Technology Review has an article on "new direct-drive turbines promise to lower the cost of offshore wind energy" - GE Grabs Gearless Wind Turbines.
With a new purchase, GE is betting on an early-stage turbine technology that could make offshore wind farms cheaper to maintain. The acquisition of ScanWind, based in Trondheim, Norway, has also secured GE a foothold in the growing offshore wind energy market. ...
In conventional wind turbines, the blades spin a shaft that is connected through a gearbox to the generator. The gearbox converts the turning speed of the blades--15 to 20 rotations per minute for a large, one-megawatt turbine--into the faster 1,800 rotations per minute that the generator needs to generate electricity. "Wind turbines are very different than any other gearbox application," says Sandy Butterfield, chief engineer of the wind program at the National Renewable Energy Laboratory in Golden, CO. "You're going from a very low speed to a high speed." Typically it's the opposite.
The multiple wheels and bearings in a wind turbine gearbox suffer tremendous stress because of wind turbulence, and a small defect in any one component can bring the turbine to a halt. This makes the gearbox the most high-maintenance part of a turbine. Gearboxes in offshore turbines, which face higher wind speeds, are even more vulnerable than those in onshore turbines. Butterfield is leading a gearbox-reliability study with turbine makers to identify design weaknesses that could be avoided.