Close up - Repower's Axel Birk on offshore turbine development

At the 2012 Hamburg Offshore Wind Conference Repower vice president of business development offshore Axel Birk shared his views on the development of offshore turbines. Windpower Monthly technology writer Eize de Vries reports.

Repower's 5MW turbine. 44 are installed offshore
Repower's 5MW turbine. 44 are installed offshore

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Repower was the world’s first turbine supplier to install a 5MW offshore-dedicated prototype called 5M onshore in the autumn in 2004. So far, 44 5Ms are operating offshore, and another 11 onshore. Evolutionary product development is key to REpower’s overall technology advancement strategy. The 6.15MW 6M model builds on the 5M. Both turbines share a functionally similar fast-speed non-integrated geared drive system and an unchanged rotor diameter.

Stressing the value of engineering capability and track record experience Birk said that the 5M-prototype still operates with the original gearbox and the first set of rotor blades. Following three 6M onshore prototypes in 2009, REpower will install 48 units in the Belgian Thornton Bank II & III wind farm during 2012-13. Another 48 turbines are planned for next year in the German Nordsee Ost project. Most REpower turbines installed offshore have been put upon four-legged jacket type foundations.

Standard technology

 Fast-speed geared wind turbines with doubly fed induction generators (DFIGs) have been standard REpower technology right from the company’s launch in 2001.  Birk said that, in general, gearbox failures only contribute to 4-8% of total turbine failures set against around 30% for electrical systems and power electronics.

Birk also believed that future offshore conditions and demands will be substantially different from the relative shallow water depths and modest distances to shore common today: "Tomorrow’s offshore wind farms will be 20-150 kilometres from shore, operate in water depths up to 60m, and face very strong average wind speeds in the 9-10.5m/s range. Such demanding physical and environmental conditions require the application of offshore-dedicated wind turbines, whereas today’s offshore wind market is dominated by onshore turbines improved for offshore use put on monopile foundations."

Peak values

Elaborating on the main reasons behind the current lifecycle-based Costs of Energy (CoE) peak values estimated at 170 €/MWh, he said: "Two key differences between early offshore wind farms and those built more recently are the water depth and distance to shore. In order to turn offshore wind power into a viable renewable-energy source these peak levels must be reduced to at least €120/MWh by 2020."  

Birk stressed that achieving this ambitious goal will require a sustained effort, including the use dedicated offshore installation vessels, better year-round turbine access, and highly reliable yield-optimised wind turbines.

Repower’s 6M flagship features the world’s largest rotor diameter at 126 metres. However, this will change soon as several competitors have in the past years introduced/announced 6MW turbine models with much larger rotor diameters in the 150-154 metre range.

Technical options

While he did not explicitly speak about specific REpower technology development plans, Birk remarked on the possible technical options to reduce CoE. He said: "Assume an unchanged investment cost level. Increasing the rotor diameter from 120m to 150m under high wind conditions would increase annual energy production by 15-18%. Another option is to increase the power rating from 6-8MW, which would yield about 15% extra annual energy. A third yield-optimising option is increasing both power rating and rotor diameter."

Another issue that continues to generate much wind-industry debate is on optimal offshore turbine power rating. Birk said in a final comment: "We believe that for future far-shore projects until about 2020 the 6-7MW range will become the standard class. Larger power ratings would require bigger foundations and another generation of larger installation vessels."


Vestas senior product manager Anders Andersen in a V164-7.0MW presentation during the same session made an interesting CoE comparison between the current turbine configuration and a downgraded 6MW version with unchanged 164-metre rotor diameter. If both versions were to be put on a jacket foundation the 7MW turbine would offer the lowest CoE, he concluded, underlining the importance of careful turbine and foundation choice and optimizing.







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