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which will dramatically reduce maintenance
costs and downtime. An overtopping device
brings also many advantages to robustness
of the design, in particular there are no endstop
problems as in larger seas the waves
will wash over the platform harmlessly.
Prototype testing in Denmark
Wave Dragon has deployed and run a successful
prototype wave energy power plant
in Nissum Bredning in Northern Denmark.
A 237 tonne all-steel built structure was
constructed and deployed March 2003. The
purpose of this test was to verify the overtopping
principle, and how much water would
overtop the structure in different sea states,
and how much of this water we could manage
to get through the turbines and thus converted
into electricity. Although the power
take off equipment is well known from river
hydro and wind turbines, a full power train
was installed to demonstrate the principle
and to get hands-on experience operating
these systems in a salt water environment.
“Our first test series ran 16,000 hours, and
we reached a 87% availability on the system,
and the power conversion was larger
than predicted from the wave tank tests and
numerical simulations. As other offshore
grid connected systems have been deployed
for less than 1,000 hours only if ever, this
was a very positive result, indeed” concludes
Lars Christensen.
Based on the experience from this 20
months testing a number of the systems
were improved and tested since April 2006.
Future developments
Next step is to demonstrate the system in
a full size, i.e. a 7 MW device. This will
be done just off South West Wales where
at the moment a large number of surveys
are carried out in order to hand in a formal
application to the DTI. The plan is to deploy
the device in 2008. This one device is the
first in a 70 MW farm being developed in
the Celtic Sea. The development is favoured
by the UK Government commitment to
develop a wave energy industry. The Wave
Dragon project receives substantial support
from the Welsh Government and the EU
R&D framework programmes.
Wave Energy
Wave energy is generated by wind that
passes over the open sea. The result is an
energy concentration: The initial solar
power level of about 1 kW/m2 is concentrated
to an average wave power level of 70
kW/m of crest length. This figure rises to
an average of 170 kW/m of crest length during
the winter and to more than 1 MW/m
during storms.
Wave energy is distributed un-evenly
around the globe. In Europe wave energy
origins from the North Atlantics mainly
and are highest at freely exposed Atlantic
coast lines from Norway in the north over
the British island through to Portugal, with
an annual average energy flux of 24 kW/m
and upwards.
In the North Sea the energy flux is from 5
to 24 kW/m.
Wave energy decreases rapidly in shallow
water. At Horns Rev a level of approximately
5 kW/m is found, according to
Wave Dragon ApS.
Following the Welsh project a 50 MW
project is developed in Portugal where a
high feed-in tariff system for wave energy
is in place.
Further general info: www.wavedragon.net
Wave Dragon specifications for a
7 MW device:
Mooring: A catenary anchor multi-leg
mooring bouy system. Type of anchor will
depend on seabed conditions. 6-8 anchors.
Structure: A mainly re-inforced concrete
structure. The central floating platform is a
131 x 97 m barge with open bottom compartments
and hosts all power equipments
etc. The floating level of this platform can
be changed to meet the changing sea states
by adjusting the air-pressure in the compartments.
The two reflectors are 144 m long.
Total height 17.5 m.
Max draught: 14 m.
Total weight 33,000 tonnes.
Power Train:
• 18 low-head axial propeller turbines.
Variable speed form 0 to 250 rpm.
• Direct-coupled Permanent Magnet Generators
– 390 kW
• Inverter control
• Step-up transformer on-board. Step-up
voltage will depend on local conditions.
O&M:
All maintenance will be carried out offshore.
Offshore Center Denmark
Newsletter ON/OFF 10 - May 2007
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