Determination of rated wind speed for maximum annual energy production of variable speed wind turbines

Rated wind speed is recognized as one of the key design factors affecting the overall power production of a wind turbine. No formulation is found in literature to relate the rated wind speed to the wind turbine power curves and the annual energy production (AEP). This paper aims to formulate the suitable rated wind speed for variable speed wind turbines continuously operating at maximum power coefficient for maximizing AEP. A capacity value is introduced which relates AEP to an integral function of the rated wind speed using Weibull distribution of wind speeds and the constant power coefficient of variable speed wind turbines. The capacity values are calculated and presented versus rated wind speeds at different wind classes and Weibull parameters. From the results, the suitable values for the rated wind speeds for maximizing AEP are found which are considerably higher than normally used values and varied from 2 to 5 times of the annual mean wind speed. For instance, for the mean annual wind speed of 4 m/s and the shape factors of k = 1.2, 1.6, 2.0, 2.4, 2.8, 3.2, and 3.6, the converged rated wind speeds are Vrate = 20, 19, 14, 12, 10, 10, and 9 m/s, respectively. On this basis, new charts for rated wind speeds are introduced for selecting suitable wind turbines for maximizing AEP. It is concluded that for some selected wind turbines operating at lower rated wind speeds, the AEP may fall below about 43% of actual achievable AEP when employing higher recommended rated wind speeds. Hence, it is shown that selecting the right rated wind speed wind turbines has great impact on overall energy production of a wind site.