Wind has become an important source of energy during the last 20 years. By 2010 there was about 200 GW of wind turbine capacity installed worldwide and the energy production reached 430 TWh. This is around 2.5% of the worldwide consumption of electric energy, and it is equivalent to more than 50 blocks of nuclear reactors. The largest turbines in use today have a 7-MW rated energy output and rotor diameters up to 125 m, both on land and in offshore installations. A description of the wind energy technology can be found in ref. [28].