For many centuries wind has been used as a reliable source of energy. It is clean and inexhaustible. Wind is used to make the job of a human simpliar. It is used for grinding grain, pumping water, and to enable sail boats to move. Though often the amount of wind power varies depending on the climate of an area.
Although wind power is only used in selected areas the recent interest in fuel conservation has sparked the development of windfarms. In the 1980s a studied showed that in order to produce one kilowatt-hour of energy cost 4 cents.
The best known horizontal axis machine in the United States is the American farm windmill which is frequently used to pump water. This machine has rotor with up to 20 blades mounted on a horizontal shaft and a tail-vane. A tail-vane is to keep the rotor facing inot the wind by swiveling the machine.
Unlike the farm windmill, the modern windmill only use four blades to generate electricity and are able to operate at high rotor speeds.
The Jacobs three blade windmill, which was used widely between 1930 and 1960, could deliver about 1 kilowatt of power at a typical wind speed of 14 miles per hour.
Lately a different type of windmill has been used. It is a large horizontal two bladed wind turbine. These were first installed in Ohio in 1975. Another type of windmill which has a rotor diameter of 400 feet and a shaft height of 250 can produce 6,200 kilowatts of power. These were first places in oahu, Hawaii.
The term wind farm is used for a large number of wind mills, that are clusters at a site with a moderately constant wind, These are generally near mountain passes. With in the united states wind farms appear in New Hampshire, California, and Hawaii. At these sites the machine capacities range from 10 to 500 kilowatts. In 1984 the total energy output of all the wind farms in the united states exceeded 150 million kilowatt-hours.