Different turbine characteristics are required for each type of application. Most electrical-power generators,
or alternators, tend to require a relatively high speed of rotation. Consequently, turbines that run fairly fast, with the
tips of the rotor moving 3-10 times the wind speed, are generally preferable to low-speed turbines because less gearing is
required between the shaft of the wind turbine and the armature of the generator. A relatively low running speed is generally
favoured for wind turbines that are directly coupled to water pumps or other mechanical loads. A striking feature of wind
turbines intended for high-speed operation is the low rotor solidity, that is, the very small blade area in proportion to
the turbine rotor's total projected area. low-speed turbines feature a high solidity (either a small number of relatively
broad blades or a large number of narrower ones).
All modern high-speed turbines and most of the low-speed units incorporate blades
designed on airfoil principles. Some inefficient low-speed machines depend on a drag effect, as does a square-rigged sailing
ship. The foregoing remarks apply equally to wind turbines that are arranged with horizontal or vertical axes of rotation.
Typical horizontal-axis high-speed wind turbines have either 2 or 3 blades and resemble aircraft propellers. The blades of
these machines are commonly arranged to vary in pitch automatically to optimize performance under conditions of varying wind
speed. The most common high-speed vertical-axis turbine is the Darrieus rotor, named for its inventor. This machine is also
known as an "egg beater" because of its characteristic appearance.
The most common form of low-speed machine is the horizontal axis,
multi bladed form often found on farms. The turbine is usually connected, via a crank, to a reciprocating water pump. A related
design is the multi bladed "bicycle-wheel" turbine, an example of a relatively low-speed turbine used for electrical-power
generation. A simple vertical-axis design, often used for water pumping, is the split cylinder configuration known as the
Savonius rotor, after its originator. In this turbine each of the 2 or 3 rotor blades consists of a semi cylinder offset radially
from the axis of rotation. The design relies, in part, on a drag effect for its operation. It is not, therefore, a particularly
efficient configuration, but is relatively simple to make.
General advantages of wind turbines are the complete absence of
air pollution and a high energy-conversion efficiency. Well-designed wind turbines can recover up to 60- 80% of the kinetic
energy from the flow passing through their rotors. However, the low energy density available in the wind typically restricts
output to a range 0.1 to 0.8 kW/m2 (kilowatts per square metre) of the rotor's projected area. The result is a large machine
size in relation to output; for example, in a machine rated at 5 MW (MW=106 W), a high output for a single wind turbine, the
rotor diameter can be as large as 100 m. This size problem results in the use of clustered machines, known as wind farms,
to extract large power outputs from individual sites. Another problem with wind-energy systems is the variability of wind
strength, which leads to substantial fluctuations in power since output is roughly proportional to the cube of the wind speed.
Furthermore, all wind-energy conversion devices incur additional costs because they must be capable of withstanding storms.