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Wind turbine

Offshore wind farm using 5MW turbines REpower 5M in the North Sea off Belgium

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.

Contents


History

James Blyth's electricity generating wind turbine photographed in 1891 Windmills were used in Persia (present-day Iran) as early as 200 B.C.[1] The windwheel of Heron of Alexandria marks one of the first known instances of wind powering a machine in history.[2][3] 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.[4] 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.[5]

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.[6] 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.[7] Some months later American inventor Charles F Brush built the first automatically operated wind turbine for electricity production in Cleveland, Ohio.[7] Although Blyth's turbine was considered uneconomical in the United Kingdom[7] electricity generation by wind turbines was more cost effective in countries with widely scattered populations.[6]

The first automatically operated wind turbine, built in Cleveland in 1887 by Charles F. Brush. It was tall, weighed 4 tons (3.6 metric tonnes) and powered a 12kW generator.[8]

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 towers with four-bladed 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.[9] 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 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.[10] 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.

The first utility grid-connected wind turbine to operate in the UK was built by John Brown & Company in 1951 in the Orkney Islands.[7][11]

Resources

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 Meters". 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 meter or less at 50 meters altitude) to Class 7 (800 to 2000 watts per square meter). 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.[12]

Efficiency

Theoretical power captured by a wind turbine

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[13] (see: Betz' law).

Practical wind turbine power

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. Example of a horizontal axis machine Wind Turbine

Types

The three primary types:VAWT Savonius, HAWT towered; VAWT Darrieus as they appear in operation

Wind turbines can rotate about either a horizontal or a vertical axis, the former being both older and more common.[14]

Horizontal axis

Components of a horizontal axis wind turbine (gearbox, rotor shaft and brake assembly) being lifted into position turbine blade]] convoy passing through Edenfield, UK

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.[15]

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 , high efficiency, and low torque ripple, which contribute to good reliability. The blades are usually colored light gray to blend in with the clouds and range in length from or more. The tubular steel towers range from tall. The blades rotate at 10 to 22 revolutions per minute. At 22 rotations per minute the tip speed exceeds .[16][17] 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 design

A vertical axis Twisted Savonius type turbine.

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.[18]

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.[19][20]

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:[21] 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.[22]

Subtypes of the vertical axis design include:

Darrieus wind turbine
"Eggbeater" turbines, or Darrieus turbines, were named after the French inventor, Georges Darrieus.[23] They have good efficiency, but produce large torque ripple and cyclical stress on the tower, which contributes to poor reliability. They also generally require some external power source, or an additional Savonius rotor to start turning, because the starting torque is very low. The torque ripple is reduced by using three or more blades which results in greater solidity of the rotor. Solidity is measured by blade area divided by the rotor area. Newer Darrieus type turbines are not held up by guy-wires but have an external superstructure connected to the top bearing.[24]
Giromill
A subtype of Darrieus turbine with straight, as opposed to curved, blades. The cycloturbine variety has variable pitch to reduce the torque pulsation and is self-starting.[25] The advantages of variable pitch are: high starting torque; a wide, relatively flat torque curve; a lower blade speed ratio; a higher coefficient of performance; more efficient operation in turbulent winds; and a lower blade speed ratio which lowers blade bending stresses. Straight, V, or curved blades may be used.[26]
Savonius wind turbine
These are drag-type devices with two (or more) scoops that are used in anemometers, Flettner vents (commonly seen on bus and van roofs), and in some high-reliability low-efficiency power turbines. They are always self-starting if there are at least three scoops.
Twisted Savonius
Twisted Savonius is a modified savonius, with long helical scoops to provide smooth torque. This is often used as a rooftop windturbine and has even been adapted for ships.[27]

Rob Varnon. Derecktor converting boat into hybrid passenger ferry, Connecticut Post website, December 2, 2010. Retrieved April 25, 2012.

Design and construction

Components of a horizontal-axis wind turbine E-70 wind turbine]] rotor hub.

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:

  • The rotor component, which is approximately 20% of the wind turbine cost, includes the blades for converting wind energy to low speed rotational energy.
  • The generator component, which is approximately 34% of the wind turbine cost, includes the electrical generator, the control electronics, and most likely a gearbox (e.g. planetary gearbox,[28] adjustable-speed drive[29] or continuously variable transmission[30]) component for converting the low speed incoming rotation to high speed rotation suitable for generating electricity.
  • The structural support component, which is approximately 15% of the wind turbine cost, includes the tower and rotor yaw mechanism.[31]

A 1.5 MW wind turbine of a type frequently seen in the United States has a tower high. The rotor assembly (blades and hub) weighs . The nacelle, which contains the generator component, weighs . The concrete base for the tower is constructed using of reinforcing steel and contains of concrete. The base is in diameter and thick near the center.[32]

Unconventional designs

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.

Wind turbines which utilise the Magnus effect have been developed.[33]

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

Gorlov type]] vertical axis wind turbine in Bristol, England. Measuring 3m in diameter and 5m high, it has a nameplate rating of 6.5kW to the grid.

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.[34] 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.

Wind turbine spacing

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[35] conducted by Charles Meneveau of the Johns Hopkins University,[36] and Johan Meyers of Leuven University in Belgium, based on computer simulations[37] 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 neighboring turbines to move in the same direction as they approach one another.[38]

Accidents

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.[39] In 2010 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.[40] Several wind turbines also collapsed.

Severe wind turbine accidents have had relatively low impacts on the communities where the accidents occurred, compared with the widespread effects of accidents at nuclear and hydroelectric power plants.

Place Date Type Nacelle height Rotor dia. Year built Reason Damage and casualties
Ellenstedt, Germany October 19, 2002 [41]
Schneebergerhof, Germany December 20, 2003 Vestas V80 80 m [41]
Wasco, Oregon, USA August 26, 2007 [42] 1 worker killed, 1 injured
Stobart Mill, UK December 30, 2007 Vestas 1982 [43]
Hornslet, Denmark February 22, 2008 Nordtank NKT 600-180 44.5 m 43 m 1996 Brake failure[44][45]
Searsburg, Vermont, USA October 16, 2008 Zond Z-P40-FS 1997 Rotor blade collided with tower during strong wind and destroyed it[46]
Altona, New York, USA March 6, 2009 [47]
Fenner, New York, USA December 27, 2009 [48]
Kirtorf, Germany June 19, 2011 DeWind D-6 68.5 m 62 m 2001 [48]
Ayrshire, Scotland December 8, 2011 [49]

Records

Fuhrl nder Wind Turbine Laasow, the world's tallest wind turbine ole, the largest vertical axis wind turbine, in Cap-Chat, Quebec

Largest capacity
The Enercon E-126 has a rated capacity of 7.58 MW,[50] has an overall height of 198 m (650 ft), a diameter of 126 m (413 ft), and is the world's largest-capacity wind turbine since its introduction in 2007.[51] At least five companies are working on the development of a 10MW turbine.
Largest swept area
The turbine with the largest swept area is a prototype installed by Gamesa at Jaul n, Zaragoza, Spain in 2009. The G10X 4.5 MW has a rotor diameter of 128m.

[52]

Tallest
The tallest wind turbine is Fuhrl nder Wind Turbine Laasow. Its axis is 160 meters above ground and its rotor tips can reach a height of 205 meters. It is the only wind turbine in the world taller than 200 meters.[53]
Largest vertical-axis
Le Nordais wind farm in Cap-Chat, Quebec has a vertical axis wind turbine (VAWT) named ole, which is the world's largest at 110 m.[54] It has a nameplate capacity of 3.8MW.[55]
Most southerly
The turbines currently operating closest to the South Pole are three Enercon E-33 in Antarctica, powering New Zealand's Scott Base and the United States' McMurdo Station since December 2009[56][57] although a modified HR3 turbine from Northern Power Systems operated at the Amundsen-Scott South Pole Station in 1997 and 1998.[58] In March 2010 CITEDEF designed, built and installed a wind turbine in Argentine Marambio Base.[59]
Most productive
Four turbines at R nland wind farm in Denmark share the record for the most productive wind turbines, with each having generated 63.2 GWh by June 2010[60]
Highest-situated
The world's highest-situated wind turbine is made by DeWind installed by the Seawind Group and located in the Andes, Argentina around above sea level. The site uses a type D8.2 - 2000 kW / 50 Hz turbine. This turbine has a new drive train concept with a special torque converter (WinDrive) made by Voith and a synchronous generator. The WKA was put into operation in December 2007 and has supplied the Veladero mine of Barrick Gold with electricity since then.[61]
Largest floating wind turbine
The world's largest and also the first operational deep-water large-capacity floating wind turbine is the 2.3 MW Hywind currently operating offshore in 220-meter-deep water, southwest of Karm y, Norway. The turbine began operating in September 2009 and utilizes a Siemens 2.3 MW turbine[62][63]

See also

References

Further reading

  • Tony Burton, David Sharpe, Nick Jenkins, Ervin Bossanyi: Wind Energy Handbook, John Wiley & Sons, 1st edition (2001), ISBN 0-471-48997-2
  • Darrell, Dodge, Early History Through 1875, TeloNet Web Development, Copyright 1996 2001
  • David, Macaulay, New Way Things Work, Houghton Mifflin Company, Boston, Copyright 1994 1999, pg.41-42
  • Erich Hau Wind turbines: fundamentals, technologies, application, economics Birkh user, 2006 ISBN 3-540-24240-6 (preview on Google Books)
  • David Spera (ed,) Wind Turbine Technology: Fundamental Concepts in Wind Turbine Engineering, Second Edition (2009), ASME Press, ISBN #: 9780791802601

External links

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