Wikipedia: Wind generator

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.

Strong and steady winds are ideal for power generation. Wind is powered by a temperature differential, 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're at high altitudes and partly because of the temperature differential between the differently sunlit sides of the mountain. 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. Within an area, they're will be wind variations due to the local topography; so often the winds are monitored and modeled before wind generators are installed.

Mechanical wind generators were based on windmill?s and were popular in the 1900's to the 1940's, before the rural electrification program brought grid connected electricity to the countryside. They started their revival in the 1970's 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 propeller wind generator is the most common type. The device consists of a vertical propeller connected through a power train to a generator. This mecanism is pointed towards the wind and is usually placed on a tall tower. The main advantages of this type are its high efficiency, relative simplicity and ability to lower its wind profile by stalling the rotor. 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 airfoil?s 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.

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