Question: Can anyone tell me why the turbines have to be the size that they are? Years ago they generated electricity and pumped water with a small regular windmill, that didn't clobber birds as it spun around, while standing 1,000 ft. high with blades 800 ft. long.
The answer is that it all comes down to economics. Obviously, wind generators are removing power from the air to generate electricity. Basically, the amount of power in the wind is related to the air density and the speed of the wind. The formula is:
P = (air density x wind speed3)
2
The key component here is the wind speed because the speed of the wind is cubed, or multiplied times itself three times. Wind speed increases as you go higher. The wind speed at 100’ is approximately 26% greater than it is at 20’ above the ground. Since the wind speed is cubed to determine the wind power, a 100’ level will have double the wind power compared to a 20’ level (1.26 times 1.26 times 1.26). Doubling the wind speed will increase the wind power by 8 times (2 times 2 times 2). So wind tower height makes a large difference in the performance of the wind generator and the cost of the electricity that it produces.
With today’s fixed speed AC machines, virtually none of the wind gust gets converted to electricity as the blade speed cannot change. Thus the force of the wind gust gets converted mostly to mechanical stress on the wind generator, the turbine blades, and the tower structure. Turbulence from wind gusts and other sources create mechanical stresses that cause many of the maintenance and failure problems with wind turbines. Minimizing physical stress is very important.
Most wind turbines today (even the arctic models) will shut down at a temperature of -22º F. to -26º F. to reduce the potential mechanical stresses. Estimates are given that the wear and stress on the components at those extreme temperatures is about 50 percent greater than at warmer temperatures.
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