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The nameplate capacity of a wind turbine is somewhat arbitrary. Therefore, so is the capacity factor. |
The power available in the wind is proportional to the cube of the windspeed. With an excellent wind turbine of 50% efficiency, the hypothetical power output can be caluclated by multiplying the square of the radius (in meters) by the cube of the windspeed (in meters per second). Most wind turbines have a cutout-speed of about 55 to 60 miles per hour (25 meters/second), according to a correspondent who works in the field. Consider, for example, a wind turbine of 23-meter radius. In a 25-m/s wind, it could hypothetically produce 8.3 MW of electricity. The Enron Type Z-750 wind turbine has a 23-m radius. It is rated at 750 kW, not 8.3 MW. The machine is designed to produce 750 kW of electrical power for all speeds in excess of about 11 or 12 m/s. Its expected capacity factor in a wind farm in Minnesota is about 35%. If the wind turbine were rated at 14 m/s, the nameplate power would be about 1.5 MW instead, but the power delivered to the grid would still be about the same. Its capacity factor would be 17% (possibly a bit higher, depending upon windspeed distribution) instead of 35%. Early wind turbine manufacturers (and proponents)
tended to emphasize nameplate power. More recently, they have gone
for relatively constant power (at least in higher winds) and higher capacity
factors.
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