This paper reports the experimental study on passive yaw behavior of a horizontal axis small wind turbines.The yaw angle velocity in a small wind turbine is one of the significant design values and IEC61400-2 Ed.2 prescribes its maximum value should be less than 3 rad/s. However, in practice, in the case of small wind turbines with diameters under 2 m, the yaw angular velocity may exceed the limit of 3 rad/s. The authors conducted a theoretical evaluation as well as a wind tunnel test to examine the possible maximum yaw velocity. The model wind turbine has a diameter of 1 m, blade number of 5, solidity of 0.26 and 2 types of tail fins (large and small). For each tail fin size, the yaw behavior was studied for a range of rotational speeds and wind speeds.The results show that the yaw angular velocity becomes larger with increasing wind velocity and tail fin area. The larger the rotor rotational speed, the smaller the yaw angular velocity. Furthermore, a maximum yaw angular velocity of 3.84 rad/s was observed at a wind speed of 10 m/s, which exceeds the conventional design value of 3 rad/s.

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.

This paper reports experiments and theory on some small wind turbines of rotor diameter 100 cm and less. Discrepancies in power performance have been found from predictions of blade element theory. Wind tunnel experiments showed that performance depends on the solidity of the fixed diameter rotor due to the dependence of Reynolds number on chord length. In particular, power output is lower than theory predicts on wind speed less than about 10m/s. For turbine rotors of constant diameter, output depends on solidity. In general, at low wind speeds, rotors with greater solidity give a higher output than with less solidity. This is due to the difference in Reynolds number, which itself is associated with chord length. The optimum value of Reynolds number for such small turbines is 100,000.

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