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.
Instead of gearboxes, ScanWind uses a novel direct-drive generator technology in its 3.5-megawatt turbines. This makes the turbines more reliable, the company says, by cutting downtime and repair costs–an especially important consideration for turbines offshore, where it’s more expensive to send technicians for maintenance. ScanWind has been testing the turbines on the Norwegian coast since 2003.
GE, based in Fairfield, CT, is the world’s second-largest maker of wind turbines, with more than 12,000 turbines installed globally. But GE’s offshore wind energy portfolio has been minimal so far, and the company wants to expand its offshore offerings. By acquiring ScanWind, transferring its expertise and understanding of onshore wind, and adding technologies such as remote monitoring and sensing, GE hopes it can make a solid, cost-effective offshore wind product.
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.
ScanWind’s turbine design gets rid of the gearbox completely. Instead, the rotor shaft is attached directly to the generator, which spins at the same speed as the blades.
In a turbine generator, magnets spin around a coil to produce current–the faster the magnets spin, the more current is induced in the coil. To make up for a direct drive generator’s slower spinning rate, the radius of rotation is increased, effectively increasing the speed with which the magnets move around the coil.
“Eliminating the gearbox from the wind turbine [removes] the technically most complicated part of the machine, inherently improving reliability,” says Henrik Stiesdal, chief technology officer of Siemens AG. Furthermore, if a permanent magnet is used in the generator, as is the case with newer turbines, the efficiency goes up even more. That’s because, unlike today’s electromagnetic generators, permanent magnets don’t need power.
Direct-drive generators currently cost more than geared systems and are 15 to 20 percent heavier. Still, GE’s decision to buy ScanWind is smart, says Butterfield. “Offshore machines are so expensive in terms of maintenance that some people are thinking the tradeoff tilts in favor of direct-drive generators,” he says. “I am optimistic that there is technology out there that’s going to help bring direct-drive generators down in parity with the weight and cost of geared systems.”
Siemens certainly believes so. The leader in offshore wind energy has been testing two 3.6-megawatt proof-of-concept direct-drive turbines near Denmark for over a year now. Stiesdal says that the technology has proven to work just as well as gearboxes in terms of power, vibrations, temperature, noise, and reliability. Now it’s a matter of bringing down its cost.
GE, meanwhile, expects to have a market-ready product by late 2012. It is targeting the European market initially because nearly all of the 1,473 megawatts of offshore wind power currently available come from installations along European coasts. According to industry analysts, this capacity must reach 30,000 megawatts by 2020 if the European Union is to meet its renewable-energy targets. One of the reasons for choosing ScanWind, says GE, is because of the company’s footprint in the Nordic countries, which, along with the U.K. and Germany, are the brightest spots for offshore wind energy.