Wind turbine

(CC) Photo: Natalie Meister
Wind farm turbines in San Gorgonia Pass, Southern California

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 the various types of wind turbines used for electric power generation, as well as the relative advantages and disadvantages of each specific type. (See Renewable energy for a discussion of the current (2010) developmental status of wind power as a renewable energy source.)

History

(CC) Photo: Michiel Verbeek
Windmill in Budel, North Brobant, The Netherlands (circa perhaps 12th century).

Wind 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

The 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:

$P={\begin{matrix}{\frac {1}{2}}\end{matrix}}\alpha \rho \pi r^{2}v^{3}$

where P = power in watts, α = an efficiency factor determined by the design of the turbine, ρ = the mass density of air in kilograms per cubic meter, π = pi = 3.14159, 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

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. 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.

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 139.434 Feet (42.5 metres) 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 65–130 feet (19.8–39.6 metres) or more. The tubular steel towers range from about 200–300 feet (61–91.4 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.
• Ability to pitch the rotor blades in a storm, to minimize damage.
• Tall tower allows access to stronger wind in sites with wind shear. In some wind shear sites, every ten meters up, the wind speed can increase by 20% and the power output by 34%.

HAWT disadvantages

• HAWTs have difficulty operating in near ground, turbulent winds.
• The tall towers and long blades up to 90 meters long are difficult to transport on the sea and on land. Transportation can now cost 20% of equipment costs.
• Tall HAWTs are difficult to install, needing very tall and expensive cranes and skilled operators.
• The FAA has raised concerns about tall HAWTs effects on radar near Air Force bases.
• Their height can create local opposition based on impacts to viewsheds.
• Downwind variants suffer from fatigue and structural failure caused by turbulence.

Cyclic stresses and vibration

Cyclic stresses fatigue the blade, axle and bearing material failures 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.

When the turbine turns to face the wind, the rotating blades act like a gyroscope. As it pivots, gyroscopic precession tries to twist the turbine into a forward or backward somersault. 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. This cyclic twisting can quickly fatigue and crack the blade roots, hub and axle of the turbines.

Vertical axis

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. VAWTs can utilize winds from varying directions.

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. Drag may be created when the blade rotates into the wind.

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 generally 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 live. However, when a turbine is mounted on a rooftop, the building generally redirects wind over the roof and this often doubles 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.

VAWT subtypes

(CC) Photo: Guilliaume Paumier
World's tallest Darrieus wind turbine, Gaspé peninsula, Quebec, Canada.

Darrieus wind turbine

"Eggbeater" turbines. They have good efficiency, but produce large torque ripple and cyclic stress on the tower, which contributes to poor reliability. Also, they 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 3 or more blades which results in a higher solidity for the rotor. Solidity is measured by blade area over the rotor area. Newer Darrieus type turbines are not held up by guy wires but have an external superstructure connected to the top bearing.

Gorlov helical turbine

Essentially a Darrieus turbine in a helical configuration. Patented in 2001. It solves most of the problems of the Darrieus rotor. It is self-starting, has lower torque ripple, low vibration and noise, and low cyclic stress. High reliability is expected from tested or matured designs. At least two wind turbine products are on the market as of 2002, including the Turby wind turbine and the Quietrevolution wind turbine. It is up to 35% efficient, which is competitive with the most efficient VAWT's.

Giromill

A subtype of Darrieus turbine with straight, as opposed to curved, blades. The cycloturbine variety have variable pitch to reduce the torque pulsation and are self-starting [1]. 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. Recently , this type of turbine has been advanced by former Russian rocket scientists who claim to have increased the efficiency of the VAWT up to 38% . A company , SRC Vertical Ltd.[2] has been formed , and has begun selling the new turbine .

Savonius wind turbine

These are drag-type devices with two- (or more) scoops that are used in anemometers, the 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. The Banesh rotor and especially the Rahai rotor improve efficiency with blades shaped to produce significant lift as well as drag. A new variety uses sails that can open or close with changes in wind speed.

VAWT advantages

• Can be easier to maintain if the moving parts are located near the ground.
• As the rotor blades are vertical, a yaw device is not needed, reducing cost.
• VAWTs have a higher airfoil pitch angle, giving improved aerodynamics while decreasing drag at low and high pressures.
• Straight bladed VAWT designs with a square or rectangular crossection have a larger swept area for a given diameter than the circular swept area of HAWTs.
• Mesas, hilltops, ridgelines and passes can have faster winds near the ground because the wind is forced up a slope or funnelled into a pass and into the path of VAWTs situated close to the ground.
• Low height useful where laws do not permit structures to be placed high.
• Does not need a free standing tower so is much less expensive and stronger in high winds that are close to the ground.
• Usually have a lower Tip-Speed ratio so less likely to break in high winds.
• Does not need to turn to face the wind if the wind direction changes making them ideal in turbulent wind conditions.
• They can potentially be built to a far larger size than HAWT's , for instance floating VAWT's hundreds of meters in diameter where the entire vessel rotates , can eliminate the need for a large and expensive bearing.
• There may be a height limitation to how tall a vertical wind turbine can be built and how much sweep area it can have. However, this can be overcome by connecting a multiple number of turbines together in a triangular pattern with bracing across the top of the structure . Thus reducing the need for such strong vertical support, and allowing the turbine blades to be made much longer.

VAWT disadvantages

• Most VAWTs produce energy at only 50% of the efficiency of HAWTs in large part because of the additional drag that they have as their blades rotate into the wind. This can be overcome by using structures to funnel more and align the wind into the rotor (e.g. "stators" on early Windstar turbines) or the "vortex" effect of placing straight bladed VAWTs closely together (e.g. Patent # 6784566).
• Most VAWTS need to be installed on a relatively flat piece of land and some sites could be too steep for them but are still usable by HAWTs.
• Most VAWTs have low starting torque, and may require energy to start the turning.
• 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.
• While VAWTs' parts are located on the ground, they are also located under the weight of the structure above it, which can make changing out parts near impossible without dismantling the structure if not designed properly.

Turbine design and construction

Wind turbines can also be classified by the location in which they are to be used, namely onshore, offshore, or even aerial wind turbines. Each of these has unique design characteristics and are designed to exploit the wind energy that exists at a specific location. Aerodynamic modeling is used to determine the optimum tower height, control systems, number of blades, and blade shape.

Virtually all modern wind turbines convert wind energy to electricity for energy distribution. The turbine 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 rotational energy to electricity. The structural support component, which is approximately 15% of the wind turbine cost, includes the tower and rotor pointing mechanism.

Special wind turbines

One E-66 wind turbine at Windpark Holtriem, Germany carries an observation deck, open for visitors to see. Another turbine of the same type, with an observation deck, is located in Swaffham, England.

A series of lighter-than-air wind turbines are in development in Canada by Magenn Power. They deliver power to the ground by a tether system.

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.

Variable pitch wind turbines are another special (yet low-cost) design. Designs such as the Jacobs are said to be inexpensive, highly efficient and usable in diy-construction.

Small wind turbines

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.

A small wind turbine can be installed on a roof. Installation issues then include the strength of the roof, vibration, and the turbulence caused by the roof ledge. A small-scale, rooftop wind turbine is said to be able to generate power from 10% to up to 25% of the electricity requirements of a regular house.

Small scale turbines for residential-scale use are available that are approximately 7 feet (2 m) to 25 Feet (7.62 metres) in diameter and produce electricity at a rate of 900 watts to 10,000 watts at their tested wind speed. Some units are designed to be very lightweight, e.g. 16 kilograms (35 lb), allowing rapid response to wind gusts typical of urban settings and easy mounting much like a television antenna. It is claimed that they are inaudible even a few feet under the turbine. Dynamic braking regulates the speed by dumping excess energy, so that the turbine continues to produce electricity even in high winds. The dynamic braking resistor may be installed inside the building to provide heat (during high winds when more heat is lost by the building, while more heat is also produced by the braking resistor). The location makes low voltage (around 12 volt) distribution practical.

In the United States, residential wind turbines with outputs of 2-10 kW, typically cost between $12,000 and $55,000 installed ($6 per watt), although there are incentives and rebates available in 19 states that can reduce the purchase price for homeowners by up to 50 percent, to ($3 per watt). The US manufacturer "Southwest Windpower," estimates a turbine to pay for itself in energy savings in 5 to 10 years.

The American Wind Energy Association has released several studies on the small wind turbine market in the U.S. and abroad, showing that the U.S. continues to dominate the Small Wind industry.[3] According to another organization, the World Wind Energy Association, it is difficult to assess the total number or capacity of small-scaled wind turbines, but in China alone, there are roughly 300,000 small-scale wind turbines generating electricity.

The dominant models on the market, especially in the United States, are horizontal-axis wind turbines (HAWT).

There have been a number of recent developments of mini-turbines which could be adapted to home use, including:

• The AeroTecture vertical-axis turbine
• The AeroVironment Architectural Wind Project
• The piezoelectric windmill project
• The Swift home wind turbine. The Swift project peaked in 2004 and has had some implementation difficulties while promising to be a low-noise/safe roof-mount/low-cost alternative
• The Motorwave micro-wind turbine

DIY Wind turbines

Some hobbyists have built wind turbines from kits, sourced components, or from scratch. DIY-wind turbine construction has been made popular by magazines such as OtherPower and Home Power, websites as Instructables, and by TV-series as Jericho and The Time Machine. DIY-made wind turbines are usually smaller (rooftop) turbines of ~ 1kW or less. These small wind turbines are usually tilt-up or fixed/guyed towers. However, larger (freestanding) and more powerful windtubines are sometimes built as well. The latter can generate power of up to 10 kW. In addition, people are also showing interest in DIY-construction of wind turbines with special designs as the Savonius, Panemone, wind turbine to boost power generation. When compared to similar sized commercial wind turbines, these DIY turbines tend to be cheaper. Through the internet, the community is now able to obtain plans to construct DIY-wind turbines. and there is a growing trend toward building them for domestic requirements. The DIY-wind turbines are now being used both in developed countries and in developing countries, to help power residences and small businesses. At present, organizations as Practical Action have designed DIY wind turbines that can be easily built by communities in developing nations and are supplying concrete documents on how to do so. To assist people in the developing countries, and hobbyists alike, several projects have been open-sourced (e.g. the Jua Kali wind turbine, Hugh Piggot's wind turbine, ForceField Wind Turbine, etc.).

Record-holding turbines

The world's largest turbines are manufactured by the Northern German companies Enercon and REpower. The Enercon E-126 delivers up to 6 MW, has an overall height of 198 m (650 ft) and a diameter of 126 meters (413 ft). The Repower 5M delivers up to 5 MW, has an overall height of 183 m (600 ft) and has a diameter of 126 m (413 ft).

The turbine closest to the North Pole is a Nordex N-80 in Havoygelvan near Hammerfest, Norway. The ones closest to the South Pole are two Enarcan E-30 in Antarctica, used to power the Australian Research Division's Mawson Station.

Health concerns

Researcher and pediatrician Nina Pierpont has allegedly discovered an illness she calls 'wind turbine syndrome,' which may be affecting those living near wind turbines. These residents report symptoms such as: sleep problems, headaches, exhaustion, irritability, and dizziness, possibly due to the constant sound. Several researchers suggest that any loud wind turbines be installed a mile or more away from residential areas. Audiologist Kenneth Smith, a fellow with the American Academy of Audiology, says low-frequency sounds can cause health disorders, but he cautions that much more study needs to be done on wind turbines before drawing conclusions. "The wind industry says the evidence so far is only anecdotal."

Provenance

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