Specific design for offshore use

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In buying into Multibrid Entwicklungsgesellschaft, French nuclear conglomerate Areva has landed 51% of a one-horse wind turbine maker with the single goal of taking its 5 MW offshore machine to full commercialisation. The unit is best described as a hybrid between the two main commercial wind turbine designs: traditional turbines with gearboxes and the less common direct drive systems without a gearbox.

The Multibrid has a gearbox, but it steps up the rotor speed by no more than 10:1 to drive a slow speed, synchronous, permanent magnet generator at speeds up to 147 rpm. Traditional machines with gearboxes have higher step-up ratios to drive generators typically at 1000 rpm or 1500 rpm. The Multibrid's main rotor bearing is integrated with the gearbox and generator. The approach provides for an unusually compact drive train, which shortens load paths between all the components, allowing for fewer moving parts, says the company.

The turbine's electrical converter system, transformer and switch gear are placed on the outside of the tower to reduce tower-head weight. This will facilitate offshore erection, says the company. To protect the components from the marine climate, the unit operates an air filtration system that creates positive air pressure within the nacelle, after removing all salt, moisture and other particulates, to keep moist saline air away from sensitive parts.

The Multibrid approach exemplifies how to let a design process mature before a push to commercialisation, says Randy Tinkerman of Deutsche WindGuard, a German wind turbine consultancy. "The obvious discussion is that offshore technology has not proven itself yet and there are competing designs, but Multibrid certainly is a leading candidate to join the industry with its first effort," says Tinkerman. "One can only assume that Areva's due diligence used performance data as a major characteristic in coming to their decision."

The original design for the Multibrid was by Germany's Aerodyn. A similar Aerodyn concept has been applied to Finnish WinWind's 1 MW machine and Aerodyn also lies behind the design of the Russian owned Bard 5 MW machine, also being developed in northern Germany (Windpower Monthly, June 2007).

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