Experimental comparison of passive adaptive blade pitch control strategies for an axial-flow current turbine

Two passive blade pitch control strategies for an axial-flow current turbine were developed and tested experimentally in a recirculating flume. The goal of the control is to regulate mechanical power, while limiting rotor loads, when flow conditions exceed the rated condition. Both strategies use blades fabricated with unidirectional carbon fiber oriented off-axis, such that the blades twist passively as they deflect in response to loading. One control strategy combines passive adaptive blades with overspeed control (operating at a rotational speed above the tip-speed ratio corresponding to peak efficiency) while the other combines passive adaptive blades with active pitch control (actuating blade pitch using motors at the blade root). Both strategies were implemented with a 0.45-m diameter turbine in linearly increasing inflow from 0.7 to 0.8 m/s and compared to control strategies using rigid, aluminum blades under the same flow conditions. The passive adaptive blades combined with active pitch control saw no improvement in steady-state load reductions relative to rigid blades used with active pitch control. However, the passive adaptive blades combined with overspeed control successfully produced constant torque with an only 12