Windmill Farms answers:
Windmills have been around for a long time. The first windmill had vertical shafts and were reportedly built in Persia around the 7th century BC for the second caliph, Umar Ibn Al-Khttab. Made of six to twelve sails covered in fabric or palm leaves, they were used to grind corn and draw up water. A similar type of vertical shaft windmill can also be found in 13th century China.
A windmill in Haarlem, Netherlands
The windmills of Kinderdijk, the NetherlandsIn Europe, windmills were developed in the Middle Ages. The earliest mills were probably grinding mills. They were mounted on city walls and could not be turned into the wind. The earliest known examples date from early 12th century Paris. Because fixed mills did not suffice for regions with changing wind directions, mill types that could be turned into the wind were developed. With some subsequent development mills became versatile in windy regions for all kind of industry, most notably grain grinding mills, sawmills (late 16th century), threshing, and, by applying Archimedes’ screws, pumping mills.
They soon became the major energy source in the low lands, where the older watermills could hardly operate due to the lack of height difference in the water ways. The pumping mills allowed the drainage of the Dutch wetlands to claim new land, polders. By continuously pumping water out to the rivers, land below sea level could be created. The earliest Dutch polders date from the middle ages, the first lake was emptied by Jan Leeghwater from 1607. To claim ever more land it became necessary to build series of mills (molengang, mill pace), because an Archimedes screw can only efficiently pump water for a limited height. Each mill pumps water into a higher reservoir, with the last pumping it out to the river. In the 18th century several molendriegangen (3 mills), and molenviergangen (4 mills), were built. The largest preserved mill pace in Kinderdijk was awarded world heritage status in 1997.
Spanish Windmills at La Mancha.Another region well-known for its windmills is La Mancha, Spain. The windmills of La Mancha were made particularly famous by a scene in Cervantes’ Don Quixote de La Mancha where the title character mistakes them for giants sent by an evil enchanter, giving rise to the phrase “tilting at windmills”.
With the industrial revolution, the importance of windmills as primary industrial energy source was replaced by steam and internal combustion engines. Polder mills were replaced by steam, or diesel engines. More recently historic windmills are being preserved for their historic value, which requires regular use because the wooden machinery is likely to be destroyed by maggots when the mill remains stationery for too long. With increasing environmental concern, and approaching limits to fossil fuel consumption, wind power has regained interest as a renewable energy source. This new generation of wind mills produce electric power and are more generally referred to as wind turbines.The development of the water-pumping windmill in the USA was the major factor in allowing the farming and ranching of vast areas of North America, which were otherwise devoid of readily accessible water. They contributed to the expansion of rail transport systems throughout the world, by pumping water from wells to supply the needs of the steam locomotives of those early times. They are still used today for the same purpose in some areas of the world where a connection to electric power lines is not a realistic option.
The multi-bladed wind turbine atop a lattice tower made of wood or steel was, for many years, a fixture of the landscape throughout rural America. These mills, made by a variety of manufacturers, featured a large number of blades so that they would turn slowly but with considerable torque in low winds and be self regulating in high winds. A tower-top gearbox and crankshaft converted the rotary motion into reciprocating strokes carried downward through a pole or rod to the pump cylinder below.
In areas not prone to freezing weather, a pump jack (or standard) was frequently mounted at the top of the well in the center of the base off the tower. This was the connection between the windmill and the pump rod, which generally went through the drop pipe to the cylinder below. The pump jack provided a means for manual operation of the pump when the wind was not blowing. Some pump jacks provided a sealed connection, allowing water to be forced out under pressure allowing a tank at a higher elevation to provide water for a home and other uses, but many had a simple spout allowing water to flow away in a trough by gravity.
A modern day windmill as seen on the Rotar farm in California.
The drop pipe and pump rod continued down deep into the well, terminating at the pump cylinder below the lowest likely groundwater level. A suction tube usually continued a short distance more. This arrangement allowed wells as deep as 1200 feet (370 m) to be constructed, though most were much more shallow.
Windmills and related equipment are still manufactured and installed today on farms and ranches, usually in remote parts of the western United States where electric power is not readily available. The arrival of electricity in rural areas, brought by the Rural Electrification Administration (REA) in the 1930s through 1950s, contributed to the decline in the use of windmills in the US. Today, with increases in energy prices and the expense of replacing electric pumps, has led to an increase in the repair, restoration and installation of new windmills.
On the Other hand………
With increasing energy demands, wind has come to the fore once more as a source of renewable energy. All over the world wind farms are being constructed to create electricity from the wind.A wind turbine is a machine for converting the mechanical energy in wind into electrical 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 conversion machinery. See the broader article on wind power for more on turbine placement 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.
For a machine that generates wind. For an unusual way to induce a voltage using an aerosol of ionised water,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. 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 can also be used in conjuction with a solar power tower to extract the energy due to air heated by the Sun and rising through a large vertical solar chimney. The first commercial solar power tower of this type is in the early stages of construction in Australia. Another prototype application is in a Wave power plant.
Horizontal Axis Wind Turbines (HAWT) 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 are pointed by 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 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:
These four- (or more) bladed squat structures, usually with wooden shutters or fabric sails, were pointed into the wind manually or via a tail-fan. These windmills, generally 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 prairie farmers to generate electricity and to pump water. 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 rural electrification connected many farms to centrally-generated power and replaced individual windmills as a primary source of farm power in the 1950s. Such devices are still used in locations where it is too costly to bring in commercial power.
Wind turbines near Aalborg, DenmarkCommon 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.
Still something of a research project, the ducted rotor consists of a turbine inside a duct which flares outwards at the back. 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 the mechanical portion 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
Counter-rotating horizontal axis turbines
Counter rotating turbines can be used to increase the rotation speed of the electrical generator.
As of 2005, no large practical counter-rotating HAWTs are commercially sold. When the counter rotating turbines are on the same side of the tower, the blades in front are angled forwards slightly so as to avoid hitting the rear ones. If the turbine blades are on opposite sides of the tower, it is best that the blades at the back be smaller than the blades at the front and set to stall at a higher wind speed. 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 turn at speeds with few common factors, for example 7:3 speed ratio. 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.
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 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 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 produced during each revolution; and the difficulty of mounting vertical axis turbines on towers, meaning they must operate in the slower, more turbulent air flow near the ground, with lower energy extraction efficiency.
H-Darrieus-turbineDarrieus 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.
These lift-type devices have vertical blades. The cycloturbine variety has variable pitch, to reduce the torque pulsation and self-start . The helical type has smooth torque, and can also use the vertical air flow component in turbulent or rising winds above buildings or cliffs .
Savonius wind turbine
These are the familiar two- (or more) scoop drag-type devices used in anemometers and some high-reliability low-efficiency power turbines, and always self-start (if at least three scoops). They can sometimes have long helical scoops, to give smooth torque.
Offshore wind turbines are considered to be 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, the average wind speed is usually higher over open water. This 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, and give good service – Denmark’s wind generation provides about 12-15% 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, though as of now Denmark is a net importer of electricity.
The offshore environment is, however, more expensive. Offshore towers are generally taller than onshore towers once one includes the submerged height, and offshore foundations are generally more difficult to build and more expensive as well. 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 is also corrosive and abrasive. Repairs and maintenance are much more difficult, and much more costly than on onshore turbines. Offshore wind turbines are outfitted with extensive corrosion protection measures like coatings and cathodic protection.
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 reduce the marginal cost of many of the difficulties of offshore operation.
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 .
Main article: Airborne wind turbine
It has been suggested that wind turbines might be flown in high speed winds at high altitude. No such systems currently exist in the marketplace. However, an Ontario based company called Magenn Power Inc. Has developed a turbine called the Magenn Power Air Rotor System (MAPS). The MAPS system uses a horizontal rotor in a helium suspended apparatus which is tethered to a transformer on the ground. Magenn states that their technology provides high torque, low starting speeds, and superior overall efficiency thanks to its ability to deploy higher in comparison to non-aerial solutions. Magenn is putting the first of the MAPS product line on the market in 2006.
It should be noted, however, that the idea of airborne wind turbines reappears in the industry every few years, and seldom (if ever) gets off the drawing board.
Turbine design and construction
Horizontal Axis Wind Turbine Aerodynamics
The aerodynamics of a horizontal axis wind turbine are not as straight forward as one would think. The air flow at the blades is not the same as the airflow far away from the turbine. In fact the wind turbine actually deflects some of the air away from the turbine. This is completely unavoidable. The very nature of the way in which energy is extracted from the air also causes air to be deflected by the turbine. In addition the aerodymanics of a wind turbine up close at the rotor surface exibhit phenomena that are rarely seen in other aerodynamic fields.
Energy in fluid is contained in four differenct forms. Gravitational potential energy, thermodynamic pressure, kinetic energy from the velocity and finally thermal energy. Gravitational and thermal energy have a negligble affect on the energy extraction process. From a macroscopic point of view the air flow about the wind turbine is at atmospheric pressure. If pressure is constant than only kinetic energy is extracted. However up close near the rotor itself the air velocity is constant as it passes through the rotor plane. This is because of conservation of mass. The air that passes through the rotor cannot slow down because it needs to stay out of the way of the air behind it. So at the rotor the energy is extracted by a pressure drop. The air directly behind the wind turbine is sub-atmospheric. Where as the air in front is greater than atmospheric pressure. It is this high pressure in front of the wind turbine that deflects some of the upstream air around the turbine.
Betz was amongst the first to tackle this phenomenon. He notably described the Betz limit to wind turbine performance. This is derived by looking at the axial momentum of the air passing through the wind turbine. As stated above some of the air is deflected away from the turbine. This causes the air close to the turbine to be less than the air far from the turbine. The degree at which air at the turbine is less than the air far away from the turbine is? Called the axial induction factor. It is defined as below.
A is the axial induction factor. U1 is the wind speed far away from the rotor. U2 is the wind speed at the rotor.
The first step to deriving the Betz limit is applying conservation of axial momentum. As stated above, far away from the turbine, the wind is being losing speed after the wind turbine. This would violate the conservation of momentum if the wind turbine was not applying a thrust force on the flow. This thrust force manifests itself through the pressure drop across the rotor. The front operates at high pressure while the back operates at low pressure. The pressure diference from the front to back causes the thrust force. The momentum lost in the turbine is balanced by the thrust force.
Axial momentum relates the wake flow to the pressure difference at the rotor. Another equation is needed to relate the pressure difference to the velocity of the flow near the turbine. Here the bernoulli equation is used between for field flow and the flow near the wind turbine. There is one limitation to the bernoulli equation. The equation cannot be applied to fluid passing through the wind turbine. Instead conservation of mass is used to relate the incoming air to the outlet air. Betz used these equations and managed to solve the velocities of the flow in the far wake and near the wind turbine in terms of the far field flow and the axial induction factor. The velocities are given below.
U2 = U1(1 ? A) U4 = U1(1 ? 2a)
U4 is introduced here as the wind velocity in the far wake.This is important because the power extracted from the turbine is defined by the following equation. However the Betz limit is given in terms of the coeficient of power. The coeficient of power is similar to efficiency but not the same. The formula for the coeficient of power is given beneath the formula for power.
Betz was able to develop an expression for Cp in terms of only the induction factors. This is done by the velocity relations beinge substitutuded into power and power is substituded into the coefficient of power definition. The relationship Betz developed is given below.
Cp = 4a(1 ? A)2
The Betz limited is defined by the maximum value that can be given by the above formula. This is found by taking the derivative with respect to the axial induction factor , setting it to zero and solving for the axial induction factor. Betz was able to show that the optimum axial induction factor is one third. The optimum axial induction factor was then used to find the maximum coefficient of power. This maximum coefficient is the Betz limit. Betz was able to show that the maximum coefficient of power of a wind turbine is 16/27. Airflow operating at higher thrust will cause the axial induction factor to rise above the optimum value. Higher thrust cause more air to be deflected away from the turbine. When the axial induction factor falls below the optimum value the wind turbine is not extracting all the energy it can. This reduces pressure around the turbine and allows more air to pass through the turbine.
The derivation of the Betz limit shows a simple analysis of wind turbine aerodynamics. In reality there is a lot more. A more rigorous analysis would include wake rotation, the effect of variable geometry. The affect of air foils on the flow is a major component of wind turbine aerodymaics. Within airfoils alone the wind turbine aerodynamicist has to consider the affect of surface roughness, dynamic stall among other problems.
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, in daytime 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%. Doubling the tower height generally requires doubling the diameter as well, increasing the amount of material by a factor of eight. In night time, or better: when the atmosphere becomes stable, wind speed close to the ground usually subsides whereas at turbine hub altitude it does not decrease that much or may even increase. As a result the wind speed is higher and a turbine will produce more power than expected from the 1/7th power law: doubling the altitude may increase wind speed by 20% to 60%. A stable atmosphere is caused by radiative cooling of the surface and is common in a temperate climate: it usually occurs when there is a (partly) clear sky at night. When the (high altitude) wind is strong (10 meter wind speed higher than approximately 6 to 7 m/s) the stable atmosphere is disrupted because of friction turbulence and the atmosphere will turn neutral. A daytime atmosphere is either neutral (no net radiation; usually with strong winds and/or heavy clouding) or unstable (rising air because of ground heating -by the sun). Here again the 1/7th power law applies or is at least a good approximation of the wind profile.
For HAWTs, tower heights approximately twice to triple the blade length have been found to balance material costs of the tower against better utilisation of the more expensive active components.
Number of blades
For small (novelty or urban) HAWT turbines manufacturers typically ship three-bladed turbines with three separate blades that must be assembled onsite, into a central hub. Without careful assembly ensuring accurate dynamic balance of the blades, the turbine can shake itself apart.Most wind turbines have three blades. Very small turbines may use two blades for ease of construction and installation. Vibration intensity decreases with larger numbers of blades. Noise and wear are generally lower, and efficiency higher, with three instead of two blades.
Turbines with larger numbers of smaller blades operate at a lower Reynolds number and so are less efficient. Small turbines with 4 or more blades suffer further losses as each blade operates partly in the wake of the other blades. Also, the cost of the turbine usually increases with the number of blades.
Tip speed ratio
The ratio between the speed of the wind and the speed of the tips of the blades of a wind turbine. High efficiency 3-blade-turbines have tip speed ratios of 6-7.
Modern wind turbines are designed to spin at varying speeds (a consequence of their generator design, see below). Use of aluminum and composites in their blades has contributed to low rotational inertia, which means that newer wind turbines can accelerate quickly if the winds pick up, keeping the tip speed ratio more nearly constant. Operating closer to their optimal tip speed ratio during energetic gusts of wind allows wind turbines to improve energy capture from sudden gusts that are typical in urban settings.
In contrast, older style wind turbines were designed with heavier steel blades, which have higher inertia, and rotated at speeds governed by the AC frequency of the power lines. The high inertia buffered the changes in rotation speed and thus made power output more stable.
The speed and torque at which a wind turbine rotates must be controlled for several reasons:
To optimize the aerodynamic efficiency of the rotor in light winds.
To keep the generator within its speed and torque limits.
To keep the rotor and hub within their centripetal force limits. The centripetal force from the spinning rotors increases as the square of the rotation speed, which makes this structure sensitive to overspeed.
To keep the rotor and tower within their strength limits. Because the power of the wind increases as the cube of the wind speed, 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 stress on the tower) when they are producing torque, most wind turbines have ways of reducing torque in high winds.
To enable maintenance; because it is 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.
To reduce noise; 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, the tip speed can be limited to approximately 60 m/s.
Overspeed control is exerted in two main ways: aerodynamic stalling or furling, and mechanical braking. Furling is the preferred method of slowing wind turbines.
Stalling and furling
Stalling works by increasing the angle at which the relative wind strikes 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. A fully stalled turbine blade, when stopped, has the flat side of the blade facing directly into the wind.
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 fully furled turbine blade, when stopped, has the edge of the blade facing into the wind.
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 realised 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 pitch angle control which is only cost-effective on very large turbines. Many turbines use hydraulic systems. These systems are usually spring loaded, so that if hydraulic power fails, the blades automatically furl. Other turbines use an electric servomotor for every rotor blade. They have a small battery-reserve in case of an electric-grid breakdown.
Dynamic braking resistor for wind turbine.Braking of a turbine can also be done by dumping energy from the generator into a resistor bank, converting the kinetic energy of the turbine rotation into heat. This method is useful if the connected load on the generator is suddenly reduced or is too small to keep the turbine speed within its allowed limit. Cyclically braking causes 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.
A mechanical drum brake or disk brake is used to hold the turbine at rest for maintenance. Such brakes are usually applied only after blade furling and electromagnetic braking have reduced the turbine speed, as the mechanical brakes would wear quickly if used to stop the turbine from full speed.
a person standing beside modern turbine blades, illustrating their size.For a given survivable wind speed, the mass of a turbine is approximately proportional to the cube of its blade-length. Wind power 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 requirements.
Typical modern wind turbines have diameters of 40 to 90 meters and are rated between 500 kW and 2 megawatts. Currently (2005) the most powerful turbine is rated at 6 MW.
For large, commercial size horizontal-axis wind turbines, the generator is mounted in a nacelle at the top of a tower, behind the hub of the turbine rotor. A speed increasing gearbox may be inserted between the rotor hub and the generator, so that the generator cost and weight can be reduced.
Commercial size generators have a rotor carrying a field winding so that a rotating magnetic field is produced inside a set of windings called the stator. While the rotating field winding consumes a fraction of a percent of the generator output, adjustment of the field current allows good control over the generator output voltage. Very small wind generators (a few watts to perhaps a kilowatt in output) may use permanent magnets but these are too costly to use in large machines and do not allow convenient regulation of the generator voltage.
Electrical generators inherently produce AC power. Older style wind generators rotate at a constant speed, to match power line frequency, which allowed the use of less costly induction generators. Newer wind turbines often turn at whatever speed generates electricity most efficiently. The variable frequency current is then converted to DC and then back to AC, matching the line frequency and voltage. Although the two conversions require costly equipment and cause power loss, the turbine can capture a significantly larger amount of the annual wind energy. In some cases, especially when turbines are sited offshore, the DC energy will be transmitted from the turbine to a central (onshore) inverter for connection to the grid.
One of the strongest construction materials available (in 2006) is graphite-fibre in epoxy, but it is very expensive and only used by some manufactures for special load-bearing parts of the rotor blades. Modern rotor blades (up to 126 m diameter) are made of lightweight pultruded glass-reinforced plastic, smaller ones also from aluminum, or sometimes laminated wood.
Wood and canvas sails were originally used on early windmills. Unfortunately they require much maintenance over their service life. Also, they have a relatively high drag (low aerodynamic efficiency) for the force they capture. For these reasons they were superseded with solid airfoils.
One wind turbine of the type E-66 at Windpark Holtriem/Germany carries an observation deck, open for visitors.
High-efficiency wind turbines (foreground) win out over traditional windmills (background) in most new installations.Wind machines were used for grinding grain in Persia as early as 200 B.C. This type of machine spread throughout the Islamic world and were introduced by Crusaders into Europe in the 13th century. 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 Denmark in 1890, and in 1908 there were 72 wind-driven electric generators from 5 kW to 25 kW. The largest machines were on 24 m towers with four-bladed 23 m 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 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.
In 1941 the world’s first megawatt-size wind turbine was connected to the local electrical distribution system on Grandpa’s Knob in Vermont, USA. This 1.25 MW Smith-Putnam turbine operated for 1100 hours before a blade failed at a known weak point, which had not been reinforced due to war-time material shortages. In the 1940s, the U.S. Had a rural electrification project that killed the natural market for wind-generated power, since network power distribution provided a farm with more dependable usable energy for a given amount of capital investment.
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. These machines, gathered in large wind parks such as at Altamont Pass would be considered small and un-economic by modern wind power development standards.
As aesthetics and durability became more important, turbines were placed atop steel or reinforced concrete towers. 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, sometimes transmitting power back to land using high voltage submarine cable. Since wind turbines are a renewable means of generating electricity, they are being widely deployed, but their cost is often subsidised by taxpayers, 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.
Companies in wind turbine industry
World market for wind energy plants in 2003Eirbyte(Supplier of small turbines in Ireland)
Airtricity (only operator of turbines)
Suzlon Energy Ltd
Det Norske Veritas – Certification of wind turbines and wind turbine projects
EMD A/S – WindPRO software package for project design and planning of turbines
Enercon GmbH, Germany – wind turbines up to 6 MW
Garrad Hassan and Partners Ltd.
General Electric, through its subsidiary GE Energy
LM Glasfiber A/S – Rotor blades ranging from 13.4 to 61.5 m
Natural Power – International wind energy consultancy services
NEG Micon – Merged with Vestas in 2004
Northern Power Systems
REpower, Germany – wind turbines up to 5 MW
Siemens Wind Power A/S (formerly Bonus Energy A/S)
Southwest Windpower 
Valmont Wind Energy, Inc. – modular tower systems for MW wind turbines.
Vestas, Denmark – wind turbines up to 4.5 MW 
Wind Prospect – Independent wind energy developer in the UK and Australia
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 and a diameter of 114 m, the REpower 5M delivers up to 5 MW, has an overall height of 183 m and a diameter of 126 m.
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 on the Antarctica, used to power the Australian Research Division’s Mawson Bay station.
The highest located turbine is at 2300 m on the mountain Gütsch near Andermatt, Switzerland. Originally a prototype of the Dutch company Lagerwey was tested there, however, it failed to meet the expectations and was demolished in 2002. Since October 2004 an Enercon E-40 produces electricity there.