Plasma actuation for active control of wind turbine power

This research evaluates, optimizes and experimentally demonstrates a new method to reduce the lift and increase the drag of a wind turbine blade for controlling the turbine power at high wind speeds. The method consists of applying electro-fluidic plasma actuation to accelerate the separation of boundary layer on the wind turbine blade. The effect of plasma actuation on a two-dimensional wind turbine profile is first assessed numerically at low Reynolds numbers Re using a commercial CFD code to which a plasma actuator model is integrated. The results from these simulations indicate the existence of preferred operating conditions for the plasma actuator in terms of parameters such as its strength, the free-stream velocity and its chordwise position on the blade. With respect to the actuation strength, it is found that there exists a threshold strength beyond which there is a sudden jump in the lift reduction obtained. A similar jump in the actuation effect is observed when the free-stream Reynolds number is reduced past a certain limit. The existence of optimal positions for the actuator on the suction side of the blade is also observed. The optimum is situated near the airfoil's trailing edge for a pre-stall angle of attack and it is displaced upstream at stall and post-stall angles of attack. All the simulations indicate that the position of the actuator relative to the point of separation of the boundary layer is the key element in the actuator's effect on lift and drag. These simulations are validated by testing for the same flow conditions in wind tunnel. The wind tunnel measurements of lift and drag replicate the trends seen in the simulations. The experimentally validated CFD tool is then used to simulate wind turbine flows at a realistic Reynolds number. This study shows that the current level of plasma actuation strength is incapable of exerting any discernable influence on the performance of a wind turbine blade. The blade element momentum theory is used to determine the amount of lift drop that would be required for rated power operation and this requirement is compared with the impact of the current generation of plasma actuators. An attempt is made at estimating the actuation strength that would be necessary to bring about the power reduction required for rated operation. This analysis shows that actuator strength of two orders of magnitude higher would be required for the concept to work on its own to limit wind turbine power. This implies that it must be coupled with another method such as rotor speed control to have a realistic chance of application in the near future.