Power and thrust control by passive pitch for tidal turbines

Tidal turbines operate in unsteady and non-uniform flows and thus experience large load fluctuations. In this study, we explore the effects of changes in freestream velocity and yaw misalignments on the performance of a turbine equipped with passively pitching blades. We compare the experimental results from a 1.2-m diameter turbine in a recirculating open channel facility and numerical results based on blade-element momentum theory. The thrust remains approximately constant with increasing free stream speed, meaning that the structural load on the turbine is minimised. While not held constant, the power increases at less than half of the rate of a turbine with fixed pitch blades. This finding shows that power can be controlled effectively by changing the tip speed ratio, allowing, for example, the power to be capped at a rated value by marginally increasing the angular velocity. Furthermore, the power and thrust are found to be negligibly sensitive to yawed incoming flow compared to a fixed-pitch turbine. Overall, these results demonstrate that passive pitch is a viable alternative to active pitch, and pave the way for further development of this technology.