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Offshore Wave Energy By Lars Christensen, Wave Dragon ApS An international race to develop and commercialise technologies that can utilise the abundant wave energy has accelerated within the last couple of years. Wave energy is a highly concentrated energy source, an order of magnitude more powerful than wind. About 10-50% of Europe’s electricity demand is ready to be supplied from technically available wave energy resources. Following two decades of mainly academic interests, wave energy has now also entered the focus of governments and investors, and this powerful combination of interests has speeded-up commercialisation and created a firm belief amongst producers, that a wave industry comparable with the wind power industry will emerge. That is, if appropriate wave energy converting technologies can be made commercially available. Fundamental challenges Wave power faces some fundamental challenges: • efficiently converting wave motion into electricity... generally speaking, wave power is available in low-speed (average wave frequency approximately one wave 8 Offshore Center Denmark Newsletter ON/OFF 10 - May 2007 every 10 seconds), high forces (more than 1MW per meter wave front in storms) and motion is not in a single direction (like in wind). • constructing devices that can survive storm damage and salt-water corrosion. • low total cost of electricity • public acceptance, i.e. low visual impact and minimal impact on coastal processes An overwhelming variety of different technologies are developed these years around the world. Investors, developers and governments worldwide can agree on two things only: Large wave energy power plants will be offshore and floating technologies; as that is where the wave energy is, and it causes less visual and coastal process impacts. The Dragon One of the widely recognised leading wave energy technologies in the world is the Wave Dragon. Unlike most other devices it does not oscillate with the waves, it gathers the wave energy passively by utilising the Overtopping principle. The front face of the device is a curved ramp, oncoming waves surge up it, as if it were a beach. Behind the crest of this ramp lies a reservoir where the water “overtopping” the ramp which now has higher potential energy than the surrounding water. The effect of Wave Dragon is amplified by long reflector wings. Mounted to the reservoir, they channel the waves towards the ramp. The energy is extracted as the water drains back to the sea through low head hydro turbines within the reservoir. The Wave Dragon is designed as a floating offshore device to be placed in water depths above 20 m. Big is beautiful “Wave Dragon is by far the largest envisaged wave energy converter today” states Lars Christensen. Each unit will have a rated power of 4-15 MW depending on how energetic the wave climate is at the deployment site. “The large size brings many advantages” Lars Christensen continues. The device will respond minimally to waves, reducing fatigue problems. Also as the device is large and stable, it will be possible to work on board it most of the year,
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 9
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Page 11 and 12: The expected lifetime of the demons
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