The increased output coupled with weight reduction has been achieved through a series of technology advances, the most significant being an integrated gear box, with a bearing which carries the rotor. The result is a nacelle housing that is 30 centimetres shorter than that of the 2 MW turbine, while its weight remains roughly the same.
The concept has raised the eyebrows of chief technical consultant for Denmark's association of wind turbine owners, Strange Skriver, who hesitates to pronounce on the consequences of letting the integrated gear box bear the rotor. "There's no doubt that first and foremost this is an economic optimisation. I'd say it was adventurous on such a large turbine to suddenly go over to a whole new construction, which in principle was used on wind turbine types of the past," says Skriver.
Vestas' Svend Sigaard says the design has undergone comprehensive testing, including load life time tests of individual components, all of which yielded positive results. Design lifetime is 20 years, regarded by Sigaard as a minimum. He expects the turbine's working life to be longer than that.
Further weight reduction has been achieved by partially replacing glass fibre in the blades with carbon fibre, with the result that the 44 metre blades for the 3 MW model are lighter than the 2 MW turbine's 39 metre blades. Use of carbon fibre, a lightning conductor, has required development of a new lightning protection system. The blades have a more dirt resistant profile, says Vestas.
The tower, built from stronger steel, is a new construction that makes it both lighter and easier to transport than the 2 MW tower. The bigger turbine also has its own crane, enabling the blades, generator and gearbox to be replaced without the use of an external crane.
Last but far from least, the 3 MW can operate with only one annual service and maintenance visit, instead of the normal two. The improvements, says Vestas, more than make up for the extra cost of the 3 MW turbine and operating costs will be lower. The machine is to go into series production next year. Eight prototypes are now being tested in Denmark, Germany and Sweden (Windpower Monthly, December 2002).