
660
620
580
540
500
460
420
380
340
300
The possible range of each variable has then been tested in the economic model (with an Equity
Discount Rate of 10%) to assess the effect on the outturn power production cost. The results have
been plotted. (Appendix E and F shows the sensitivity of the economic model and the influencing
factors to a range of discount rates).
Shoreline Improvement Sensitivity
5
3
2
1
0
45.0
40.0
35.0
30.0
25.0
20.0
15.0
10.0
5.0
-1
-2
-3
Improvement rank
4
-4
-5
p/kWh
Civil Cost
Turbine Efficiency
Wave Resource
Generator Efficiency
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Near-shore Improvement Sensitivity
5
3
2
1
0
25.0
20.0
15.0
10.0
5.0
-1
-2
-3
Improvement rank
4
-4
-5
p/kWh
Civil Cost
Turbine Efficiency
Wave Resource
Generator Efficiency
Capture Efficiency
BoP Cost
The flatter the curve the greater the influence on the power production cost. Therefore, from the graph
it can be seen that changes within the possible improvement range of capture efficiency and available
resource have a greater effect than reductions in CAPEX or turbine / generator efficiencies.
10.3
Improvement priorities / targets
10.3.1
Priorities
Improvements with the biggest impact on the power production cost, requiring the least effort to
implement have been identified above, and could be considered as the priority actions to undertake.
Capture efficiency has the biggest impact on the developments economic viability. A greater
understanding of collector performance through CFD modelling and tank tests may lead to greater
efficiency.
While having less of an impact on the power production cost, many of the methods of reducing the
civil cost require less development effort. Developing a generic contract strategy to better manage
cost, risk and programme of these projects is a low effort improvement.
10.3.2
Targets
In order to summarize the future viability of this technology, a power production cost target has been
chosen for each device type, and the viability of the improvements required to achieve that target
assessed. These should be considered as medium-term targets that could be achieved by 2008-2012.
10.3.2.1
Shoreline
Due to the more limited market resource and the lower CAPEX and power production cost for smaller
developments (less than 2MW) this device type should be considered for remote power supply to
small communities. This would be as an alternative or supplement to small diesel generator power
production. As such a target power production cost of 8 to 10 p/kWhr would be a comparable cost
target.
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Our current assessment of this technology predicts a power production cost of approximately 17.5
p/kWhr (with a 10% EDR). In order to reach the above target a number of simultaneous
improvements would have to be made:
•
Yearly average capture efficiency up from 42% to 52%
•
Turbine efficiency up from 65% to 67.5%
•
Generator efficiency up from 91% to 94%
•
Civil cost reduction of 50%
This would result in an 8 p/kWhr production cost.
10.3.2.2
Near-shore
As shown by the previous project scale relationship, near-shore development is better suited for power
supply above 1 MW with greater economies as the project increases to the 10 MW scale. This type of
development should be compared with the current offshore wind technology, as such targeting a 5
p/kWhr power production cost.
Our current assessment of this technology predicts a power production cost of approximately 9.6
p/kWhr (with a 10% EDR). In order to reach the above target a number of improvements would have
to be made:
•
Yearly average capture efficiency up from 42% to 55%
•
Turbine efficiency up from 65% to 67.5%
•
Generator efficiency up from 91% to 94%
•
Balance of plant cost reduction from £500/kW to £300/kW
•
Civil cost reduction of 20%
•
No capital cost contingency (previously 10%)
This would result in a 5 p/kWhr production cost.
10.4
Learning Rates
The approach taken within this report is one of ‘engineering assessment’ [74]. The technological
maturity of OWCs has been assessed by a ‘technology stretch’ process whereby the potential for
refinement and development of sub-systems and components has been evaluated. The potential that
can be realised through this method is dependent on the judgement of the people involved and, as
such, is open to interpretation and manipulation. Hence, in order to view the potential for
improvements, an alternative empirical approach has been considered.
The main alternative to engineering assessment for assessing potential future cost reductions is so
called learning, learning by doing, or experience, curves. Evidence from a wide range of technologies
demonstrates a clear relationship between production and cost [75]. A learning rate can be defined
which is the rate at which cost decreases with each doubling of cumulative production. Learning rates
in renewable energy technologies have been assessed to be 10-30% [76]. Onshore wind in Europe
was assessed to have an 18% learning rate.
To assess the possible learning rate that might be applicable to near-shore OWCs, the rate of
progress achieved on Arup dry-built offshore concrete platforms was examined. This form of
construction is directly comparable to the future vision for the civil construction of a near-shore OWC.
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US$/cu. m concrete
10000
7.3% Learning Rate
Ravenspurn
Wandoo
Malampaya
1000
1
Figure 10.1
10
100
Learning Rates for dry-built offshore concrete platforms
The assessed learning rate is only 7.3%, which reflects the fact that marine concrete design and
construction is a relatively mature field so learning rates can be expected to be lower. The learning
rate present for the civil works in the future vision near-shore OWC is 11%. The productivity increases
and cost reductions that led to this 11% learning rate were all felt to be achievable.
The balance of plant learning rate already present in the future vision is 10%. This seems low
compared to the wind energy example (18%) and can probably be increased.
The other elements that make up the composite OWC learning rate are the performance
improvements expected in the wave-to-wire efficiency.
Overall, since the lower rate of improvement foreseen in the large civil works cost element will
dominate the composite learning rate, a learning rate of 13% is suggested as being applicable to
OWC technology.
10.4.1
Shoreline Targets
To achieve the 8p/kWh cost of power target above, a 50% civil cost reduction was considered to be
one of the improvements required. At a learning rate of 7.3% for civil works alone, this would not be
achieved until 500 OWCs had been constructed, more than the forecast UK market.
If the suggested overall learning rate of 13% is applied, the cost of power will decrease from
th
17.5p/kWh to 8p/kWh by the time the 50 unit has been constructed. This would amount to 12.5% of
the total market foreseen in the UK for shoreline OWCs, or a capital expenditure approaching £80m. It
is considered unlikely that the government would provide the necessary financial support to allow this
quantity of units to be constructed. The capital cost is approaching twice the level of funding currently
being made available through the Dti for all wet renewable power demonstration projects. This
analysis suggests that efforts are best directed at researching designs that may achieve substantial
cost savings through ‘concept shift’ such as Wavegen’s roof-only LIMPET.
10.4.2
Near-shore Targets
The target cost of power for near-shore was 5p/kWh compared to a future vision cost of 9.6p/kWh.
This would require 250 number 82.5m long units to be installed at a rated capacity of 562MW and a
capital cost of approximately £1bn. These would comprise 15% of the total market foreseen for nearshore OWCs around the UK. By comparison, this is the size of five of the UK round 1 offshore
windfarms for which some capital grant funding was available. It is considered unlikely that this volume
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of construction would secure the level of financial support to make it attractive to investors, especially
considering that developments on this scale would merely permit near-shore OWCs to reach the level
of current power cost performance of offshore wind.
Hence, although the target improvements are considered achievable, the economics still appear
unattractive. Consequently, a greater range of improvements must be sought or a greater rate of
learning achieved. Increasing the learning rate to 18% would mean that 5p/kWh was achieved by the
th
production of the 100 unit. Such learning rates seem essential for OWCs to have a viable future.
10.4.3
Port Kembla
The Port Kembla OWC team has been evaluating the improvements that might be possible through an
engineering assessment and the degree of technology stretch they can foresee. The team expect
wave-to-wire efficiency to improve from 33% to 55% and capital costs to decrease by 75% by the time
th
the 1000 unit is constructed. This compares with the medium-term target wave-to-wire efficiency of
49% forecast in this report, based on energy at the collector lip (35% based on 20m contour energy
flux). Simultaneously, the team expect to move the device to locations with a much greater wave
energy resource.
th
The Port Kembla team forecasts a cost of power of around 2.5p/kWh by the time the 50 unit is
constructed. This has been assessed to be a learning rate of 16.5%. The suggested rate of learning
for the Port Kembla device appears optimistic and the evidence available from the Port Kembla team
to date does not demonstrate that the economics envisaged in this report can be bettered.
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Research & Development
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11
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Areas for Research and Development
11.1
Wave Climate
The wave climate can be confidently predicted for anywhere around the UK in deep water away from
the coastline. The challenge for developers working near-shore and at the shoreline is to characterise
the wave parameters expected there. The transformation of offshore data to inshore locations is a
relatively manhour intensive activity that is reliant on skilled practitioners. Data gathering with a wave
rider buoy is more reliable than analysis but the long deployment periods needed to gather
comprehensive design data inhibit project development.
To make development easier, inshore wave climates should be determined for a number of target
locations for OWC deployment. This could be an extension to the Marine Renewable Energy Atlas
recently produced for UK waters.
11.2
Wave Collector/Capture
Improving the capture efficiency of the wave collector has been shown to have the greatest impact on
the economics of OWCs. The geometrical factors affecting capture efficiency are well known but they
have not been translated into a set of sizing rules that can be followed by designers on new projects.
Likewise, the loss in efficiency due to internal slosh and wave wash down the front wall has been
recognized but this has not been translated into a body of learning that can be drawn upon when
designing the next generation of OWCs.
The selection of collector size, geometrical form, inclination angle, lip depth and profile has hitherto
been done on something of a trial-and-error basis supported by flume or wave tank testing. The
knowledge that has been gained is becoming more valued by developers as they begin to exploit their
intellectual property in order to secure funding for future projects. However, the rather closed approach
of developers, striving to secure financial backing, can be somewhat self-defeating as it can decrease
the learning rate and hence the rate at which the improved economic performance can be delivered.
To overcome this, parametric analysis of collector chambers is recommended. This should be
performed using CFD tools in conjunction with tank testing. The CFD model may have to treat the
turbine as a simple damper initially, but ultimately, complete CFD models of the wave excitation, air
chamber response and turbine interaction will be tractable. The CFD tools themselves warrant some
fundamental development so that wave time histories can be readily generated by commercially
available packages. The use of CFD would help overcome some of the disadvantages in relying on
the linear wave theory that has been the foundation for much of the research on OWCs to date despite
the knowledge that non-linear behaviour was having a significant bearing on results.
The parametric exercise would distil the geometrical arrangements, both inside and outside the
chamber, that lead to greatest capture efficiency for a range of environmental conditions. The exercise
should also include an assessment of the effects of guide walls compared to continuous lines of
capture chambers.
The management of such exercises should be through engineering companies, rather than
developers, as engineering companies have the requisite skills to devise and then draw out the
lessons from parametric work and can bring experience in related technology fields to bear to best
effect.
11.3
Construction and Installation
Comprising typically 70% of OWC cost, it was evident that changes to the cost of construction and
installation of devices would show the highest impact on economics after capture efficiency.
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Bottom-mounted OWCs are faced with the challenges of resisting extreme wave loading without
structural or foundation failure to be able to capitalise on the more energetic locations. The best sites
for wave resource are the worst in terms of offering suitable weather windows in which to deploy
devices.
There is some potential to lower the factors of safety commonly specified for offshore structures and
this would translate directly into lower cost. The new DnV guidelines should be tested for OWCs and
the consequences and likelihood of damage or complete loss of devices should be understood.
The response of caisson structures and breakwaters to wave impact loading is a developing field of
research in marine coastal engineering. OWCs could benefit from, or extend, this research to help
assess the true loading that OWCs must resist.
The Port Kembla device has departed significantly from conventional offshore wisdom by adopting
short taut moorings and a lightweight structure that offers many sites for impact pressures to be
experienced. However, if successful, it could stimulate further research that might improve the concept
so that better economic performance can be delivered. The approach of trial-and-error by individual
developers is a painful one though, as witnessed by the OSPREY team.
The OWC future vision in this report has chosen to follow the conventional design practice for offshore
oil & gas gravity structures. From an examination of learning rates in this sector, the rate of
improvement seems insufficient to attract investors. To help overcome this, a design competition is
suggested. Teams of designers, fabricators and installers should be formed with a brief of delivering a
prototype near-shore OWC for deployment at a designated site such as EMEC on Orkney. Their brief
would be to brainstorm and then develop an outline design for a new OWC. The teams should include
experts in conventional marine structure design and construction, power engineers, installation
contractors, specialists in rock anchoring or piling as well as the developers. Specific use of new
materials such as fibre-reinforced concrete or construction methods could be explored within this
programme. The design shown to be the most economic and have the most future potential to an
independent adjudicator could then proceed to deployment. The unsuccessful teams would have their
costs covered to ensure a positive attitude was fostered towards the competition. Such an approach is
in keeping with the aims of the Marine Energy Challenge and is supported by the Wavenet study [8].
A further mechanism that would help drive down costs would be to develop a renewable energy
project development contract. Such a contract would aim to place the development risk with those
best able to bear it, thereby encouraging contractors to reduce their risk provision in tenders. Some
form of capped liability arrangement could be considered, when the government was providing
financial support, whereby the government would take the role of ultimate project guarantor. Such
arrangements would, of course, need careful consideration to ensure the fledgling industry was being
supported to a point where it could grow independently rather than being just subsidised.
11.4
Turbine
The design process for oscillating water column (OWC) installations would benefit from the
development of comprehensive, generic CFD tools for analysis of complete systems to enable better
and more effective matching of turbines to collector structures. It is desirable that the CFD tools should
be validated and calibrated against as wide a variety of experimental data as possible, particularly
from large-scale prototype machines.
There has already been much theoretical and experimental work on a wide variety of possible blade
and vane configurations, relief valves and control methods. The majority of this work has concentrated
on the Wells turbine but there has apparently been little effort into direct comparison of different
designs on a like-for-like basis and the alternative impulse design has been somewhat overlooked. A
comprehensive and qualitative comparison of a variety of vane and blade configurations on a like-forlike basis is required and will assist in the determination of the optimum configuration and turbine type,
which may be different for different wave climates. The comparison should initially be undertaken
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using CFD and backed up by wind tunnel tests on a significant scale (at least ¼ full size). Both Wells
and impulse type designs should be considered.
Development of standards for defining performance characteristics of OWC installations would greatly
enhance the ability to compare different designs on a like-for-like basis.
A range of standard turbines with known characteristics should be developed for use in shoreline
systems where the collector system can be custom built to provide the required input air flow. A
corresponding range of matched turbine and collector structures for near-shore deployment over a
range of sea states should also be developed.
Efforts to better understand turbine and relief valve noise production and suppression would also have
merit.
11.5
Generator and Power Take Off
The generator, power electronic converter and inverter will be state-of-the-art technology adopted from
current industrial and wind industry standard technology. There may be design considerations that are
application specific to wave energy particularly as a result of the pulsating nature of the power.
However, it is not envisaged that there will need to be any specific technology development required in
this area.
11.6
Controls
Probably the most productive control area for further research is that of throttle valve control. The
throttle valve can be controlled to prevent stall and allow operation in high sea states. The valve
position also affects damping. Its control should be considered following on from full CFD modelling.
Some academic work has been done into the use of controls to tune the OWC to changing wave
period; however this has been far from comprehensive. The turbine acts as a damping element to the
OWC and hence the full CFD modelling of the OWC chamber and turbine will need to come before
active tuning for changing wave climate can be comprehensively considered. The full potential of fixed
parameter designs should be achieved first but there is scope to investigate the potential for active
tuning at the later stages of development; this should be considered together with throttle valve
control.
Although we are recommending that the full potential of fixed blade turbine designs are investigated
first, there is scope to investigate the area of variable pitch turbine blade control, in order to optimise
efficiency and also maybe to adjust damping.
11.7
Grid Connection
Grid connection is a site-specific issue and is not specific to OWCs or wave energy. In recent years a
number of studies have been carried out in order to quantify the cost of grid connecting wind farms
and embedded generators in order to meet government targets. The general order of grid connection
costs for OWC sites has been set out in this report, and further useful work would be detailed
connection design studies which would normally be carried out as part of a commercial project.
11.8
Power System Dynamics
The main power system issue with the OWC technology is that of sub- synchronous resonance that
can arise from the pulsating nature of the power when connected into some networks. There would be
merit in carrying out detailed studies based on specific sites to establish the nature of the issue. Some
work could be done using existing prototype data provided suitable turbine models were available.
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11.9
Marine Energy Challenge
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Business Development and Partnerships
To ensure that commercially viable technologies are developed, the creation of strategic partnerships
between technology developers and potential volume manufacturers should be encouraged.
The majority of the OWC installations have been developed on a prototype basis, often by academic
institutions or small research and development companies. These organisations have built up a
significant level of knowledge and expertise but, in order to move the development forward, expertise
from other key technology areas needs to be incorporated. These key areas are civil and structural
engineering, turbine design and manufacture, CFD and mathematical modelling and electrical power
engineering. If such technology development is to be publicly funded then engineering consultants
with power industry experience should also be appointed.
Although the LIMPET site has its limitations from a wave climate point of view, it does have significant
advantages of access and is currently the only operational OWC site in the world. The facility would
benefit from some investment; so that it could become a more commercial test bed facility particularly
for developing turbine and power take off designs. A similar argument could be made for the Pico site,
however despite its more favourable wave climate the difficulty of access and poor operational track
record suggests that costs could be higher.
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Conclusions
OWCs have the longest track-record of deployment of any of the wet renewable technologies under
consideration in the Marine Energy Challenge. Grid-connected facilities have been operational now for
several years. Small-scale OWCs in navigation buoys have proven economic and reliable. Yet despite
this history, an OWC market has not been established and other wet renewable technologies are
threatening to supplant the predominant position OWCs have had.
Two companies, Wavegen and Energetech, have begun to make some progress towards
commercialising the technology but multi-megawatt projects are probably still five years away from
being realised.
If OWCs are to have a long-term future, their rate of development must be increased. The Marine
Energy Challenge was devised as a way of pushing wet renewable technology forward. This report for
the Challenge demonstrates that near-shore OWCs can, within a reasonable timescale, reach a point
where they can work within the current UK framework for renewable power generation without needing
additional government support. The improvements reported here in performance and cost to reach this
position do seem achievable. However, the OWC industry will not deliver these improvements without
a more coherent approach and some financial assistance. The best use of future funding would be to
stimulate a rate of learning that will deliver the progress rate of successful renewable energy
technologies such as onshore wind.
Shoreline OWC technology will never achieve as low a power price as near-shore OWCs.
Nonetheless, it can still play a role in serving isolated communities where there is power price
premium. Efforts commensurate with the scale of resource should continue, to help the shoreline
OWCs reduce costs and improve performance, so that they can be a viable niche power provider.
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Supporting Information
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Appendix A
References
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A1.1
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Heath, T., Whittaker, T.J.T & Boake, C.B.; The design, construction and operation of the LIMPET
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polychromatic wave spectra, Wave Energy 2000, 2000
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Marine Energy Challenge
Oscillating Water Column Wave Energy Converter Evaluation Report
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76] McDonald, A & Schrattenholzer, L.; Learning Rates for Energy Technologies, Energy Policy 29,
pp255-261, 2001
77] Setoguchi,T., Santhakumar, S., Takao, M. T. H., Kim, T.H. & Kaneko, K.; A modified Wells
turbine for wave energy conversion, Renewable Energy, Volume 28, Issue 1, Pages 79-91,
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78] Brito-Melo, A., Gato, L.M.C. & Sarmento, A.J.N.A.; Analysis of Wells turbine design parameters
by numerical simulation of the OWC performance, Ocean Engineering, Volume 29, Issue 12,
Pages 1463-1477, September 2002
79] Govardhan, M. & Dhanasekaran, T.S.; Effect of guide vanes on the performance of a selfrectifying air turbine with constant and variable chord rotors, Renewable Energy, Volume 26,
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80] EMEC, Performance Assessment for Wave Energy Conversion Systems in Open Sea Test
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81] Department of Trade and Industry, Atlas of Marine Renewable Energy Resources, Report
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82] Kim, T.H., Takao, M., Setogucchi, T., Kaneko, K. and Inoue, M.; Performance comparison of
turbines for wave power conversion, International Journal of Thermal Sciences, Volume 40 Issue
7, Pages 681-689, July-August 2001.
83] Falcao, A.F. de O., and Justino, P.A.P.; OWC wave energy devices with air flow control, Ocean
Engineering, Volume 26, Issue 12, Pages 1275-1295, December 1999.
84] Korde, U.A.; Efficient primary energy conversion in irregular waves. Ocean Engineering, Volume
26, Issue 7, Pages 625-651, July 1999
85] Ghosh, K.; Cascade wind turbines for the oscillating water column wave energy device: Part1,
Renewable Energy, Volume 9, Issues 1-4, Pages 1219-1222, September-December 1996.
86] Korde, U.A.; Development of a reactive control apparatus for a fixed tow-dimensional oscillating
water column wave energy device, Ocean Engineering, Volume 18, Issue 5, Pages 465-483,
1991.
87] Sarmento, A.J.N.A., Gato, L.M.C and de O Falcao, A.F.; Turbine-controlled wave energy
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Appendix B
OWC Construction
Programmes
The Carbon Trust
Marine Energy Challenge
Oscillating Water Column Wave Energy Converter Evaluation Report
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Appendix C
OWC Cost Estimates
Page 1
Shoreline cost
GENERIC SHORELINE OWC CIVIL COSTS
Item
Short description
Quantity
Unit
Unit rate
Amount
£
£
1
GENERAL SITE WORKS
1.1
excavate (stripping) topsoil 50 cm including storage for re-use,
haul maximum 1 km
13,000
m²
5.00
65,000
1.2
Access Road - layer of sand/gravel - 300mm
3,000
m²
20.00
60,000
1.3
install fence for protecting site including material supply,
height 2 m, material wire mesh
400
m
50.00
Total General Site Works
2
EXCAVATION & PREP
2.1
excavate rock
2.2
20,000
145,000
5,400
m3
Rock bund prep and height extension
1
item
20,000
2.3
install dewatering system
1
item
10,000
2.4
provide and install pumping equipment, raiser and discharge
pipes etc.,
1
item
20,000
2.5
pump out water down to excavation base
1
item
1,000
2.6
50mm blinding concrete on dock floor
500
m²
5.00
2,500
2.7
dewatering during construction period including time for
dewatering and foundation works
12
month
500.00
6,000
30.00
Total Excavation and Prep
162,000
221,500
3
PLANT AND FACILITIES
3.1
site offices, toilets, washrooms
item
15,000
3.2
generators and power distribution system
item
5,000
3.3
electrical/mechanical/welding/carpentry and formwork
workshop
item
2,000
3.4
store and laydown areas
item
2,000
3.5
concrete batch plant
1
no
12,000.00
3.6
water storage tanks
1
no
1,000.00
Total Plant and Facilities
4
DEVICE CONSTRUCTION
4.1
base slab - formwork
100
4.2
base slab - reinforcement supply
60
4.3
base slab - reinforcement fix
4.4
base slab - concrete supply
400
base slab - concrete place
4.6
base slab - concrete top form / slope allowance
4.7
walls - formwork
4.8
walls - reinforcement supply
1,000
37,000
SITE WORK TOTAL
4.5
12,000
2
403,500
m
Te
30.00
3,000
500.00
30,000
60
Te
250.00
15,000
400
m3
55.00
22,000
m
3
24.00
9,600
100
m
2
50.00
5,000
1,260
m2
50.00
63,000
70
Te
500.00
35,000
4.9
walls - reinforcement fix
70
Te
300.00
21,000
4.10
walls - concrete supply
441
m3
55.00
24,255
4.11
walls - concrete place
441
m3
24.00
10,584
4.12
walls - formwork - permanent steel (e.g. lip)
10
Te
2,000.00
20,000
4.13
roof - prepare precast beds (4 No)
55
m2
100.00
5,500
4.14
roof - pre-cast panel reinforcement supply
25
Te
500.00
12,500
4.15
roof - pre-cast panel reinforcement fix
25
Te
300.00
7,500
4.16
roof - pre-cast panel concrete supply
150
m3
55.00
8,250
4.17
roof - pre-cast panel concrete place
150
12.00
1,800
4.18
roof - curing per panel
90
m3
no
25.00
2,250
4.19
roof - erection of panels
90
no
100.00
9,000
4.20
roof - concrete supply topping
265
m3
55.00
14,575
3
4.21
roof - concrete place topping
265
m
24.00
6,360
4.22
roof - topping reinforcement supply
40
Te
500.00
20,000
4.23
roof - topping reinforcement fix
40
Te
300.00
12,000
4.24
turbine hall & attentuation - slab formwork
25
m2
30.00
750
4.25
turbine hall & attenuation - reinforcement
15
Te
500.00
7,500
4.26
turbine hall & attentuation - reinforcement fix
15
Te
250.00
3,750
4.27
turbine hall & attentuation - concrete supply
100
m3
55.00
5,500
4.28
turbine hall & attentuation - concrete place
100
m
3
24.00
2,400
4.29
turbine hall & attentuation - blockwork
300
m2
35.00
10,500
4.30
turbine hall & attentuation - roof steelwork
9
Te
2,000.00
18,000
4.31
turbine hall & attentuation - concrete roof
30
m3
350.00
10,500
4.32
Turbine Hall Architectural fit out (water tight door, etc.)
1
item
5,000.00
5,000
4.34
Turbine Hall - Lifting beam
1
te
2,000.00
2,000
DEVICE CONSTRUCTION SUB-TOTAL (excl indirects)
424,074
5
CONSTRUCTION INDIRECTS
5.1
detailed design
item
15%
5.2
site data collection
item
10%
79,057
5.3
management and supervision
item
20%
158,115
5.4
plant &equipment
item
15%
118,586
5.5
craneage
item
15%
118,586
5.6
small tools
item
1.8%
14,230
5.7
indirect labour
item
10%
79,057
5.8
freight
item
0.25%
5.9
profit & overheads
item
15%
Total Construction Indirects
118,586
1,976
118,586
806,781
DEVICE CONSTRUCTION TOTAL
1,230,855
6
FINAL FINISHING
6.1
clearing and demolition of bund including haulage and
disposal of material
2,000
m²
30.00
60,000
6.2
Landscaping
10,500
m³
6.50
68,250
6.3
dredging operation
0
m3
6.00
TOTAL FINISHING
7
BOP
7.1
Power Train Cost Estimate
7.2
Grid Connection
0
128,250
£/kW
500
kW
500.00
item
250,000
60,000
TOTAL BOP
310,000
CIVIL TOTAL
BOP Estimate
£1,762,605
£310,000
GRAND TOTAL
£2,072,605
COST / m FRONT
1
£103,630
OWC Cost & Prog rev 09
Near Shore cost
NEAR SHORE CONSTRUCTION
Item
SINGLE DEVICE MEASURE
Short description
Quantity
Unit
1
SITE PREP AND TEMPORARY WORKS
1.1
Dry dock modification / refurbishment
1
item
1.2
Dry dock rent
8
month
1.3
Offshore SI
item
Page 2
SCALE COST COMPARISON
Unit rate
Cost per device
Cost per generator
Cost for 10 devices
% Productivity
£
£
£
£
Rate Reduction
m Front
82.5
27.5
825
Factored 10 device
£
825
45,000
45,000
45,000
0%
45,000
50,000.00
400,000
400,000
4,000,000
40%
2,400,000
125,000
125,000
125,000
250,000
0%
250,000
570,000
570,000
4,295,000
Total Dry Dock Cost
SITE WORK TOTAL
2
2,695,000
DEVICE CONSTRUCTION
m2
Te
30.00
4,500
1,500
45,000
0%
45,000
500.00
30,000
9,998
300,000
0%
300,000
60
Te
200.00
12,000
3,999
120,000
40%
72,000
400
m3
55.00
22,000
7,332
220,000
10%
198,000
400
m3
12.00
4,800
1,600
48,000
0%
48,000
400
m3
10.00
4,000
1,333
40,000
0%
40,000
m2
Te
100.00
320,000
106,640
3,200,000
10%
2,880,000
500.00
180,000
59,985
1,800,000
0%
1,800,000
2.1
base slab - formwork
150
2.2
base slab - reinforcement supply
60
2.3
base slab - reinforcement fix
2.4
base slab - concrete supply
2.5
base slab - concrete place
2.6
base slab - concrete pumping allowance
3,200
2.7
ballast tank walls - prepare slipform
2.8
ballast tank walls - reinforcement supply
360
2.9
ballast tank walls - reinforcement fix
360
Te
250.00
90,000
29,993
900,000
20%
720,000
2.10
ballast tank walls - concrete supply
2,400
m3
55.00
132,000
43,989
1,320,000
10%
1,188,000
2.11
ballast tank walls - concrete place
2,400
m3
12.00
28,800
9,598
288,000
0%
288,000
2.12
ballast tank slab - formwork
760
30.00
22,800
7,598
228,000
0%
228,000
2.13
ballast tank slab - reinforcement supply
32
m2
Te
500.00
16,000
5,332
160,000
0%
160,000
2.14
ballast tank slab - reinforcement fix
32
Te
200.00
6,400
2,133
64,000
20%
51,200
2.15
ballast tank slab - concrete supply
220
m3
55.00
12,100
4,032
121,000
10%
108,900
2.16
ballast tank slab - concrete place
220
m3
12.00
2,640
880
26,400
0%
26,400
2.17
ballast tank slab - concrete pumping allowance
220
m3
10.00
2,200
733
22,000
0%
22,000
2.18
chamber rear slab (sloped) - formwork
400
m2
50.00
20,000
6,667
60,000
0%
60,000
2.19
chamber rear slab (sloped) - reinforcement supply
100
Te
500.00
50,000
16,667
500,000
0%
500,000
2.20
chamber rear slab (sloped) - reinforcement fix
100
Te
250.00
25,000
8,333
250,000
20%
200,000
2.21
chamber rear slab (sloped) - concrete supply
610
m3
55.00
33,550
11,183
335,500
10%
301,950
2.22
chamber rear slab (sloped) - concrete place
610
m3
12.00
7,320
2,440
73,200
0%
73,200
2.23
chamber rear slab (sloped) - e/o for top formwork
400
m2
50.00
20,000
6,667
60,000
0%
60,000
2.24
chamber walls - formwork
2,200
110,000
36,667
1,100,000
0%
1,100,000
chamber walls - reinforcement supply
80
m2
Te
50.00
2.25
500.00
40,000
13,333
400,000
0%
400,000
2.26
chamber walls - reinforcement fix
80
Te
250.00
20,000
6,667
200,000
20%
160,000
2.27
chamber walls - concrete supply
540
m3
55.00
29,700
9,900
297,000
10%
267,300
2.28
chamber walls - concrete place
540
12.00
6,480
2,160
64,800
0%
64,800
2.29
chamber walls - formwork - permanent steel (e.g. lip)
25
m3
Te
2,000.00
50,000
16,667
500,000
0%
500,000
2.30
roof - formwork
835
41,750
13,917
417,500
0%
417,500
roof - reinforcement supply
62
m2
Te
50.00
2.31
500.00
31,000
10,333
310,000
0%
310,000
2.32
roof - reinforcement fix
62
Te
200.00
12,400
4,133
124,000
40%
74,400
2.34
roof - concrete supply
425
m3
55.00
23,375
7,792
233,750
10%
210,375
2.35
roof - concrete place
425
m3
12.00
5,100
1,700
51,000
0%
51,000
2.36
roof - e/o for top formwork
835
m2
50.00
41,750
13,917
417,500
0%
417,500
2.37
Steel skirt
45
te
1,500.00
67,500
22,500
675,000
10%
607,500
2.38
Plant Penetrations
12
te
5,000.00
60,000
20,000
600,000
0%
600,000
2.39
Towing Fittings, rubbing strips
10
te
15,000.00
0%
DEVICE CONSTRUCTION SUB-TOTAL (excl indirects)
150,000
50,000
1,500,000
1,735,165
578,314
17,071,650
1,500,000
16,051,025
3
DEVICE CONSTRUCTION INDIRECTS
3.1
detailed design
item
20%
347,033
115,678
694,066
0%
694,066
3.2
management and supervision
item
20%
347,033
115,678
3,470,330
20%
2,776,264
3.3
plant &equipment
item
10%
173,517
57,839
1,735,165
20%
1,388,132
3.4
craneage
item
10%
173,517
57,839
1,735,165
10%
1,561,649
3.5
small tools
item
1.8%
31,233
10,411
312,330
0%
312,330
3.6
indirect labour
item
10%
173,517
57,839
1,735,165
15%
1,474,890
3.7
freight
item
0.25%
3.8
profit & overheads
item
15%
Total Construction Indirects
DEVICE CONSTRUCTION TOTAL
4,338
1,446
43,379
0%
43,379
260,275
86,758
2,602,748
30%
1,821,923
1,510,461
503,487
12,328,347
10,072,633
3,245,626
1,081,801
29,399,997
26,123,658
4
MARINE OPERATIONS
4.1
tug mobilisation/demobilisation
4
days
14,000.00
56,000
56,000
56,000
0%
56,000
4.2
tow to field
5
days
14,000.00
70,000
70,000
700,000
0%
700,000
4.3
installation
2
days
14,000.00
28,000
28,000
280,000
20%
224,000
4.4
Flotation bags
item
30,000
30,000
60,000
0%
60,000
4.5
winches, dead-anchors, rigging
item
50,000
50,000
100,000
10%
90,000
4.6
mobilise/demobilise dredger / rock dumper
350,000
350,000
350,000
0%
350,000
4.7
Scour protection
2,000
m3
35.00
70,000
23,333
700,000
20%
560,000
4.8
ballast - pumped sand
13,400
m3
15.00
201,000
67,000
2,010,000
20%
1,608,000
855,000
674,333
4,256,000
3,648,000
855,000
674,333
4,256,000
3,648,000
item
Total Marine Operations
MARINE OPERATIONS SUB-TOTAL (excl indirects)
5
MARINE OPERATIONS INDIRECTS
5.1
management and supervision
item
10%
85,500
85,500
855,000
80%
171,000
5.2
profit & overheads
item
15%
128,250
128,250
1,282,500
50%
641,250
213,750
213,750
2,137,500
812,250
1,068,750
888,083
6,393,500
4,460,250
Total Marine Operation Indirects
MARINE OPERATIONS TOTAL
6
BOP
6.1
Power Train Cost Estimate
6.2
Offshore cable (installed)
6.3
Grid Connection
£/kW
2,250
kW
700.00
1,575,000
1,575,000
15,750,000
29%
250
m
400.00
100,000
100,000
500,000
0%
500,000
120,000
120,000
1,000,000
0%
1,000,000
1,795,000
1,795,000
17,250,000
12,750,000
CIVIL TOTAL
BOP Estimate
£4,884,376
£1,795,000
£2,539,885
£1,795,000
£40,088,497
£17,250,000
£33,278,908
£12,750,000
GRAND TOTAL
£6,679,376
£4,334,885
£57,338,497
£46,028,908
COST / 250kW
£742,153
£481,654
£637,094
£511,432
£2,968,612
£17,339,538
£2,548,378
£2,045,729
£80,962
£157,632
£69,501
£55,793
item
TOTAL BOP
COST / MW
COST / m FRONT
2
11,250,000
OWC Cost & Prog rev 09
Appendix D
Economic Model
OWC Cost & Prog rev 01
Nearshore Economic Model
Year Ending
Power in the Sea
Frontage per unit
Units per device
Number of devices
Capture efficiency
Turbine efficiency
Generator efficiency
Device Output
Total Output
transmission and transformer losses
Capacity after losses (MW)
2007
0
kW/m
34.0
m
27.5
no.
3.0
no.
10.0
%
42%
%
65.0%
%
MW
2%
MW
6.8
£/MWhr
Income
£ / year
£/yr
2013
6
2014
7
2015
8
2016
9
2017
10
2018
11
2019
12
2020
13
2021
14
2022
15
2023
16
2024
17
2025
18
2026
19
2027
20
2028
21
2029
22
2030
23
2031
24
2032
25
8760
8,760
8,784
8,760
8,760
8,760
8,784
8,760
8,760
8,760
8,784
8,760
8,760
8,760
8,784
8,760
8,760
8,760
8,784
8,760
8,760
8,760
8,784
8,760
8,760
3%
263
264
263
263
263
264
263
263
263
264
263
263
263
264
263
263
263
264
263
263
263
264
263
263
263
8,497
8,520
8,497
8,497
8,497
8,520
8,497
8,497
8,497
8,520
8,497
8,497
8,497
8,520
8,497
8,497
8,497
8,520
8,497
8,497
8,497
8,520
8,497
8,497
8,497
8,760
2.5%
35
37.7
38.6
39.6
40.6
41.6
42.6
43.7
44.8
45.9
47.1
48.2
49.5
50.7
52.0
53.3
54.6
56.0
57.4
58.8
60.3
61.8
63.3
64.9
66.5
68.2
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
10.8
11.6
11.9
12.2
12.5
12.8
13.2
13.5
13.8
14.2
14.5
14.9
15.3
15.6
16.0
16.4
16.8
17.3
17.7
18.1
18.6
19.1
19.5
20.0
20.5
21.0
21.6
69.9
53.62
54.85
56.12
57.41
58.74
60.10
61.50
62.93
64.39
65.90
67.44
69.01
70.63
72.29
73.99
75.73
77.52
79.35
81.22
83.15
85.12
87.14
89.21
91.33
93.51
95.74
3,248,075
3,332,015
3,399,619
3,478,244
3,558,835
3,651,417
3,726,111
3,812,898
3,901,856
4,003,976
4,086,497
4,182,294
4,280,486
4,393,136
4,484,296
4,590,038
4,698,424
4,822,696
4,923,392
5,040,111
5,159,749
5,296,849
5,408,071
5,536,908
5,668,965
5.01
50.10
£/kW
kW
500.00
750.00
(£)
(£)
(£)
11,250,000
23.9%
33,278,908
70.7%
1,500,000
3.2%
2.0%
920,578
2.0%
3.0%
97,442
0.2%
(£)
47,046,928
0%
10%
(£)
51,499,819
- 51,499,819
2.50%
108,000
-
2.00%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
-
-
2.50%
2.50%
2.50%
110,700
113,468
116,304
119,212
122,192
125,247
128,378
131,588
134,877
138,249
141,705
145,248
148,879
152,601
156,416
160,327
164,335
168,443
172,654
176,971
181,395
185,930
190,578
195,342
64,961
66,640
67,992
69,565
71,177
73,028
74,522
76,258
78,037
80,080
81,730
83,646
85,610
87,863
89,686
91,801
93,968
96,454
98,468
100,802
103,195
105,937
108,161
110,738
10%
%
(£) @£/yr
2.50%
200,226
113,379
8,722,237
1.5%
£168,750
172,969
177,293
181,725
186,268
190,925
195,698
200,591
205,605
210,746
216,014
221,415
226,950
232,624
238,439
244,400
250,510
256,773
263,192
269,772
276,517
283,429
290,515
297,778
305,223
312,853
680,760
0.80%
376,375
385,785
395,429
405,315
415,448
425,834
436,480
447,392
458,577
470,041
481,792
493,837
506,183
518,838
531,809
545,104
558,731
572,700
587,017
601,693
616,735
632,153
647,957
664,156
2.0%
64,961
66,640
67,992
69,565
71,177
73,028
74,522
76,258
78,037
80,080
81,730
83,646
85,610
87,863
89,686
91,801
93,968
96,454
98,468
100,802
103,195
105,937
108,161
110,738
113,379
102,500
105,063
107,689
110,381
113,141
115,969
118,869
121,840
124,886
128,008
131,209
134,489
137,851
141,297
144,830
148,451
152,162
155,966
159,865
163,862
167,958
172,157
176,461
180,873
185,394
892,467
914,888
937,133
960,306
984,059
1,008,805
1,033,362
1,058,941
1,085,160
1,112,472
1,139,581
1,167,816
1,196,757
1,226,901
1,256,827
1,287,993
1,319,938
1,353,209
1,386,244
1,420,646
1,455,907
1,492,629
1,529,097
1,567,070
10,328,230
- 51,499,819
2,355,608
2,417,127
2,462,486
2,517,938
2,574,775
2,642,611
2,692,749
2,753,957
2,816,695
2,891,504
2,946,916
3,014,479
3,083,730
3,166,235
3,227,469
3,302,045
3,378,486
3,469,487
3,537,147
3,619,465
3,703,841
3,804,220
3,878,974
3,969,838 -
1
0.9000
0.8100
0.7290
0.6561
0.5905
0.5314
0.4783
0.4305
0.3874
0.3487
0.3138
0.2824
0.2542
0.2288
0.2059
0.1853
0.1668
0.1501
0.1351
0.1216
0.1094
0.0985
0.0886
0.0798
0.0718
- 51,499,819
2,120,047
1,957,873
1,795,152
1,652,019
1,520,379
1,404,392
1,287,934
1,185,488
1,091,245
1,008,205
924,774
851,378
783,843
724,333
664,507
611,876
563,436
520,751
477,816
440,042
405,271
374,629
343,791
316,660 -
334,488
-2,059,993
-2,059,993
-2,059,993
-2,059,993
-2,059,993
-2,059,993
-2,059,993
-2,059,993
-2,059,993
-2,059,993
-2,059,993
-2,059,993
-2,059,993
-2,059,993
-2,059,993
-2,059,993
-2,059,993
-2,059,993
-2,059,993
-2,059,993
-2,059,993
-2,059,993
£100,000
-
cash
EBITDA
Equity Discount rate
Asset Life - Straight Line (years)
Salvage Value (£)
Depreciation Rate Tax - Declining Balance
2012
5
(28,808.5)
Power Price (£ per MWh)
Present Value
Project NPV (£'000)
IRR
2011
4
7.0
%
£/MWhr
£/MWhr
£/MWhr
p/kWhr
Insurance Cost
Operational (£/yr) - % of CAPEX
Business interruption (£/yr) - % of revenue
3rd Party (£/yr)
2010
3
696.8
RPI increase
Renewable Obligations Certificate
Climate Change Levy (2002 prices)
Power price (standard market price comparison)
Total
Operating Cost
Operating Costs - (£) @£/yr
Crown Estate rent
@%of energy revenue
Decommission
Maintenance Cost
% of BOP cost
2009
2
91%
kW
Hours Available
Production hours in year
Maintenance hrs-during summer ie forced outage
CAPEX
BoP cost / kW
Unit turbine
BoP cost
foundations
electrical, grid connection
Contractors all risk (£) - one off on CAPEX
Cost overun (£) - % of 1st year revenue
total capital expenditure
capital grant
contingency
Capital Expenditures
2008
1
10.0%
(£'000)
4,659,265
(28,808.5)
2.473%
25.0
-2,059,993
0%
18:03
on 24/09/2004
-2,059,993
OWC Cost & Prog rev 01
Shoreline Economic Model
Year Ending
Power in the Sea
Frontage per unit
Units per device
Number of devices
Capture efficiency
Turbine efficiency
Generator efficiency
Device Output
Total Output
transmission and transformer losses
Capacity after losses (MW)
2007
0
kW/m
34.0
m
20.0
no.
1.0
no.
42%
%
65%
%
2%
MW
0.2
2013
6
2014
7
2015
8
2016
9
2017
10
2018
11
2019
12
2020
13
2021
14
2022
15
2023
16
2024
17
2025
18
2026
19
2027
20
2028
21
2029
22
2030
23
2031
24
2032
25
(1,178.1)
8760
8,760
3%
£/MWhr
Income
£ / year
£/yr
8,784
8,760
8,760
8,760
8,784
8,760
8,760
8,760
8,784
8,760
8,760
8,760
8,784
8,760
8,760
8,760
8,784
8,760
8,760
8,760
8,784
8,760
8,760
8,760
263
264
263
263
263
264
263
263
263
264
263
263
263
264
263
263
263
264
263
263
263
264
263
263
263
8,497
8,520
8,497
8,497
8,497
8,520
8,497
8,497
8,497
8,520
8,497
8,497
8,497
8,520
8,497
8,497
8,497
8,520
8,497
8,497
8,497
8,520
8,497
8,497
8,497
2.5%
35
37.7
38.6
39.6
40.6
41.6
42.6
43.7
44.8
45.9
47.1
48.2
49.5
50.7
52.0
53.3
54.6
56.0
57.4
58.8
60.3
61.8
63.3
64.9
66.5
68.2
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
60.7
65.4
67.0
68.7
70.4
72.2
74.0
75.8
77.7
79.6
81.6
83.7
85.8
87.9
90.1
92.4
94.7
97.0
99.5
102.0
104.5
107.1
109.8
112.5
115.3
118.2
121.2
69.9
107.36
109.93
112.58
115.28
118.06
120.90
123.82
126.80
129.87
133.01
136.22
139.52
142.90
146.37
149.92
153.56
157.29
161.12
165.04
169.05
173.17
177.40
181.72
186.16
190.70
195.36
157,806
162,040
165,483
169,466
173,548
178,219
182,021
186,417
190,924
196,078
200,277
205,129
210,103
215,791
220,427
225,783
231,274
237,550
242,670
248,582
254,642
261,569
267,221
273,747
280,437
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2.50%
2,050
2,101
2,154
2,208
2,263
2,319
2,377
2,437
2,498
2,560
2,624
2,690
2,757
2,826
2,897
2,969
3,043
3,119
3,197
3,277
3,359
3,443
3,529
3,617
3,708
3,156
3,241
3,310
3,389
3,471
3,564
3,640
3,728
3,818
3,922
4,006
4,103
4,202
4,316
4,409
4,516
4,625
4,751
4,853
4,972
5,093
5,231
5,344
5,475
5,609
12,813
13,133
13,461
13,798
14,143
14,496
14,859
15,230
15,611
16,001
16,401
16,811
17,231
17,662
18,104
18,556
19,020
19,496
19,983
20,483
20,995
21,520
22,058
22,609
23,174
30,590
10
100.00
£/kW
kW
500.00
500.00
total capital expenditure
capital grant
contingency
Capital Expenditures
(£)
(£)
(£)
250,000
11.8%
1,762,605
83.4%
60,000
2.8%
2.0%
41,452
2.0%
(£)
2,114,057
0%
10%
(£)
2,325,463
-
2,000
2,325,463
-
2.00%
10%
%
(£) @£/yr
391,934
5.0%
£12,500
0.80%
16,912
17,335
17,769
18,213
18,668
19,135
19,613
20,104
20,606
21,121
21,649
22,191
22,745
23,314
23,897
24,494
25,107
25,734
26,378
27,037
27,713
28,406
29,116
29,844
2.0%
3,156
3,241
3,310
3,389
3,471
3,564
3,640
3,728
3,818
3,922
4,006
4,103
4,202
4,316
4,409
4,516
4,625
4,751
4,853
4,972
5,093
5,231
5,344
5,475
£2,000
-
EBITDA
Equity Discount rate
10.0%
-
(£'000)
5,609
2,050
2,101
2,154
2,208
2,263
2,319
2,377
2,437
2,498
2,560
2,624
2,690
2,757
2,826
2,897
2,969
3,043
3,119
3,197
3,277
3,359
3,443
3,529
3,617
3,708
40,137
41,152
42,157
43,204
44,278
45,399
46,507
47,664
48,849
50,086
51,310
52,586
53,895
55,260
56,611
58,020
59,464
60,971
62,462
64,018
65,612
67,275
68,921
70,638
464,332
2,325,463
117,669
120,888
123,326
126,261
129,270
132,821
135,514
138,754
142,074
145,992
148,967
152,543
156,208
160,531
163,816
167,764
171,809
176,580
180,207
184,564
189,030
194,294
198,300
203,109 -
183,895
1
0.9000
0.8100
0.7290
0.6561
0.5905
0.5314
0.4783
0.4305
0.3874
0.3487
0.3138
0.2824
0.2542
0.2288
0.2059
0.1853
0.1668
0.1501
0.1351
0.1216
0.1094
0.0985
0.0886
0.0798
0.0718
2,325,463
105,902
97,919
89,905
82,840
76,332
70,586
64,816
59,729
55,042
50,904
46,747
43,083
39,706
36,724
33,728
31,087
28,653
26,504
24,343
22,439
20,684
19,134
17,575
16,201 -
13,202
-93,019
-93,019
-93,019
-93,019
-93,019
-93,019
-93,019
-93,019
-93,019
-93,019
-93,019
-93,019
-93,019
-93,019
-93,019
-93,019
-93,019
-93,019
-93,019
-93,019
-93,019
-93,019
-93,019
-93,019
cash
Asset Life - Straight Line (years)
Salvage Value (£)
Depreciation Rate Tax - Declining Balance
2012
5
0.2
%
Power Price (£ per MWh)
Present Value
Project NPV (£'000)
IRR
2011
4
168.9
MW
£/MWhr
£/MWhr
£/MWhr
p/kWhr
Insurance Cost
Operational (£/yr) - % of CAPEX
Business interuption (£/yr) - % of revenue
3rd Party (£/yr)
2010
3
91%
kW
RPI increase
Renewable Obligations Certificate
Climate Change Levy (2002 prices)
Power price (standard market price comparison)
Total
Operating Cost
Operating Costs - (£) @£/yr
Crown Estate rent
@%of energy revenue
Decommission
Maintenance Cost
% of BOP cost
2009
2
1.0
%
Hours Available
Production hours in year
Maintenance hrs-during summer ie forced outage
CAPEX
BoP cost / kW
Unit turbine
BoP cost
foundations
electrical, grid connection
Contractors all risk (£) - one off on CAPEX
2008
1
(1,178.1)
3.564%
25.0
0%
18:03
on 24/09/2004
Appendix E
Discount Rate
Sensitivity
The Carbon Trust
E1
Marine Energy Challenge
Oscillating Water Column Wave Energy Converter Evaluation Report
Introduction
This appendix looks at the economic viability of shoreline and near shore OWCs by assessing the
future development’s Net Present Value (NPV) compared with a range of equity discount rates (EDR).
The range of particular project variable is set out across the top of the following tables, and shows the
effect on the NPV. The base case variables are as defined in the economic model in Appendix D. In
line with section 10.3, the shoreline project is assessed using a 10p/kWhr production cost, and the
near-shore using a current possible production cost of 5p/kWhr.
E2
Shoreline EDR Sensitivities
E2.1
Project CapEx
Our current best estimate of the total construction cost for a project of this scale is £2.3 million. In a
remote location allowing for a power production cost, including ROCs and CCL of 10p/kWhr the
project would still not be economically viable. However if the CapEx figure could be reduced by 50%,
then a positive NPV could be achieved at approximately 10% EDR.
E2.2
Power Production cost
In a remote location allowing for a power production cost, including ROCs and CCL of 10p/kWhr the
project, again the project would still not be economically viable. However, if the total power production
value including ROCs and CCL were increased to £180/MWhr an equity discount rate of
approximately 10% would achieve a positive NPV.
Arup
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The Carbon Trust
E2.3
Marine Energy Challenge
Oscillating Water Column Wave Energy Converter Evaluation Report
Wave Resource
The table shows that the OWC would need to be sited in the best wave resource available around the
UK of 51kW/m to achieve a positive NPV with a discount rate of 8%.
E2.4
Capture efficiency
A minimum of 60% pneumatic capture efficiency would be required to achieve a positive NPV.
E2.5
Capital Grant
The table below gives an illustration of the level of capital grant required to achieve a positive NPV.
Arup
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Page E2
The Carbon Trust
E3
Marine Energy Challenge
Oscillating Water Column Wave Energy Converter Evaluation Report
Near-shore Sensitivities
E3.1
Project CapEx
Our current best estimate of the total construction cost for a project of this scale is £40.8 million (all in,
including 10% contingency – see economic model). In the UK context with a power production cost,
including ROCs and CCL of 6p/kWhr the project would require an equity discount rate of
approximately 7.5% to achieve a positive NPV. The CapEx figure could realistically vary by plus or
minus 20%. This at best would give a positive NPV at approximately 10% EDR, and at worst would
leave the project far from viable.
E3.2
Power Production cost
The target power production cost of £60/MWhr assumes a power trading value of £21/MWhr, a ROC
value of £35/MWhr and CCL of £4/MWhr. This should be considered the current best estimate and
lower bound figure, and again would require an equity discount rate of approximately 7.5% to achieve
a positive NPV. However, if the total power production value including ROCs and CCL were increased
to an assumed upper bound of £95/MWhr (It is currently forecast that the value of ROCs may increase
to £55/MWhr in the run up to 2010) an equity discount rate of approximately 10% would achieve a
positive NPV.
Arup
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Page E3
The Carbon Trust
E3.3
Marine Energy Challenge
Oscillating Water Column Wave Energy Converter Evaluation Report
Wave Resource
The table shows that the OWC would need to be sited in the best wave resource available around the
UK of 51kW/m to achieve a positive NPV with a discount rate of 7%.
E3.4
Capture efficiency
An optimistic future vision of 60% would require an equity discount rate of approximately 6.5% to
achieve a positive NPV.
E3.5
Capital Grant
The table below gives an illustration of the level of capital grant required to achieve a positive NPV.
Arup
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Page E4
Appendix F
Improvement Variables
Discount Rate
Sensitivity
The Carbon Trust
F1.1
Marine Energy Challenge
Oscillating Water Column Wave Energy Converter Evaluation Report
Shoreline OWC Improvement Variables
Arup
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The Carbon Trust
F1.2
Marine Energy Challenge
Oscillating Water Column Wave Energy Converter Evaluation Report
Near-shore Improvement Variables
Arup
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Page F2
Appendix G
Peer Review
Responses
OWC Peer Review Form
Arup Ref Number:
115214-00
Powertech Ref Number:
Principal Reviewer:
Date
Graeme Mackie
11 October 2004
Company
Discipline
WAVEGEN
PLEASE COMMENT OF THE FOLLOWING ASPECTS OF THE REPORT, USING YOUR OWN
EXPERIENCE, DATA AND RESULTS WHERE APPROPRIATE TO ILLUSTRATE COMMENTS
MADE
STATE OF THE ART
ACTIONS TAKEN / RESPONSES
Note: E-On responses are in red
Missing data
Recent advances
Economics of Near-Shore OWC
Wavegen’s current projections are 5.7p/kWh based on UK pricing for a
fully developed system in a large scale development.
A further review of economics has
been made. This report forecasts
5p/kWh as a target in near term.
General comments
Executive Summary (page 1)
6th para. Replace ‘actual wave resource’ with ‘average wave resource’.
The distribution of wave heights is such that for any exposed site the will
always be occasions when there is sufficient energy to drive an oversized
turbine/generator but there may be insufficient of those occasions in
relation to the annual average wave environment.
Generic Future Vision – a near-shore concrete caisson OWC (page 2)
Do sandy bottom sites exist where there is 34kW/m average wave power?
From British Geological Survey maps we would expect rock bottom with
1 to 2 m sand or gravel which would be fugitive in nature. If deep sand
sites are not available does this change the solution or economics?
Power Production Costs (page 3)
For the Near-shore device the annual energy produced per device is
6100MWh. To arrive at this figure assumes an input power of 34kW/m.
However, this is the incident power at 20m water depth. The incident
power at 9 – 11m water depth will be less.
Agreed.
Importance is firm, acceptably level
sea-bed.
Efficiency of pneumatic capture is
taken from the 20m contour value.
3.1.1 Concept (page 11)
LIMPET was commissioned at the end of 2000 (November 2000).
Report changed.
3.1.4.1 (Page 12) – first para.
The benefit of the contra-rotating unit on LIMPET was to allow the
downstream blade to recover energy from the swirl introduced by the
upstream blade. Under steady state conditions on a test rig this gave
broader bandwidth than a monoplane turbine. However, in practice the
Added to end of first para..“However,
in practice the cyclical motion does not
allow the system to stabilise and
energy recovery is minimal.”
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cyclical motion does not allow the system to stabilise and energy recovery
is minimal.
3.1.4.1 (Page 12) – third para.
There is no flow induced vibration problem with the butterfly valve.
3.1.4.1 (Page 12) – forth para.
Modulation of the radial valve to achieve flow control is feasible. The
problem on LIMPET relates to a lack of instrumentation which is
currently being addressed.
3.1.4.2 (Page 13) – fifth para.
The induction machine using resistors to dissipate rotor energy was the
75kW QUB machine, not LIMPET. Remove 5th, add below:The original induction machine drive design used resistors to dissipate
rotor energy and provide variable speed operation this would as with the
75kW prototype give rise to poor efficiencies. Replacement with an
industrial standard drive system should have given much better electrical
efficiencies than those reported. If the figure of 32% electrical efficiency
is correct than it is probably due to a combination of rotor resistance
losses, over rating of the machine and operation in low sea states.
3.1.4.2 (Page 13) – sixth para.
LIMPET is very much under active development. In 2004 Wavegen
installed and tested a new small modular turbine with improved controls
on LIMPET’s north outlet. The main unit has been reconfigured to a
250kW monoplane unit and additional instrumentation has been added to
transmit measurements of flow and noise in real time. Modifications are
planned to the acoustic attenuation solution. Detailed analysis of the flow
regime has been carried out using state of the art CFD software in order to
align predicted flow improvements with mathematical modelling and site
measurements. LIMPET has a full programme of tests through to 2006
involving a range of turbine solutions.
3.1.4.3 (Page 13) – fifth bullet point
Deluge of the turbine hall by overtopping is an expected event and the
structure has been designed to survive it. There was one incident where a
door was pushed in by the external head of water but the door has been
replaced by a more substantial one.
3.1.4.4 (Page 14) – Successes
Could also add:-
Checked with E-ON
No change - Reference is made to the
claim that these problems have been
over come.
Removed “However, read across…”
Added.. “This is reported to be due to
the lack of instrumentation which is
currently being addressed”
Checked with E-ON. – Changed 5th
paragraph to that indicated.
Report changed to say LIMPET still
under development.
There is still one standard wooden
fabricated door that is a turbine room
access and is totally unsuitable for the
environment.
Report changed.
OK
•
•
•
•
•
Reliability of basic system
Performance matches mathematical model
Performance matches tank model after correct bathymetry applied
Successful integration to local grid
Demonstrates the potential of future optimally designed units
3.1.4.5 (Page 14) – Performance
Did we really quote that we expected 100% generator efficiency?
Yes.
[Ref.5] – P35, Fig 29
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Estimated Efficiency
80%
60%
90%
43%
3.2.3 Design (assumptions and working principles)
Geometry: Total height above MWL is 15m
Blade form: 3D symmetrical
Rated voltage: 400V
Inertia: 600kg.m^2
13.4kW/m wave resource was based on 4 years of site measurement.
3.2.4.1 Problems
As a note on the flooding of the control room, the control room has now
been re-positioned separate from the device. Reasons for doing this were
to improve access and remove equipment from the splash zone
3.4.5.1 The Denniss-Auld Turbine
Text does not mention that this is a variable pitch device.
3.7 Other OWC installations
QUB’s 75kW device on Islay was dismantled in 1999.
3.9 Risk Matrix
LIMPET Schedule inaccuracy – Medium (was commissioned Nov 2000,
not end 2001)
LIMPET Operation & Maintenance Deluge of equipment room – Should
be Low. The equipment room is behind a bund and has never suffered
deluge. The turbine room is regularly overtopped but is designed for such.
- If marine standard doors were fitted throughout turbine building, but
they are not. Maybe change this to Control and Equipment Room to avoid
confusion. The fact that it was moved is an indication of the high risk.
Pico’s problems also illustrates the point.
3.10.1
LIMPET was commissioned Nov 2000.
3.10.2
Presumably annual average output for Port Kembla is ‘estimated’
Items added to report. OK
Noted. No change needed.
Note added on moving control room.
OK
Note added that it is variable pitch. OK
Report changed. OK
Considered a high risk compared to
other projects.
Report re-worded saying equipment
room not deluged
Suggest leaving it at High. Done.
Report amended.
Report amended.
OK
Noted. Recognition of Wavegen given
elsewhere. Yes – Added – “estimated”
7.1.4
2. edit text to read ‘Wavegen’s roof only OWC concept using pre-cast
…..’
Report amended. OK
7.1.4.1
Roof only OWC
Text to read “A roof only OWC is a concept developed by Wavegen that
utilizes the ….”
OK
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OWC Peer Review Form
Arup Ref Number:
115214-00
Powertech Ref Number:
Principal Reviewer:
Date
Graeme Mackie
4 November 2004
Company
Discipline
Wavegen
PLEASE COMMENT OF THE FOLLOWING ASPECTS OF THE REPORT, USING YOUR OWN
EXPERIENCE, DATA AND RESULTS WHERE APPROPRIATE TO ILLUSTRATE COMMENTS
MADE
STATE OF THE ART
ACTIONS TAKEN / RESPONSES
Note: E-On responses in red.
Discussed further with Wavegen who
Executive Summary – Generic Future Vision – a nearshore OWC
now accept the efficiencies quoted by
The report indicates 42% wave to pneumatic conversion efficiency.
Arup/E-on. There was a
Wavegen achieves this level of conversion efficiency in its wave tank
misunderstanding in that Arup/E-on
with devices having low freeboard wing walls and would expect better
are quoting efficiencies based on 20m
than 42% for the geometry shown in the report. The NEL OWC
contour values whereas Wavegen were
breakwater device “NEL OWC Breakwater Device 2GW Power Station,
using lip values. When the expected
Report 103/80 dated 1980” quotes a measured wave-to-pneumatic
efficiencies are normalised they are not
conversion efficiency of 71% based on a PM Spectrum representative of
too dissimilar.
west of Lewis seas. Wavegen would expect about 60% to be achievable
OK
with the Arup-E.ON arrangement.
3.8
Wavegen is also targeting the new breakwater market with small (20kW –
35kW) Wells turbine-generator units, one of which is currently being
tested in LIMPET. (see also paragraph 9.4.1)
3.10.1 – LIMPET
LIMPET cost was £1,746,577 including Wavegen R&D and project
management of £300,000 and grid connection of £129,000. Changing
from £2.2M to £1.75M reduces the actual cost of power in column six to
39p/kWh. With the improved turbine efficiency now being achieved (48%
instead of 22%) this brings the cost of power down to 18p/kWh. Were
LIMPET to experience the designed 20kW/m input power then the cost
would further fall to 11p/kWh (at zero interest rate as in your table).
6.1 - 1st para.
With reference to the statement, “skill required in the project execution
has been relatively low”, just because the projects executed to date have
been small it does not follow that the skills required to execute them have
been any less than for big projects. In some ways a tightly funded small
project can offer more challenges than a large well funded project.
7.1.4.1 – in situ construction (2nd para)
Secondary dredging operations on LIMPET have been relatively
inexpensive, i.e. about 1.5% of project cost.
Text added covering this development.
OK
Costs have been escalated to 2004
values. Arup/E-on also believe that
Wavegen’s recorded costs are lower
because contractual risks were passed
down to a subcontractor who was
unable to perform. The true costs of
project execution are believed to have
not been passed through to Wavegen
because of this.
OK
Text amended to say that a ‘small
scale’ low overhead methodology may
be appropriate and cheapest for smaller
projects but production projects will
need a different approach.
OK
‘Difficult’ has been removed.
OK
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7.1.4.1 – Cliff face tunnel OWC – 3rd para.
Wavegen is in a joint venture with the local Utility and performs
assessments and design on behalf of the JV. Wavegen is reporting costs in
the range 9 to 15p/kWh for this project.
Text amended to reflect jv
arrangement and economics.
OK
7.1.4.3 – 4th bullet point
Blasting and dredging proved to be a low cost means of bund removal.
Water column blockage has not been a significant issue.
Bullet point deleted.
OK
8.2.2 – last para
Neither LIMPET nor Pico had bi-planar turbines. The QUB 75kW device
on Islay tested a bi-planar turbine and found it to be marginally superior
to a monoplane unit. A bi-planar turbine is a simple solution for
introducing increased solidity and damping and Wavegen is finding the
performance of its small 20kW bi-planar unit currently being tested on
LIMPET gives whole cycle blade efficiencies approaching 60%. Strictly
speaking this is a twin rotor solution as the blade separation is greater
than 1.5 chord lengths. Further tests are planned with a true bi-planar set
up. Replace first 2 sentences with:….The contra-rotating design has not
proved successful at Limpet and simpler monoplane and bi-plane designs
appear to be favoured for the future. The disappointing performance of
contra-rotating designs is believed to be due to having too much damping
for the OWC. However ……etc
8.3.1 – 2nd para.
Pico blades are also aluminium alloy. The stator blades however are
stainless steel. Change 2nd paragraph: To date …etc….or stainless steel
(Port Kembla) or both (Pico).
8.3.1 – 4th para.
Wavegen requests that the reference to 6000 hours between blade changes
be removed from the report. While cracks were detected on some of the
blades the change out of blades was also linked to a reconfiguration of the
turbine to run as a single monoplane unit and the wish to introduce stress
monitoring on the new blades. The stress corrosion fatigue is as much
exposure time related as stress cycle related. The blades were in-service
for 30,000 hours. Mechanisms that might lead to stress corrosion fatigue
are currently under investigation at Wavegen. Solutions to extend blade
life are being evaluated including changes to blade manufacturing
process, improved protective coatings, leading edge and root protection
from abrasion and improved air flow to reduce turbulence inducing stress
variations. Change last sentence to:It was suggested that the blades would not be re-useable after 6,000 hours
running in 30,000 hours of service.
8.4.5 – 2nd para.
Wavegen recently (August 2004) installed a new small twin rotor Wells
turbine unit at LIMPET. This unit has an in-line silencer and with this
configuration no difference in performance between the inhale and exhale
cycles is evident from initial trial results. Early results with this unit have
demonstrated overall pneumatic-to-wire (turbine + generator + inverter
drive) efficiencies of just under 50% which, after deduction of electrical
losses, gives a whole cycle turbine efficiency in excess of 60%.
E-on commented:
Changed first 2 sentences of paragraph
as shown opposite.
E-on commented: Pico blades were
reported to us as stainless steel by IST.
We were aware of the confusion at the
time of writing and the report
comments still stand.
We were told a number of blades were
unsuitable to be reused after the stated
period this is the case and the comment
is relevant to the point made.
Changed end of paragraph as
indicated.
E-on commented: This prototype was
witnessed during our site visit and only
ran for a day, before a clutch throttle
valve mechanism failed. The OWC is
not matched to this 20kW turbine, and
hence the forward and reverse flow
efficiencies may well not be typical of
matched OWC and turbine. This level
of expect turbine efficiency has been
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assumed in the Future Vision.
Comments in this section reflect the
view that Limpet’s reported
performance can be improved.
8.6 – 1st para.
Loss of grid does lead to a loss of availability as explained in your second
paragraph and this is linked to the fact that LIMPET is not operated with
24 hour / 7 day coverage and therefore grid events can lead to, at worst
case over a weekend, up to 2 days loss of production. Design is also an
attributable factor in that there is no redundancy built in to any of
LIMPET’s systems, but this is forced by financial constraints on an
ambitious prototype project run by an SME. Therefore failure of a fairly
minor component such as a sensor or PLC cards can lead to lengthy
downtime while spares are procured and fitted. More recently we have
introduced a planned maintenance scheme to maximise system
availability. It also has to be considered that LIMPET is a demonstration
device and as such is linked to a programme of research involving
changes to systems, particularly data capture systems, and testing of
alternative control strategies. All of these activities involve shut down
while being implemented. As such, 65% plant availability is considered
quite a good figure by Wavegen. The reliability data acquired on key
components confirms that, for a commercial project properly manned on a
24/7 basis and with first line spares and on-call maintenance team, 90%
availability is quite achievable with the current design.
9.2.3 & 9.3.2
The prime sites identified in the report (Shetland, Islay, Orkney) will
more likely require a nearshore device to be placed on rock with a thin
fugitive covering of sand or gravel. While charts might show sand,
geological survey maps show this to be a thin covering over rock as
would be expected in areas of high wave energy.
9.4.1 - 1st para.
The 20kW turbine referred to has specific application to breakwater
projects of relatively low incident wave energy. The economics for such
schemes can be very attractive as most of the civil costs are incurred
irrespective of the installation of wave energy device. Breakwaters can
extend up to more than a kilometre in length at large ports and a scheme
experiencing only 8kW/m could involve an installed capacity of around
4MW made up of 200 x 20kW turbines. The attraction of this scheme is
the low manufacturing and installation cost for the mass produced
turbine-generator modules and their easy of change-out for maintenance.
In addition, the output from a number of small units provides a smoother
integrated power supply than the output from a single large unit.
10.4 – last para.
Someone stepping straight to the final conclusion paragraph of this report
without understanding the nuances of the terms shoreline and nearshore
might read this paragraph and conclude that it applies to OWC devices in
total, particularly as the previous bullet points refer to nearshore devices.
E-On commented: We are aware of all
these points and we agree, and have
made the point that an availability of
great than 90% should be achievable if
auto restart following loss of grid is
provided as would be expected with a
commercial design
Report amended to clarify that rock
sites can be developed too.
OK
This section of the report has been
removed.
The points made in the (original)
section are valid. The maximum unit
size should read 1MW, at the end of
paragraph 2.
The comments about economies of
scale apply. Large scale
commercialisation of this technology
is not likely at 20kW even in
breakwaters. Recommend keeping
section and amend if required. Note
section has been removed.
This section has been replaced for the
final draft.
OK
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OWC Peer Review Form
Arup Ref Number:
115214-00
Powertech Ref Number:
Principal Reviewer:
Tom Thorpe
Date
14 October 2004
Company
Energetech
Discipline
PLEASE COMMENT OF THE FOLLOWING ASPECTS OF THE REPORT, USING YOUR OWN
EXPERIENCE, DATA AND RESULTS WHERE APPROPRIATE TO ILLUSTRATE COMMENTS
MADE
Action taken / Response
General Comment
Note: E-On comments in red
Before moving onto addressing your report using the structure you
propose, some initial orientation is required. Hence I have moved
the ‘General Comments’ section to here.
The report tries to address a difficult question that I have worked on
for some years, namely given what we know of the immature
technology known as wave energy, what are the likely future
generating costs. The methodology you have used is an intriguing
one but, given the time allowed for the feedback, it is not one that I
can assess in detail given the information presented in the report.
Nevertheless, I am in agreement with what you have set down and I
congratulate you on devising such a methodology.
It is difficult to be clear about the Economic model used in Section
10.1 and the Appendices (this might be attributable to having lots of
information with little time to review it – what is the discount rate
used in the calculation? This approach is not the one normally used
by the DTI and so I would encourage discussions with the Carbon
Trust on agreeing an economic model to be used by all the
consultancies in the Marine Energy Challenge.
Equity discount rate is 10% in a ‘with
inflation’ cost model.
My main point is that you have, of necessity, used two generic types
of wave power device and one type of turbine to calculate the likely
mature technology costs. Your analysis here shows that, in general,
shoreline devices do not appear to be economically viable, even with
elevated power purchase price, and nearshore devices would only
become viable with discount rates of 7.5% or less and then at a
power purchase price of £60/MWh. I would generally agree with
these values and they confirm the finding of R122 from ETSU that
shoreline and nearshore devices have unfavourable economics, even
when the nearshore device has a wind turbine on top – Wavegen
might wish you to consider that option.
However, the Energetech OWC design is very different from the
LIMPET/OSPREY concept for the following reasons:
• It is fabricated from steel
• It is a floating device held in place by mooring lines (it does
have ‘feet’ mounted on concrete pads but the main mooring
method is taut lines)
The assessed ‘learning rate’ from the
future costings supplied for the
Energetech device appears to be around
16.5% compared to 10% stated in the
paper.
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•
•
It has a ‘parabolic’ wall to focus the waves on the central
OWC area (not a spot since the walls only approximate to a
parabola)
It does not use a Wells turbine but rather a variable pitch
turbine
You have noted all these difference within various parts of the report
but they have not been included in the economic calculations and
‘Future Visions. However, it is far from clear whether it is possible
to do for the Energetech system what you did for concrete devices –
we ourselves are only now considering the design of our next device
for deeper, more energetic sites. Nevertheless, it means that several
of the assumptions made and predicted results from the report are not
applicable to this technology.
This will be a general R&D focus for any
OWC.
The turbine may prove better than Wells
but improved efficiency is already
factored into the economics so should also
hold for Energetech. - YES
The report will be widened a little in its
scope to be more inclusive for the
Energetech approach. Text added on
learning curves and potential
improvements that could be made.
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General comments - continued
P23 – Section 3.4.2:
• First para – crest to a single area not point
• The end of last para should read ‘the lower the level of wave
energy entering the device’ (a cosθ term) not the greater the
energy capture loss.
P24 – 3.4.3
• End of first para. This sentence should read ‘The relatively
shallow slope of the sea bed would ensure that the breaking
wave is of the ‘spilling’ type1, which would reduce loadings
considerably. Nevertheless, the device has been designed
against a breaking wave.
P24 – 3.4.4
The meaning of the bullet points is unclear. The parabolic wall has
been designed against breaking waves; all the steelwork has been
fabricated; the device is moored using taut moorings and legs to
ensure the correct chamber level; the anchoring system has no access
problems and the feet rest on concrete pads.
P 35 – 3.9, Table
• The rock integrity is at least a M risk – it governs the piling
for the mooring system, so we conduct extensive drilling for
cores
• Scour is probably a L or M risk – we use concrete pads
• Turbine efficiency is probably an M risk – it has been
studied extensively, so it should be similar to the others
P 36 – 3.9, Table
• Productivity gain should be better than the rest because it is a
steel construction, which offers some benefit from
replication: e.g. we would employ reusable bags to provide
additional buoyancy rather than build in steel buoyancy
tanks
• Ballast placement should be an L – the device is floating and
moored against its buoyancy
P 44 – 4.5.6
• Most turbines have some inertia to smooth out speed
variations, others have a flywheel attached for this purpose
P45 – 4.7
• There are also hundreds of OWCs operating with minimal
maintenance as power sources for navigational buoys
P69 – 6.1
• Para 1 – it is true that small designers and contractors make
for low skill levels in construction – that is why we use J P
Kenny and experienced offshore fabricators and installers
• Para 3 – we don’t have sub-divisions, because we have a
different shape of ‘wave front’ approaching the OWC
chamber - a converging ‘circular’ wave front. We believe
that sub-divisions are not needed but the Port Kembla device
will confirm or deny this.
Report amended.
Report amended.
Reflector wording amended.
Other points noted but no changes
proposed.
Report amended.
Report unchanged as the concrete pads
might be undermined.
No change as the turbine type is still a
prototype whereas Wells turbines had
been used in each of the other OWCs
Agreed - no available data yet.
Noted – no change made. Concrete also
has repeatability benefits and PK is
already shown as being better than
LIMPET and Pico.
Report amended to ‘not applicable’.
Comment added that flywheels are used.
Yes, there are different approaches. This
section is about control systems.
Report amended. OK
Comment added about small scale vs
large scale.
Report amended.
P 79 – 7.2.1.3
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• It would make more sense to utilise an existing dry dock.
final.doc
The steel structure has allowed Energetech to use an existing
fab yard.
P82 – 8
• Queens University Belfast has lots of information on the
topics covered here, which might be better to use. It seems
ambitious to develop a Grand Unified Theory of OWCs as
part of this exercise. For instance, J P Kenny looked at
AQWA for our system but found out that its accuracy was
low in shallow water systems.
• Nearly all this Section is not applicable to the Port Kembla
system.
QUB were involved in an interactive
workshop on OWC’s with Arup &
Wavegen where the procedure for
assessment shown here was presented
without undue comment.
Text widened a little to avoid comments
that it is not relevant to the PK system.
P99-9
• For reasons noted above, several parts of this Section are not
relevant to the Energetech device.
Ditto, above point.
P 102 – 9.2.3
• We have assessed both a concrete and steel structure for the
same duties at Port Kembla and the steel structure turned out
cheaper due to the vast size of the caissons needed for the
concrete structure.
Importing steel structures from Indonesia
is likely to be cheaper than Australian
concrete construction. No change made.
1
Hedges, TS, (2001). “Wave Breaking and Reflection”, Technical Note, University of Liverpool, Dept. of Civil Engineering.
Henderson, AR, (2001). “Breaking Wave Loads on Offshore Wind Turbines”, EWEA Special Topic Conf. On Offshore Wind
Turbines, Brussels, December 2001.
Holmes, P, (2001). “Coastal and Offshore Structures”, in a Course on Coastal Defence Systems, University of West Indies, July 18-21, 2001
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STATE OF THE ART
Missing data
P17- End of 3.2.4.1: There were additional problems that I think
have significant implications for wave energy.
• The fabrication and installation was at a remote site with
little infrastructure
• There was often a single source for construction work
These observations confirm your recommendation to build at a
central fabrication yard and then tow and install.
P 27 - 3.4.3:
• Total height above MWL – 12 m
• Arrangement – horizontal axis
• Duty type – 50 Hz, 12 pole
• Rated speed – 500 rpm
• Max test speed – 750 rpm
• Hub tip ratio – 0.75
• Blade chord - mm
P24 – 3.4.5
• Wave to pneumatic – 67%
• Pneumatic to mechanical – peak 80%, average 54%
• Mechanical to electrical – 90%
• Overall – 32.4%
Report amended.
Data added to table.
Data added to table.
OK
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Recent advances
P94 – 8.4.5
• The latest predictions of turbine efficiency are shown below.
Noted
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final.doc
RESOURCE ASSESSMENT
Data Sources
Method of Data Acquisition
UK Resource / Development Site Ranking
Definition of energy input
Shoreline v Nearshore
Energy Calculation
•
ETSU report R-120 gives a shoreline resource of 400
GWh/year (v Arup 230) and nearshore of 2.1 TWh
per year (v Arup’s 7.8). A reasonably good
agreement considering the very different
methodologies used.
Notes added to s5.1 & conclusions.
DEVICE MODELLING
Methods used
Model limitations
Specific modelling tools used
Modelling of damping
TANK TESTING
Accuracy / limitations
Facilities used
Damping considerations
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final.doc
EFFICIENCIES
Wave capture calculation
Turbine efficiency
P 41 – 4.3
The addition of parabolic walls makes the Port Kembla device
very different. You are no longer dealing with a wave front
impacting on a rectangular OWC chamber but with a nearly
circular wave surge approaching the column from the front
and sides. This behaviour will have different characteristics
(e.g. effect of column inclination and lip) to a conventional
system. I agree with you that this system needs optimisation.
P41 – 4.4.1
In the penultimate para, a control system is cited to have poor
reliability and high maintenance requirements but in the next
para an actuated valve has none of these downsides. The
control system on the turbine is an actuator – out of the air
flow.
Text expanded to cover Port Kembla case in greater
detail.
E-ON commented: We think these are very different
systems. Consideration needs to be given to the duty
of the device. A rotating mechanical control system
that is continually operative is somewhat different to
a static device that only operates occasionally.
However a blow off value and throttle valve are still
challenging systems to design.
Generator efficiency
COSTS
BOP £/kW
Grid connection
Operation & Maintenance
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final.doc
ECONOMIC MODEL
Power Price
P78 – 7.2
If only it did generate at 5 p/kWh!!! The wave regime at Port
Kembla is low, as befits an initial device and, therefore, it is
unlikely to generate at 5 p/kWh. We have made no prediction
of the cost as yet, because we have seen how all previous
device predictions have been incorrect.
We must have misunderstood previous discussions
– the section has been deleted.
Likewise
Insurance costs
IMPROVEMENTS
Priorities
Possibilities / Future vision
CONCLUSIONS
ANY OTHER COMMENTS
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OWC Peer Review Form
Arup Ref Number:
115214-00
Powertech Ref Number:
Principal Reviewer:
Date
António F. de O. Falcão
16 October 2004
Company
Discipline
IST, Portugal
PLEASE COMMENT OF THE FOLLOWING ASPECTS OF THE REPORT, USING YOUR OWN
EXPERIENCE, DATA AND RESULTS WHERE APPROPRIATE TO ILLUSTRATE COMMENTS
MADE
STATE OF THE ART
ACTIONS TAKEN / RESPONSES
Note: E-On responses in red.
1. General comments
The report is a very valuable contribution to the assessment and to the Arup/E-on deliberately chose to
review the more recent papers starting
development of OWC technology for wave energy conversion.
around 1993. The reference list is, of
Most of the assessment conclusions in the Report coincide with my points
course, not exhaustive but we consider
of view (and in many cases my intuition) and with my own experience
that enough has been reviewed to gain
with wave energy in general and OWC technology in particular (I was the
a picture of OWC technology.
co-ordinator of the several projects within whose framework the design
and construction of the Pico OWC plant took place).
EON reviewed over 100 papers
As could be expected, a large part of the report is devoted to the special applicable to the areas of the turbine,
case of how to exploit the UK wave energy resource with OWC OWC control, generator and controls
converters. Since my own experience is rather with the Portuguese case, I and prototype project reports. Not all
abstain from commenting such sections.
of these are included in the references.
On what concerns the technology, understandably the Report relies We are adding another 9 references
heavily on the UK experience, with important roles played by that are relevant to the report.
Although there has been a lot of
WAVEGEN and the Queen´s University of Belfast.
academic research carried out, the
Although the number of references (about 70) is by no means outcomes are often far from
insignificant, one should say that a very large portion of published conclusive. Similarly attempts have
material relevant to OWC technology is not mentioned (was it overlooked been made with many prototype
or simply not quoted?).
devices but reliable published data is
elusive.
The authors of the report rightly regard the air turbine as a key piece of
OWC plant equipment. The turbine is responsible for a large part of the
losses that occur in the energy conversion chain. The Wells turbine
(especially its fixed-pitch version) is known to exhibit a relatively modest
time-averaged efficiency. Unfortunately, the competing types of selfrectifying turbines are also far from highly efficient. Nevertheless, I
would have expected more attention devoted to alternative types of
turbine; for example, a relatively large number of easily available papers
on the impulse turbine, reporting R&D work in Japan and Ireland, seem to
have been overlooked. (The situation is different for the Dennis-Auld
turbine, on which (reliable) published information is scarce.)
E-On commented & checked out
Japanese or Irish research:
See above comments. Papers on the
research carried out on the impulse
turbine have been covered, and
comments are made about the
improvements that can be made with
fixed blade designs. Impulse turbine is
referred to throughout the report and
the technical issues are discussed in
8.2.1, 8.2.2 and 8.4.3.
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2. Detailed comments
Pages 16, Table on Pico Plant:
•
Hub to tip ratio: 0.59
•
Blade form: NACA 0012 at tip, NACA 0015 at root
•
Depth: 8m at lip, 8-10m approach
•
Rated speed: 1400 rpm (note that the torque is zero at synchronous
speed 750 rpm)
•
Max test speed: 1500 rpm
Details added to report. OK
Page 17, on the Pico plant. Your statement is confusing: "The assumed E-On commented on time-averaged
turbine peak efficiency of 75% seems optimistic for a fixed pitch machine efficiency: Change paragraph as
in variable bi-directional flow". Indeed 75% seems to be attainable (from indicated.
testing on 0.6m diameter Wells turbine models) as peak efficiency in
steady flow, but obviously not as time-averaged efficiency under
randomly-varying pressure head. Has there been a misunderstanding on
information supplied by the Portuguese team? Yes, apparently. Replace
paragraph with:The assumed turbine peak efficiency of 75% is that obtained in a steady
flow. The time averaged efficiency in a bi-directional flow will clearly be
less than this at around 50%.
Page 44. The title 4.6 Mechanical Efficiency is clearly inadequate.
Changed to Power Conversion
OK
Page 70, on the differences between measured and estimated efficiencies. Agreed
The low figure of 10% (which I believe concerns to LIMPET) should not
be taken as representative of present state-of-the-art OWC technology. We also agree.
Indeed, I believe this to be largely due, in LIMPET: (i) to the
(surprisingly?) small water depth in front of the structure; and (ii) to the
large distortion in the velocity inward-flow distribution at turbine inlet,
produced by the (inadequately designed) noise-suppressing device.
Page 85. Section 8.2.1. Your conclusion (from the LIMPET experience) E-On commented:
should not be made general. Indeed, there is no evidence (on the contrary) You could take that line. Unfortunately
due to operational and funding issues
that such mismatching occurs at the Pico plant.
at Pico, there is insufficient data to
make the case that Pico is different. As
the turbines are from the same design
team, our comments reflect our
assessment of that process.
Page 86 (last 7 lines). The problem of the guide vanes, that become E-On commented:
alternately inlet and exit guide vanes, is very different in the Wells turbine We are aware of these issues and have
as compared with the impulse turbine. From the aerodynamic point of stated as such in the text.
view, this is known to be a serious problem in the case of the impulse
turbine (a major limitation to its peak efficiency) but not for the Wells
turbine (see a detailed analysis in L.M.C. Gato, A.F. de O. Falcão,
"Performance of Wells turbine with double row of guide vanes".
International Journal of Japan Society of Mechanical Engineers, series II,
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vol. 33, p. 265-271 (1990)).
Page 87 (mid-page). Note that the Pico turbine is neither bi-plane nor
E-On commented:
contra-rotating (it is a mono-plane rotor with double row of guide vanes):
This has been corrected following
Your conclusion may apply to LIMPET and OSPREY, but not to Pico.
comments received from Wavegen.
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OWC Peer Review Form
Arup Ref Number:
115214-00
Powertech Ref Number:
Principal Reviewer:
RCT Rainey
Date
18/11/04
Company
Atkins
Discipline
Structural
PLEASE COMMENT OF THE FOLLOWING ASPECTS OF THE REPORT, USING YOUR OWN
EXPERIENCE, DATA AND RESULTS WHERE APPROPRIATE TO ILLUSTRATE COMMENTS MADE
STATE OF THE ART
Missing data
Recent advances
General comments
1. Survivability of the Generic OWC Devices
The proposed offshore WECs appear to be about 10% of the weight of an equivalent breakwater – the author is
effectively suggesting that breakwaters could be made much lighter. The Alderney breakwater (see
http://www.alderney.gov.gg/pimages/gallery_lg/143.jpg), for example, has a similar wave climate, and is of similar
dimensions, but is much heavier because it is solid rather than hollow. It has repeatedly been breached by storms,
however, and is not considered to be over-designed, but rather under-designed (see
http://www.alderney.gov.gg/index.php/pid/114 contact: [email protected]).
There is an ongoing EPSRC-funded research project into the failure mechanism (contact:
[email protected]). Model tests have revealed extremely high wave impact pressures (3MPa at model
scale), which are absorbed by the inertia of the breakwater, as it shakes in response. Making the structure lighter will
tend to increase this dynamic response (reduced inertia implies increased accelerations), and thus make the cracking of
the structure more severe.
The rubble mound on which the breakwater is built is also vulnerable at Alderney - typical UK sites for the proposed
WECs (e.g. Benbecula) would likewise require such mounds, because the seabed is uneven. Thus there would appear
to be significant uncertainties over the survivability of the concept.
(Note: A representative rate for offshore concrete construction for the North Sea is about £2000/cubic metre. The unit
cost of the generic OWC equates to £55,793/metre in total, with 72% dedicated to the structural costs i.e.
£40,170/metre. In broad terms, this could equate to a two metre thick concrete section, ten metres high.)
2. Installation
Could the author comment on the installation methodology for the generic OWC caisson units at the 9m water depth
contour? In our experience, this zone is inaccessible to conventional handling vessels and land-based machinery.
Special equipment e.g. jack-up barges could be used, but costs of this equipment would need to be accounted for in the
economics.
RESOURCE ASSESSMENT
Data Sources
Method of Data Acquisition
Summary of Comments on Microsoft Word Peer Review Form_RCTR_Review.doc
Page: 2
Author: gordon.jackson
Subject: Note
Date: 21/01/2005 14:57:20
The OWC has been designed to gravity substructure standards. It is not a masonry breakwater with earth filling as Alderney.
Author: gordon.jackson
Subject: Note
Date: 21/01/2005 14:57:20
Concrete elements will be designed for impact pressures following current practice. The ability of the foundation to resist impact
factors takes account of the relatively large natural period of the structure compared to individual impact events.
Author: gordon.jackson
Subject: Note
Date: 21/01/2005 14:57:20
Scour protection is used on sand sites. Where rock is present at the surface, either underbase grouting or rock anchoring can be
considered. Such foundation types would not be susceptible to undermining.
Author: gordon.jackson
Subject: Note
Date: 21/01/2005 14:57:20
The cost estimate has been built up using labour, plant and equipment norms as used on offshore oil and gas concrete platforms.
The lower unit cost of the OWCs reflects the lower reinforcement densities, greater productivities and reduced preliminaries
requirement for the OWC.
Author: gordon.jackson
Subject: Note
Date: 21/01/2005 14:57:20
The OWC units are floated into position and set down by water ballasting the compartments. The only vessels required are tugs
and a dredger to discharge sand ballast into the structure. Both will have reasonable operability in the target summer installation
windows.
UK Resource / Development Site Ranking
1. Foundation Conditions
Could the author comment on the proportion of UK sites that are thought to comply with the foundation requirements
for the generic OWC caisson i.e. level uniform sand or clay combined with the power capture requirement for a
steeply sloping seabed?
At Benbecula, for example, which is typical of the Hebrides, the seabed is characterised as metamorphic bedrock, with
randomly distributed depressions and gullies up to 10m deep, filled with loose sand and gravels. The slope trend is
approximately 1:100, but there are significant local variations. See Figure 1 below:
Figure 1 – Typical Hebridean bathymetric contours
Definition of energy input
Shoreline v Nearshore
Page: 3
Author: gordon.jackson
Subject: Note
Date: 21/01/2005 14:58:33
The final report enlarges on the possibility of rock sites.
Energy Calculation
Page: 4
1. Plant availability
It is noted that the effects of plant availability are excluded from the production cost assessment i.e. no outages are
assumed in calculation.
Author: gordon.jackson
Subject: Note
Date: 21/01/2005 14:57:20
3% planned outage is taken. No allowance is made for un-planned outages.
DEVICE MODELLING
Methods used
Model limitations
Specific modelling tools used
OWC Peer Review Form
Arup Ref Number:
115214-00
Powertech Ref Number:
Principal Reviewer:
Date
Professor Peter White
15 December 2004
Company
Discipline
University of Coventry
Wells Turbines & Thermofluids
PLEASE COMMENT OF THE FOLLOWING ASPECTS OF THE REPORT, USING YOUR OWN
EXPERIENCE, DATA AND RESULTS WHERE APPROPRIATE TO ILLUSTRATE COMMENTS
MADE
STATE OF THE ART
ACTIONS TAKEN / RESPONSES
1. General
Note: E-On comments in red
The report gives a fair and accurate description of the world wide design,
construction and operation of Oscillating Water Column (OWC) wave
energy conversion devices. The Executive Summary highlights the major
problems encountered especially for shoreline devices which are unique
to the location, have been provided with inadequate temporary works and
as a result suffered from extended construction phases. Low wave to wire
conversion rates have been experienced largely due to inadequate
knowledge of the local wave resource, and poor matching of the
turbogenerator, of which more details are given in section3. It is
interesting to note that construction costs are still the major factor
affecting viability, and is the same as the outcome of the ETSU 1992
report which quoted construction costs and discount rates as having major
impacts on the cost of power generation.
Turbine
Throughout the report, and starting in the viability of OWCs section of
the Executive Summary, values of turbine efficiency are quoted, and it is
not clear whether these are peak uni-directional steady state values, or
overall values in the random flows resulting from the interaction of the
OWC with the wave resource, as it is of course the latter value which is of
interest. To reinforce this the table in section3.1.4.5 quotes estimated and
measured efficiencies of the LIMPET without stating if they are peak
values or taken over a one year operating period.
In section 3.2.4.2 the meaning of efficiency is also unclear as the
estimated efficiency of the turbine is quoted as 75%, and the overall
efficiency as 35% whereas below the table is a comment that the assumed
turbine peak efficiency of 75% seems optimistic. Clarity is required
wherever efficiency is referred to within the document, and should
certainly be specified in section 10.1.1.1. The general conclusion in
3.1.4.1 concerning turbine energy capture that a variable pitch turbine is
required needs to be considered with the implication this will have on
generator sizing, as the turbine will be delivering a wide range of power
levels, and large power ranges are not conducive to high generator
ffi i i
h
i t bl 3 1 4 5
Clarified as peak and average as
appropriate.
This point has also been made by Prof
Falcao. The efficiencies in the table
should refer to average values that
could be obtained in the actual
designed for sea state. Where this is
not the case it should be clearly stated.
The paragraph about a peak efficiency
of 75% has been changed, to make this
clear.
Added 3rd paragraph to 10.1.1:The assumed efficiencies below are the
average values that could be obtained
in the designed sea state.
The issues of low generator efficiency
are largely to do with correct sizing of
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efficiencies as shown in table 3.1.4.5.
In sections 4.2 and 8.2.1 Computational Fluid Dynamics (CFD) is
suggested as a means of modelling wave to wire performance. It should
be noted that CFD predictions of Wells turbine performance are
comparable to measured data until the turbine stalls, after which they
diverge.
the turbine & generator and the “cut in
speed” in low sea states.
Noted.
In section 4.4.1 a potential weakness of the Wells turbine is identified as
the turbine consuming power after it has fully stalled, however, what is
much more relevant to overall output is that the turbine does not produce
any power until the airflow reaches a particular value. Thus at the point of
flow reversal in every wave the turbine produces no power, and requires
power input to maintain the speed of rotation. This is a major factor in
turbine optimisation and sizing.
Changed 2nd Paragraph last sentence
thus:- “…., particularly at low or high
stall air velocity when the turbine may
actually consume power to maintain
rotation.”
The importance of matching the whole system should be emphasised in
section 4.5.3. In section 4.6 the improvement of wave to wire efficiencies
is discussed with reference to correct sizing, blade design, and best
practice from other turbine types. An idea of these effects can be obtained
using the work of Lockett, Webster and White which uses a frequency
domain model incorporating wave data, OWC conversion rates, and
steady state turbine efficiency curves. The programme output is an
efficiency of the turbine from air to shaft power over the yearly sea states
optimised to give maximum efficiency. For example a Wells turbine with
NACA 0015 blades in an OWC subjected to South Uist sea data has an
overall efficiency over the year of 47% from a measured peak efficiency
of 58% in steady unidirectional air flow. This work can be used to test the
comparison between Wells and impulse turbines discussed in section
8.2.1. – A useful tool for further research, work.
Noted.
Fixed guide vanes on a Wells turbine as mentioned in section 8.2.2 have
been shown to improve measured efficiencies by 5-10%.
Noted
The figures quoted in sections 8.4.3 and 10.2.1 of peak turbine
efficiencies of 60-70% hopefully giving overall efficiencies over a range
of sea states of circa 50% are born out by the frequency domain
predictions mentioned above.
ok
Added a new paragraph to the end of
4.5.3. Thus:It should be noted that whether the
design is considered as a single system
optimisation problem or a series of
smaller sub-systems to be optimised,
each sub-system’s design and
performance is dependent on the
others.
Work has already been carried out on investigating some different blade
profiles for use in Wells turbines, and in order to incorporate the effects of Noted.
the blades operating on a rotor which is a cascade with a 90o stagger angle
tests need to be carried out on A Wells turbine. The tests must be carried
out at the same Reynolds number as the full scale Wells turbine, as the
operating Reynolds number has a significant effect on the blade and hence
turbine performance.
Priorities
From the aspects of the report discussed the priorities for improving the
performance of OWCs are:
•
Obtain accurate sea wave data for the site being considered. This
is especially crucial for shoreline devices
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•
•
•
•
is especially crucial for shoreline devices.
Carry out matching of the wave data to the OWC, the pneumatic
output of the OWC to the turbine and the shaft output of the
turbine to the generator, optimising each component to
maximise the electrical output over the year.
Use CFD to ensure that the air flow reaching the turbine is
uniformly distributed and equal in both directions.
Utilise wave to wire predictive models which can assess the
effects of turbine type, guide vanes, blade profiles, blow off
valves, and any other idea to improve performance before a
device is built.
Collect data from working OWCs which can be used to verify
predictive models. This may necessitate academic institutions
working more closely with constructors and operators.
We would agree and this is what we
have suggested in the scope for
improvement and future vision.
These priorities are also set out in the
R&D section, which is in the final
draft.
We would also add the use of CFD for
optimising the design of turbines as
there is still much work to do here.
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