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Wind turbines near Aalborg, Denmark. For scale, a standard doorway can be seen at the base of the pylon.A wind turbine is a rotating machine that 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) or wind energy converter (WEC).
This article discusses electric power generation machinery. Windmill discusses machines used for grain-grinding, water pumping, etc. The article on wind power describes turbine placement, economics, public concerns, and controversy. The wind energy section of that article describes the distribution of wind energy over time, and how that affects wind-turbine design. See environmental concerns with electricity generation for discussion of environmental problems with wind-energy production.
2 Potential turbine power
3 Types of wind turbines
3.1 Horizontal axis
3.1.1 HAWT Subtypes
3.1.2 HAWT advantages
3.1.3 HAWT disadvantages
3.1.4 Cyclic stresses and vibration
3.1.5 VAWT subtypes
3.1.6 VAWT advantages
3.1.7 VAWT disadvantages
5 Turbine design and construction
6 Special wind turbines
7 Small wind turbines
7.1 DIY Wind turbines
8 Record-holding turbines
9 Health concerns
10 See also
12 Further reading
13 External links
13.1 General wind turbine information
13.2 Wind turbine manufacturers
Main article: History of wind power
The world’s first megawatt wind turbine at Castleton, VermontWind machines were used for grinding grain in Persia as early as 200 B.C. This type of machine was introduced into the Roman Empire by 250 A.D. By the 14th century Dutch windmills were in use to drain areas of the Rhine River delta. In Denmark by 1900 there were about 2500 windmills for mechanical loads such as pumps and mills, producing an estimated combined peak power of about 30 MW. The first windmill for electricity production was built in Cleveland, Ohio by Charles F Brush in 1888, and in 1908 there were 72 wind-driven electric generators from 5 kW to 25 kW. The largest machines were on 24 m (79 ft) towers with four-bladed 23 m (75 ft) diameter rotors. Around the time of World War I, American windmill makers were producing 100,000 farm windmills each year, most for water-pumping. By the 1930s windmills for electricity were common on farms, mostly in the United States where distribution systems had not yet been installed. In this period, high-tensile steel was cheap, and windmills were placed atop prefabricated open steel lattice towers.
A forerunner of modern horizontal-axis wind generators was in service at Yalta, USSR in 1931. This was a 100 kW generator on a 30 m (100 ft) tower, connected to the local 6.3 kV distribution system. It was reported to have an annual capacity factor of 32 per cent, not much different from current wind machines.
The very first electricity generating windmill operated in the UK was a battery charging machine installed in 1887 by James Blyth in Scotland. The first utility grid-connected wind turbine operated in the UK was built by the John Brown Company in 1954 in the Orkney Islands. It had an 18 metre diameter, three-bladed rotor and a rated output of 100 kW.
 Potential turbine power
Main article: Wind turbine design
Wind Turbine Power CoefficentThe amount of power transferred to a wind turbine is directly proportional to the density of the air, the area swept out by the rotor, and the cube of the wind speed.
The usable power P available in the wind is given by:
where P = power in watts, ? = an efficiency factor determined by the design of the turbine, ? = mass density of air in kilograms per cubic meter, r = radius of the wind turbine in meters, and v = velocity of the air in meters per second.
As the wind turbine extracts energy from the air flow, the air is slowed down, which causes it to spread out. Albert Betz, a German physicist, determined in 1919 (see Betz’ law) that a wind turbine can extract at most 59% of the energy that would otherwise flow through the turbine’s cross section, that is ? Can never be higher than 0.59 in the above equation. The Betz limit applies regardless of the design of the turbine.
This equation shows the effects of the mass rate of flow of air traveling through the turbine, and the energy of each unit mass of air flow due to its velocity. As an example, on a cool 15 °C (59 °F) day at sea level, air density is 1.225 kilograms per cubic metre. An 8 m/s (28.8 km/h or 18 mi/h) breeze blowing through a 100 meter diameter rotor would move almost 77,000 kilograms of air per second through the swept area. The total power of the example breeze through a 100 meter diameter rotor would be about 2.5 megawatts. Betz’ law states that no more than 1.5 megawatts could be extracted.
 Types of wind turbines
Wind turbines can be separated into two types based by the axis in which the turbine rotates. Turbines that rotate around a horizontal axis are more common. Vertical-axis turbines are less frequently used.
 Horizontal axis
Horizontal-axis wind turbines (HAWT) have the main rotor shaft and electrical generator at the top of a tower, and must be pointed into the wind. Small turbines are pointed by a simple wind vane, while large turbines generally use a wind sensor coupled with a servo motor. Most have a gearbox, which turns the slow rotation of the blades into a quicker rotation that is more suitable to drive a generator.
Since a tower produces turbulence behind it, the turbine is usually pointed upwind of the tower. Turbine blades are made stiff to prevent the blades from being pushed into the tower by high winds. Additionally, the blades are placed a considerable distance in front of the tower and are sometimes tilted up a small amount.
Downwind machines have been built, despite the problem of turbulence, because they don’t need an additional mechanism for keeping them in line with the wind, and because in high winds, the blades can be allowed to bend which reduces their swept area and thus their wind resistance. Since turbulence leads to fatigue failures, and reliability is so important, most HAWTs are upwind machines.
 HAWT Subtypes
Doesburger windmill, Ede, The Netherlands.There are several types of HAWT:
These squat structures, typically (at-least) four-bladed, usually with wooden shutters or fabric sails, were developed in Europe. These windmills were pointed into the wind manually or via a tail-fan and were typically used to grind grain. In the Netherlands they were also used to pump water from low-lying land, and were instrumental in keeping its polders dry. Windmills were also located throughout the USA, especially in the Northeastern region.
Modern Rural Windmills
The Eclipse windmill factory was set up around 1866 in Beloit. Wisconsin and soon became a huge success building mills for farm water pumping and railroad tank filling. Other firms like Star, Dempster, and Aeromotor also entered the market. Hundreds of thousands of these mills were produced before rural electrification and small numbers continue to be made. They typically had many blades, operated at tip speed ratios (defined below) not better than one, and had good starting torque. Some had small direct-current generators used to charge storage batteries, to provide a few lights, or to operate a radio receiver. The American rural electrification connected many farms to centrally-generated power and replaced individual windmills as a primary source of farm power by the 1950s. They were also produced in other countries like South Africa and Australia (where an American design was copied in 1876). Such devices are still used in locations where it is too costly to bring in commercial power.
Water pumping rural windmill in Germany.In Schiedam, the Netherlands, a traditional style windmill (the Noletmolen) was built in 2005 to generate electricity. The mill is one of the tallest Tower mills in the world, being some 42.5 metres (139 ft) tall.
Common modern wind turbines
Turbines used in wind farms for commercial production of electric power are usually three-bladed and pointed into the wind by computer-controlled motors. This type is produced by Danish and other manufacturers. These have high tip speeds of up to six times the wind speed, high efficiency, and low torque ripple which contributes to good reliability. The blades are usually colored light gray to blend in with the clouds and range in length from 20 to 40 metres (65 to 130 ft) or more. The tubular steel towers range from about 200 to 300 feet (60 to 90 metres) high. The blades rotate at 10-22 revolutions per minute. A gear box is commonly used to step up the speed of the generator, though there are also designs that use direct drive of an annular generator. Some models operate at constant speed, but more energy can be collected by variable-speed turbines which use a solid-state power converter to interface to the transmission system. All turbines are equipped with high wind shut down features to avoid over speed damage.
 HAWT advantages
Blades are to the side of the turbine’s center of gravity, helping stability.
Ability to wing warp, which gives the turbine blades the best angle of attack. Allowing the angle of attack to be remotely adjusted gives greater control, so the turbine collects the maximum amount of wind energy for the time of day and season.