Effects of Stall
In addition to the reduction in power output caused by tip losses and viscous drag, the effects of stall on the performance of the turbine must be considered. Stall generally occurs at high wind speeds (low tip speed ratios) and/or with high blade pitch angles. Static airfoil characteristics, such as the nonlinear changes in the lift with the onset of flow separation, can be incorporated into the БЕМТ theory using a look-up table or other such strategy, such as described in Section 7.11.3 for the helicopter rotor. Approaches specifically for the wind turbine problem are also described by Tangier (2002) and Coton et al. (2002). Representative results of power output showing the effects of nonlinear aerodynamics, and the effects of stall in particular, are shown in Fig. 13.17. While stall is often used on wind turbines as a means of power regulation (these are called “stall-regulated” turbines), it generally has a deleterious effect on power output if it occurs at other operating conditions. Notice that after peak efficiency is attained the effect of stall tends to cause the power output to drop much more sharply with increasing wind speed than if no stall was present. This is consistent with experimental measurements of power output on wind turbines and illustrates the need for good stall models if the power output is to be predicted accurately over a wide range of wind speeds. The numerous difficulties of representing nonlinear aerodynamics and stall effects has historically limited the accuracy of predictions of blade loads and the power output from wind turbines.