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Prototype turbine throws blade -- Not a design flaw says DOE

Research on the experimental 500 kW downwind turbine in southern California developed by The Wind Turbine Company (WTC) has hit a set back after the prototype threw a blade in late May. But both company and federal government research officials say the turbine's weight-reducing design is not at fault. The mishap, however, has caused the National Renewable Energy Laboratory, which finished testing the company's 250 kW turbine last year and is funding and overseeing research on the 500 kW machine, to put further studies on hold until it knows exactly what happened and why. Officials do know the blade was thrown after it struck the turbine tower.

"The 250 kW turbine's performance was exemplary," says Peter Goldman, who heads wind research at the Department of Energy (DOE). "And, we're confident we're moving in the right direction with the 500 kW turbine. This is R&D."

WTC president Larry Miles says the fault was in the pitch drive control system. "One blade was performing normally and producing power," he says. "The other blade pitched into a deep feather, driving the blade into an upwind direction." The safety control system failed to detect the problem in time (it takes just one turn of the rotor) and the blade tip deflected into the tower. "Our only consolation is that there are other turbines out there using the electrical pitch drive system and it could have happened to any one of them," Miles says. "For us it was an unfortunate convergence of bad events."

WTC, an engineering firm in Bellevue, Washington, is focusing on a two-blade downwind design -- which differs dramatically from the common European three-blade upwind design -- to cut weight and ultimately the cost of energy. As well as standing with its back to the wind the WTC turbine has hinged blades to further lower stress on the entire structure and allow downsizing of components, including using smaller guyed towers made of natural gas pipeline.

"Each blade is hinged and cones downwind to alleviate loads," says Stan Calvert, the DOE's team leader for wind energy. "The design allows for some forward coning and leads to more complicated dynamics, but our experience so far has validated the design." Miles says he has been using blades designed for upwind turbines, but his company is working on longer and skinnier blades specifically for the downwind machine. Their design could be complete by the end of the year and will bring the turbine capacity to 650 kW. Miles says he plans to notch that up to 750 kW by mid-2003.

Testing of the 500 kW turbine began in January at a site 20 miles from Tehachapi in Los Angeles County (Windpower Monthly, February 2002). It will be about 60% of the weight of a Vestas 660 kW and cost 20-25% less, says Miles. He expects the turbine to produce electricity cheap enough to compete with other energy resources without needing a federal Production Tax Credit subsidy.

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