Modelling Passive Yawing Motion of Horizontal Axis Small Wind Turbine: Derivation of New Simplified Equation for Maximum Yaw Rate

Abstract

A large number of small wind turbines are installed in an urban area or on a rooftop where they experience sudden changes of wind speed and wind direction. For small wind turbines, the yawing load is one of the important design drivers because of its severity. Typically, a passive yaw system using a tail fin is employed for these small wind turbines. IEC61400-2 ed.2 provides a simplified equation for the maximum yaw angular velocity, or yaw rate, for use in estimating the yawing load. However, it is unclear whether this yaw rate considers the sudden change of wind direction. Furthermore, this simplified equation of yaw rate is only function of a rotor radius. In this paper, in order to consider the sudden change of wind direction, the authors derived the theoretical equation of the yawing motion to calculate yaw rate up to yaw angle of 180 degrees. This equation embraces the design variables including rotor radius, design tip speed ratio, tail fin area, and moment of inertia about the yaw axis. This theoretical equation was verified by comparing with wind tunnel test results. A number of theoretical calculations were undertaken varying all the design variables to find the relationship between the design variables and the yaw rate. Then, a new simplified equation, to calculate the yaw rate for a small wind turbine with a passive yaw system, is derived. This simplified equation is more practical than the existing IEC standard because it includes all design variables. This new equation enables to the yawing load to be estimated even for large change of wind direction.