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Wind turbine

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A wind turbine, windmill or wind generator is a device for converting wind power to mechanical rotation with a low velocity turbine designed for compressible fluids. For a machine that generates wind, see wind machine[?].

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Location is critical

Wind generators are impractical in many areas. The available power grows as the cube of the average wind speed. A site with prevailing winds of 30 kph is eight times as valuable as a site with only 15 kph. As a general rule, wind generators are practical where the average windspeed is greater than 12 mph (19 kph).

The normal way of prospecting for wind-power sites is to look for trees or vegetation that is 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 this is less reliable.

In typical land-based installations, a tower lifting the bottom of the turbine 30 meters will pay for itself by placing the turbine in faster air.

In areas with dramatic topography, moving a generator 30m can sometimes double its output. Often the winds are monitored and modeled before wind generators are installed.

Wind power is practical in most areas of the North American great plains, and the central Eurasian plains, as well as selected ridges of major mountain-chains. Some authorities claim that the mountain ridges alone have enough wind energy to power their respective continents. In areas with storms, it often practical to replace or supplement solar cells with a wind-gnerator. The greatest reservoir of wind energy is in the open oceans, especially around 40 degrees south.

Offshore wind turbines are less unsightly, and can save money by using shorter towers. In stormy areas with extended shallow continental shelfs (such as Denmark), they are reasonably easy to install, and give good service.

Wind is powered by a temperature differential. It is slowed by obstructions. and is generally stronger at high altitudes. Plains have high winds because they have few obstructions. Mountain passes have high winds mostly because they funnel high-altitude winds. Some passes have winds powered by a temperature differential between the sides of the ridges. Coasts have high winds because water has few obstructions and because of the temperature difference between the land and the sea. The ocean, since there aren't any obstructions for a large distance around, also generally has high winds.

Anatomy

A wind turbine strongly resembles a propeller, but has subtly differences. The turbine is perpendicular to the wind, mounted on a tower. With 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.

Power output from a wind generator is proportional to the cube of the wind speed. As wind speed doubles, the capacity of wind generators increases eightfold.

There is usually a means of stalling the turbine's blades to reduce its wind resistance when the wind is extremely strong.

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 slowly with increasing turbine size, so given all these factors, to minimize costs, wind farm turbines are basically limited by the strength of materials, and siting. One of the best construction materials available in 2001 is graphite-fiber in epoxy. Graphite composites enable turbines of sixty meters radius to be built, enough to tap a few megawatts of power. Smaller turbines can be made of lightweight fiberglass, aluminum, or sometimes laminated wood.

Small machines are pointed into the wind by a vane. Large machines have a wind-sensor driving a servomotor.

When it turns to face the wind, the turbine acts like a gyroscope. When the turbine pivots to face the wind, 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.

To reduce the precessive stresses, modern turbines have exactly three blades, only one of which is in a maximum stress position (vertical) at a time. The major historic design defect is to have an even number of blades, so that two blades are vertical at the same time. Two-bladed turbines have the highest cyclic stresses.

When there are four or more blades, the blades of a high-speed, high efficiency turbine start stalling in the disturbed air from the previous blade.

There are a number of vibrations that decrease in peak intensity as the number of blades increases. Some of the vibrations, besides wearing out the machine, are also audible. However, fewer, larger blades operate at a higher Reynolds number and are therefore more efficient. Also, the cost of the turbine increases with the number of blades, so the optimum number of blades turns out to be three.

Since a tower produces turbulence behind it, the turbine is usually placed in front. The turbine has to be placed a considerable distance in front and sometimes tilted up a small amount to ensure that the lower blade doesn't impact the tower. Downwind machines are occasionally built despite the problem of turbulence because they don't need an additional pointing device and in high winds, the blades can be allowed to bend which reduces their wind resistance.

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[?].

When a turbine is spun by the wind, it adds a rotation to the wind, increasing the apparent wind on the blade. Since blades are really designed to work like an airplane wing, this increases the torque produced by the turbine. But this also increases the force in the wind direction on the blade and therefore on the tower. The mechanical stress is significantly higher when the turbine rotates. That's why wind turbines are stopped during high wind.

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 one in front is angled inwards slightly so as to never hit the rear one. 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.

Counter rotating turbines can be on opposite sides of the tower. In this case it is best that the one in back be smaller than the one in 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.

Putting a turbine in back helps pulls its side downwind. Since the rear turbine will be at a considerable distance behind the front, it provides considerable leverage for a fin placed there, which means no servo is necessary to point the machine into the wind.

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

Techniques

Wind has been used to grind grain, pump water, heat water (with a churn), an dproduce electricity. In modern times, almost all turbines either pump water or generate electricity.

A wind generator usually consists of an aerodynamic mechanism for converting the movement of air into a mechanical motion which is then converted with a generator into electrical power. Some designs try to convert wind power into electrostatic power by spraying water, which is charged by facing a toroidal charged electrode, with the wind which is then blown upon a mesh.

Wind turbines have been used for household electricity generation in conjunction with battery storage over many decades in remote areas. Generator units of more than 1 MWe are now functioning in several countries. The power output is a function of the cube of the wind speed, so such turbines require a wind in the range 3 to 25 metres/second (11 - 90 km/hr), and in practice relatively few areas have significant prevailing winds. Like solar, wind power requires alternative power sources to cope with calmer periods.

However, there are now many thousands of wind turbines operating in various parts of the world, with a total capacity of over 31,000 MWe of which Europe accounts for 75% (ultimo 2002). This has been the most rapidly-growing means of electricity generation at the turn of the century and provides a valuable complement to large-scale base-load power stations. Denmark gets over 10% of its electricity from wind, whereas wind accounts for 0.4% of the total electricity production on a global scale (ultimo 2002). The most economical and practical size of commercial wind turbines seems to be around 600 kWe to 1 MWe, grouped into wind farms up to 6 MWe. Most turbines operate at about 25% load factor over the course of a year, but some reach 35%.

Wind is variable, so to provide constant power, wind generators need storage batteries or need to be supplemented by an auxiliary means of electricity generation. In remote areas this source of power is usually photovoltaic or diesel. With a grid connection, the auxiliary power is often from gas turbines or hydropower. Birds can be killed by running into the blades or by being electrocuted by the power lines. At a small extra cost, power lines can be buried to eliminated the danger from electrocution. To reduce bird deaths, wind farms should be out of bird migration routes.

Mechanical wind generators were based on windmills and were popular in the 1900s to the 1940s, before the rural electrification program brought grid connected electricity to the countryside. They started their revival in the 1970s as the price of oil increased. 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.

Though wind speed varies, the frequency and voltage output of the generator must remain constant. The two most common ways of doing this are to use an induction generator[?] which turns slightly faster than the utility frequency, usually about five percent faster. It can handle small variations in speed and still provide power at the correct frequency. A more sophisticated way is to use high power electronics to transform current from a generator to utility frequency[?], this is currently more expensive has a rougher waveform; but, can handle a wider variation in the generator rotation rate. Induction generators work best at one speed while electronic generators can work at a wide speed range; so, to get the advantages of both they could be combined, a low power electronic generator working at all speeds and a high power induction generator cutting in at a medium wind speed. This would overall be able to tap the same wind range as a pure electronic generator; but, have a cost somewhat between an electronic generator and a pure induction generator.

The wind-turbine is the most common type. The main disadvantage is the fact that the entire mecanism has to be able to turn on a tower.

The ducted rotor consists of a propeller and generator inside a duct which flares outwards at the back. The air is accelerated inside the duct and the propeller spins 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 can be smaller. A disadvantage is that it is more complicated than the unducted propeller and the duct is heavy, which puts a greater load on the tower.

The simplest type of wind generator is the savonius rotor. It consists of two vertical curved airfoils[?] mounted between two disks. Whenever wind blows horizontally through this device, the disk turns driving a generator. The main advantage of this type is the device itself doesn't have to be turned into the wind, so no set of slip rings or yaw mecanism is necessary. Also, the heavy generator is on the ground, the airfoils can be made from a pipe section and there is only one moving part. The disavantages of this type are its low efficiency and the fact that the wind profile can't be reduced in high winds.

Darrieus wind generators look like a wire supported eggbeater. It consists of thin vertical airfoils which meet at their tips and bend out at the middle. Like the savonius rotor; the main advantage of this type is the device itself doesn't have to be turned into the wind, so no set of slip rings or yaw mecanism is necessary. Also, the heavy generator is on the ground. The disavantages of this type are the difficulty of making the low efficiency, the fact that it needs a starter and the fact that it has huge side loads at the top which usually has to be braced with wires.

Schemes have been bandied about to loft wind generators from kites and / or balloons in order to harness powerful high altitude winds. This has the advantage of being able to tap an almost constant wind and doing so without a set of slip rings or yaw mechanism. The main disadvantage is that kites come down when there is insufficient wind. Balloons can be added to the mix to keep the contraption up without wind; but, balloons leak slowly and have to be at least resupplied with lifting gas, possibly patched as well. Also this scheme requires a very long power cable and an aircraft exclusion zone.

Electrostatic wind generators work by spraying water from a nozzle facing a toroidal charged electrode. This induces an opposite charge in the water and when the water flows out of the nozzle, each drop carries a small amount of charge. These water droplets are them blown by the wind, going through the center of the charged toroid without touching it. The droplets then hit a fine mesh, adding to its charge. The main advantage of this system is that it has no rapidly moving parts. The disadvantages are that it won't work in the rain, it needs a constant supply of water, its wind profile can't be reduced, it requires many small parts, the whole device has to be able to turn and it has to be well crafted to reduce corona discharge losses.

Wind generators range from small four hundred watt generators for residential use to several megawatt machines for wind farms and offshore. The small ones have direct drive generators, direct current output, aeroelastic blades[?], lifetime bearings and use a fin to point into the wind; while the larger ones generally have geared power trains, alternating current output, flaps and are actively pointed into the wind. As technology progresses, large generators are becoming as simple as small generators. Direct drive generators and aeroelastic blades for large wind turbines are being researched and direct current generators are sometimes used. Wind is an important renewable source of electrical power and the rate of installation is growing by about twenty five percent per year. In combination with solar power, it is an excellent source of electricity.

History

Windmills were first used to pump water and mill grain. The first wind generators were placed atop brick towers, or other buildings.

By the 1930s they were mainly used to generate electricity on farms. The most famous make was the Jacobs Electric. Jacobs discovered and pioneered the modern three-blade, high-speed wind-turbine, with an integrated, low-speed, ungeared generator. In this period, high tensile steel was cheap, and windmills were placed atop prefabricated open steel lattice towers.

The most famous application of the Jacobs wind-generator was to power the radio for the second polar year expedition to the south pole. Nearly thirty years later, during the IGY, explorers found the Jacobs windmill still turning, despite off-the-scale readings on a maximum-measuring wind-speed meter left as an experiment.

In the 1940s, the U.S. had a rural electrification project killed the practical market for wind-generated power. The techniques were almost lost.

In the 1970s many persons 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 from using wood and automobile parts. Most persons 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 expecially sought-for.

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 power transmission, wind generators are now often on wind farms in windy locations and huge ones are being built offshore. Since they're an inexpensive means of generating electricity they are being widely deployed.

There is resistance to the establishment of windfarms due to perceptions that they are noisy and contribute to "visual pollution" i.e. they are considered to be eyesores. Siting them offshore should address these objections.

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