Physical suitability of sites for community-scale tidal power generation
Tidal stream energy offers a renewable energy resource in response to global climate change. Most work has focused on the development of large-scale arrays. However, community-scale projects present a more economically viable option with greater flexibility for grid connection and reduced environmental impacts. This thesis examines the physical suitability of sites for community-scale tidal energy extraction, with examples taken from western Scotland and the northern parts of Ireland. An improved regional assessment of the tidal stream resource was conducted, using high resolution, 3D modelled velocity data combined with real turbine operational thresholds. 33 sites were found to show potential for tidal energy extraction, 15 in previously unrecognised locations. Detailed site-specific assessments were then carried out for two of the highlighted sites: (1) Cuan Sound, a narrow channel between two islands and (2) the Falls of Lora, tidal rapids over a shallow sill at the mouth of a sea loch (fjord). A series of matlab functions have been developed, allowing large-scale particle image velocimetry (LSPIV) to be applied to georectified images of these tidally energetic sites. This provides a novel site assessment tool, using the advected surface signature of turbulent structures to infer a highly-resolved, site-wide measurement of surface velocity that is difficult, if not impossible, to obtain by other assessment methodologies. Previous particle image velocimetry techniques, typically applied in a laboratory or riverine system, have been adapted to better suit the larger-scale, tidally energetic sites of interest here. The suite of functions produced includes: (1) image pre-processing tailored specifically to increasing the seeding density and reducing the noise of the more complex interrogation areas used for LSPIV in this marine context; (2) cross correlation coefficient calculation via normalised two dimensional cross correlation, which was found to perform more accurately and efficiently than previous techniques using convolution or fast Fourier transforms; (3) post-processing of the resultant high-resolution velocity field, allowing the derivation of higher-order flow properties, cross-axial transect extraction and site-wide power productivity estimates. A combination of LSPIV, Lagrangian drifter deployments, georectified imagery, hydrodynamic modelling and vessel-mounted ADCP transects allowed a comprehensive picture of peak ebb and flood hydrodynamics at Cuan Sound and the Falls of Lora. Horizontally expanding boundary layers and flow separation from the coast drove a strong sink-jet asymmetry across Cuan Sound, whereas the interaction between stratification and topographic constriction at the Falls of Lora drove turbulent jets with complex vertical structure. A turbine siting assessment based on velocity, horizontal shear, vorticity, asymmetry, water depth, bed slope and estimated power production, suggests suitable conditions within Cuan Sound for small-scale tidal stream generation warranting further depth-resolved investigation. In the Falls of Lora, the highly-localised jetting flows and density-driven flood-ebb asymmetry pose challenges for the available technology, suggesting, more generally, that sites with density-driven dynamics will be problematic and should be treated with caution when flagged in regional assessments.
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