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ENERGYWind Turbines Shed Their GearsBoth Siemens and GE bet on direct-drive generators.

TUESDAY, APRIL 27, 2010BY PETER FAIRLEY E-mail Audio PrintWind turbine manufacturers are turning away from the industry-standard gearboxesand generators in a bid to boost the reliability and reduce the cost of wind po

wer.

Siemens has begun selling a three-megawatt turbine using a so-called direct-drive system that replaces the conventional high-speed generator with a low-speed generator that eliminates the need for a gearbox. And last month, General Electricannounced an investment of 340 million euros in manufacturing facilities to bui

ld its own four-megawatt direct-drive turbines for offshore wind farms.

Most observers say the industry's shift to direct-drive is a response to highlypublicized gearbox failures. But Henrik Stiesdal, chief technology officer of Siemens's wind power unit, says that gearbox problems are overblown. He says Siemens is adopting direct-drive as a means of generating more energy at lower cost.

"Turbines can be made more competitive through direct-drive," says Stiesdal.

Siemens's plans hinge on a new design that reduces the weight of the system's generator. In conventional wind turbines, the gearbox increases the speed of the wind-driven rotor several hundred fold, which radically reduces the size of the generator required. Direct-drive generators operate at the same speed as the turbine's blades and must therefore be much bigger--over four meters in diameter forSiemens's three-megawatt turbine. Yet Siemens claims that the turbine's entire

nacelle weighs just 73 metric tons--12 tons less than that on its less powerful,gear-driven 2.3-megawatt turbines.

Much of the weight reduction comes from the use of permanent magnets in the generators' rotor--a trick that GE is also using. Conventional turbine generators us

e electromagnets--copper coils fed with electricity from the generator itself. Henk Polinder, an expert in permanent-magnet generators at Holland's Delft University of Technology, says that a 15-millimeter-thick segment of permanent magnetscan generate the same magnetic field as a 10- to 15-centimeter section of coppe

r coils.

Stiesdal says Siemens reduced weight further by inverting its generator's design. Rather than a steel rotor covered with permanent magnets spinning inside a stationary doughnut-shaped stator (the design GE is using in its four-megawatt direct-drive turbine) Siemens's rotor is a steel cylinder with permanent magnets onthe inside, and this rotor spins around a column-like stator.

Siemens erected a prototype of its machine in Brande, Denmark, in December and plans to install 10 more this year, primarily in Denmark, before beginning mass production in 2011. GE's technology, which it acquired with the purchase of Norwegian turbine producer ScanWind last year, is being demonstrated at a test site in Norway; commercialization of its four-megawatt machine is slated for 2012.

More competition is on the way. Venture capital firm New Enterprise Associates is backing a Boulder, CO-based startup called Boulder Wind Power, which is developing a 1.5-megawatt direct-drive turbine. The firm was founded in December by Sandy Butterfield, who was chief engineer for the U.S. National Renewable Energy Laboratory's (NREL) wind technology center, where he led a major study of the gearbox design process.

Whether gearbox failures are an industrywide problem remains a matter of some contention. NREL initiated its study in 2007, when there were several failures: aU.S.-based company, Clipper Windpower, experienced serious gearbox problems with

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in months of installing the first of its 2.5-megawatt turbines at a wind farm inLackawanna, NY, while gearboxes in the 30 Vestas Wind Systems turbines forming

the U.K.'s offshore Kentish Flats wind farm had to be replaced after just two years of operation. NREL concluded that most wind turbine gearboxes would fail "well before" their 20-year design life.

Stiesdal says Siemens's own studies show that gearboxes are quite reliable, over

all. A 2008 analysis of Siemens machines installed from 1983 to 1989 in the U.S.found that the "vast majority" were still operating with their original gearbox

es. But he does expect increased reliability from the direct-drive system, whichhas about half as many parts as a conventional turbine.

Direct-drive systems do introduce one potential problem, however. There are ongoing concerns regarding the future supply of the rare earth metals used to make permanent magnets. "That's a serious issue," says Stiesdal