Experimental study of tidal turbines loading and performance in oblique waves and currents

Tidal energy exploitation is at an early development stage. World wide, installed capacity for tidal energy is 18 MW. In Europe only, installed capacity for wind is 168 GW. Levelised Cost of Energy (LCOE) projections for tidal energy are esti mated at 328 £/MWh by 2025. In contrast, LCOE for onshore and offshore wind are 60 £/MWh and 100 £/MWh, respectively. These costs need to be reduced to improve the long-term economic viability of the sector. High costs of tidal turbines are, in part, the result of load uncertainties caused by the highly variable flows tur bines experience. Turbine performance and loading are measured for a 1:15 scale model tested in the FloWave circular, combined wave and current basin at the Uni versity of Edinburgh, UK. Three different flow directions were tested. Each of them were also combined with three different regular waves in four different directions. Fifteen physical quantities were measured including flow velocity, wave height, ro tational speed and rotor and foundation loads. It was found that wave height or period have no significant impact on the average loading of the turbine. However, waves not-normal to the rotor plane showed a lower influence on the turbine aver age loading than waves normal to the rotor plane. In addition, the presence of waves increases the load fluctuations in the rotor by almost a factor of three. Rotor loading relative to the rotor's angular position experience three dips during a single rotation associated to the passage of the blades in front of the support tower. Analysis in the frequency domain to identify key frequencies with potential impact is presented. The frequency of the wave-induced loads can be observed on the rotor load spectra, where the peaks associated with waves have the highest amplitude, followed by the frequency of the blades passing in front of the support tower.