The cover article in the April 2010 Popular Science is about how GE’s latest design in wind turbines will attempt to leapfrog the rest of the industry. (I don’t usually buy Popular Science because their articles usually do not cover a subject in sufficient depth to answer my questions. But once in a while they have a cover piece that is just so intriguing I can’t resist.) But before you look at this article, let me explain just what the GE people are probably trying to beat. Right now, the best overall design is probably the Clipper, which is built in Cedar Rapids, Iowa. This is not surprising, because this company was formed by a small group of entrepreneurs who each had wind turbine manufacturing experience going back 25 years or more. By pooling their patents and their capital, they hoped to finally build the kind of turbine which they had always wanted to build, but which no one had ever built. The design improvements they wanted were mainly these:
First, scale it up large enough so that it’s up high where the wind is. This is important because the amount of energy you get out of a turbine equals the cube of the wind speed. That means that if by going higher you can get twice the velocity, then you get 8 times the output. So they built a machine that has a blade “like a rotating football field” with its axle 350 feet above ground.
Second, don’t generate AC directly off the turbine shaft. Doing this leaves you with a shaft speed phase-locked to the power line frequency. Better to make DC, letting the shaft speed vary with wind speed. Then use inverters to convert the DC to whatever AC the power line wants.
Third, make the turbine maintainable. Maintaining these machines is highly technical, and it takes a while to train a maintenance technician. But with 300 foot ladders to climb every day, knees would start to wear out by age thirty-five. You’d have to stop doing this work almost as soon as you got a good start. Clipper’s answer is to put in an electric man-lift.
Finally, don’t use one large generator--use four small ones. If you use one big generator, then if it ever goes bad, you have to bring in a humongous crane to lift it down.
Clipper solves the problem like this: Like most big wind turbines, Clipper has a gear-box to step up the very low shaft speed (15-60 rpm) to at least a few hundred rpm. The Clipper gear-box has one bull gear turning four pinions. The pinions are arranged in a circle, and each is permanently mounted in the gearbox with its own set of bearings and its own oil seal where the pinion shaft sticks out of the case. Each pinion shaft is splined, and about 2 inches in diameter. Each generator rotor just has a hollow shaft, internally splined, which fits over this arbor. And since the generators use rotating fields using permanent magnets, there are no brushes either. The rotor is simply a collar that holds magnets, which fits over the arbor. The wiring is all in the stator, which slides over the rotor and is easily removable. The stator housing has a flange on one end which bolts onto the gear box. It’s light enough so that the built-in jib-crane can handle and it can be lowered to the ground with the built-in power winch.
But now GE is taking a different approach: They eliminate the gear-box entirely by mounting a single rotor wheel directly on the prop shaft, with diameter so large that the permanent magnet poles are moving through the stator windings fast enough even without a step-up in rpm. The only part which might ever need replacing is the stator winding. But this is made in small removable sections. It seems to me that both the Clipper design and the new GE design would provide an ultra-low-maintenance machine. As the wind turbine market matures and buyers begin to “price in” the advantage of maintainability, both of these designs are positioned to capture an increasing share of the market.
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