Experimental comparison of blade pitch and speed control strategies for horizontal-axis current turbines

Abstract

The majority of utility-scale horizontal-axis current turbines use either speed or pitch control to maintain a constant power output once the currents exceed a certain threshold: the turbine-specific “rated speed”. In this study, we experimentally characterized power performance and turbine loading over a range of blade pitch settings and tip-speed ratios for a threebladed horizontal-axis turbine. We then implemented a control strategy to maintain power output in time-varying currents using blade pitch control and compare the turbine performance under this control strategy to “overspeed” and “underspeed” control strategies for a fixed pitch turbine. The experiments were conducted with a laboratory-scale 0.45-m diameter turbine in an open channel flume with a 35% blockage ratio. During pitch characterization experiments, inflow velocity was maintained at 0.8m/s with 4% turbulence intensity. During time-varying inflow experiments, currents varied from 0.7 to 0.8m/s over a 20- min period, while a proportional controller regulated either blade pitch or rotor speed, and we recorded turbine power output and turbine loads. In this velocity range, where turbine performance is independent of Reynolds number, we demonstrated that pitch control substantially reduced torque requirements relative to underspeed control and turbine loads relative to overspeed control. Additional tests were conducted for underspeed control and pitch control in a Reynolds-dependent regime with time-varying inflow between 0.4–0.5 and 0.5–0.6 m/s. These cases suggest that blade pitch control could provide even greater benefits relative to speed control in small-scale applications.