Abstract
Before tidal stream energy is exploited, tidal power resource and environmental assessments must be undertaken. This thesis explores limits to power extraction for tidal sites defined by a strait between an island and landmass. Numerical simulations provided by Fluidity are used to analyse power extraction from locations in the strait and around the island for an idealised island-landmass domain and an actual coastal site.
The numerical model is verified by comparing predictions with analytical solutions for inviscid flow past a circular cylinder located in the centre of a channel and in the vicinity of a wall. The model is then validated against laboratory measurements of flow patterns for impulsively-started flow past a submerged circular cylinder, and for flow past a surfacepiercing circular cylinder in oscillatory laminar shallow flow. It is demonstrated that the numerical method captures satisfactorily the mechanisms of early wake formation, which indicates the model can be applied to assess tidal stream resource within the coastal geometries considered herein. Finally, the methodology to account for power extraction is satisfactorily verified for bounded and unbounded flows.
Contrary to current practices, results from a parameter study for different idealised coastal sites reveal that the maximum power extracted in the strait is not well approximated by either the power extracted naturally at the seabed or the undisturbed kinetic power. Moreover, an analytical channel model underpredicts the maximum power extracted in the strait due to its inability to account for changes in the driving head resulting from power extraction and flow diversion offshore of the island. An exception is found for islands with large aspect ratios, with the larger dimension extending parallel to the landmass; i.e. the island-landmass geometry approaching that of a channel. In this case, the extracted power is satisfactorily approximated by the power naturally dissipated at the seabed and there is good agreement with the analytical model. The maximum power extracted in the strait is shown to decrease when water depths offshore are greater than in the strait, underlining the importance of fully understanding the wider bathymetry of a given site. A similar conclusion is reached when strait blockage is reduced. The power extraction in the strait is found to be sensitive to both viscosity and seabed friction, and these parameters need to be properly estimated during the setup and calibration of models in order to reduce uncertainty. Power extraction increases when turbines are sited simultaneously both in the strait and offshore.
Tidal power assessment is performed for Rathlin Sound, off the coast of Northern Ireland. Again, no clear relationship is found between maximum power extracted in the strait and iii either the power dissipated naturally at the seabed or the undisturbed kinetic power. A similar ratio of power extracted to undisturbed kinetic power is obtained as for the equivalent idealised model. The analytical channel model underpredicts the maximum power extracted. The actual and idealised coastal site models indicate similar responses to changes in seabed friction, and similar reduction in power extraction with decreasing strait blockage.