ON THE FREE YAW BEHAVIOR OF HORIZONTAL AXIS WIND TURBINES

Torque associated with rotor stall is shown to be an important factor in the yaw behaviour of fixed pitch horizontal axis wind turbines (HAWT). For a given operating machine, the best performance occurs when the plane of rotation is perpendicular to the wind velocity for all wind speeds, V, less than a fixed value, V//0. For V greater than V//0 a velocity dependent torque yaws a free rotor to more efficient energy gathering positions provided the yaw torque exceeds the corresponding machine frictional torque. The optimum angular positions (yaw trajectories) computed from dynamic equilibrium considerations are compared with, and shown to be in satisfactory agreement with, solutions furnished by a model based on a postulated energy gathering function. The postulational approach developed is particularly useful because of its generality and simplicity in describing the performance of a HAWT.