New approach to tidal stream energy analysis at sites in the English Channel

Blunden, L.S. (2009) New approach to tidal stream energy analysis at sites in the English Channel. University of Southampton, School of Civil Engineering and the Environment, Doctoral Thesis, 295pp.

Record type: Thesis (Doctoral)

Abstract

Tidal stream power generation offers the prospect of predictable, low-CO2
power at a number of locations around the UK and the world. Previous
assessments of tidal energy resources have taken the form of desk studies
based on simplified navigational data. Where numerical model data has been
used it has been at too low a resolution to capture high velocity tidal flows
constrained by coastal topography. Analytical solutions for maximum energy
extraction in simple tidal channels have been produced, but they have not
been extended to more complex open-boundary cases such as flow around
headlands and islands. There is therefore a role for site-specific numerical
modelling, which when validated, offers the twin advantages of a
high-resolution picture of the resource and allowing simulation of momentum
extraction within the model to take place.

In order to parameterize the sub-grid-scale momentum extraction in such
models, a new analytical model of the velocity reduction in a large array of
tidal turbines has been derived. The model extends previous models of large
wind turbine arrays and uses analogies with flow through submerged
vegetation. It provides an equivalent added drag coefficient suitable for use in
a 2-D coastal numerical model.

A numerical model of the flows in the region of the Portland Bill headland
has been produced, forced by tidal elevations at the free boundary. A site
selection exercise was carried out for the Portland Bill location and an area of
around 12 km2 was identified as having a high potential for development
using mean cubed speed found through tidal analysis of model results
without energy extraction.

A large tidal stream generator array has also been simulated within the
Portland Bill model—linked to the new model for momentum extraction—and
was found to have a significant effect on the tidal parameters in the locality.
This was the first time that a large tidal array has been simulated in a realistic
coastal domain of large extent, with a parameterization that takes into account
the interaction of the turbines with the rough-wall flow in the natural state.
Results predict that there is a region downstream of the array extending
approximately 5–10 km around the simulated tidal stream turbine array in
which the tidal stream ellipse major axis is reduced by at least 5%. In the area
of momentum extraction the principal semi-diurnal tidal stream ellipse major
axis length was reduced by 10–15%.