Wind power is the direct conversion of mechanical wind energy into electrical energy. Wind turbines can be used in wind farms on land, as offshore power generators, small residential power generators and more. At the end of 2008, worldwide capacity of wind-powered generators was 121.2 giga watts. Wind power has the capability of producing about 1.5% of worldwide electricity. Use of wind power doubled between 2005 and 2008, and now about 80 countries around the world use wind power on a commercial basis. Wind farms are typically connected to local grid networks. Smaller wind turbines are used to provide electricity to isolated locations and for residential use. Small domestic wind turbines encourage utility companies to buy back surplus electricity. Wind energy is attractive because of its renewable qualities. It is extremely plentiful, is widely distributed and very clean. One downside is the cost of constructing wind turbine farms. As with many other forms of power generation, the farms are not universally welcomed due to their visual impact. Turbine wind power is non-dispatchable, which means that the available output of solar power must be taken and used when it is available since it is not possible to store the energy produced by wind power. Other sources, including hydropower, must be used to match supply with the demand. Wind power is considered to be intermittent, which causes problems when wind power is low in supply. When wind is used for a moderate fraction of demand, additional costs for the intermittency are low. Wind power began to be used approximately 5,500 years to sail ships and produce wind-driven natural ventilation. The first practical windmills were built in the 1st century AD. While contemporary windmills have a horizontal axis, early windmills had a vertical axis. The first horizontal axis windmills were not developed until around 1180. Windmills and the "water-pumping windmill" allowed for the expansion of farming, ranching, and eventually the rail transport systems around the world. Wind varies, which means that averages per location do not indicate the exact amount of energy a wind turbine could produce there. To decide how much wind occurs at a location, it is necessary to use the Weibull model, which closely mirrors the actual distribution of hourly winds at many locations. The Rayleigh Distribution system can be used as well, but is also considered to be much less accurate. The maximum amount of power is generated by high wind speed, and much of the energy comes in short wind bursts. In fact, large amounts of wind energy can be collected in a mere 15% of operating time. Wind energy is renewable, and while it may be intermittent, this rarely occurs and any problems can be balanced with other forms of energy. Wind energy does not deplete fossil fuels, continues to be clean, and produces no greenhouse emissions.
