Wind Power

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Wind Power Not all the kinetic energy present in winds can be extracted by a wind Hub Blades Housing Tower High-Speed Shaft Generator Main Shaft Gear-Box Transmission Brake wind Figure 3-9 Principle of operation of a wind turbine. 8 It can be shown that the theoretical maximum efficiency of wind turbines is 59%, known as the Betz limit. 51 Chapter 3 - Wind Energy turbine. In addition to various losses, such as slippage and drag, some of this energy must also be carried downstream of the turbine in order to maintain the air flow. The power generated by a wind turbine is proportional to the kinetic energy of the mass of air swept through its rotor. The mass itself increases with the air density, the size of the rotor, and the speed of the wind. As a result, total power generated by a wind turbine increases with the air density, the cube of the wind velocity, and rotor area, that is, the square of the rotor diameter (See Box “Power and Torque”). Wind speed increases with height above the ground, so the height of the tower on which the rotor is mounted indirectly affects power production. Figure 3-10 shows the increase in power with the height of the tower. For example, a wind turbine mounted on a tower 120’ (36 m) tall will be exposed to wind speeds twice that of a tower 30’ (9 m) tall. This alone results in the production of eight times more power. Air density (and power output) decreases with altitude at the rate of roughly 2.5% for every 1000’ above sea level. Temperature affects the power derived from wind energy in two ways. First, they affect wind patterns and intensity. Second, they affect the air density and thus the mass of air drawn through the turbine. In desert and open areas, where most wind turbines are installed, temperature differences as high as 60oC are possible between summer and winter seasons. As a result, as much as 20% more power can be extracted in winter when the air is denser.


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