33M-VS wind turbine
DaIe Osborn, U.S. Windpower & Edgar DeMeo, EPRI
It’s hard not to be mesmerized while watching a wind farm generate electricity--a sea of blades pinwheeling madly or twirling lazily as the breeze picks up and slackens. Stand below the spinning rotors, and as the blades rev up to a whine and slow to a whisper you can hear the wind’s capriciousness.
And therein lies the weakness of the wind turbine. Until recently, turbines were designed to operate at a single, ideal speed. The two- or three-blade rotors are geared to spin the shaft of a generator at a fixed clip, so that it cranks out an alternating current at 60 cycles per second, the standard utility-grid frequency. If a sudden gust tries to whirl the blades any faster, the generator shaft strains against the added torque. Not only does wind energy go to waste, but the unwanted torque shortens the machinery’s lifetime.
This drawback has forced engineers to either build wind turbines out of costly heavy-duty components or keep replacing parts. That’s one key reason electric utilities have shied away from the wind turbine’s nonpolluting, renewable energy.
But technology has finally caught up with the wind. A new variable-speed wind turbine, the 33M-VS, is proving that wind farms can compete with conventional fuel-burning power plants. The turbine was developed by the Palo Alto-based Electric Power Research Institute (EPRI), the research and development arm of the electric utility industry, and U.S. Windpower.
Unlike previous turbines, the 33M-VS is rigged to roll with the wind’s punches. When gusts whip the rotor, the generator shaft is free to speed up in response. As the shaft’s rotation speed changes with the wind, the alternating current that flows from the generator swings up and down in frequency.
But between the generator and the utility grid lies an electronic power converter. This device first converts the variable-frequency current to direct current, then switches it back to alternating current at a fixed 60 cycles per second. So the generator feeds an even current to the utility grid. And the wind gust problems--wear and tear and wasted energy--have all but blown away.
Edgar DeMeo, who manages EPRI’s research efforts in solar and wind power, realized in the early 1980s that power converters could smooth out the wind turbine’s growing pains. Back then, as tax credits nurtured the technology’s initial development spurt, engineers were coming to grips with the mechanical fatigue that plagued conventional turbines. But the cost of power converters--made from extra-large silicon chips to handle high currents--was prohibitive.
Like so many electronic products, however, power converters began to tumble in price. In the mid-eighties, recalls DeMeo, wind turbine technology was coming along, and with big advances in power electronics, we asked if it made sense to look at a marriage. EPRI had already played a role in developing power converters for huge adjustable-speed industrial motors. To bootstrap the technology onto wind turbines, in 1988 EPRI joined forces with Oakland-based U.S. Windpower, the nation’s biggest wind-turbine manufacturer. In 1990, after some successful tests, EPRI brought the Pacific Gas & Electric Company and Niagara Mohawk Power Corporation into the consortium.
In late 1990, the consortium--known as the Variable-Speed Wind Turbine Development Alliance--began redesigning U.S. Windpower’s existing 100-kilowatt turbine to demonstrate the new variable-speed technology. Its blades sweeping more than three times the area of the 100-kilowatt model, the prototype was designed to generate between 300 and 400 kilowatts-- enough to power 150 or so typical homes for a year.
By the end of 1991, the turbine was spinning away at U.S. Windpower’s headquarters near blustery Altamont Pass, just east of San Francisco Bay. EPRI calculates that if planted in a spot where annual wind speeds average 16 miles per hour, the turbine could generate electricity at five cents per kilowatt-hour. That’s about on par with the generating costs of a newly built coal- or gas-fired plant, and two to four cents cheaper than traditional wind turbines. Last year U.S. Windpower ran 22 more test models for a season at Altamont Pass, then took them apart to look for mechanical wear. We believe we’re beyond the threshold of competing economically with traditional fossil-fuel technologies, says company president Dale Osborn. And that means the world’s windiest landscapes may soon sprout fields of wheeling, whispering rotors.
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