Design and Demonstration of a Passive Pitch System for Tidal Turbines

Tidal turbines operate in unsteady and nonuniform flows, conditions that give rise to large fluctuations in the loads generated. Previous studies [1], [2] have shown that allowing for the blades of a tidal turbine to passively pitch around a longitudinal axis can reduce the fluctuations in thrust and torque due to shear and turbulence. In this work, we investigate the effects of steady changes in the inflow velocity and direction on the loads generated by a turbine equipped with passively pitching blades. We do so by presenting the results of an experimental campaign conducted in the large recirculating water channel at the Institute of Marine Engineering in Rome, in which the turbine was subject to flows with different freestream speeds and yaw angles. Results show that a turbine equipped with passively pitching blades produces more power than one with rigid blades when the freestream velocity is lower than rated and vice-versa. In addition, we show that the thrust generated by the turbine remains constant despite significant changes in the freestream velocity. Moreover, the power and thrust generated by such a turbine are observed to be insensitive to yaw, suggesting that this technology can be used to overcome issues arising from misalignment between the turbine and the incoming current. Finally, we use these results to simulate the performance of two turbines in response to low-frequency changes such as those encountered over a tidal period: one equipped with an industry-standard pitch control system, and one with passively pitching blades. The comparison demonstrates that the turbine equipped with passively pitching blades results in the same energy yields as the turbine with active pitch, with the additional benefit of limiting the maximum thrust. Overall, these results suggest that passive pitch control can provide better performances than active pitch control, with an overall simpler and potentially more reliable technology.