wind turbine

A wind turbine is a machine for converting 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.

This article discusses the energy conversion machinery. See the broader article on wind power for more on turbine placement, economics, public concerns, and controversy, and in particular see the wind energy section of that article for an understanding of the temporal distribution of wind energy and how that affects wind turbine design. See environmental concerns with electricity generation for discussion of environmental problems with wind energy production.

For a machine that generates wind, see wind machine. For an unusual way to induce a voltage using an aerosol of ionised water, see vaneless ion wind generator.

Types of wind turbines

Small-scale wind power in Marshall County, Indiana.

Wind turbines can be separated into two types based on the axis about which the turbine rotates. Turbines that rotate around a horizontal axis are more common. Vertical-axis turbines are less frequently used.

Wind turbines can also be classified by the location in which they are to be used. Onshore, offshore, or even aerial wind turbines have unique design characteristics, which are explained in more detail in the section on turbine design and construction.

Wind turbines may also be used in conjunction with a solar collector to extract the energy due to air heated by the Sun and rising through a large vertical solar updraft tower.

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 for generating electricity.

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. Because turbulence leads to fatigue failures and reliability is so important, most HAWTs are upwind machines.

There are several types of HAWT:

Windmill Doesburgermolen, Ede, The Netherlands

Windmills 

These four- (or more) bladed squat structures, 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 windmill in Germany.

Modern Rural Windmills 

These windmills, invented in 1876 by Griffiths Bros and Co (Australia), were used by Australian and later American farmers to pump water and to generate electricity. 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 in the 1950’s. Such devices are still used in locations where it is too costly to bring in commercial power.

Wind turbines near Aalborg, Denmark

Common modern wind turbines 

Usually three-bladed, sometimes two-bladed or even one-bladed (and counterbalanced), and pointed into the wind by computer-controlled motors. The rugged three-bladed turbine type has been championed by Danish turbine manufacturers. These have high tip speeds of up to 6x wind speed, high efficiency, and low torque ripple which contributes to good reliability. This is the type of turbine that is used commercially to produce electricity. The blades are usually colored white and range in length from 20 to 40 metres (60 to 120 feet) or more.

Cyclic stresses and vibration

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

Vertical axis

30 m Darrieus wind turbine in the Magdalen Islands

Vertical-axis wind turbines (or VAWTs) have the main rotor shaft running vertically. The advantages of this arrangement are that the generator and/or gearbox can be placed at the bottom, near the ground, so the tower doesn’t need to support it, and that the turbine doesn’t need to be pointed into the wind. Drawbacks are usually the pulsating torque that can be produced during each revolution and the drag created when the blade rotates into the wind. It is also difficult to mount vertical-axis turbines on towers, meaning they must operate in the often slower, more turbulent air flow near the ground, with resulting lower energy extraction efficiency.

Windmill with rotational sails

This windmill has sails of variable surface. Windmill starts making electricity from windspeed of 2 m/s. Its sails contract and expand as the wind speed changes. The speed is controlled through a magnetic rev counter whiche according to windspeed expands or contracts the sails. Control unit(microprocessor type) controls sails either manually or automatically. On automatic control the sails expand when RPM is less then 5 and contract when more than 9 RPM. In case of control unit failure, strong wind would tear the sails but the construction would remain intact.

H-Darrieus-turbine

Darrieus wind turbine 

These are the “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.

Giromill

A type of Darrieus turbine, these lift-type devices have vertical blades. The cycloturbine variety have variable pitch, to reduce the torque pulsation and self-start. 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.

Savonius wind turbine 

These are the two- (or more) scoop drag-type devices used in anemometers and in the Flettner vents commonly seen on bus and van roofs, and some high-reliability low-efficiency power turbines. They always self-start (if there are at least three scoops). They can sometimes have long helical scoops, to give smooth torque. The Banesh rotor and especially the Rahai rotor improve efficiency by shaping the blades to produce significant lift as well as drag.

Windstar 530G Array in Palm Springs

Windstar turbines 

These lift-type devices by Wind Harvest have straight, extruded aluminum blades attached at each end to a central rotating shaft and are operated as Linear Array Vortex Turbine Systems (LAVTS). Vertical-axis rotors each with their own 50-75kW generator are placed in three to any number of rotors in linear arrays with each rotor’s blades passing within two feet of its neighbor. In this configuration, the center rotors gain an increase in output and efficiency (reaching the high efficiencies of HAWTs). This increased efficiency is protected under patent (number 6784566) as the “vortex effect”. Each rotor unit has a dual braking system of pneumatic disc brakes and blade pitch. The newest Windstar LAVTS stand 50 feet tall, have 1500 and 3000 square feet of swept area per rotor and are designed to be placed in the turbulent winds within the understory of wind farms.

Offshore

Offshore wind turbines near Copenhagen

Offshore wind development zones are generally considered to be ten kilometers or more from land. Offshore wind turbines are less obtrusive than turbines on land, as their apparent size and noise can be mitigated by distance. Because water has less surface roughness than land (especially deeper water), the average wind speed is usually considerably higher over open water. Capacity factors (utilisation rates) are considerably higher than for onshore and near-shore locations which allows offshore turbines to use shorter towers, making them less visible.

In stormy areas with extended shallow continental shelves (such as Denmark), turbines are practical to install — Denmark’s wind generation provides about 25-30% of total electricity demand in the country, with many offshore windfarms. Denmark plans to increase wind energy’s contribution to as much as half of its electrical supply.

Locations have begun to be developed in the Great Lakes – with one project by Trillium Power approximately 20 km from shore and over 700 MW in size. Ontario, Canada is aggressively pursuing wind power development and has many onshore wind farms and several proposed near-shore locations but presently only one offshore development.

In most cases offshore environment is more expensive than onshore. Offshore towers are generally taller than onshore towers once the submerged height is included, and offshore foundations are more difficult to build and more expensive. Power transmission from offshore turbines is generally through undersea cable, which is more expensive to install than cables on land, and may use high voltage direct current operation if significant distance is to be covered — which then requires yet more equipment. The offshore environment can also be corrosive and abrasive in salt water locations but locations such as the Great Lakes are in fresh water and do not have many of the issues found in the ocean or sea. Repairs and maintenance are usually much more difficult, and generally more costly, than on onshore turbines. Offshore wind turbines are outfitted with extensive corrosion protection measures like coatings and cathodic protection however some of these measures may not be required in fresh water locations.

While there is a significant market for small land-based windmills, offshore wind turbines have recently been and will probably continue to be the largest wind turbines in operation, because larger turbines allow for the spread of the high fixed costs involved in offshore operation over a greater quantity of generation, reducing the average cost. For similar reasons, offshore wind farms tend to be quite large—often involving over 100 turbines—as opposed to onshore wind farms which can operate competitively even with much smaller installations.

There are some conceptual designs that might make use of the unique offshore environment. For example, a floating turbine might orient itself downwind of its anchor, and thus avoid the need for a yawing mechanism. One concept for offshore turbines has them generate rain, instead of electricity. The turbines would create a fine aerosol, which is envisioned to increase evaporation and induce rainfall, hopefully on land.

Near-shore

Near-shore turbines are generally considered to be within a zone that is on land three kilometers of a shoreline and on water within ten kilometers of land. Wind speeds in these zones share wind speed characteristics of both onshore wind and offshore wind. Issues that are shared within near-shore wind development zones are ornithological (including bird migration and nesting), aquatic habitat, transportation (including shipping and boating) and visual aesthetics.

Sea shores also tend to be windy areas and good sites for turbine installation, because a primary source of wind is convection from the differential heating and cooling of land and sea over the course of day and night. Winds at sea level carry somewhat more energy than winds of the same speed in mountainous areas because the air at sea level is denser.
Near-shore wind farm siting can sometimes be highly controversial as coastal sites are often picturesque and environmentally sensitive (for instance, having substantial bird life).

Onshore

Wind turbines near Walla Walla in Washington

Onshore turbine installations in hilly or mountainous regions tend to be on ridgelines generally three kilometers or more inland from the nearest shoreline. This is done to exploit the topographic acceleration where the hill or ridge causes the wind to accelerate as it is forced over it. The additional wind speeds gained in this way make large differences to the amount of energy that is produced. Great attention must be paid to the exact positions of the turbines (a process known as micro-siting) because a difference of 30 m can sometimes mean a doubling in output. Local winds are often monitored for a year or more with anemometers and detailed wind maps constructed before wind generators are installed.

For smaller installations where such data collection is too expensive or time consuming, the normal way of prospecting for wind-power sites is to directly look for trees or vegetation that are permanently “cast” or deformed by the prevailing winds. Another way is to use a wind-speed survey map, or historical data from a nearby meteorological station, although these methods are less reliable.

Wind farm siting can sometimes be controversial, particularly as the hilltop, often coastal sites preferred are often picturesque and environmentally sensitive (for instance, having substantial bird life). Local residents in a number of potential sites have strongly opposed the installation of wind farms, and political support has resulted in the blocking of construction of some installations.

History

The world’s first megawatt-sized wind turbine on Grandpa’s Knob, Castleton, Vermont

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.

By the 1930s windmills were mainly used to generate electricity 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 load factor of 32 per cent, not much different from current wind machines.

Records

The world’s largest turbines are manufactured by the Northern German companies Enercon and REpower. The Enercon E112 delivers up to 6 MW , has an overall height of 186 m (610 ft) and a diameter of 114 m (374 ft). The REpower 5M delivers up to 5 MW , has an overall height of 183 m (600 ft) and a diameter of 126 m (413 ft).

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

The highest turbine is at 2300 m (7,500 ft) on the Gütsch mountain near Andermatt, Switzerland. Originally a prototype from a Dutch company was tested there, but it was demolished in 2002. Since October 2004, an Enercon E-40 has been producing electricity there