Tidal Stream Energy Resource – a case study at grid-connected Morlais, Irish Sea

A renewable energy resource with much potential, yet which is often overlooked in energy roadmaps, is tidal energy (both kinetic energy – tidal stream, and potential energy – tidal range). Tidal energy is particularly attractive in the energy mix due to its predictability. However, it is as yet under-developed globally, particularly the tidal stream resource. Morlais (“voice of the sea” in Welsh) is a 35 km² grid-connected tidal energy site in the Irish Sea, UK. Although the site has the potential for 240 MW of tidal stream energy, currently developers have agreed 38 MW of electricity at a Strike Price of £261/MWh. To aid development of the site, we have conducted measurement campaigns over the last decade, including complete multibeam coverage of the 35 km² site and the deployment of eleven acoustic Doppler current profiles (ADCPs), along with additional wave buoy and meteorological measurements.

Peak (spring) undisturbed power density exceeds 10 kW/m² over much of Morlais, with the most energetic locations closest to the shore — facilitating power export to the grid. There is a large submerged sand bank extending from a major headland (South Stack) which is responsible for some of the most energetic tidal streams. This sand bank has a width of around 300 m, rises around 20 m compared to the surrounding sea bed, and there is evidence that it produces secondary flows that have been observed at many of the ADCP moorings. There is significant interaction of waves and currents across Morlais. However, this mainly influences wave properties, which could affect maintenance of moorings or devices (due to increased wave steepness), rather than directly influencing the tidal energy resource. There are large variations in flood/ebb asymmetry across the site, and this can largely be explained by the phase relationship between the principal lunar semidiurnal constituent M2 and its first harmonic, M4. Although prominent tidal energy test sites (e.g. EMEC in Orkney) also exhibit strong tidal asymmetry, it could be more of an issue for a commercial site like Morlais since it affects the timing of power export to the grid.