Wind power
Wind Power
Wind is a clean and inexhaustible source of energy that has been used for many centuries to grind grain, pump water, propel sailing ships, and perform other work. The amount of wind power available, however, varies depending upon the weather. Thus the windmills used to pump water have been largely replaced by electrically powered pumps.
Today wind power is used primarily in remote areas, but recent interest in fuel conservation has spurred development of modern wind machines for generating electricity. In the 1980s, producing a kilowatt-hour of wind energy cost about four cents. Most wind machines have a horizontal shaft axis though some have a vertical axis. The best known horizontal axis machine in the United States is the American farm windmill frequently used to pump water. It consists of a rotor with up to 20 blades mounted on a horizontal shaft and a tail-vane to keep the rotor facing into the wind by swiveling the whole assembly. A set of gears and linkages connect to the pump rod, which moves vertically up and down. At a wind speed of 15 miles per hour (24 kilometers per hour), the pump delivers 10 gallons per minute (3.8 liters per minute) to a height of 100 feet (30 meters). Relatively inefficient, these pumps convert only a small portion of the wind stream energy to work as it passes through the rotor.
Unlike the traditional farm windmill, the modern efficient machines used to generate electricity have only one to four blades and operate at high rotor speeds. The blades look like the twisted airfoils of an airplane propeller. The Jacobs three-blade windmill, used widely between 1930 and 1960, could deliver about 1 kilowatt of power at a typical wind speed of 14 miles per hour (23 kilometers per hour).
More recently, large horizontal two-bladed wind turbines have been developed. The first of these, installed in Sandusky, Ohio, in 1975, had a 38-foot (12-meter) diameter rotor and was rated at 100 kilowatts. Another model, with a rotor diameter of 400 feet (122 meters) and a shaft height of 250 feet (76 meters), produces 6,200 kilowatts of power at a wind speed of 29 miles per hour (47 kilometers per hour). The first of these wind turbines was erected in Oahu, Hawaii.
First used in antiquity, vertical axis machines fell out of favor until the Savonius rotor arrived in the 1920s. It consists of S-shaped blades built from little more than an oil drum cut in half. An advanced version of this machine, coupled to a generator, produces 5 kilowatts of power in a 27 mile-per-hour (43 kilometer-per-hour) wind.
Based on a 1931 patent by Darreus, the Sandia Laboratories in New Mexico in 1974 built a device with two aluminum rotor blades. The blades were tied to the shaft at the top and bottom and bowed out in the middle in such a way that they resembled the blades on a food mixer. The machine produced 60 kilowatts of power in a 28 mile-per-hour (45 kilometer-per-hour) wind. Several models of this machine were built in 1980.
Wind farm is the term used for a large number of wind machines clustered at a site with persistent favorable winds, generally near mountain passes. Wind farms have been erected in New Hampshire, in the Tehachapi Mountains and at Altamont Pass in California, and at various sites in Hawaii. Machine capacities range from 10 to 500 kilowatts. In 1984 the total energy output of all wind farms in the United States exceeded 150 million kilowatt-hours.
The earliest vertical windmills were used in Persia more than 2,000 years ago for the grinding of grain. They subsequently spread throughout the Middle East and were brought to Europe by the Crusaders in the 13th century. Holland then improved the design of the horizontal axis machines. Windmills were used to drain marshes, to pump water, and to drive the machinery used for milling grain, sawing wood, and producing paper.
Windmills were adopted for pumping water in North America by the middle of the 19th century, and for generating electricity by the beginning of the 20th century. Their use declined drastically in the 1930s when inexpensive electricity reached the rural areas. A renewed interest in the use of wind power to generate electricity followed the energy crisis of the 1970s. A program of the United States Department of Energy encouraged the development of new machines, the construction of wind farms, and an evaluation of the economic effect of a large-scale use of wind power. (See also Waterpower)
Contributed by Gabor Karadi
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