Discussing optimum size -- How big can they go?

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As each successive generation of new wind turbines gets larger, the debate continues as to how much further the trend can continue. A researcher at Denmark's national laboratory at Risø, Thomas Krogh, suggests that a 10 MW turbine is feasible.

The rotor diameter would be 160 metres and the tower height 100-140 metres. Annual energy production, at a site with a hub height wind speed of 7.5 m/s, would be 28 GWh. The machine would be a pitch controlled horizontal axis upwind design for offshore use and have built-in health monitoring. The tower would be a hybrid concrete/steel type and the rotor blades would need to be manufactured in more than one piece. Such a concept may use new gear/generator designs, materials and production methods.

Krogh points out that despite the rapid increase in turbine size, improvements in design have enabled tower head masses to be kept within reasonable limits. The tower head mass of the Vestas 4.5 MW machine is 210 tonnes -- significantly lower than that of the Enercon 4.5 MW, Repower 5 MW or Multibrid 5 MW machines. The weight reduction trend was also highlighted at a wind research and development workshop in London last month. Wood epoxy and carbon fibre blades of 49 metres in length are only one tonne heavier (7.5 tonnes compared to 6.5 tonnes) than the 31 metre blades manufactured in 1996, according to Mark Hancock of Vestas.

With turbine companies slowing down their technology development, the workshop felt that the prospect of 10 MW wind turbines has receded. Bill Leithead of Strathclyde University, England, suggested that 7 MW might be where a plateau is reached. There seemed little doubt that continuing weight and cost reductions -- despite the recent reversal of the steadily downward cost trend -- would be achieved. The consensus was that research and market development needs to proceed in parallel and that the cost reductions that have been realised so far are due in equal measure to technology improvement and market stimulation.

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