Effects of waves on BEM theory in a marine tidal turbine environment

Blade Element Momentum (BEM) theory is a well understood and proven method for modeling blade loads and determining steady state performance characteristics of wind turbines. Recently this theory has successfully been applied to horizontal axis marine current turbines when incorporating modifications that lead to better predictions of turbine performance for a range of operating conditions. Relatively little work exists in the implementation of BEM theory in a marine environment with surface gravity waves. A better understanding of the effects of waves on tidal turbines is necessary to predict fatigue loading that can eventually lead to blade failure. This paper presents a BEM numerical model that incorporates the unsteady velocities due to the presence of waves and assesses the effects of waves on tidal turbine performance. Numerical results of the coefficient of power (CP) and coefficient of thrust (CT) match closely with experimental results for the mean Cp and CT for a range of tip speed ratios. The model is also able to predict the instantaneous amplitudes of the performance characteristics as compared with the measured values except for the BEM calculated thrust which shows a 20% reduction in amplitude.