Wake behind a wind turbine operating in yaw

Abstract

The object of this study is primarily to understand the behavior of the wake behind a yawed turbine, and if yaw can be a method for actively controlling the direction of the wake, and thereby controlling the power output of downstream turbines. Two model wind turbines were tested experimentally in the wind tunnel at NTNU. First of all, the performance of a single upstream turbine operating at fixed rotational speed with varying yaw angles was examined. Further, the aim was to understand to what extent the side force created by the yawed turbine affects the wake, thus the velocity deficits and turbulence intensities in the downstream flow field were experimented on. In addition, the performance and dynamic loads experienced by a second turbine operating at 3D downstream of the upstream turbine were examined. Finally, the overall efficiency of the wind farm was found for the different yaw scenarios.The study confirms that when a turbine is operating in yaw, both the power and thrust coefficient will decrease significantly with increasing yaw angle. Yawing the upstream turbine will also affect the behavior of the wake to a great extent, as the wake is deformed and deflected sideways. When the upstream turbine is yawed 40°, the width of the wake at 3D downstream is decreased to half its size of un-yawed condition and is shifted about 0.5D sideways. The performance of the downstream turbine increases with increasing yaw angle of the upstream turbine. When the upstream turbine is yawed 50°, the downstream turbine obtains a power gain of 24% compared to the un-yawed condition, resulting in a maximum power coefficient of 33%. This confirms that the second turbine experiences less interaction with an upstream turbine operating under yawed conditions.It was found that the optimal wind farm efficiency of the two model turbines occurs when the upstream turbine is yawed between 0° to 30°, resulting in a wind farm efficiency of approximately 54% for all three conditions. The power loss experienced by the upstream turbine is offset by the corresponding power gain of the downstream turbine. However, fatigue loads were found to act on both the turbine operating in yaw and the downstream turbine partly exposed to the wake, which will eventually reduce their longevity. Therefore, the result of the study carried out on two wind turbines in the wind tunnel suggests that it will not be beneficial to use yaw as a mechanism for controlling the wake direction and thereby increase the wind farm efficiency since the power gain of the downstream turbine will be offset by the power loss of the yawed turbine. Tailoring the blade design of the yawed turbines may, however, have a positive impact on the overall wind farm efficiency and also reduce fatigue loads.