Conclusion
The physics of windmills

The implementation of wind turbine electricity generation must be analysed at many levels.

The first is at blade level , which essentially consists in analysing the external forces acting on it: the force of the wind, the force of air resistance, and the resistance due to generator coupling. The rotational speed of a wind turbine is limited by the appearance of aerodynamic instabilities at the blade tip, i.e. around 350 km/h. This speed corresponds to an optimal wind speed of around 66 km/h. The calculated efficiency is in the order of 25% of the Betz limit, itself equal to 60% of the wind power intercepted by the area swept by the blades. The overall efficiency of large wind turbines is thus in the order of 15%. This optimal value is obtained for wind speeds from 15 to 20 m/s. Typically a 4 MW wind turbine rotates at 20 rpm...