Assessing tidal energy resource in the Iroise Sea using High Frequency radar and in situ velocity measurements

A year-long surface current velocity time series recorded by high frequency radars were used to study circulation and hydro-kinetic resource in the Iroise Sea. The study is focused on the analysis of tidal currents around the Ushant Island which is a promising site for tidal energy generation. The analysis reveals current velocities of 4 m/s northwest of the Ushant Island and in the Fromveur Strait, with 1 m/s threshold exceeded 60% and 70% of the time respectively. A pronounced asymmetry of the surface currents was diagnosed between the flood and ebb flow around the Ushant Island. The asymmetry ranges between 0.5 and 2.5 with the strongest variation in the Fromveur Strait. Radar derived velocities were compared with velocity profiles recorded by ADCPs in the radar coverage zone. The comparison demonstrates a very good overall agreement: with the correlation of 0.86 and zero phase lag. The ratio of the depth averaged to surface velocity is found to be 0.8. When averaged over the 15 m bottom layer, the velocity magnitude ratio is 0.7. Analysis of the bottom mounted ADCP data allows accurate quantification of the current velocity variation with depth, which increases confidence in hydro-kinetic resource assessment from the radar data. A method for optimizing the locations of energy conversion devices is presented. It offers a possibility to provide a balanced power generation in a region of opposing flood- versus ebb-dominated asymmetry if the asymmetry occurs in a region of a limited spatial scale (Fromveur Strait).