Direct-drive turbines to propel offshore wind

April 30, 2010 7:00 AM PDT

by Martin LaMonica

With projected growth in offshore wind farms, turbine makers are adopting direct-drive generators, a technology that could help address concerns over cost and reliability of offshore wind.

Siemens Energy's first direct-drive turbine is now available for both onshore and offshore applications, the company said last week. Siemens has been testing a prototype since last year and the results have been good, according to the company.

Inside Siemens' 3-megawatt direct-drive turbine.

(Credit: Siemens Energy) Rated at 3 megawatts of capacity, the SWT-3.0-101 is a lower height and weight than Siemens' 2.3-megawatt machine and has half the number of parts. It's more compact than generators that use a gearbox and is efficient at low wind speeds.

In a traditional wind turbine, a gearbox and a rotor are connected to a generator to get a flow of current. The Siemens direct-drive technology has a permanent magnet attached to the rotor, which connects directly to a generator, eliminating the need for a gearbox.

Siemens has been working on the direct-drive turbines for over a decade and expects to commercialize it for large turbines in the coming years. (Offshore wind developer Cape Wind has a contract to use traditional GE turbines.)

General Electric, too, is pursuing direct-drive turbines specifically for offshore applications in Europe. GE last year purchased ScanWind, a Norwegian company that specializes in direct-drive generators for wind turbines.

Direct-drive turbines are already used in smaller turbines. Northern Power, for example, specializes in turbines with a capacity of 100 kilowatts, sized to power a school or office building. Later this year, though, it's planning on building a utility-scale turbine rated at 2.2 megawatts.

Another company entering the field is Boulder Wind Power, notes Technology Review. That company was founded by researchers at the National Renewable Energy Laboratories who studied gearboxes and predicted that many will fail before their anticipated life.

Although there are technical advantages to direct-drive turbines, using permanent magnets rather than copper coils and gearboxes does pose a materials problem. Permanent magnets require rare earth metals, most of which are sourced in China, and there are growing concerns over the availability of supply.