Wind turbine types

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

Horizontal Axis Wind Turbine

    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 to drive a generator.
    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. Since turbulence leads to fatigue failures, and reliability is so important, most HAWTs are upwind machines.

Netherlands windmill

    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.

Rural windmill

    The Eclipse windmill factory was set up around 1866 in Beloit, Wisconsin and soon became a huge success building mills for farm water pumping and railroad tank filling. Other firms like Star, Dempster, and Aeromotor also entered the market. Hundreds of thousands of these mills were produced before rural electrification and small numbers continue to be made. 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 by the 1950's. They were also produced in other countries like South Africa and Australia (where an American design was copied in 1876). Such devices are still used in locations where it is too costly to bring in commercial power.

Modern wind turbine

    Turbines used in wind farms for commercial production of electric power are usually three-bladed and pointed into the wind by computer-controlled motors. This type is produced by Danish and other manufacturers. These have high tip speeds of up to six times the wind speed, high efficiency, and low torque ripple which contributes to good reliability. The blades are usually colored light gray to blend in with the clouds and range in length from 20 to 40 metres (65 to 130 ft) or more. The tubular steel towers range from about 200 to 300 feet (60 to 90 metres) high. The blades rotate at 16-22 revolutions per minute, and a speed increaser gear box steps up the speed of the generator. Some models operate at constant speed, but more energy can be collected by variable-speed turbines which use a solid-state power converter to interface to the transmission system. All turbines are equipped with high wind shut down features to avoid over speed damage.

Vertical Axis Wind Turbine

    Vertical-axis wind turbines (or VAWTs) have the main rotor shaft running vertically. Key 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 pulsating torque that can be produced during each revolution and 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, resulting in lower energy extraction efficiency.

Darrieus wind turbine

    "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.

Gorlov helical turbine

    Essentially a darrieus turbine in a helical configuration. Patented in 2001. It solves most of the problems of the Darrieus rotor. It is self-starting, has lower torque ripple, low vibration and noise, and low cyclic stress. High reliability is expected from tested or matured designs. At least two wind turbine products are on the market as of 2007, including the Turby wind turbine and the Quietrevolution wind turbine. Most importantly, the GHT is an excellent turbine for zero-head hydropower, and appears to be a much needed ecologically benign and affordable solution for micro-hydropower. It is up to 35% efficient, which is competitive with the most efficient VAWT's.

Giromill

A subtype of Darrieus turbine with straight, as opposed to curved, blades. The cycloturbine variety have variable pitch to reduce the torque pulsation and are self-starting. 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. Recently , this type of turbine has been advanced by former Russian rocket scientists who claim to have increased the efficiency of the VAWT up to 38% . A company , SRC Vertical Ltd. has been formed , and has begun selling the new turbine.

    Savonius turbine

    These are drag-type devices with two (or more) scoops that are used in anemometers, the Flettner vents (commonly seen on bus and van roofs), and in some high-reliability low-efficiency power turbines. They are always self-starting if there are at least three scoops. They sometimes have long helical scoops to give a smooth torque. The Banesh rotor and especially the Rahai rotor improve efficiency with blades shaped to produce significant lift as well as drag. A new variety uses sails that can open or close with changes in wind speed.
        (src: wikipedia)