Flume testing of passively adaptive composite tidal turbine blades under combined wave and current loading

Elastic bend-twist coupling of composite tidal turbine bladesenables the blades to adapt to the hydrodynamic loads, passively feathering with increasing thrust. Previous numerical and experimental work has demonstrated that this passive feathering can decrease blade loads and shed excess power under steady flow conditions. It is also important to consider the effects of dynamic loading on the blades, as waves will frequently be encountered in the field. This paper investigates the performance of a scaled tidal turbine with bend-twist composite blades compared to a turbine with equivalent rigid blades under current and co-directional wave-current conditions. Experimental testing in the flume at IFREMER demonstrated that for a turbine operating in current at the optimum tip speed ratio the mean thrust and power could be reduced by as much as 11% and 14% respectively by using bend-twist composite blades compared to rigid aluminum blades. The average loads acting on the aluminum and composite blades were found to be similar in the wave-current condition tested. However, the magnitudes of the thrust and torque fluctuations per wave cycle were reduced by up to 10% and 14% respectively, indicating that the composite bend-twist blades dampened the oscillating wave loads acting on the turbine system.