Abstract
The majority of utility-scale horizontal-axis tidal turbines use speed control or pitch control to maintain power output once the currents exceed a threshold value (the “rated speed”) for the device. In this study, we experimentally characterize power performance and structural loading over a wide range of blade pitch settings and tip speed ratios for a three-bladed horizontal-axis turbine. We then implement a control strategy to maintain power output in unsteady currents using blade pitch control and compare its performance and loading profiles against those from overspeed and underspeed control for a fixed pitch turbine. The experiments are conducted with a 0.45- diameter turbine in an open channel flume with 35% blockage. An acoustic Doppler velocimeter was placed 3- diameters upstream of the rotor plane to measure free stream velocity. During pitch characterization experiments, inflow velocity was maintained at 0.8 m/s (Reynolds-independent) with 4% turbulence intensity. A six-axis load cell at the blade root allowed fluctuations in blade loads to be characterized separately from overall loads on the rotor, which were measured by a similar six-axis cell in the rotor hub. To demonstrate the effectiveness of pitch control in maintaining power output in time-varying inflow, a proportional feedback controller actuating blade pitch was implemented in oscillating flow 0.7-0.8 m/s with a 20- minute period and the resulting fluctuations in power and structural loading were recorded. A similar controller actuating rotor speed was also implemented, and the comparison with the pitch controller demonstrates that pitch control substantially reduces torque or thrust relative to underspeed or overspeed control, respectively.