Wind turbine

This article discusses wind-powered electrical generators. See windmill for wind-powered machinery used to grind grain or pump water.

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Wind farm in the North Sea off Belgium

A wind turbine is a rotary device that extracts energy from the wind. If the mechanical energy is used directly by machinery, such as for pumping water, cutting lumber or grinding stones, the machine is called a windmill. If the mechanical energy is instead converted to electricity, the machine is called a wind generator, wind turbine, wind turbine generator (WTG), wind power unit (WPU), wind energy converter (WEC), or aerogenerator.

History

Main article: History of wind power

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Wind turbines near Aalborg, Denmark

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The world's first automatically operated wind turbine was built in Cleveland in 1888 by Charles F. Brush. It was 60 feet tall, weighed four tons and had a 12kW turbine.

Wind machines were used in Persia 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 practical windmills were built in Sistan, a region between Afghanistan and Iran, from the 7th century. These were vertical axle windmills, which had long vertical driveshafts with rectangle-shaped blades. Made of six to twelve sails covered in reed matting or cloth material, these windmills were used to grind corn and draw up water, and were used in the gristmilling and sugarcane industries.

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 known electricity generating windmill operated, was a battery charging machine installed in 1887 by James Blyth in Scotland. The first windmill for electricity production in the United States 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, mostly 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. 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 1100 hours. Due to war time material shortages the unit was not repaired.

The first utility grid-connected wind turbine operated in the UK was built by John Brown & Company in 1954 in the Orkney Islands. It had an 18 meter diameter, three-bladed rotor and a rated output of 100 kW.

Resources

Main article: Wind power

Wind turbines in locations with constantly high wind speeds bring best return on investment. With a wind resource assessment it is possible to estimate the amount of energy the wind turbine will produce.

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 U.S. National Renewable Energy Lab 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/square meter or less at 50 meters altitude) to Class 7 (800 to 2000 watts/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.

Types

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

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The 3 primary types of HAWT and VAWT as they appear in operation.

Horizontal axis

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Components of a horizontal axis wind turbine (gearbox, rotor shaft and brake assembly) being lifted into position

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 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 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 cyclic (that is repetitive) turbulence may lead to fatigue failures most HAWTs are upwind machines.

Subtypes

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Doesburger windmill, Ede, The Netherlands.

12th-century windmills

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.

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.

19th-century windmills

The Eclipse windmill factory was set up around 1866 in Beloit, Wisconsin and soon became successful building mills for pumping water on farms and for filling railroad tanks. 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 not better than one, and had good starting torque. Some had small direct-current generators used to charge storage batteries, to provide power to 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.

Modern wind turbines

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Three bladed wind turbine

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 miles per hour), 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 20 to 40 metres (65 to 130 ft) or more. The tubular steel towers range from 60 to 90 metres (200 to 300 feet) tall. The blades rotate at 10-22 revolutions per minute. At 22 rotations per minute the tip speed exceeds 300 ft per second. A gear box is commonly used to step 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 shut-down features to avoid damage at high wind speeds.

Advantages

• Variable blade pitch, which gives the turbine blades the optimum 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.
• The tall tower base allows access to stronger wind in sites with wind shear. In some wind shear sites, the wind speed can increase by 20% and the power output by 34% for every 10 metres in elevation.
• High efficiency, since the blades always move perpendicular to the wind, receiving power through the whole rotation. In contrast, all vertical axis wind turbines, and most proposed airborne wind turbine designs, involve various types of reciprocating actions, requiring airfoil surfaces to backtrack against the wind for part of the cycle. Backtracking against the wind leads to inherently lower efficiency.
• The face of a horizontal axis blade is struck by the wind at a consistent angle regardless of the position in its rotation. This results in a consistent lateral wind loading over the course of a rotation, reducing vibration and audible noise coupled to the tower or mount.

Disadvantages

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Turbine blade convoy passing through Edenfield in the UK

• The tall towers and blades up to 45 meters long are difficult to transport. Transportation can now amount to 20% of equipment costs.
• Tall HAWTs are difficult to install, needing very tall and expensive cranes and skilled operators.
• Massive tower construction is required to support the heavy blades, gearbox, and generator.
• Reflections from tall HAWTs may affect side lobes of radar installations creating signal clutter, although filtering can suppress it.
• Their height makes them obtrusively visible across large areas, disrupting the appearance of the landscape and sometimes creating local opposition.
• Downwind variants suffer from fatigue and structural failure caused by turbulence when a blade passes through the tower's wind shadow (for this reason, the majority of HAWTs use an upwind design, with the rotor facing the wind in front of the tower).
• HAWTs require an additional yaw control mechanism to turn the blades and nacelle toward the wind.
• In order to minimize fatigue loads due to wake turbulence, wind turbines are usually sited a distance of 5 rotor diameters away from each other, but the spacing depends on the manufacturer and the turbine model.

Cyclic stresses and vibration

Cyclic stresses fatigue the blade, axle and bearing resulting in material failures that were a major cause of turbine failure for many years. Because wind velocity often increases at higher altitudes, the backward force and torque on a horizontal-axis wind turbine (HAWT) blade peaks as it turns through the highest point in its circle. The tower hinders the airflow at the lowest point in the circle, which produces a local dip in force and torque. These effects produce a cyclic twist on the main bearings of a HAWT. The combined twist is worst in machines with an even number of blades, where one is straight up when another is straight down. To improve reliability, teetering hubs have been used which allow the main shaft to rock through a few degrees, so that the main bearings do not have to resist the torque peaks.

The rotating blades of a wind turbine act like a gyroscope. As it pivots along its vertical axis to face the wind, gyroscopic precession tries to twist the turbine disc along its horizontal axis. For each blade on a wind generator's turbine, precessive force is at a minimum when the blade is horizontal and at a maximum when the blade is vertical. The cyclic loading affects the design of the mechanical elements, structure, and foundation of the wind turbine.

Vertical axis design

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.

With a vertical axis, the generator and gearbox can be placed near the ground, so the tower doesn't need to support it, and it is more accessible for maintenance. Drawbacks are that some designs produce pulsating torque.

It is difficult to mount vertical-axis turbines on towers, meaning they are often installed nearer to the base on which they rest, such as the ground or a building rooftop. The wind speed is slower at a lower altitude, so less wind energy is available for a given size turbine. Air flow near the ground and other objects can create turbulent flow, which can introduce issues of vibration, including noise and bearing wear which may increase the maintenance or shorten the service life. However, 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.

Subtypes

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30 m Darrieus wind turbine in the Magdalen Islands

Darrieus wind turbine 

"Eggbeater" turbines, or Darrieus turbines, were named after the French inventor, Georges Darrieus. 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 a higher solidity for 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.

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A helical twisted VAWT.

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. 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.

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Windmill with rotating sails

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. They sometimes have long helical scoops to give a smooth torque.

Advantages

• A massive tower structure is less frequently used, as VAWTs are more frequently mounted with the lower bearing mounted near the ground.
• Designs without yaw mechanisms are possible with fixed pitch rotor designs.
• The generator of a VAWT can be located nearer the ground, making it easier to maintain the moving parts.
• VAWTs have lower wind startup speeds than HAWTs. Typically, they start creating electricity at 6 m.p.h. (10 km/h).
• VAWTs may be built at locations where taller structures are prohibited.
• VAWTs situated close to the ground can take advantage of locations where mesas, hilltops, ridgelines, and passes funnel the wind and increase wind velocity.
• VAWTs may have a lower noise signature.

Disadvantages

• A VAWT that uses guy-wires to hold it in place puts stress on the bottom bearing as all the weight of the rotor is on the bearing. Guy wires attached to the top bearing increase downward thrust in wind gusts. Solving this problem requires a superstructure to hold a top bearing in place to eliminate the downward thrusts of gust events in guy wired models.
• The stress in each blade due to wind loading changes sign twice during each revolution as the apparent wind direction moves through 360 degrees. This reversal of the stress increases the likelihood of blade failure by fatigue.
• While VAWTs' components are located on the ground, they are also located under the weight of the structure above it, which can make changing out parts very difficult if the structure is not designed properly.
• Having rotors located close to the ground where wind speeds are lower due to the ground's surface drag, VAWTs may not produce as much energy at a given site as a HAWT with the same footprint or height.

Turbine design and construction

Main article: Wind turbine design

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Components of a horizontal-axis wind turbine

Wind turbines are designed to exploit the wind energy that exists at a location. Aerodynamic modeling 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 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.

Unconventional wind turbines

Main article: Unconventional wind turbines

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.[1]

Small wind turbines

Main article: Small wind turbine

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A small wind turbine being used at the Riverina Environmental Education Centre near Wagga Wagga, New South Wales, Australia

Small wind turbines may be as small as a fifty-watt generator for boat or caravan use. 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.

Record-holding turbines

Largest capacity

The Enercon E-126 has a rated capacity of 7.58 MW , 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.

At least four companies are working on the development of a 10MW turbine:

• American Superconductor
• Arup are developing a 10 MW VAWT, the Aerogenerator X
• Clipper Windpower are developing the Britannia 10 MW HAWT
• Sway announced the proposed development of a prototype 10 MW wind turbine with a height of 162.5 m (533 ft) and a rotor diameter of 145 m (475 ft).

Largest swept area

The turbine with the largest swept area is a prototype installed by Gamesa at Jaulín, Zaragoza, Spain in 2009. The G10X – 4.5 MW has a rotor diameter of 128m.

Tallest

The tallest wind turbine is Fuhrländer Wind Turbine Laasow. Its axis is 160 metres above ground and its rotor tips can reach a height of 205 metres. It is the only wind turbine taller than 200 metres in the world.

Largest vertical-axis

Le Nordais wind farm in Cap-Chat, Quebec has a vertical axis wind turbine (VAWT) named Éole, which is the world's largest at 110 m. It has a nameplate capacity of 3.8MW.

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 although a modified HR3 turbine from Northern Power Systems operated at the Amundsen-Scott South Pole Station in 1997 and 1998. In March 2010 CITEDEF designed, built and installed a wind turbine in Argentine Marambio Base.

Most productive

Four turbines at Rønland wind farm in Denmark share the record for the most productive wind turbines, with each having generated 63.2 GWh by June 2010

Highest-situated

The world's highest-situated wind turbine is made by DeWind and located in the Andes, Argentina around 4,100 metres (13,500 ft) 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.

Gallery of record-holders

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  Enercon E-126, highest rated capacity

• Windkraftanlage Laasow.jpg

  Fuhrländer Wind Turbine Laasow, world's tallest

• Quebecturbine.JPG

  Éole, the largest vertical axis wind turbine, in Cap-Chat, Quebec

• Veladero 01.png

  Highest-situated wind turbine, at the Veladero mine in San Juan Province, Argentina

• Rønland Windpark.jpg

  Rønland, most productive turbines, in Denmark

See also

• Airborne wind turbine
• American Wind Energy Association
• Atmospheric icing
• Darrieus wind turbine
• Environmental concerns with electricity generation
• Environmental effects of wind power
• Electrical generator
• Éolienne Bollée
• Floating wind turbine
• GE 1.5 MW wind turbine
• Green energy
• High altitude wind power
• Hybrid power source

• List of wind turbine manufacturers
• List of wind turbines
• Maglev wind turbine
• Microgeneration
• Renewable energy
• Savonius wind turbine
• Thomas O. Perry
• Windmill
• Wind power
• Windpump
• Wind turbine design
• Wind turbines (UK domestic)

Further reading

• BBC News,"Wind farms 'must take root in UK, BBC News, 19 May 2005
• 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 Birkhäuser, 2006 ISBN 3540242406 (preview on Google Books)
• Wind Power Myths vs. Facts, American Wind Energy Association
• Germany Inaugurates 5MW Wind Turbine Prototype, RenewableEnergyWorld.com website.

External links

• Photo journal and tutorial for 1.5kw residential wind turbine
• Time-lapse video of wind turbine installation at the WMRA Deer Island Wastewater Treatment Plant
• Wind Projects
• Guided tour on wind energy
• Wind Energy Technology World Wind Energy Association
• Wind turbine simulation, National Geographic
• D.I.Y. Wind Generator Plans Reviewed
• November 3, 2009 "Power Paths" Coal to Wind Turbines on PBS's Independent Lens [2]
• Building a robust path for wind turbine engineering Movie and information on wind turbine engineering

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