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Marine current energy
conversion: the dawn of a new
era in electricity production
AbuBakr S. Bahaj
rsta.royalsocietypublishing.org
Research
Cite this article: Bahaj AS. 2013 Marine
current energy conversion: the dawn of a new
era in electricity production. Phil Trans R Soc A
371: 20120500.
http://dx.doi.org/10.1098/rsta.2012.0500
One contribution of 14 to a Theme Issue
‘New research in tidal current energy’.
Subject Areas:
ocean engineering, energy, power and
energy systems
Keywords:
tidal energy, marine currents, ocean energy,
renewables
Author for correspondence:
AbuBakr S. Bahaj
e-mail: [email protected]
Sustainable Energy Research Group, Energy and Climate Change
Division, Faculty of Engineering and the Environment,
University of Southampton, Southampton SO17 1BJ, UK
Marine currents can carry large amounts of energy,
largely driven by the tides, which are a consequence
of the gravitational effects of the planetary motion
of the Earth, the Moon and the Sun. Augmented
flow velocities can be found where the underwater
topography (bathymetry) in straits between islands
and the mainland or in shallows around headlands
plays a major role in enhancing the flow velocities,
resulting in appreciable kinetic energy. At some
of these sites where practical flows are more
than 1 m s−1 , marine current energy conversion is
considered to be economically viable. This study
describes the salient issues related to the exploitation
of marine currents for electricity production, resource
assessment, the conversion technologies and the
status of leading projects in the field. This study
also summarizes important issues related to site
development and some of the approaches currently
being undertaken to inform device and array
development. This study concludes that, given the
highlighted commitments to establish favourable
regulatory and incentive regimes as well as the
aspiration for energy independence and combating
climate change, the progress to multi-megawatt arrays
will be much faster than that achieved for wind energy
development.
1. Introduction
The exploitation of natural and sustainable resources
to meet our energy needs is paramount if societies are
to reduce their ever-increasing emissions and pollution
stemming from energy supplies derived from fossil fuels.
Extensive activities around the globe are gearing up to
reduce our reliance on such fuels by increasing the share
c 2013 The Author(s) Published by the Royal Society. All rights reserved.
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Marine currents driven by the tides can carry large amounts of energy, especially around the
UK’s shores. Other forms of marine currents stem from global oceanic circulation such as those
encountered in the Florida Current in the Atlantic Ocean or the Kuroshio Current in the Pacific
Ocean and those caused by variations in sea water density—such as in the Bosphorus Channel
(Turkey). In many ocean areas, marine currents are considered to be too slow to exploit. However,
augmented flow velocities can be found where the underwater topography (bathymetry) in
straits between islands and the mainland and shallows around headlands plays a major role in
enhancing the flow velocities, resulting in appreciable kinetic energy. Some of these sites, where
practical flows are more than 1 m s−1 , are considered to be economically viable for marine current
energy extraction.
The tidal current resource is a consequence of the gravitational effects of the planetary motion
of the Earth, the Moon and the Sun, which produce the tides that drive such currents. Hence, the
resource is highly predictable, albeit variable in intensity. The consequence of this is that projects
......................................................
2. Resource characteristics and assessment
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of energy generated from renewable resources such as biomass, geothermal, ocean and wind
energy. Major programmes in many countries are centred on the deployment of large-scale
wind energy, be it onshore or offshore. If we consider the latter, the emphasis here is on the
development of large-capacity turbines of more than 6 MW, and the installations on platforms
geared to progress wind farm development further towards deeper water where the resource is
larger. Such progress is likely to continue at a fast pace driven by resource availability, higher
yields and available subsidies and the need to satisfy energy supply security and to meet
international emission targets.
Moving further offshore, wind energy creates other opportunities to develop additional
renewable resources to support the needs highlighted earlier. Our oceans, which cover more than
two-thirds of the planet, present a vast amount of energy available in the form of waves, currents
as well as temperature and salinity gradients. As examples, energy conversion from waves was
considered by Clément et al. [1] and Falcão [2], whereas consideration of temperature and salinity
gradients can be found in Damy & Marvaldi [3] and Brauns [4], respectively.
In this study, we limit the discussion to the consideration of marine currents or tidal
stream energy conversion, which, in effect, exploits ocean resources and has some similarities
in application to wind energy conversion. These similarities lie in the operating environment,
weather variations, aspects of power capture and take-off, supply chain, and operation and
maintenance to name but a few.
Exploitation of tides for the generation of electrical power is now a reality. Methods of
extracting tidal energy fall into two categories: (i) barrages—water is impounded behind a barrier
and then discharged in a controlled fashion through a conventional hydropower plant mounted
within the barrier—this category includes barrages across tidal estuaries such as the 50 year
old La Rance scheme [5] and the several discarded proposals [6] for a similar scheme on
the River Severn; (ii) tidal current (stream) generators [7]—water flow is interrupted by some
arrangements of hydrofoils to generate torque that drives a generator. The conversion device
may be a ducted or non-ducted arrangement. Many of the current prototype designs have
taken a similar configuration to wind turbines, comprising non-ducted, low-solidity, horizontal
axis rotors. Such turbines are well suited to high sea flow conditions and can be configured
as ‘fences’ or arrays of turbines sited across channels within appropriate and energetic sites in
the oceans.
This study describes the salient issues related to the exploitation of marine currents for
electricity production, highlighting aspects of resource assessment, conversion technologies and
the status of leading projects in the field. The study also summarizes the important issues related
to site development and some of the approaches currently being undertaken to inform device and
array development.
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north
3
2.0
0.5
2.0
1.5
1.0
0.5
0.5
1.0
1.5
2.0
east
0.5
1.0
velocity (m s–1)
1.5
baseline
energy extraction
2.0
Figure 1. Complex effect of array upon both flow speed and flow direction: plotted on the hodograph are simulated flow velocity
at a particular point in the domain (point indicated in inset map), in the natural state and with energy extraction (energy is
extracted in the rectangular area on the inset map). (Online version in colour.)
that rely on such a resource offer an advantage over other renewable energy technologies—such
as wind or wave energy—by providing quantifiable long-term energy yields that can be planned
for and managed with the electrical grid.
In the UK, the resource was mapped around 2004 using support provided by the predecessor
to the current Department of Energy and Climate Change in the form of an Atlas of UK Marine
Renewable Energy Resources [8]. During this period, another more detailed study estimated that
the UK has a total resource of around 90 TWh per annum, emphasizing that only about 20 per cent
(18 TWh) could be exploited [9]. Other similar resource studies have been conducted for different
countries, some of the results of which have been documented in the annual reports produced
by Ocean Energy Systems [10]. It must be noted, however, that, in many of these estimates, there
are considerable uncertainties in the quality of the data produced. However, such studies are
constructive as, in the absence of detailed measurements at prospective sites, they set out an
indicative assessment of the potential of the resource and the required market development and
potential for its conversion technologies.
Resource assessment is critical for both estimating energy yield and assessing extreme design
conditions over the expected lifetime of devices. In addition, a full understanding of site flow
conditions is required so that energy extraction from the site can be quantified and any impacts
of the energy extraction can be assessed [11]. This understanding can be gained through a
combination of survey data and numerical modelling. For example, an ongoing project is being
carried out at the University of Southampton for the Alderney Commission for Renewable Energy
(ACRE) [12] in which the possible impact of large arrays of tidal turbines on local sandbanks is
being examined. Figure 1 shows the impact on simulated flow velocity at a point where energy is
extracted from a site (indicated by the rectangle on the inset map) close to Alderney, Channel
Islands, UK. Tidal flows and energy extraction were simulated in TELEMAC-2D following
the methodology outlined in Blunden & Bahaj [13], with validation provided by four acoustic
Doppler current profiler deployments commissioned by ACRE. A snapshot of the spatial pattern
of the impact of the array installation upon tidal current velocity magnitudes is given in figure 2.
The plot in figure 2 shows differences in velocity at a particular stage of the tide, between when
energy is extracted, compared with the natural state. In the analysis, only those differences larger
than ±0.1 m s−1 are included.
......................................................
1.0
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1.5
flood
EBB
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4
Figure 2. Impact of array installation upon velocity magnitudes over a sandbank (dotted line) close to the Channel Islands. The
visualization shows differences in flow speed at a particular stage of the tide, when energy is extracted (in the rectangular area),
compared with the natural state. (Online version in colour.)
Site measurement surveys of flow conditions and bathymetry will need to be conducted
in order to identify secondary effects such as turbulence, unsteady flows and wave/current
interactions that may not be apparent from available coarse data or marine atlases. Careful
planning of site array layouts is very important so that turbulence and wake effects generated
by the devices and their support structures are addressed, and thus optimum energy yields
can be achieved. As there are no marine current energy conversion (MCEC) arrays in existence
today, there is a knowledge gap in the information needed for designs at a scale that can
facilitate planning and installation of optimized configurations of arrays within a site. In order
to capture this required information, it is anticipated that the first installations of arrays will be
fully instrumented so that data at scale can be captured. This will provide the necessary data
to allow the development and validation of array planning and design tools. In the absence
of this, the industry is currently relying on knowledge generated through simulations and the
studies of model turbines in circulating water channels [14,15]. The results from these can be
used in conjunction with numerical simulations to appraise current array designs for future
implementation at appropriate sites [16].
3. Technology development
(a) Energy conversion
Marine currents are akin to the wind energy resource, where the kinetic energy of the moving fluid
can be extracted in a similar fashion using suitable turbine rotors. In most cases, wind energy
conversion analysis and tools can be used to characterize the extraction of energy from marine
......................................................
0.25
0.20
0.15
0.10
0.05
0
–0.05
–0.10
–0.15
–0.20
–0.25
–0.30
–0.35
–0.40
–0.45
–0.50
–0.55
–0.60
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velocity
difference
(m s–1)
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(b) Conversion technology status
As indicated earlier, in most cases, the physical principles of marine current turbines have much
in common with those developed for converting wind energy, but with the driving fluid being
water rather than air. Most MCEC devices fall within the following types: horizontal axis turbines
(similar to wind turbines), orthogonal flow turbines (axis can be vertical or horizontal), Venturi
devices, shrouded variants of these aimed at concentrating the incident flow to a rotor and
oscillating hydrofoils. The devices can be fixed rigidly to the seabed, mounted on a mooring or
fixed to a moored platform.
Serious development of prototypes was initiated in the UK starting around the year 2000
through support from the government’s Technology Programme, run by the Department of Trade
and Industry (DTI), and eventually passed to the new Department of Energy and Climate Change.
This programme has now evolved and falls under the Technology Strategy Board’s (TSB) research
and development-funding portfolio [18]. In 2010, the TSB set up the Marine Renewable Proving
Fund (MRPF) with £60 M for both wave and tidal energy. The front runners benefiting from
the early UK government programme are given in table 1, whereas table 2 shows global lead
contenders.
It must be noted that government funding under the Technology Programme (now being
run under the TSB banner) was limited to 50 per cent of the cost of the projects for device
development, as highlighted in table 1. The companies Lunar Energy and SMD Hydrovision
(TidEL machine) and their associates were unable to secure the required matching funding and
hence did not progress with their projects. This is in contrast to Marine Current Turbines Ltd
(MCT) and Tidal Generation Ltd (TGL), who are the UK’s leaders in certain aspects of device
readiness and deployments in the UK. Details on international projects can be found in Ocean
Energy Systems [10].
(c) Routes to array deployment
(i) Site development
The data shown in table 1 represent the development of single devices, which in most cases was
started by small-scale research and development companies. As indicated in table 1, some of the
UK devices are at the prototype scale with device assessment being undertaken at test sites such
as the European Marine Energy Centre (EMEC) in the Orkney Islands, UK [19]. Such assessments
......................................................
where (kg m−3 ) is the density of the fluid, A (m2 ) is the cross-sectional area of the rotor under
consideration and vo (m s−1 ) is the unperturbed speed of the fluid.
The Po is proportional to the cube of the fluid velocity; hence, the energy yield is highly
sensitive to variations in velocity. There are, however, some subtleties when considering MCEC,
when compared with wind energy conversion. The power in the flow is proportional to the fluid
density, which, for water, is about 800 times that of air. Hence, the power density (kW m−2 ) for
marine current energy converters will be appreciably higher than that produced by wind energy
converters when considered at appropriately rated speeds for both technologies [17]. This also
means that smaller and hence more manageable converters can be installed within the water layer
where the bathymetry (water depth) is favourable for an optimal size of turbines to be deployed
within the site. It must be noted that, unlike wind energy, in marine current energy the bathymetry
places a constraint on the maximum size and hence the rated power of a marine energy converter
or turbine.
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currents using, for example, a two- or three-bladed horizontal axis turbine. The power Po (W)
available from a marine current (in the absence of significant changes in depth or elevation) is
given by
1
(3.1)
Po = Avo3 ,
2
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history
initial funding
Marine current turbines (MCT): founded
Sea Flow grants: DTI £1.6 M and
1999. Now owned by Siemens’s Solar
EC £800 k
and Hydro Division 2012
Key SeaGen grants: DTI grant £4.27 M
first machine: SeaFlow 300 kW installed in
and MRPF grant £2.15 M
Lynmouth, Bristol Channel, UK, 2003
second machine: SeaGen 1.2 MW installed,
Strangford Lough, UK, 2008
(www.marineturbines.com)
..........................................................................................................................................................................................................
Lunar Energy: founded 2001.
DTI: £2.25 M
Rotech tidal turbine 1 MW capacity. Tested awarded £3.5 m in total. Not fulfilled
at various scales in towing tanks
owing to lack of match funding
(www.rotech.co.uk)
..........................................................................................................................................................................................................
SMD Hydrovision: founded 2005
Twin 500 kW counter-rotating generators.
2006 tested 10th scale at NAREC
(www.smd.co.uk/products/
renewables)
DTI: £2678 M
not fulfilled owing to lack of
match funding
..........................................................................................................................................................................................................
Pulse: founded 2007
first machine: pulse stream 100 kW
installed in 2009
new design 1 MW, possible deployment in
20 m water (www.pulsetidal.com)
DTI: £878 k
Npower Juice: £108 k
European Union (FP7) grant: £3.2 M
..........................................................................................................................................................................................................
TGL: founded 2005. Now subsidiary of
Alstom 2012
first 0.5 MW machine tested for a few
months at EMEC in 2010
second 1 MW machine in development
(www.tidalgeneration.co.uk)
local development agency:
approx. £75 k
DTI/TSB: approximately £3.7 M,
also undisclosed support from the
UK’s Energy Technology Institute
..........................................................................................................................................................................................................
under real conditions are devised to subject turbine design philosophies to various in situ testing
regimes. Others have been undertaken independently by the companies themselves, such as the
MCT’s SeaGen project at Strangford Lough, Northern Ireland, labelled as commercial prototypes,
which already has over 2 years of operational experience [20].
......................................................
device
6
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Table 1. Lead devices in the UK and support provided by the UK government programmes. (1) This photograph is reproduced
with the permission of Marine Current Turbines Ltd, a Siemens Business. (2) Image reproduced courtesy of Rotech Engineering
Ltd. (3) Image reproduced courtesy of SMD Ltd. (4) Image reproduced courtesy of Pulse Tidal Ltd. (5) Artist impression of TGL
turbine.
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history
initial funding
Ocean Renewable Power Company: founded 2004
TidGEN 180 kW (www.orpc.co)
total grants approx. £5 M
..........................................................................................................................................................................................................
Verdant Power: founded 2001
unknown
1 MW pilot project with 30 turbines
(www.verdantpower.com)
..........................................................................................................................................................................................................
Kawasaki Heavy Industries Ltd: founded 1878
unknown
tidal development from 2010? 1 MW turbine
planned EMEC berth 3 (www.khi.co.jp)
..........................................................................................................................................................................................................
Notwithstanding the important strides highlighted earlier and the successes in current
technology development, experience in deployment and operation will now need to migrate to
multiple devices of what can appropriately be termed first-generation technologies. However,
array device deployment, although significant in the development of the technology, forms only
one part of the complex interactions within a project scope that aims to develop a specific site.
Site development tools are also needed to encompass many of the requirements of legislation;
environmental analysis; stakeholder interaction; issues related to the technology deployment,
commissioning, operation and maintenance; and proximity to ports and the grid. Some of these
steps are summarized in figure 3.
(ii) Support mechanisms
Projects encompassing marine energy conversion deployment at array scale have been driven
by fiscal instruments. In the UK, the government’s main market support mechanism is through
the Renewables Obligation scheme [7,21]. Such incentives provide Renewables Obligation
Certificates (ROCs) in the UK, whereas in other countries such as Canada a feed-in tariff (FiT)-type
mechanism is promoted [22]. For the UK, enhanced support for marine energy projects (wave and
tidal) will start to take effect on 1 April 2013 and was set at the five ROCs band with a 30 MW
project capacity cap for deployment in the period leading up to 2017. This band is around two
and half times the current support level given under the same scheme for offshore wind. In other
countries such as Canada, the FiT is currently under discussion [22].
......................................................
device
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Table 2. Depicts non-UK global leaders in marine energy conversion in terms of deployment. (1) Image reproduced courtesy
of Ocean Renewable Power Co. (2) Image reproduced courtesy of Verdant Kris Unger/Verdant Power Inc. (3) Image reproduced
with the permission of Kawasaki Heavy Industries, Ltd.
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site characterization
— resource assessment
— environmental impact
studies
site
development
site planning
installation
— vessel availability
— foundations
— cable routes
— commissioning
— power agreement
— device specification
— array layout
— health and safety
— insurance
site infrastructure
— deployment strategies
— ports and transport
— cabling and grid
Figure 3. Site development steps giving a brief indication of the scope of work to be undertaken by project developers.
(iii) Project zones
The above-mentioned fiscal support mechanisms are aided by the creation of project zones aimed
at developments that encompass multi-megawatt and multi-device arrays. Prime examples of
these are (i) the Pentland Firth in the UK where such zones are championed by the Crown Estate
(which owns the seabed around the UK shores) [7,23] and (ii) in Canada where the development
of the Bay of Fundy project is driven by the Nova Scotia government [24]. The projects and
deployment in the latter are currently under development. Initial indications estimate that single
turbines will be deployed within the first ‘learning phase’ followed by arrays where cabling and
power capacities allow. The planned development for a proposed 1000 MW of marine current
arrays to be installed in the UK by 2020 is given in table 3. Table 3 also gives time lines and an
indication of the project phases undertaken by site developers to fulfil the requirements for arrayscale deployment.
(d) Informing array deployment
Although the projects mentioned earlier are extremely important for enhancing the status of
technology development, currently no real data exist on large devices deployed in arrays in
the sea. As mentioned earlier, project developers are currently relying on small-scale research
and development routes to facilitate and inform designs of arrays [13,25,26]. Such knowledge
will inform optimized site array designs, including consideration of wakes, the influences of
turbulence and their impact on device spacing. This is crucial as well-planned and optimized
sites will lead to projects that will result in the needed energy yields which are appropriate for a
return on investment.
In addition, deployment in arrays can also be informed by simulation studies linked to
laboratory-scale tests and analysis from single devices. Figure 3 shows such an approach that
is structured to give an overarching connectivity between the various aspects encountered in
the design and optimization of an array. Such analyses are used to predict the performance and
energy yields including devices under yaw. To illustrate this approach, we consider an analysis
......................................................
permitting and
consent
— legal processes
— stakeholder
engagement
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operation and
maintenance
— routine planning
— contingencies
— weather events
— decommissioning
8
capacity (MW)
400
– in 2012: 253 MW of grid capacity secured. Consent applications for phase 1 submitted. Transmission
lines upgrade started
........................................................................................................................................................................
– phase 2: deploy the rest after experience from phase 1
........................................................................................................................................................................
– phase 1b: 65 devices (+65 MW)
........................................................................................................................................................................
landings; substation and grid connection locations depend on environmental sensitivities and
landfall
– electrical connection: either buried beach or horizontal directional drilling through rock cable
........................................................................................................................................................................
– foundations: gravity-based or piled; type will be assessed and one will be used for both designs
........................................................................................................................................................................
– phase 1a: 20 devices (20 MW) primarily for array demonstration purposes
........................................................................................................................................................................
project development and status
– projected capacity: 389 1 MW devices, technology Atlantis and TGL
200
– all the above is subject to consenting
(Continued.)
........................................................................................................................................................................
– phase 2: is planned to be completed by 2020
........................................................................................................................................................................
– phase 1: 30–45 MW, possibly with different technologies. Consent and licence applications for
phase 1 to be submitted by late 2013, with installation planned to begin 2016 (onshore
infrastructure in late 2014)
........................................................................................................................................................................
planning activities, while in parallel commencing the Environmental Impact Assessment (EIA) and
Navigational Risk Assessment (NRA) processes
– scoping report October 2011. Currently undertaking site investigation and project development
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......................................................
.............................................................................................................................................................................................................................................................................................................................................
Westray South: SSE Renewables Developments (UK) Ltd
.............................................................................................................................................................................................................................................................................................................................................
Ltd., International Power Marine Developments Ltd and
Morgan Stanley Capital Group Incorporated)
project site and developer
Inner Sound: MeyGen Ltd (Atlantis Resources Corporation Pte
Table 3. Planned development for a proposed 1000 MW of marine current installations by 2020 in the Pentland Firth, UK.
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9
capacity (MW)
200
– possible offshore substation
............................................................................................................................................................
within 1 h
– fast turbine deployment using specialist barge to deploy both turbine and gravity base
............................................................................................................................................................
– technology: Open Hydro’s Open-Centre Turbine with a 16 or 20 m diameter
............................................................................................................................................................
– phase 2: increase to 200 MW by end of 2020
............................................................................................................................................................
– phase 1: 30 MW from 2016/2017
............................................................................................................................................................
– currently undergoing project development and site investigation activities, as well as
preparing for EIA and NRA processes
............................................................................................................................................................
project development and status
– project briefing document published in April 2012
100
– phase 1 of deployment planned to start 2017
............................................................................................................................................................
– plan to (i) secure planning and environment consents for the site by 2015 and (ii) start
construction in 2016
............................................................................................................................................................
– 66 devices over three phases over 4 years in an area of 4.3 km−2 (99 MW)
100
– no clear timeline or recent information available online
............................................................................................................................................................
– the tidal resource and seabed surveys have been completed and the EIA has been submitted
............................................................................................................................................................
– Technology Hammerfest Strom UK and Scottish Power Renewables (submerged 1 MW
device)
............................................................................................................................................................
– plan for phase 1 at 30 MW capacity (no date given)
............................................................................................................................................................
– potential 95 MW capacity project
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......................................................
.............................................................................................................................................................................................................................................................................................................................................
Ness of Duncansby: Scottish Power Renewables UK Ltd
.............................................................................................................................................................................................................................................................................................................................................
Brough Ness: SeaGeneration Ltd (Marine Current Turbines Ltd)
.............................................................................................................................................................................................................................................................................................................................................
project site and developer
Cantick Head: Cantick Head Tidal Development Ltd (SSE Renewables
Holdings (UK) Ltd & OpenHydro Site Development Ltd)
Table 3. (Continued.)
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10
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Table 4. Turbine assumptions for the analysis.
11
tidal current turbine rated power
0.5–2.5 MW
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
turbine gear box and seals efficiency
constant 97% (considered optimistic)
generator speed
constant r.p.m.
generator efficiency
constant 95% (considered optimistic)
power and thrust coefficients
experiment curve fits
power control
blades pitch above rated speed
design tip speed ratio (TSR)
TSR = 4
velocity profile
uniform across turbine
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
based on (i) results determined from towing tank experiments on a scale model of an 800 mm
diameter horizontal axis tidal turbine [27] and (ii) site tidal data extracted from tidal diamonds
data at both Portland Bill, UK (AC 2615 F) and at the Fall of Warness, Orkney Islands (AC 2250 E)
using the analysis described in Blunden et al. [28]. The latter site is close to the EMEC test site in the
Orkney Islands and represents what is assumed to be the expected result for turbines tested at the
EMEC site. It must be noted that predictions at Portland Bill forecast a strong constant southerly
flow component, making the flow conditions non-rectilinear (sometimes referred to incorrectly
as ‘bi-directional’) in contrast to that at the EMEC site. Applying the assumptions summarized in
table 4, the predicted turbine design conditions and power output are presented in figures 4 and 5.
Figure 4, for Portland Bill, represents an interconnected depiction of the predicted thrust on
the turbine, the expected load factors and the resultant annual energy yields as a function of
turbine diameter and rated power. The analysis has been performed for both (i) a non-yawing
turbine with blades that pitch 180◦ when the tide changes direction and (ii) a yawing turbine that
physically turns to maintain its direction perpendicular to the flow.
The predictions demonstrate a methodology for combining site conditions and the properties
of turbines to give the expected energy yields from a specific site. This can be used to inform
design approaches and for creating investment vehicles that can be combined with the fiscal
support available to allow funds to flow for the exploitation of specified sites. In addition,
decisions can be made about turbine design parameters that can exploit site conditions using
either a conservative design and/or optimized revenue generation. For instance, it is possible to
oversize a turbine for a particular site: if, because of depth restraints, a turbine with diameter
of 16 m were chosen, then by increasing the rated power from 1.0 to 1.5 MW an extra 250 MWh
would be produced (figure 4d) but the maximum load experienced would rise from 496 to 651 kN
(figure 4b), and the load factor would be reduced from 0.31 to 0.23 MW (figure 4c). Obviously,
these have implications for foundations and other system costs that will need to be put in place
to allow for such a change and decisions would need to be made on whether such an approach is
economically favourable.
The same analysis was conducted for the EMEC site, and the results are shown truncated in
figure 5. Again, load factors and energy yields at this site are presented only for the case of a fixed
orientation turbine. The results indicate that at this site, which is strongly rectilinear, a variable
orientation turbine (which yaws to face the flow) is predicted to produce less than 1 per cent extra
energy when compared with a fixed turbine. Hence, quantifying energy generation and income
will inform whether such an approach is worthwhile and economic. In addition, the maximum
energy yield for the simulated 16 m diameter turbine at the site is approximately 5 per cent less
than that produced at Portland Bill owing to the EMEC site being less energetic.
......................................................
constraint
10–20 m
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parameter
tidal current turbine diameter
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design speed (m s–1)
12
(b)
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400
14
600
16
500
3.6
3.8
4.0
3.4
700
3.2
0
0
3.
90
8
2.
800
18
diameter (m)
maximum thrust (kN)
20
2.4
2.6
(a)
4.43
12
30
0
10
fixed: load factor
(c)
0
0
5
0.2
0.3
16
5
0.1
0
3
0.
0
0.1
14
12
325
300 0
2750 0
250
225 0
0
20
00
175
0
.25
18
diameter (m)
fixed: annual energy (MWh)
(d)
20
5
0.1
1250
1000
rotating: annual energy (MWh)
0
0.1
rotating: load factor
16
14
0.5
0 0
0.3 .40 .45 0
5
diameter (m)
(f)
20
18
3750
1500
0
0.2
10
(e)
3500
4500
4 4250
3750 000
3500
3
300 250
27 0
250 50
0
2250
.30
0
5
0.2
0.20
5
0.1
12 0.30 25 0
0. 0.2 15
0.
10
1.0
1.5
2.0
0.5
rated power (MW)
2.5
0.5
2000
1750
1500
1250
1.0
2.0
1.5
rated power (MW)
2.5
Figure 4. Predictions for the turbine design speeds, thrusts and performance for both fixed orientation and yawing Portland
Bill. (Online version in colour.)
Such analyses are invaluable for both linking device and site characteristics while keeping an
eye on the energy yield and hence the economics of a project, and the investment necessary to
exploit a site. Furthermore, array spatial planning will also have an influence on the achieved
yield as device density and its resultant impedance within a specific site will also have an impact.
As indicated in §2, resource assessment and the creation of a hydrodynamic model of the site will,
in the absence of real site data, inform optimization of the spatial planning of arrays.
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rotating: load factor
rotating: annual energy (MWh)
(a) 20
(b)
0
0.3 0.40 .45
5
diameter (m)
14
0.2
0.15
0
12 0.3 .25 .20
0 0
5
0.1
10
0.5
1.0
1.5
2.0
rated power (MW)
2000
1750
1500
1250
2.5
0.5
1.0
1.5
2.0
rated power (MW)
2.5
Figure 5. Predictions of load factor and energy yield at EMEC for a fixed orientation turbine. (Online version in colour.)
4. Conclusions
As highlighted in this study, the marine energy community has made major strides, achieving
a lot in a very short period of time. Many seasoned observers believe that the present status of
the technology is comparable to that of the emerging wind energy development in the 1980s.
Therefore, in order to develop marine energy technology, deployment pathways will need to
be found to reach the levels of acceptable multi-megawatt arrays. It is clear from projected
deployment intentions that initial phases of deployments will be in sheltered sites. However,
most of these sites will need the appropriate infrastructure such as proximity to the grid, ports
and buy-in from stakeholders. The cost of such support is demonstrated within the Pentland
Firth and Orkney Islands waters ‘round 1’ leases. In addition to £4 billion estimated cost of the
1.6 GW of potential capacity for different technologies (600 MW wave energy devices, 1000 MW
tidal current devices), £1 billion will be required from public sources to develop and build
new grid connections, harbours and other supporting infrastructure in the Orkney Islands and
Caithness [23].
As highlighted within this study, plans for implementations at multi-megawatt array-scale
development are forging ahead. In the UK and other countries such as Canada, there is also
concerted support for the technology at ministerial and local government levels. Hence, given
the highlighted commitments to establish favourable regulatory and incentive regimes as well as
the aspiration for energy independence and combating climate change, the progress should be
much faster than that achieved for wind energy development.
In the UK, however, there are further complications in relation to ROCs as, beyond 2017, the
scheme will be replaced by a new government scheme enveloped within the Electricity Market
Reform (EMR). Within the latter, there is a risk that marine energy (wave and tidal) will not
get the same level of support as the current five ROCs. This is because the EMR is likely to
be technology blind as it attempts to concentrate resources on technologies that maximize the
amount of renewables/low carbon generation for the least cost. In particular, the need to achieve
the European Union’s target of 15 per cent electricity from renewables by 2020 is anticipated to
result in the expansion of offshore wind to up to 30 000 MW, compared with the few hundred
megawatts likely to be achieved by the marine energy industry within that time frame [29].
Hence, a debate is now being generated to establish that marine energy technologies should be
considered as having ‘other compelling arguments’ to warrant support at the higher levels [30].
Hence, there is a danger that leaving technology development entirely to market forces is likely
to result in slower progress in large-scale deployment. Nevertheless, it is anticipated that the
market incentives based on energy produced and the announced enhanced ROCs/FiT are likely
to provide a viable industry in due course, provided that the technology is reliable at scale and
within the operating environment.
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16
5
0.2
4000
37
3500 50
32
3 50
275 000
250 0
0
2250
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0
0.3
18
13
4250
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This work forms part of the Sustainable Energy Research Group’s studies in the ocean energy area. Full details
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