A wind turbine is a rotating machine which converts the kinetic energy in wind into mechanical energy. If the mechanical energy is used directly by machinery, such as a pump or grinding stones, the machine is usually called a windmill. If the mechanical energy is then converted to electricity, the machine is called a wind generator, wind turbine, wind power unit (WPU), wind energy converter (WEC), or aerogenerator.
The turbine can be divided into three components. The rotor component, which is approximately 20% of the wind turbine cost, includes the blades for converting wind energy to low speed rotational energy. The generator component, which is approximately 34% of the wind turbine cost, includes the electrical generator, the control electronics, and most likely a gearbox component for converting the low speed incoming rotation to high speed rotation suitable for generating electricity. The structural support component, which is approximately 15% of the wind turbine cost, includes the tower and rotor pointing mechanism.
As of December 2008, these are some of the largest wind farms in the United States:
Tehachapi Pass Wind Farm - The east and south area of the Tehachapi Pass has one of California's larger Wind farms, generating electricity. The turbines have been in place since the early 1980s and have been upgraded through the years. The original wind turbines were much smaller than the much taller and larger new version turbines now sited for use.
In a recent move, Southern California Edison plans to secure 1,500 megawatts (MW) or more of power generated from new projects to be built in the Tehachapi area.
Sweetwater Wind Farm - Construction of the Sweetwater Wind Farm has proceeded in five stages and some generating capacity is still being built. Sweetwater stage 4 employs 135 Mitsubishi 1.0 megawatt turbines and 46 Siemens 2.3 megawatt turbines. Its output is being sold to San Antonio’s CPS Energy under a 20-year purchase agreement. Construction of Sweetwater stage 5 began in February 2007, with completion expected by December 2007. Using 35 Siemens turbines, Sweetwater 5 will have a capacity of 80 MW.
San Gorgonio Pass Wind Farm - is the most famous sight on San Gorgonio Pass on its eastern slope, as it marks the gateway into the Coachella Valley. The "farm" is just west-southwest of White Water, California. The pass is one of the windiest places in Southern California, and it is one of three major wind farms in California, along with the Altamont Pass Wind Farm and the Tehachapi Pass Wind Farm.
As of January 2008 the farm consists of 3218 units delivering 615 MW.
Horse Hollow Wind Energy Center - is the world's largest wind farm at 735.5 megawatt (MW) capacity. It consists of 291 GE Energy 1.5 MW wind turbines and 130 Siemens 2.3 MW wind turbines spread over nearly 47,000 acres (190 km²) of land in Taylor and Nolan County, Texas.
The first phase of the Horse Hollow Wind Energy Center consisted of 213 MW and was completed in late 2005; phase two consisted of 223.5 MW and was completed in the second quarter of 2006; phase three which consisted of 299 MW, was completed by the end of 2006.
Fowler Ridge Wind Farm - is currently under construction in Benton and Tippecanoe Counties, near Earl Park around 90 miles northwest of Indianapolis. The wind farm will be completed in two phases and will have a nameplate capacity of 750 MW total, making it one of the largest in the world.
The first phase of the project, consisting of 222 wind turbines,will bring the first 400 MW on-line by the end of 2008. Phase 2 (350 MW) could begin in early 2009.
Altamont Pass Wind Farm - (37°44'13" North, 121°39'5" West), a mountainous pass in Northern California, is home to one of the oldest wind farms in the U.S. and the largest concentration of wind turbines in the world.
Altamont Pass is located one hour east of San Francisco, California. Altamont Pass wind farm construction began in 1981 in response to favorable federal and state legislation that resulted from the energy price increases of the 1970s.
The Altamont wind farm consists of about 4,800 small wind turbines with a capacity of 576 megawatts (MW) annual generation of about 1.1 terawatt-hours (TWh) of electricity.
An advantage of this particular site is that under hot inland (California Central Valley) conditions a thermal low is developed that brings in cool coastal marine air through this pass, driving the turbines at a time of maximum need. Unfortunately this is not always reliable and with an inland high pressure condition the entire region can be both hot and windless.
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