Performance evaluation and fast prediction of a pitched horizontal-axis tidal turbine under wave-current conditions using a variable-speed control strategy

Abstract

A horizontal-axis tidal turbine (HATT) connected to a floating carrier experiences six degrees of freedom due to surface waves, significantly deviating the relative operating velocity and performance. To improve the output power, the HATT must employ a variable rather than conventional fixed speed control strategy, although the outcomes and disadvantages remain unclear. Therefore, this paper develops a CFD model to estimate the response of the HATT in wave-current states, under pitch motion using a speed control strategy. The results indicate that the speed control strategy can effectively improve the output power of the HATT during the pitching motion. The performance is approximated as the sum of a constant and pitch-dependent terms: damping and added mass. The best-fitted performance coefficients of load, power and moment are investigated with the pitch amplitude and period effect. The results indicate that the amplitudes of fluctuations are related with the amplitude and frequency of the pitch due to larger interference in the HATT operating velocities and dynamics of blade performance. The added mass is smaller than the damping term and can be ignored. The findings can be useful for the implementation of modern turbine control systems, to improve the cost-effectiveness of floating tidal energy systems.