Wind power has evolved rapidly in the last two decades with regard to both power rating and size of wind turbines that are currently being installed. In the past few years, several developments have taken place. There has not only been a continuous increase in the rated power of wind turbines being installed, but also in the development of reliable wind turbines rated around 2-10 MW which are grid safety code compliant and suitable for different installation environments— onshore and offshore, as it can be seen in the commercial products currently in the market.
Broadly, wind turbines (Manwell et al. 2010) can be classified into two groups: horizontal axis wind turbines, which are more efficient in capturing wind power particularly when it is concentrated in a particular direction and when wind speeds are relatively high, and vertical axis wind turbines, which are suitable for capturing wind power at low wind speeds and when the wind is omni-directional. Vertical axis wind turbines are mainly the Darrieus and Savonius types which either depend on the rotor blades generating lift as in the first case or on the impulse or drag of the rotor blades as in the second. Most Darrieus machines have their blades in an eggbeater configuration although several new designs including helical rotors are being introduced.