Effects of extreme wave-current interactions on the performance of tidal stream turbines

The main objective of this paper is to analyse extreme cases of wave-current interactions on tidal stream energy converters. Experiments were undertaken in the INSEAN tow tank facility where carriage speeds of 0.5 and 1m/s were used with and without waves. The waves studied in this testing campaign had wave heights of 0.2 to 0.4m with a 2s wave period in a stationary reference frame. These wave conditions were considered extreme cases considering the use of a turbine with a rotor diameter of 0.5m. The turbine was equipped with a torque transducer, an encoder and a strain gauge to measure both the rotor torque and the forces on a single blade root. The results of the experiments showed that extreme wave-current cases can result in significant variations in power. Investigating the time histories of the blade root loading in wave-current conditions illuminated the importance of the relationships between the wave phase and blade angular position, and the number of blade rotational periods in a wave period. These affected the loading patterns and also the loading range seen by the blade, both of which have important implications for the fatigue life of the blade.