Rated wind speed reality or myth for optimization in design of wind turbines

The rotor design procedure of wind turbines starts first with adopting a rated output power and a rated wind speed. There is lack of modelling to determine an optimized rated wind speed and no evidences are observed to suggest the best rated wind speed to produce maximum output power annually. For example in the market of small wind turbines, it is frequently observed that the rated wind speed is taken from small value of 8 m/s to large value of 20 m/s. To examine overall performance of these wind turbines, it is required to develop mathematical models to relate the annual power production of the wind turbines to the given rated wind speeds. By examining power curves of some small European wind turbines from 100 to 900 Watt with constant speed generators, a simplified mathematical model for power curves is introduced and combined with Weibull distribution of wind speeds. Results of the model based on a capacity factor are presented versus the rated wind speed using wind characteristics of an optional region. It is observed that between the cut-in and cut-out wind speeds, there is an optimum rated wind speed above which the annual output power production of the wind turbine remains unchanged.