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A wind turbine is a device that converts kinetic energy from the wind, also called wind energy, into mechanical energy; a process known as wind power. If the mechanical energy is used to produce electricity, the device may be called wind turbine or wind power plant. If the mechanical energy is used to drive machinery, such as for grinding grain or pumping water, the device is called a windmill or wind pump. Similarly, it may be called wind charger when it is used to charge batteries.
The result of over a millennium of windmill development and modern engineering, today's wind turbines are manufactured in a wide range of vertical and horizontal axis types. The smallest turbines are used for applications such as battery charging or auxiliary power on boats; while large grid-connected arrays of turbines are becoming an increasingly important source of wind power-produced commercial electricity.
Windmills were used in Persia (present-day Iran) as early as 200 B.C. The windwheel of Heron of Alexandria marks one of the first known instances of wind powering a machine in history. However, the first known practical windmills were built in Sistan, a region between Afghanistan and Iran, from the 7th century. These "Panemone" were vertical axle windmills, which had long vertical driveshafts with rectangular blades. Made of six to twelve sails covered in reed matting or cloth material, these windmills were used to grind grain or draw up water, and were used in the gristmilling and sugarcane industries.
Windmills first appeared in Europe during the middle ages. The first historical records of their use in England date to the 11th or 12th centuries and there are reports of German crusaders taking their windmill-making skills to Syria around 1190. By the 14th century, Dutch windmills were in use to drain areas of the Rhine delta.
The first electricity-generating wind turbine was a battery charging machine installed in July 1887 by Scottish academic James Blyth to light his holiday home in Marykirk, Scotland. Some months later American inventor Charles F Brush built the first automatically operated wind turbine for electricity production in Cleveland, Ohio. Although Blyth's turbine was considered uneconomical in the United Kingdom electricity generation by wind turbines was more cost effective in countries with widely scattered populations.
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 largest machines were on 24-metre (79 ft) towers with four-bladed 23-metre (75 ft) diameter rotors. By 1908 there were 72 wind-driven electric generators operating in the US from 5 kW to 25 kW. Around the time of World War I, American windmill makers were producing 100,000 farm windmills each year, mostly for water-pumping. By the 1930s, wind generators 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 the generators 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-metre (98 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. In the fall of 1941, the first megawatt-class wind turbine was synchronized to a utility grid in Vermont. The Smith-Putnam wind turbine only ran for 1,100 hours before suffering a critical failure. The unit was not repaired because of shortage of materials during the war.
As of 2012, Danish company Vestas is the world's biggest wind-turbine manufacturer.
A quantitative measure of the wind energy available at any location is called the Wind Power Density (WPD) It is a calculation of the mean annual power available per square meter of swept area of a turbine, and is tabulated for different heights above ground. Calculation of wind power density includes the effect of wind velocity and air density. Color-coded maps are prepared for a particular area described, for example, as "Mean Annual Power Density at 50 Metres". In the United States, the results of the above calculation are included in an index developed by the National Renewable Energy Laboratory and referred to as "NREL CLASS". The larger the WPD calculation, the higher it is rated by class. Classes range from Class 1 (200 watts per square metre or less at 50 m altitude) to Class 7 (800 to 2000 watts per square m). Commercial wind farms generally are sited in Class 3 or higher areas, although isolated points in an otherwise Class 1 area may be practical to exploit.
Total wind power could be captured only if the wind velocity is reduced to zero. In a realistic wind turbine this is impossible, as the captured air must also leave the turbine. A relation between the input and output wind velocity must be considered. Using the concept of stream tube, the maximal achievable extraction of wind power by a wind turbine is 59% of the total theoretical wind power (see: Betz' law).
Further insufficiencies, such as rotor blade friction and drag, gearbox losses, generator and converter losses, reduce the power delivered by a wind turbine. The basic relation that the turbine power is (approximately) proportional to the third power of velocity remains.
Wind turbines can rotate about either a horizontal or a vertical axis, the former being both older and more common.
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 an electrical generator.
Since a tower produces turbulence behind it, the turbine is usually positioned upwind of its supporting 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 forward into the wind a small amount.
Downwind machines have been built, despite the problem of turbulence (mast wake), 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 cyclical (that is repetitive) turbulence may lead to fatigue failures, most HAWTs are of upwind design.
Turbines used in wind farms for commercial production of electric power are usually three-bladed and pointed into the wind by computer-controlled motors. These have high tip speeds of over 320 km/h (200 mph), high efficiency, and low torque ripple, which contribute to good reliability. The blades are usually colored white for daytime visibility by aircraft and range in length from 20 to 40 metres (66 to 130 ft) or more. The tubular steel towers range from 60 to 90 metres (200 to 300 ft) tall. The blades rotate at 10 to 22 revolutions per minute. At 22 rotations per minute the tip speed exceeds 90 metres per second (300 ft/s). A gear box is commonly used for stepping up the speed of the generator, although designs may also 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 protective features to avoid damage at high wind speeds, by feathering the blades into the wind which ceases their rotation, supplemented by brakes.
Vertical-axis wind turbines (or VAWTs) have the main rotor shaft arranged vertically. Key advantages of this arrangement are that the turbine does not need to be pointed into the wind to be effective. This is an advantage on sites where the wind direction is highly variable, for example when integrated into buildings. The key disadvantages include the low rotational speed with the consequential higher torque and hence higher cost of the drive train, the inherently lower power coefficient, the 360 degree rotation of the aerofoil within the wind flow during each cycle and hence the highly dynamic loading on the blade, the pulsating torque generated by some rotor designs on the drive train, and the difficulty of modelling the wind flow accurately and hence the challenges of analysing and designing the rotor prior to fabricating a prototype.
With a vertical axis, the generator and gearbox can be placed near the ground, using a direct drive from the rotor assembly to the ground-based gearbox, hence improving accessibility for maintenance.
When a turbine is mounted on a rooftop, the building generally redirects wind over the roof and this can double the wind speed at the turbine. If the height of the rooftop mounted turbine tower is approximately 50% of the building height, this is near the optimum for maximum wind energy and minimum wind turbulence. It should be borne in mind that wind speeds within the built environment are generally much lower than at exposed rural sites, noise may be a concern and an existing house may not adequately resist the additional stress.
Another type of vertical axis is the Parallel turbine similar to the crossflow fan or centrifugal fan it uses the ground effect. Vertical axis turbines of this type have been tried for many years: a large unit producing up to 10 kW was built by Israeli wind pioneer Bruce Brill in 1980s: the device is mentioned in Dr. Moshe Dan Hirsch's 1990 report, which decided the Israeli energy department investments and support in the next 20 years. The Magenn WindKite blimp uses this configuration as well, chosen because of the ease of running.
Subtypes of the vertical axis design include:
Wind turbines are designed to exploit the wind energy that exists at a location. Aerodynamic modelling is used to determine the optimum tower height, control systems, number of blades and blade shape.
Wind turbines convert wind energy to electricity for distribution. Conventional horizontal axis turbines can be divided into three components:
A 1.5 MW wind turbine of a type frequently seen in the United States has a tower 80 metres (260 ft) high. The rotor assembly (blades and hub) weighs 48,000 pounds (22,000 kg). The nacelle, which contains the generator component, weighs 115,000 pounds (52,000 kg). The concrete base for the tower is constructed using 58,000 pounds (26,000 kg) of reinforcing steel and contains 250 cubic yards (190 m3) of concrete. The base is 50 ft (15 m) in diameter and 8 ft (2.4 m) thick near the center.
One E-66 wind turbine at Windpark Holtriem, Germany, carries an observation deck, open for visitors. Another turbine of the same type, with an observation deck, is located in Swaffham, England. Airborne wind turbines have been investigated many times but have yet to produce significant energy. Conceptually, wind turbines may also be used in conjunction with a large vertical solar updraft tower to extract the energy due to air heated by the sun.
The ram air turbine is a specialist form of small turbine that is fitted to some aircraft. When deployed, the RAT is spun by the airstream going past the aircraft and can provide power for the most essential systems if there is a loss of all on–board electrical power.
Small wind turbines may be used for a variety of applications including on- or off-grid residences, telecom towers, offshore platforms, rural schools and clinics, remote monitoring and other purposes that require energy where there is no electric grid, or where the grid is unstable. Small wind turbines may be as small as a fifty-watt generator for boat or caravan use. The U.S. Department of Energy's National Renewable Energy Laboratory (NREL) defines small wind turbines as those smaller than or equal to 100 kilowatts. Small units often have direct drive generators, direct current output, aeroelastic blades, lifetime bearings and use a vane to point into the wind.
Larger, more costly turbines generally have geared power trains, alternating current output, flaps and are actively pointed into the wind. Direct drive generators and aeroelastic blades for large wind turbines are being researched.
On most horizontal windturbine farms, a spacing of about 6-10 times the rotor diameter is often upheld. However, for large wind farms distances of about 15 rotor diameters should be more economically optimal, taking into account typical wind turbine and land costs. This conclusion has been reached by research conducted by Charles Meneveau of the Johns Hopkins University, and Johan Meyers of Leuven University in Belgium, based on computer simulations that take into account the detailed interactions among wind turbines (wakes) as well as with the entire turbulent atmospheric boundary layer. Moreover, recent research by John Dabiri of Caltech suggests that vertical wind turbines may be placed much more closely together so long as an alternating pattern of rotation is created allowing blades of neighbouring turbines to move in the same direction as they approach one another.
Several cases occurred where the housings of wind turbines caught fire. As housings are normally out of the range of standard fire extinguishing equipment, it is nearly impossible to extinguish such fires on older turbine units which lack fire suppression systems. In several cases one or more blades were damaged or torn away. In 2010 70 mph (110 km/h; 61 kn) storm winds damaged some blades, prompting blade removal and inspection of all 25 wind turbines in Campo Indian Reservation in the US State of California. Several wind turbines also collapsed.
|Place||Date||Type||Nacelle height||Rotor dia.||Year built||Reason||Damage and casualties|
|Ellenstedt, Germany||October 19, 2002|||
|Schneebergerhof, Germany||December 20, 2003||Vestas V80||80 m|||
|Wasco, Oregon, USA||August 25, 2007||Siemens||Human error: turbine restarted while blades were locked in maximum wind-resistance mode||1 worker killed, 1 injured|
|Stobart Mill, UK||December 30, 2007||Vestas||1982|||
|Hornslet, Denmark||February 22, 2008||Nordtank NKT 600-180||44.5 m||43 m||1996||Brake failure|
|Searsburg, Vermont, USA||October 16, 2008||Zond Z-P40-FS||1997||Rotor blade collided with tower during strong wind and destroyed it|
|Altona, New York, USA||March 6, 2009||Lightning likely |
|Fenner, New York, USA||December 27, 2009|||
|Kirtorf, Germany||June 19, 2011||DeWind D-6||68.5 m||62 m||2001|
|Ayrshire, Scotland||December 8, 2011|||
A list over the different models of wind turbines from the top 10 wind turbine manufacturers by annual market share (installed capacity):
|MW||Name||Manufactor||Marked date||Offshore||Swept area||Rotor diameter||Geared|
|7.0 MW||V164-7.0 MW||Vestas||2015 Q1||x|
|6.0 MW||SWT-6.0-154||Siemens Windpower||?||-|
|5.0 MW||G128-5.0 MW||Gamesa||?||x|
|4.5 MW||G136-4.5 MW||Gamesa||?||-|
|4.5 MW||G128-4.5 MW||Gamesa||?||-|
|3.6 MW||SWT-3.6-120||Siemens Windpower||?||-|
|3.6 MW||SWT-3.6-107||Siemens Windpower||?||-|
|3.0 MW||SWT-3.0-113||Siemens Windpower||?||-|
|3.0 MW||SWT-3.0-108||Siemens Windpower||?||-|
|3.0 MW||SWT-3.0-101||Siemens Windpower||?||-|
|3.0 MW||V112-3.0 MW||Vestas||?||-|
|3.0 MW||V112-3.0 MW Offshore||Vestas||?||x|
|3.0 MW||V90-3.0 MW||Vestas||?||-|
|3.0 MW||V90-3.0 MW Offshore||Vestas||?||x|
|3.0 MW||E-82 E3||Enercon||?||-|
|2.6 MW||V100-2.6 MW||Vestas||?||-|
|2.3 MW||E-82 E2||Enercon||?||-|
|2.3 MW||SWT-2.3-113||Siemens Windpower||?||-|
|2.3 MW||SWT-2.3-108||Siemens Windpower||?||-|
|2.3 MW||SWT-2.3-101||Siemens Windpower||?||-|
|2.3 MW||SWT-2.3-93||Siemens Windpower||?||-|
|2.3 MW||SWT-2.3-82||Siemens Windpower||?||-|
|2.25 MW||S88 MARK II DFIG 2.25 MW||Suzlon||?||-|
|2.1 MW||S88-2.1 MW||Suzlon||?||-|
|2.0 MW||E-82 E2||Enercon||?||-|
|2.0 MW||G114-2.0 MW||Gamesa||?||-|
|2.0 MW||G97-2.0 MW||Gamesa||?||-|
|2.0 MW||G90-2.0 MW||Gamesa||?||-|
|2.0 MW||G87-2.0 MW||Gamesa||?||-|
|2.0 MW||G80-2.0 MW||Gamesa||?||-|
|1.8/2.0 MW||V100-1.8/2.0 MW||Vestas||?||-|
|1.8 MW||V100-1.8 MW||Vestas||?||-|
|1.8/2.0 MW||V90-1.8/2.0 MW||Vestas||?||-|
|2.0 MW||V80-2.0 MW||Vestas||?||-|
|1.5 MW||S82-1.5 MW||Suzlon||?||-|
|1.25 MW||S66-1.25 MW||Suzlon||?||-|
|1.25 MW||S64-1.25 MW||Suzlon||?||-|
|0.85 MW||G58-0.85 MW||Gamesa||?||-|
|0.85 MW||G52-0.85 MW||Gamesa||?||-|
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