Numerical models to predict the performance of tidal stream turbines working under off-design conditions

As previously experienced by the wind industry, it is envisaged that tidal stream turbine blades will presentmisalignments or blade deformations over time as they are constantly working under harsh and highlyunsteady environments. Blade misalignment will affect the power capture of a tidal stream turbine andif not detected in time could affect other components of the drive train. Therefore, the aim of this paperis to compare the use of two numerical modelling techniques to predict the performance of a tidal streamturbine working under off-design conditions, in this case, the misalignment of one or more blades. Thetechniques used in this study are Blade Element Momentum Theory and Computational Fluid Dynamics.The numerical models simulate the performance of a three-bladed horizontal axis tidal stream turbine withone or two blades offset from the optimum pitch setting. The simulations were undertaken at 1.0 m/s flowspeeds. The results demonstrated that both unsteady BEMT and steady or transient CFD are able topredict power coefficients when there is a certain level of misalignment in one or even two blades. However,both techniques failed to accurately predict a loss of power performance at high rotational speeds.