operating in a near-optimum wind speed of 10 m/sec (36 km/h) at hub height will create more than 1.4 MW of electricity; in eight hours it will produce the amount of electricity used by the average U.S. household in one year (about 10,600 kilowatt-hour [kWh]).

There is an upper theoretical limit (the Betz limit of 59%) to how much of the available energy in the wind a wind turbine can actually capture or convert to usable electricity. Modern wind turbines potentially can reach an efficiency of 50%. Almost all wind turbines operating today have a three-bladed rotor mounted upwind of the hub containing the turbine. The blades have an aerodynamic profile like the wing of an aircraft. The force created by the lift on the blades result in a torque on the axis; the forces are transmitted through a gearbox, and a generator is used to transform the rotation into electrical energy, which is then distributed through the transmission grid (Figure 1-3).

Human use of wind energy has a long history (the following summary is taken from Pasqualetti et al. 2004). Wind energy has been used for sailing vessels at least since 3100 BC. Windmills were used to lift water and grind grain as early as the 10th century AD. The first practical wind turbine was built by Charles Brush in 1886; it provided enough electricity for 100 incandescent light bulbs, three arc lights, and several electric motors. However, the turbine was too expensive at that time for commercial development.

By the 1920s, some farms in the United States generated electricity by wind turbines, and by the 1940s wind turbines sold by Sears Roebuck and Company were providing electricity for small appliances in rural American homes; in Denmark, 40 wind turbines were generating electricity. The first wind-powered turbine to provide electricity into an American electrical transmission grid was in October 1941 in Vermont. However, significant electricity generation from wind in the United States began only in the 1980s in California. Today (2006), it amounts to less than 1% of U.S. electricity generation.

There has been a rapid evolution of wind-turbine design over the past 25 years. Thus, modern turbines are different in many ways from the turbines that were installed in California’s three large installations at Altamont Pass, Tehachapi, and San Gorgonio (Palm Springs) in the early 1980s. A typical turbine structure consists of a pylon (tower or monopole) that can produce electricity at wind speeds as low as 12-14 km/h (3.3-3.9 m/sec). Generators typically reach peak efficiency at wind speeds of approximately 45 km/h (12.5 m/sec) and shift to a safety mode when the wind exceeds a particular speed, often on the order of 80-100 km/h (22-28 m/sec). Smaller generators are used for individual buildings or other uses.

This report is concerned with utility-scale clusters of generators or wind-energy installations (often referred to as “wind farms”), not with small turbines used for individual agricultural farms or houses. Some of



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