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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.

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Savonius

 is a type of vertical axis wind turbine  (VAWT) generator. The Savonius is adrag-type VAWT which operates in the same way as a cup anemometer Savonius wind turbines typically only have an efficiency of around 15% - i.e. just 15% of the wind energy hitting the rotor is turned into rotational mechanical energy. This is much less than can be achieved with a Darrieus wind turbine (which useslift rather than drag. 

The speed of the cups of a cup anemometer (and aSavonius wind turbine) cannot rotate faster than the speed of the wind and so they have a tip speed ratio(TSR) of 1 or below. Therefore Savonius type vertical axis wind turbines turn slowly but generate a high torque.

 This does not make them very suitable for electricity generation since turbine generators need to be turned at hundreds of RPM to generate high voltages and currents. A gearbox could be employed but the added resistence would leave the Savonius requiring a very strong wind to get spinning - i.e. it typically would not self-start.

Savonius wind turbines 

are ideally suited to applications such as pumping water and grinding grain for which slow rotation and high torque are essential. Because of the torque yield of a Savonius wind turbine, the bearings used must be very sturdy and may require servicing every couple of year

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Darrieus Wind Turbine

A Darrieus is a type of vertical axis wind turbine (1) (VAWT) generator. Unlike the Savonius wind turbine (2), the Darrieusis a lift-type VAWT. Rather than collecting the wind in cupsdragging the turbine around, a Darrieus uses lift forcesgenerated by the wind hittingaerofoil’s to create rotation.

Benefits of the Darrieus Wind Turbine

A Darrieus wind turbine can spin at many times the speed of the wind hitting it (i.e. the tip speed ratio (TSR) is greater than 1). Hence a Darrieus wind turbine generates less torque than a Savonius but it rotates much faster. This makes Darrieus wind turbines much better suited to electricity generation rather than water pumping and similar activities. The centrifugal forces generated by a Darrieus turbine are very large and act on the turbine blades which therefore have to be very strong - however the forces on the bearings and generator are usually lower than are the case with a Savonius.

 Darrieus wind turbines are not self-starting. Therefore a small powered motor is required to start off the rotation, and then when it has enough speed the wind passing across the aerofoil’s starts to generate torque and the rotor is driven around by the wind. 

An alternative is shown in the illustration above. Two smallSavonius rotors are mounted on the shaft of the Darrieus turbine to start rotation. These slow down the Darrieus turbine when it gets going however they make the whole device a lot simpler and easier to maintain


copyright @Centre for Application of Renewable Energy 2007