DNV undertook a detailed review of the measurement programme used by Marine Current Turbines (MCT) for determining the performance of the company’s SeaGen tidal energy turbine which has been operating in Northern Ireland’s Strangford Lough since 2008. DNV has verified that the measured results have been correctly obtained and interpreted.
Testing a large machine installed in the aggressive conditions of a tide race is a technically challenging process and large volumes of data are needed to be recorded and analysed. Some key results that came from this programme indicate that SeaGen has met its design goals with the peak efficiency for both rotors on both tides averaging 48% (Cp = 0.48) and the best result was a peak efficiency of 52% and worst peak was 45%. This compares favourably with the recorded efficiency claims of eading wind turbine manufacturers which function analogously albeit using a fluid that is 800 times less dense. The absolute theoretical maximum efficiency for a perfect rotor would be 59.3%: a well known limit generally attributed to the German aerodynamicist Albert Betz. SeaGen has been found to achieve from 75% to 88% of the theoretical ideal.
40%+ efficiency rate
The corresponding overall system efficiency, including all losses in the generator, gearbox and power electronics, was found to be in the range 40 to 45%; that is the proportion of energy in the flow of water intercepted by SeaGen’s rotors that can get delivered as electrical energy into the grid. Because SeaGen is accredited by OFGEM as a UK generator, the world’s first and only official tidal current power plant, and therefore sells electricity and earns Renewable Obligation Certificates, the output to the grid is also independently metered and audited.
“Our engineering team are delighted that SeaGen has been shown to deliver such good results, and DNV’s endorsement provides a valuable confirmation of the accuracy of the team’s design assumptions for a system which originally involved much uncertainty through being a world’s first,” stated Peter Fraenkel, Technical Director of MCT.
The diagram shown in the inset shows SeaGen’s power curve made up from data collected as part of this validation programme. It can be seen that there is a slight variation in performance between the ebb and the flood tides. The ebb tide performance may be slightly enhanced by the streamlined cross-arm which carries the power trains, as this is upstream of the rotors on that tide.
MCT’s 1.2-MW SeaGen has the capacity to generate power for the equivalent of about 1,500 homes. It works in principle much like an “underwater windmill” with the rotors driven by the power of the tidal currents rather than the wind. Since February 2008, MCT has partnered RWE npower renewables on plans to develop a 10-MW tidal farm in waters off Anglesey, north Wales and is working with Minas Bay Pulp & Paper to deploy a single SeaGen system in Canada’s Bay of Fundy.
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