Investigation of power performance and wakes of wind turbines under yawed flow
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Abstract
Wind turbine wakes are recognized as a key issue causing under-performance of existing wind farms. To improve the performance and reduce the cost of wind power, one approach is to develop innovative methods to improve the net capacity factor by reducing wake losses. “Smart Wind Farms” employing active wake management strategies can help in reducing the wake impacts, thus improving the overall power performance of the array of wind turbines.
In this dissertation, the output power of a utility scale wind turbine under yawed flow and characteristics of the wake measured by TTU Ka-band radars are studied to explore the possibility of steering the wake of a yawed upwind turbine so that it does not reduce the power performance of a downstream turbine. Atmospheric conditions during the measurement period were analyzed using data from a nearby 200 m meteorological tower. Further, theory adapted from helicopters in forward flight was used to develop a low fidelity model to calculate the induced velocity and wake skew angle. The average induced velocity estimated by the model was compared with field data measurements from the Ka-band radars.
In theory, power production in yawed flow is a function of mass flow rate through the rotor and the induced velocity. Also, a yawed turbine is able to deflect its wake due to a non-uniform pattern of the induced velocity. Preliminary field observations show that the power performance of a turbine does not degrade significantly under yaw conditions up to approximately 15 degrees. Additionally, a yawed wind turbine may be able to deflect its wake in the near-wake region, changing the wake trajectory downwind, with the progression of the far-wake being dependent on several atmospheric factors such as wind streaks. Changes in the blade pitch angle also affect the characteristics of the turbine wake and are examined through field observations.
The purpose of this research is to set forth a methodology to characterize the turbine performance and wake characteristics under yawed flow from various measurement platforms.