A tall tower holds a wind turbine aloft where winds are consistently stronger.

A wind turbine, windmill or wind generator is a device for converting the kinetic energy in wind into mechanical rotation with a low velocity turbine designed for compressible fluids (air).

Most of this article covers individual wind turbines, which turn the flow of wind into rotational force. This force can then be used, and is most often used, to drive and generators to produce electricity.

See the broader article on wind power for more on turbine placement and controversy.

For a machine that generates wind, see wind machine. For another way to convert wind energy to electricity, see vaneless ion wind generator.

Wind energy

Main article: Wind

Like almost all other form of terrestrial energy, wind energy ultimately comes mostly from the Sun (an argument can be made that geothermal energy contributes energy as well). An estimated 1 to 3 percent of the energy from the Sun is converted into wind energy, which, to compare, is about 50 to 100 times more energy than is converted into biomass by all the plants on earth through photosynthesis. Most of this wind energy can be found at high altitudes where continuous wind speeds of over 160 km/h (100 mph) are common (keep in mind though that the air is less dense there). Eventually, the wind energy is converted through friction into diffuse heat all through the earth's surface and atmosphere.

While the exact kinetics of wind are extremely complicated and relatively little understood, the basics of its origins are relatively simple. The earth is not heated evenly by the sun. Not only do the poles receive less energy from the sun than the equator does, but dry land heats up (and cools down) more quickly than the seas do. This powers a global atmospheric convection system reaching from the earth's surface to the stratosphere which acts as a virtual ceiling.

The change of seasons, change of day and night, the Coriolis effect, the irregular albedo (reflectivity) of land and water, humidity, and the friction of wind over different terrain are some of the many factors which complicate the flow of wind over the surface.

Energy extraction calculations

The power in the wind can be extracted by having it act on moving wings, connected to a rotor, which converts some of that power into torque on the rotor. The amount of power transferred depends on the wind speed (cubed), the swept area (linearly), and the density of the air (linearly).

The mass flow of air that travels through the swept area of a wind turbine varies with the wind speed and air density. As an example, on a cool 15 degrees C day (59 degrees F) at sea level, air density is about 1.22 kilograms per cubic metre (it gets less dense with higher humidity). An 8 m/s breeze blowing through a 100 meter diameter rotor would move about 76,000 kilograms of air per second through the swept area.

The kinetic energy of a given mass varies with the square of its velocity. Because the mass flow increases with the wind speed, the wind energy available to a wind turbine increases as the cube of the wind speed. The power of the example breeze above through the example rotor would be about 2.5 megawatts.

As the wind turbine extracts energy from the air flow, the air is slowed down, which causes it to spread out, causing it to divert around the wind turbine to some extent. A German physicist, Albert Betz, determined in 1919 that a wind turbine can extract at most 59% of the energy that would otherwise flow through the turbine's cross section. The Betz limit applies regardless of the design of the turbine. More recent work [1] by Gorlov shows a theoretical limit of about 30% for propeller-type turbines. Actual efficiencies range from 10% to 20% for propeller-type turbines, and are as high as 35% for three-dimensional vertical-axis turbines like Darrieus or Gorlov turbines (see below).

Windiness varies, and an average value for a given location is not in itself a clear indication of the amount of energy a wind turbine could yield there. The distribution model most frequently used is the Raleigh model, an example of which is plotted to the right against an actual measured dataset.

Because available power rises with the cube of wind speed, much of the average power available to a windmill comes in short bursts. The 2002 Lee Ranch sample is telling: half of the energy available arrived in just 15% of the operating time.

Turbine design and construction

The wind blows faster at higher altitudes because of the drag of the surface (sea or land) and the viscosity of the air. The variation in velocity with altitude, called wind shear is most dramatic near the surface. Typically, the variation follows the 1/7th power law, which predicts that wind speed rises proportionally to the seventh root of altitude. Doubling the altitude of a turbine, then, increases the expected wind speeds by 10% and the expected power by 34%.

Tip speed ratio 

The ratio between the speed of the wind and the speed of the tips of the blades of a wind turbine.

Wind turbines can be separated into two general types based on the axis about which the turbine rotates. Turbines that rotate around a horizontal axis are most common.

Horizontal axis

All existing HAWTs (or Horizontal Axis Wind Turbine) have the main rotor shaft and generator at the top of a tower, and must be pointed into the wind by some means. Small turbines can have a simple wind vane, while large turbines generally use a wind sensor coupled with a servomotor. Most have a gearbox too, which turns the slow rotation of the blades into a quicker rotation that is more suitable for generating electricity.

Since a tower produces turbulence behind it, the turbine is usually placed in front. This has forced wind turbines to use extremely stiff blades to prevent the blades from being pushed into the tower. On top of this, the blades are then placed a considerable distance in front of the tower and are sometimes tilted up a small amount. Downwind machines are occasionally 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.

There are several types of HAWT.

• Windmills -- These 4 bladed squat buildings, usually with wooden shutters or fabric sails, were pointed into the wind manually. These windmills, most associated with the Netherlands, were historically used to grind grain or pump water from low-lying land. They greatly accelerated shipbuilding in the Netherlands, and were instrumental in keeping its polders dry.
• American Style Farm Windmills -- These windmills were used by American prarie farmers to generate electricity, to power the water pumps with which they pumped their wells until rural electrification replaced them in the 1950s. They typically had many blades, operated at tip speed ratios not better than one, and had good starting torque.
• Common modern wind turbines -- usually three-bladed, sometimes two-bladed or even one-bladed (and counterbalanced), pointed into the wind by computer-controlled yaw motors. This turbine type has been championed by Danish turbine manufacturers. These have high tip speed ratios, high efficiency, and low torque ripple which contributes to good reliability.
• Ducted rotor -- The ducted rotor consists of a turbine inside a duct which flares outwards at the back. The narrow duct accelerates the air and so the rotor spins more quickly. The main advantage of the ducted rotor is that it can operate in a wide range of winds. Another advantage is that the generator operates at a high rotation rate, so it doesn't require a bulky gearbox, so it can be smaller and lighter. A disadvantage is that (apart from the gearbox) it is more complicated than the unducted rotor and the duct is usually quite heavy, which puts an added load on the tower.

On small wind generators the tower height is usually at least twenty meters. In the case of large generators, the tower height is about twice as great as the propeller radius.

Number of blades

Because wind velocity increases at higher altitudes, the backward force and torque on a horizontal axis wind turbine (HAWT) blade peak 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 two effects combine to 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 are 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 it turns to face the wind, the turbine acts like a gyroscope. When the turbine pivots to face the wind, 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 turbine.

Many commercially made wind turbines, such as the Whisper 175, use 2 blades because such turbines are easy to construct as they avoid the need for using a hub with linkages to individual blades, and the blade(s) can be shipped easily in one long package. Three-bladed turbines, which are much more efficient, and more quiet, require more complicated onsite assembly.

These small scale turbines can have any number of blades. There are a number of vibrations that increase in peak intensity as the number of blades decreases. Some of these vibrations, besides causing wear on the machine, are also audible. However, fewer, larger blades operate at a higher Reynolds number and are therefore more efficient. A small turbine with 4 or more blades is less efficient as each blade stalls in the wake of the other blades. Also, the cost of the turbine usually increases with the number of blades, so most (small scale) wind turbines have three blades.

Counter-rotating horizontal axis turbines

Counter rotating turbines can be used to increase the rotation speed of the electrical generator. When the counter rotating turbines are on the same side of the tower, the blades on the one in front are angled inwards slightly so as to never hit the rear ones. They are either both geared to the same generator or, more often, one is connected to the rotor and the other to the field windings. Counter rotating turbines geared to the same generator have additional gearing losses. Counter rotating turbines connected to the rotor and stator are mechanically simpler; but, the field windings need slip rings which adds complexity, wastes some electricity and wastes some mechanical power. As of 2005, no large practical counter-rotating HAWTs are commercially sold.

Counter rotating turbines can be on opposite sides of the tower. In this case it is best that the one at the back be smaller than the one at the front and set to stall at a higher wind speed. This way, at low wind speeds, both turn and the generator taps the maximum proportion of the wind's power. At intermediate speeds, the front turbine stalls; but, the rear one keeps turning, so the wind generator has a smaller wind resistance and the tower can still support the generator. At high wind speeds both turbines stall, the wind resistance is at a minimum and the tower can still support the generator. This allows the generator to function at a wider wind speed range than a single-turbine generator for a given tower.

To reduce sympathetic vibrations, the two turbines should have an irrational relative rate, (e.g. the square root of two). Overall, this is a more complicated design than the single-turbine wind generator, but it taps more of the wind's energy at a wider range of wind speeds.

Vertical axis

Vertical axis turbines (or VAWTs) have the main rotor shaft running vertically. The main advantages of this arrangement are that the generator and/or gearbox can be placed at the bottom, on or near the ground, so the tower doesn't need to support it, and the fact that the turbine doesn't need to be pointed into the wind.

• Darrieus wind turbine -- These are the "eggbeater" turbines. These were the first turbines to be scaled up to megawatt size. 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 to start turning.
• Savonius wind turbine -- These are the familar two or three scoop drag-type devices used in anemometers and some high-reliability low-efficiency power turbines.

Rotation control

Modern wind turbines are designed to spin at varying speeds (see wind turbine generator details in this article). Coupled with the fact that newer windmills generally use lightweight aerospace materials in their blades, which have low inertia, this means that newer wind turbines can suddenly speed up rotation if the winds pick up. The low inertia in newer designs allows the wind turbine to optimize the energy capture from sudden gusts of wind that are typical in urban settings.

In contrast, older style wind turbines were designed with steel blades, which have higher inertia, and were calibrated to a rotation frequency that matched the AC frequency of the power lines. The high inertia now came in handy as it buffered the changes in rotation speed and thus made power output more stable.

The speed at which wind turbines rotate must be controlled for several reasons:

• Maintainance; because it is obviously incredibly dangerous to have people working on a wind turbine while it is active, it is sometimes necessary to bring a turbine to a full stop.

• Noise reduction; As a rule of thumb, the noise from a wind turbine increases with the fifth power of the relative wind speed (as seen from the moving tip of the blades). In noise-sensitive environments (nearly all onshore installations), noise limits the tip speed to approximately 60 m/s. High efficiency turbines may have tip speed ratios of 5-6, which, for land based turbines, limits high efficiency operation to winds of just 10 m/s.

• Centripetal force reduction; as the rotational speed increases, so does the centripetal force working on the central hub or axis. When it exeeds safe limits blades could snap off, and the turbine would fail dramatically.

Overspeed control is exerted in two main ways: stalling and furling, and mechanical braking. Furling is the prefered method of braking wind turbines.

Stalling and furling

Because the power of the wind increases as the cube of the speed wind turbines have to be built to survive much higher wind loads (such as gusts of wind) than those from which they can practically generate power. Since the blades generate more downwind force (and thus put far greater strain on the tower) when they are producting torque, most wind turbines have ways of reducing torque in high winds. Usually the blades (or their tips) are either stalled or furled . Stalling works by increasing the angle at which the relative wind stikes the blades (angle of attack), and it reduces the induced drag (drag associated with lift). Stalling is simple because it can be made to happen passively (it increases automatically when the winds speed up), but it increases the cross-section of the blade face-on to the wind, and thus the ordinary drag. Furling works by decreasing the angle of attack, which reduces the induced drag from the lift of the rotor, as well as the cross-section. One major problem in designing wind turbines is getting the blades to stall or furl quickly enough should a gust of wind cause sudden acceleration.

A fixed-speed HAWT inherently increases its angle of attack at higher wind speed as the blades speed up. A natural strategy, then, is to allow the blade to stall when the wind speed increases. This technique was used on many early HAWTs, until it was realized that stalled blades generate a large amount of vibration (noise). Standard modern turbines all furl the blades in high winds. Since furling requires acting against the torque on the blade, it requires active (hydrolic) pitch angle control which is only cost-effective on very large turbines. These systems are usually spring operated, in order to work even in case of electrical power failure. When the power fails, the springs are automatically activated.

Electromagnetic braking

Braking is also sometimes done by extracting too much energy (from the wind turbine, by means of a controlled short-circuit (converting kinetic energy into electrical energy into heat). This drains energy from the turbine. Cyclically braking causes the the blades to slow down, which increases the stalling effect, reducing the efficiency of the blades. This way, the turbine's rotation can be kept at a safe speed in faster winds while maintaining (nominal) power output. While this is sometimes done in small scale household turbines, in large scale turbines, the technique is usually only used to keep the turbine from turning alltogether. When the wind turbine is at rest a short circuit is often enough to prevent the turbine from turning.

Mechanical braking

Mechanical braking is usually done only when the wind turbine is at rest, and operators would prefer it to remain so. A drum brake or disk brake would quickly wear down if it were used to slow a turbine down.

Turbine size

In the wind business, bigger is better. Construction and maintenance costs are similar for large and small turbines, so utility companies build the largest feasible turbines.

For a given survivable wind speed, the mass of a turbine (calculated from volume) is approximately proportional to the cube of its blade-length. Wind intercepted by the turbine is proportional to the square of its blade-length. The maximum blade-length of a turbine is limited by both the strength and stiffness of its material.

Labor and maintenance costs increase only gradually with increasing turbine size, so to minimize costs, wind farm turbines are basically limited by the strength of materials, and siting. Larger turbines require faster winds to operate.

Generating electricity

Wind turbines generate electricity, using an electrical generator, which employs the fact that a changing magnetic field through a conduction loop will cause a voltage across that conductor. Most large wind turbines use electromagnets in the nacelle of the turbine to generate a magnetic field. Coils connected to the rotor then rotate around the magnet, causing a continually changing magnetic field to pass through the coil. The portion of the magnet field that passes through the coil is called the flux. An increasing flux causes an increasing voltage to form across the coil. Electromagnets of course require some electricity to run, but in most designs permanent magnets are too cumbersome and expensive. Electrical generators inherently produce AC power, as the voltage changes direction (from positive to negative) whenever the poles of the magnet switch places. While older style windmills rotate at a constant speed, to match power line frequency (60 Hz in North America), newer windmills often turn at whatever speed generates electricity most efficiently. Such newer turbines therefore output at an arbitrary frequency. The AC is then rectified to DC with a three-phase bridge rectifier, and the resulting DC is inverted back to AC at exactly line frequency (e.g. 60Hz). This process of converting to DC and then back to AC allows the windmill to turn at any speed. While any conversion process brings energy losses with it, as a whole, the wind turbine will be more efficient.

Most deployed turbines produce electricity about 25% of the time (load factor 25%), but some reach 35%. The load factor is generally higher is winter.

Materials

One of the best construction materials available (in 2001) is graphite-fiber in epoxy. Graphite composites can be used to build turbines of sixty meters radius, enough to tap a few megawatts of power. Smaller household turbines can be made of lightweight fiberglass, aluminum, or sometimes laminated wood.

Sails were originally used on early windmills. Unfortunately they have a short service life. Also, they have a relatively high drag for the force they capture. They turn the generator slowly, waste much of the available wind power and have a large wind resistance for their power output, requiring a strong wind tower. For these reasons they were superseded with solid airfoils.

Offshore design

Offshore windparks generally seem to be made up the same type of turbine as land based parks, though they are generally much larger. Indeed, most the things in this article apply to them too, but there are some subtle differences. For one, the offshore environment is quite corrosive, for another, repairs and maintainance are much more difficult, and much more costly. Wind turbines are outfitted with some sort of corrosion protection, through coatings or cathodic protection.

Offshore wind turbines are considered to be less unsightly (they can be invisible from shore), and because the winds are usually more potent offshore, such turbines don´t need to reach quite as high into the air. Offshore conditions are harsh though, abrasive and corrosive, and it´s often impossible or near-impossibe to repair a broken down turbine in open waters. In stormy areas with extended shallow continental shelves (such as Denmark), turbines are reasonably easy to install, and give good service - Denmark's offshore wind generation provides about 12-15% of total electricity demand in the country.

History

High-efficiency wind turbines (foreground) win out over traditional windmills (background) in most new installations.

Windmills have been used to grind grain since around the 10th century, but they do not generate electricity. Wind turbines, machines that harness the energy of the wind to generate electrical energy, were first invented in the 19th century in Denmark. By the 1930s they were mainly used to generate electricity on farms, mostly in the United States as there was no nationwide power grid. In this period, high tensile steel was cheap, and windmills were placed atop prefabricated open steel lattice towers. In the 1940s, the U.S. had a rural electrification project that killed the natural market for wind-generated power. The techniques were almost lost. In the 1970s many people began to desire a self-sufficient life-style. Solar cells were too expensive for small-scale electrical generation, so practical people turned to windmills. At first they built ad-hoc designs using wood and automobile parts. Most people discovered that a reliable wind generator is a moderately complex engineering project, well beyond the ability of most romantics. Practical people began to search for and rebuild farm wind-generators from the 1930s. Jacobs wind generators were especially sought after. Later, in the 1980s, California provided tax rebates for ecologically harmless power. These rebates funded the first major use of wind power for utility electricity. As aesthetics and durability became more important, turbines were placed atop steel or reinforced concrete shells. Small generators are connected to the tower on the ground, then the tower is raised into position. Larger generators are hoisted into position atop the tower and there is a ladder or staircase inside the tower to allow technicians to reach and maintain the generator. Originally wind generators were built right next to where their power was needed. With the availability of long distance electric power transmission, wind generators are now often on wind farms in windy locations and huge ones are being built offshore. Since they're a renewable means of generating electricity, they are being widely deployed, but their cost is often subsidised by governments, either directly or through renewable energy credits . Much depends on the cost of alternative sources of electricity. Wind generator cost per unit power has been decreasing by about four percent per year. In the year 2001, they're one of the least expensive forms of energy, costing between two and six cents per kilowatt hour, similar to coal and methane fired plants.

Companies in wind turbine industry

• Friesian Enercon GmbH
• Kenetech Corporation
• Garrad Hassan and Partners Ltd.
• Vestas Manufactures 1.5 MW to 4.5 MW turbines
• Nordex
• Siemens Wind Power A/S (formerly Bonus Energy A/S)
• LM Glasfiber A/S Rotor blades ranging from 13.4 to 61.5 m
• Gamesa
• DeWind
• General Electric, through its subsidiary GE Energy
• REpower up to 5 MW turbines

External links

Primarily technical:

• Windturbine analysis Personal website explaining wind turbine theory and Darrieus turbines in particular
• Wind and Hydro Power, turbine basics+principle of operation
• How does it work? -- Danish Wind Industry Organisation.

Business associations:

• European Wind Energy Association (EWEA)
  • Residential Wind Power Q&A
• British Wind Energy Association
• American Wind Energy Association
• Danish Wind Industry Association
• California Wind Energy Collaborative (CWEC)
• The Canadian Wind Energy Association
• The British Columbia Wind Energy Association
• Windustry -- "How to" wind energy information

Other:

• Links to articles about windpower
• Discussion forum for Wind and other Renewable Energy sources

• On wind farms - On the relatively small impact of wind farms on birds
• Iberica 2000.org - On the serious impact of wind farms on birds.

• OtherPower.com - How to build a wind turbine from scratch (including the blade). Includes several designs.
• Scoraig Wind Electric -- Homebrew wind turbines - courses, books and free information. Lots of pics of hands on construction.