Two-Bladed Wind Turbines Make a Comeback
Wind-turbine designers are warming up to an alternative to the three-bladed rotors that have been an industry standard for the past quarter century.
Several major wind-power companies are testing a departure from the industry’s standard three-bladed turbine design by dropping one of the three blades and spinning the rotor 180 degrees to face downwind.
The design presents engineering challenges, but the hope is that it could greatly improve the economics of offshore wind power. By some estimates, two-bladed turbines could cost 20 percent less to build and install while generating the same amount of power as conventional turbines.
China’s Ming Yang Wind Power, the world’s ninth-largest wind-turbine manufacturer, recently announced plans for the largest test of the design to date. It plans to erect a six-megawatt, two-bladed turbine in China this year that will generate as much power as the largest commercial offshore turbines. Ming Yang plans to build another in Norwegian waters next year.
Others are also taking another look at the alternative approach. Last year, Hitachi and Fuji Heavy Industries collaborated to build two two-megawatt wind turbines that use the downwind design. And in March, the Dutch wind-tech developer 2-B Energy raised £26.5 million ($45 million) to fund the installation of two downwind, two-bladed, six-megawatt turbines in Scottish waters.
Offshore wind is steadier than most places on land. But installing and maintaining offshore wind turbines is expensive, costing twice as much as onshore wind turbines on average.
Two-bladed turbines cost less because they use fewer materials. The removal of one blade makes the rotor lighter, which in turn makes it possible to place the rotor on the downwind side of the tower. Conventional wind turbine rotors face the wind and must resist bending back into the turbine’s tower, but downwind rotors can use lighter and even hinged blades that bend away from heavy gusts.
Light, flexible rotors translate into further materials savings in the turbine’s gearbox, tower, and foundation. The 140-meter diameter rotor, gearbox, and generator for Ming Yang’s six-megawatt prototype weighs just 308 tons—about 40 tons less than those of offshore market leader Siemens’s conventional six-megawatt offshore turbines.
Two-bladed wind turbines are also easier to install. Whereas the three-bladed rotors spinning in today’s offshore farms must be assembled on site, two-bladed rotors can be preinstalled on the turbine’s machinery onshore; the assembled package fits more conveniently on ships and is light enough to lift onto the tower.
But the two-bladed, downwind turbine design has had problems in the past, which have kept it from the market. It’s louder, for one thing, in part because the blades spin faster, although this isn’t a problem offshore. More importantly, in some wind conditions, the flexible blades can spring back and hit the turbine tower. In fact, this caused the undoing of a major attempt to revive two-blade turbines in 2002 (see “Wind Power for Pennies”).
However, designers say they’ve figured out how to keep the blades intact in all conditions. In 2009, wind-turbine designer Peter Jamieson with Glasgow-based turbine-engineering firm Garrad Hassan & Partners identified a controlled rotor-braking strategy to mitigate the risk of a tower strike in downwind turbines. According to Jamieson, two-bladed designs could cut costs by 15 to 20 percent.
Larry Miles, CEO of the Wind Turbine Company, which made the demonstration turbine that failed in 2002, is not optimistic about the wind industry’s willingness to endure the warranty and financing risks required to truly rethink its products. For that reason, he has set his sights on designing small, 100-kilowatt turbines for distributed wind power applications, rather than the utility market.
Still, Miles thinks a Chinese firm might just have the financial wherewithal to innovate at utility-scale. “If anybody does it, it will be Ming Yang. They have enough substance to do it, and they have the Chinese market,” he says.