Lower Velocity Sites Improve the Tidal-Stream Energy Resource

Abstract:

It is essential that developers have detailed knowledge of the tidal-stream energy resource. ROMS hydrodynamic models (~1 km resolution) of key areas in northwest Europe, were used to examine the spatial and temporal distribution of the tidal-stream resource. Currently, sites with peak spring tide velocities (M2 and S2 constituents) in excess of 2.5 m/s and water depths between 25 and 50 m are preferred. When assuming this so-called “1^st generation” criteria, a limited resource with limited scope for long-term sustainability of the industry was calculated for the Irish Sea; a key area for UK development. Selecting sites that also included 20% lower velocities (>2 m/s) and deeper water locations (>25m) resulted in a seven-fold increase in the available resource (for the Irish Sea). Although new engineering challenges will be encountered (e.g. more wave exposed locations) by developing these 2^nd generation tidal-stream energy sites (>2m/s and >25m), some oceanographic challenges would be improved. For example, the flood-ebb tidal flow is not typically rectilinear at 1^st generation UK sites (a mean error from rectilinear of ~20° in this assumption), which is reduced to near-rectilinear flow (∼3° error) when including 2^nd generation sites. Analysis of our northwest European model revealed more phase diversity is offered by developing lower tidal energy sites, allowing firm and constant electricity generation. Moreover, at 1^st generation sites, we calculate significant, and unaccounted, variability in annual practical power generation. For example, mean peak spring tidal velocities can under-estimate the annual practical resource by up to 25%, for regions experiencing similar mean peak spring tidal velocities, due to the ratio of M2 and S2, together with the influence of other tidal constituents, such as K1 and O1. Therefore, based on prevalence, firm power and engineering challengers, we find a strong case for developing lower flow technologies.