4th International Conference on Ocean Energy, 17 October, Dublin
Introduction of Wavestar Wave Energy Converters at the
Danish offshore wind power plant Horns Rev 2
L. Marquis1, M.M. Kramer1,2, J. Kringelum3, J.F. Chozas2 and N.E. Helstrup4
1
2
Wave Star A/S, Park Allé 350A, 2605 Brøndby, Denmark, E-mail: [email protected]
Dept. of Civil Eng., Aalborg University, Sohngaardsholmsvej 57, 9000 Aalborg, Denmark., E-mail: [email protected]
3
4
DONG Energy, Nesa Alle 1, 2820 Gentofte, Denmark, E-mail: [email protected]
Energinet.dk, Tonne Kjærsvej 65, 7000 Fredericia, Denmark, E-mail: [email protected]

Abstract
1.
Wavestar, DONG Energy, Aalborg University
and Energinet.dk are working together to evaluate
in practice the idea of combining the energy
production from wind and wave technologies. For
that purpose, Wavestar is planning the installation
of a 600kW Wavestar Wave Energy Converter
(WEC) which is to be connected to a wind turbine at
the DONG Energy owned wind power plant Horns
Rev 2 placed off the western coast of Denmark. The
plant delivers its energy production to a
transformer station owned by Energinet.dk.
Energinet.dk has the obligation to ensure that
power is transmitted to the Danish consumers. If
Executed the project will be the first one in the
world where wind and wave power are combined at
full scale.
The goal of the project is to evaluate the
opportunities of combining wind and wave energy
production on a commercial scale and to
demonstrate the reduction of energy fluctuations
with this combination. This can increase the value of
the produced power from future wind/wave plants.
Further potential synergies of combining wind and
wave energy in the same area include increased
energy production from the available area and
sharing of infrastructure costs as well as O&M
facilities.
In a future phase, the possibility of installing
additional Wavestar WECs on the site will be
investigated. Up to five Wavestar WECs can
potentially be installed between each turbine.
Introduction
Wavestar has developed Wave Energy Converters
(WECs) of increasing size since 2003. The latest
prototype, installed at Hanstholm, Denmark, in 2009, is
a section of the 600kW WEC and has been in
continuous operation since the installation date,
delivering electricity to the Danish grid [1-2].
Wavestar has decided to install a 600kW WEC
based on the promising results achieved after two years
of intensive tests and development on the latest
prototype. Since the vision of Wavestar is to combine
wind and wave energy production at the same sites, a
natural step in the process towards the realisation of
Wavestar’s vision is the installation of a 600kW WEC
at Horns Rev 2 (HR2); an existing wind turbine farm in
Denmark.
An R&D partnership agreement was signed in
December 2011 between Wavestar and DONG Energy
with the purpose of investigating the potential
installation of a 600kW WEC near a wind turbine at
HR2 (Fig. 1).
Keywords: Combined wind and wave, wave energy,
Wavestar, WEC, wind energy
Figure 1: Simulated image of Wavestar by a wind turbine.
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4th International Conference on Ocean Energy, 17 October, Dublin
As shown in Fig. 3, the Wavestar 600kW WEC is
planned to be situated 300m north-east of turbine H7.
Permission from the authorities is in process, but not
granted yet. The location was chosen mainly based on
an accessible connection point. H7 was already from
the start equipped with an extra J-tube for the potential
connection of a wave device. Internal investigations
from DONG Energy have shown that the array cabling
of the wind farm will be able to transmit the extra
capacity. In some extreme conditions, which are
estimated to occur only a few hours a year, it will,
however, be necessary to shut down the WEC. In
addition, results from geotechnical investigations are
available at the wind turbine position H7 closest to the
planned installation of the WEC.
The analysis work will focus on establishing how
wind and wave energy harvesting of a site can
complement each other. Real data collected from the
wind turbine close to the WEC and from the WEC
itself will be used for said analysis. The focus areas
selected throughout the project are as follows:
•
Production data from the wind turbines
•
Production data from the WEC
•
Production data combining wind and wave energy
•
Wind and wave profile
•
Operation and maintenance (O&M) cost
•
Cost of energy (COE).
1.1 Project status
A pre-study of the project has been conducted by an
intensive collaboration between Wavestar and Dong
Energy. Wind and wave data recorded from HR2 were
analysed and the subjects listed below were discussed:
•
Structural design basis for the WEC
•
Geological data analysis for foundation and
location of the WEC on the HR2 site
•
Request for authorisation to implement a WEC on
the Horns Rev 2 site has been sent to the Danish
Energy Agency.
As of August 2012, the financing of the WEC is not
yet decided, but under negotiation.
2.
3.
Wavestar 600kW
The Wavestar C6-600kW WEC is designed to
deliver maximum 600kW in electrical power to the
grid. This power level is reached at approximately 2.3m
significant wave height. The WEC is planned for
unmanned operation with remote data monitoring. The
WEC is considered to be installed at a maximum water
depth of 20m including storm surge and is designed to
be in operation for a minimum of 20 years.
The WEC is equipped with 20 floats, each float
having a diameter of 6m. The individual float is
mounted on a 12m long steel arm which is hinged on
the main tube. 10 arms are placed on each side of the
tube. When the waves move a float up and down,
power is transferred by a hydraulic cylinder to the
Power Take Off (PTO) which produces the power to
the electrical grid.
The main tube contains all technological equipment,
such as PTO, and the control systems in a dry air
environment. This makes it possible to use standard
components, which are less expensive than offshore
equipment, and still ensure a long life time and
minimum maintenance.
The main tube is connected to two jacking sections,
one at each end. These jacking sections contain the
jacking mechanisms, which can keep the main tube at
production level above the sea, and lift the main
structure and floats up to a storm protection level, when
necessary.
The WEC is designed to produce energy in waves
with a maximum wave height Hmax = 8m which
corresponds to a site-specific, significant wave height
which is normally close to 4m (i.e. Hmax/2). At higher
wave conditions, the floats are automatically lifted out
of the water, and the WEC is jacked up to storm
protection level, see Fig. 4. Storm protection involves
un-ballasting the floats and retracting the hydraulic
cylinders thereby pulling the floats out of the water.
Horns Rev 2 site
The Horns Rev 2 wind power plant is owned by
DONG Energy who is one of the world leaders in terms
of development, construction and operation of offshore
wind farms. With a total capacity of 209MW in
operation since 2009, the 91 wind turbines rated at
2.3MW deliver their energy production to a transformer
station owned by Energinet.dk. A high voltage cable
exports the energy produced to shore where it is
transferred to the national grid. All communication data
is transmitted through fibre optic cables.
The site is placed approximately 30km from the
Danish west coast near Esbjerg, on a reef at a water
depth of 9-17m. Here, there is an average wind speed
of 10m/s and an average significant wave height of
1.5m. Fig. 2 show a photo of the farm [3].
Figure 2: Aerial photo of the Horns Rev 2 wind farm.
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4th International Conference on Ocean Energy, 17 October, Dublin
Figure 3: Map of Horns Rev 2 showing the wind turbine locations and the possible location of the Wavestar WEC.
Figure 4: Simulated image of a Wavestar C6-600kW in operation (left) and in storm protection mode (right).
PTO efficiency [%]
70
100% operation, array interaction, power limit and storm protection limit
Machine: Electrical power [kW]
Wave period T0,2 [s]. Range and center value on second row.
Hm0 [m]
0-1
1-2
2-3
3-4
4-5
5-6
6-7
7-8
8-9
9-10
10-11 11-12 12-13
Range
Center
Arm length [m]
12
0.0 - 0.5
0.25
0.5 - 1.0
0.75
Float diameter [m]
6
1.0 - 1.5
1.25
1.5 - 2.0
1.75
Number of floats
20
2.0 - 2.5
2.25
2.5 - 3.0
2.75
Storm protection [m]
4.00
3.0 - 3.5
3.25
3.5 - 4.0
3.75
Nominal power [kW]
600
4.0 - 4.5
4.25
4.5 -
4.75
13-14
14-15
0.5
1.5
2.5
3.5
4.5
5.5
6.5
7.5
8.5
9.5
10.5
11.5
12.5
13.5
14.5
0
0
0
0
0
0
0
46
0
71
0
85
0
89
0
88
0
84
0
79
0
74
0
69
0
66
0
62
0
59
0
0
0
0
50
95
121
231
182
339
212
381
215
374
206
350
192
322
178
296
166
274
155
254
145
237
136
222
128
208
0
0
0
0
154
228
375
552
535
600
579
600
554
600
511
600
466
600
426
563
391
516
361
475
336
441
314
412
295
386
0
0
40
53
319
425
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
593
600
550
600
513
600
480
577
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Table 1: Power matrix of generated electrical power for Wavestar C6-600kW. Values marked with grey colour are in kW.
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4th International Conference on Ocean Energy, 17 October, Dublin
The power production depends on the wave climate.
Here, the wave climate is described by the significant
wave height, which has a huge impact on production,
and the mean wave period, which also affects the
power production to some extent. If the significant
wave height is below 0.5m, the WEC is stopped due to
the calm waves, and if the significant wave height
exceeds 4.0m, the WEC shuts down and initiates storm
protection mode. The power production matrix is
shown in Table 1. The electricity production by a single
Wavestar C6-600kW placed at Horns Rev 2 is expected
to be 1.4 GWh/year.
•
•
4. Benefits from combining production of
wind and wave energy
Several studies evaluate the benefits of combining
marine renewables. The correlations between winds
and waves are assessed and the advantages of
harnessing them together to improve the reliability and
decrease the variability of the power production. The
studies are based on Ireland [4], California [5],
Denmark [6] and three potential sites in Europe [7]. A
recent study also examines the advantages of
combining the power productions from full-scale wave
converters and full-scale wind turbines [8]. The work is
based on simultaneous, real power production data
from Wavestar and from an operational wind turbine
located at the same site near Hanstholm, on the west
coast of Denmark. Results from this project are in line
with the findings of the previous studies and conclude
that the potential benefits of combining wave and wind
technologies, during typical and storm operating
conditions, are that:
• a more continuous power output than the individual
production of the technologies is provided by
reducing the zero-production
• a smoother power production than the individual
production of the technologies is achieved.
•
found when comparing the pattern of the production
from the wave converter and the wind turbine.
The combination of wave and wind power reduces
the percentage of time that the combined production
drops to zero. In Fig. 5, the wind turbine has zero
production in several time-intervals. However, the
combined production does not drop to zero at any
time.
The comparison of wind and wave power
production shows that abrupt changes in wind
power (ie increasing from a minimum to a
maximum value and vice versa) are faster than the
same changes in wave power. The evolution of the
three power curves from 01/11 to 01/12 illustrates
that the wind turbine power increases from 30% to
maximum power production in 8 hours, whereas for
Wavestar and the combined production it takes 11
hours. Hence, when wave power is included in the
system, the fast changes of wind power reduce.
In Fig. 5, it can also be seen that waves are more
constant than winds; and hence, the power output
from the wave converter is smoother than the power
output from the wind turbine. The combination of
both outputs results in an overall less-fluctuating
power, which indeed has a potential benefit for
cable losses.
5. Synergies from sharing infrastructure
and O&M
Beyond the technical benefits depicted in Section 4,
there are in general additional potential advantages
when co-locating wind and wave technologies:
• They share common synergies in relation to marine
policies, marine stakeholders, spatial constraints
and environmental impact assessments.
• They can share the costs of cable laying, installation
and maintenance activities. In hybrid solutions, the
structure costs may also be shared. This potential
depends highly on the actual structural design.
• By combining the two technologies, the
predictability of the power production is improved.
This is useful in the planning of operational and
maintenance activities, storm periods, testing of
control strategies and bids in the electricity markets.
Day-ahead electricity markets are related to the
balancing costs. Studies shows that wave power can
be beneficial in reducing the regulating costs of
wind power, since waves are 10% more predictable
than winds [9].
Fig. 5 illustrates these benefits. The upper part of the
figure presents the evolution of the significant wave
height and the wind speeds at Horns Rev 2 during a 13day period. The lower part of the figure shows the
simulated power production of Wavestar, an offshore
Siemens wind turbine and a 50/50 combination of both
technologies. The simulated power production time
series are based on the power curves shown in Fig 6.
The following comments result from the curves
presented in Fig. 5:
• The evolution of the significant wave height and the
wind speed shows a certain correlation between the
two resources. In Fig. 5, this is demonstrated in that
the blue and the red curves on the upper graph
follow the same pattern. However, whenever the
wind stops blowing, waves continue rolling for
some time afterwards thereby causing a delay
between waves and winds. Fig. 5 shows a time
delay of up to 9 hours for the significant wave
height and the wind speeds. The same delay is
For the present study at HR2, a potential, positive
synergy to be investigated is reduced infrastructure
costs for the whole energy farm. Most obviously, array
and export cabling can be shared between the two types
of renewable energy generators. At HR2, this cabling
already exists. To some extent, operational synergies
are expected regarding the common maintenance work
for the two technologies, which are using comparable
equipment (hydraulic, generators, transformers).
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4th International Conference on Ocean Energy, 17 October, Dublin
Figure 5: Time series for a 13-day period. Upper graph: Environmental conditions for waves (blue) and wind (red). Lower
graph: Power production simulations of Wavestar (blue), wind turbine (red) and a combination of both (green).
Figure 6: Power curves for the wind turbine and Wavestar used for the Horns Rev 2 power simulations. The wind turbine power
curve is approximated to a free Siemens turbine with clean rotor blades and horizontal, undisturbed air flow [10].
At HR2, crew-vessels etc are already on-site, and it
is planned to log O&M routines in detail to be able to
analyse which procedures can be done by the same
technicians as those working on the wind turbines and
which ones require specialists. Further synergies are:
• The wake effect from the WECs may potentially
reduce the general wave climate in the area with the
turbines. This can lead to reduced wave loads on the
turbine
foundations
and
increase
access
possibilities. Naturally, one WEC will not
contribute to this effect, but the possibility of
installing equipment to measure wave height behind
the WEC will be investigated.
For installation of future common wind and wave
farms, sharing of base harbours and planning
facilities/tools may be realisable. Whether or not
vessels can be shared depends highly on the current setup for installing both wind turbines and wave energy
converters. This project is expected to provide valuable
experience to further investigate this subject.
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4th International Conference on Ocean Energy, 17 October, Dublin
6.
Visions for future Wavestar
development
If the project is a success, it may be possible for
Wavestar to develop the concept further for future
projects. As the WEC is founded on the sea bed, using
the same monopile technology as wind turbine, the
Wavestar WEC can be placed inside the farm.
Theoretically, up to five WECs can be placed between
each wind turbine on every other row as illustrated in
Fig. 7. Hereby the farm capacity can be increased by
50%. In practice the number will be lower due to
constraints in the operation and layout of the wind
farm.
Figure 8: Simulated image of a hybrid Wind & Wave
platform with a 2.4MW Wavestar WEC and a 5MW wind
turbine.
References
[1] M.M. Kramer, L. Marquis and P. Frigaard (2011):
Performance Evaluation of the Wavestar Prototype.
EWTEC 2011 Proceedings, Southampton, UK.
[2] E. Vidal, R.H. Hansen and M.M. Kramer (2012): Early
Performance Assessment of the Electrical Output of
Wavestar’s Prototype. ICOE 2012 Proceedings, Dublin,
Ireland.
[3] The Horns Rev 2 site, home page:
http://www.dongenergy.com/Hornsrev2/EN/Pages/index.
aspx
Figure 7: Simulated image of Wavestar between turbines.
Free capacity in existing infrastructure is normally
limited only providing the opportunity to add a few
devices without significant extra costs for additional
export cable and substation. In the future, it will be
possible to consider offshore farms as “energy parks”,
combining two energy sources (wind and wave) and
possibly other sources, where these sources are
integrated already in the design of the farm.
[4] F. Fusco, G. Nolan and J.V. Ringwood (2010):
Variability Reduction through optimal Combination of
Wind/Wave Resources - An Irish Case Study. Energy
35.
[5] E.D. Stoutenburg, N. Jenkins and M.Z. Jacobson (2010):
Power Output Variability of Co-located Offshore Wind
Turbines and Wave Energy Converters in California.
Renewable Energy.
Furthermore, for new projects and in the long term, a
hydride solution is proposed by Wavestar where the
combination of wind and wave is completely fulfilled
on one device (Fig. 8). A 5MW wind turbine can be
implemented on the wave device structure comprising
three WECs placed on a star combination and with of a
capacity of 2.4MW. The complete device will have a
total capacity of 7.4MW.
[6] H.C. Sørensen et al. (2005): Bølgekraftanlæg ved Horns
Rev – Screening. Technical report, Rambøll.
[7] L. Cradden et al. (2011): Joint Exploitation of Wave and
Offshore Wind Power. EWTEC 2011 Proceedings,
Southampton, UK.
[8] J.F. Chozas, M.M. Kramer, H.C. Sørensen and J.P.
Kofoed (2012): Combined Production of a full-scale
Wave Converter and a full-scale Wind Turbine – a Real
Case Study. ICOE 2012 Proceedings, Dublin, Ireland.
The common structure may allow a decrease in the
production costs for the main structural parts, such as
the foundations. The specific synergy will however
depend highly on the actual structural design and loads,
which are still to be evaluated.
[9] J.F. Chozas, H.C. Sørensen and N.E. Helstrup (2012):
Economic Benefit of Combined Wave and Wind Power
Productions in the Regulating Electricity Market. ICOE
2012 Proceedings, Dublin, Ireland.
Acknowledgements
[10] Technical specification for Siemens Wind Turbine SWT2.3-93,
http://www.energy.siemens.com/mx/pool/hq/powergeneration/wind-power/E50001-W310-A102-V64A00_WS_SWT-2.3-93_US.pdf
The authors wish to acknowledge the financial
support from the FP6 EU project no 038590 and the
Danish ForskVE programme project no 2009-1-10305.
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