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small wind
Anyone shopping for a home-sized wind turbine by comparing the various models offered by different manufacturers soon realizes that there is a problem in trying to make those comparisons — in the United States, no standard method for describing wind turbine performance in advertisements. U.S. manufacturers rate their turbine output at a wind speed they choose based on their system’s design. Typical wind speeds for these ratings range from 24 mph (10.5 m/s) to 36 mph (16 m/s). In addition, some manufacturers designate the rated wind speed as the wind speed at which their turbine reaches its maximum power output. Other manufacturers use the rated wind speed to indicate the point at which their turbine begins to govern. In the latter instance, the wind generator may actually achieve a power output 30-40% higher than its rating. The power rating actually has to do with the amount of power that the wind turbine’s generator can safely produce. Rated power of different turbines is not standard at any one wind speed. Instead, manufacturers rate their turbines over a range of wind speeds (from 24 mph to 36 mph, depending on the manufacturer). Trying to make comparisons between different models is virtually impossible for the average consumer not interested in delving into power equations and conversion efficiency calculations. To compound this problem, many people use dollars-per-watt to compare different wind systems. It therefore follows that rating home-sized wind turbines by their generator size is, while not intentionally so, rather misleading to the end user trying to comparison shop. While it is unlikely that manufacturers will agree on any given rated wind speed as a standard for comparison any time soon, there is a way for consumers to more closely approximate the power outputs of different models of wind generators. This can be done by comparing the rotor diameters or swept areas of the wind turbines. An analogy can be made between the swept area of the wind generator’s rotor and a solar water heater. Solar water heaters depend on panels, called solar collectors, through which a fluid passes to collect the sun’s radiation. Typical solar collectors are four feet by eight feet, or 32 square feet. One 4 x 8 foot panel will collect and convert a given quantity of the sun’s radiation. In addition, one manufacturer’s solar collector will heat about the same amount of water as another manufacturer’s panel, making comparison shopping a matter of identifying the better product for your application. Since solar collector output is more or less standardized and based on the area of the panel exposed to sunlight, comparing collectors is quite simple. If you want to double the heat output of your solar water heater, you double the number of solar collectors in the system. In other words, 64 square feet of solar collector will convert twice as much of the sun’s energy to hot water as 32 square feet. In a wind energy system, the turbine rotor serves as the collecting device of the wind turbine. The rotor allows the wind generator to convert moving air masses into the rotating motion necessary to drive its electrical generator. Since most rotors operate at more or less the same efficiency, the only way to intercept more of the power in a given air mass is to increase the size of the wind generator’s collecting device — the rotor. This makes swept area, or even rotor diameter, a better tool for comparing different models of wind turbines than the generator’s rating. The rotor’s swept area is the area of the circle delineated by the wind generator’s rotating blades. The equation for determining area of a circle is Area = Pi x r2 The value of Pi is approximately 3.1416. The radius (r) is one half of the rotor diameter, or about the length of one blade in the rotor. It should be pointed out that since rotor area is a function of the square of the radius (or the length of one blade) small increases in blade length can result in rather large increases in swept area. In fact, doubling the rotor diameter yields a fourfold increase in swept area, with a corresponding increase in electrical generation. Since most home-sized wind turbines are operating at about the same efficiency, rotor diameter or swept area rather than generator size makes a far better yardstick for comparing the various models offered by different manufacturers. While not a perfect measure by any means, comparing swept areas is far more accurate than relying on a wind turbine’s generator capacity. For example, it can be fairly safely assumed that a wind turbine with a swept area of 200 square feet will produce about twice as much electricity as a model sporting a rotor with a swept area of only 100 square feet. To quote Paul Gipe in Wind Power For Home and Business, "Nothing says more about a wind turbine than rotor diameter. Nothing." -- Mick Sagrillo, Sagrillo Power & Light Co.
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