Wind Generators

History of Wind generators
The first known wind generators were used in Persia as early as 200 B.C. and were used for for grinding grain. The same type of wind generators were later introduced into the Roman Empire by about 250 A.D. By the 14th century the dutch were using wind generators for pumping and as mills. By 1900 the dutch had about 2500 wind generators that produced an estimate peak power of about 30 mega-watts.

The first wind generator that was used for producing electricity 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. By the 1930 wind generators were mainly used in remote areas to produce electricty as the electricity distribution grid was not widespread. The predecessor tp modern horizontal-axis wind generator 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 electricty distribution grid. It was said to have an annual load factor of 32 per cent, which isnot much different from current wind generators.

Horizontal axis wind generators
Horizontal axis wind generators have the main rotor shaft and electrical generator at the top of a tower, and must be pointed into the wind. Small generators are pointed by a simple wind vane or tail. Large generators often use a wind sensor coupled with a servomotor. Most large wind generators use a gearbox, which turns the slow rotation of the blades into a quicker rotation that is more suitable for generating electricity.

A tower produces turbulence behind it, so the wind generator is usually pointed upwind of the tower. Blades for an upwind vertical axis wind generator are made stiff to stop the blades from being pushed into the tower by high winds. Downwind wind generators have been built, despite the turbulence, because they don't need an additional mechanism for keeping them in line with the wind, and at 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 very important for remote area power systems, most horizontal axis wind generators are upwind machines.

Vertical axis wind generators
Vertical axis wind generators have the main rotor shaft running vertically. The advantages of this configuration are that the generator and/or gearbox can be placed at the bottom, near the ground, so the tower doesn't need to support the additional weight, and that the generator doesn't need to be pointed into the wind. They generally also operate at lower wind speeds. However, they are not as efficient at extracting energy from the wind.