Repower outlines offshore turbine strategy

WORLDWIDE: 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. These days, its focus is on 6M

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 the company’s track record, Repower vice president of offshore business development, Axel Birk, said that the 5M-prototype still operates with the original gearbox and its first set of rotor blades.

Following on from three 6M onshore prototypes that began operating in 2009, Repower will install 48 units at the Belgian Thornton Bank II & III wind farm during 2012-13. Another 48 6M turbines are planned for next year at the Nordsee Ost project in German waters. Most Repower turbines installed offshore have been put on four-legged jacket-type foundations.

Fast-speed geared wind turbines with doubly fed induction generators (DFIGs) have been standard Repower technology since the company's launch in 2001.

Birk said that, in general, gearbox failures only contribute 4-8% of total turbine failures set against around 30% for electrical systems and power electronics.

He also believes that future offshore conditions and demands will be substantially different from the relative shallow water depths and modest distances to shore that are common today. "Tomorrow's offshore wind farms will be 20-150 kilometres from shore, operate in water depths up to 60 metres, and face very strong average wind speeds, in the 9-10.5m/s range," he said. "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 and put on monopile foundations".

Elaborating on the main reasons behind the current lifecycle-based cost of energy (CoE) peak values, estimated at EUR170 /MWh, he said: "Two key differences between early offshore wind farms and those built more recently are 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 EUR120/MWh by 2020".

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

Repower's 6M flagship offshore turbine features the world's largest rotor diameter at 126 metres. However, this will soon be surpassed, as several competitors have introduced or announced 6MW turbine models with much larger rotor diameters, in the 150-154 metre range.

While he did not speak explicitly about specific Repower technology development plans, Birk remarked on possible technical options to reduce CoE. "Assume an unchanged investment cost level," he added. "Increasing the rotor diameter from 120-150 metres 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, said Birk. "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."

On this note, 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 an 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 optimisation.

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