Extracting energy from tidal currents: The ocean response at multiple space and time scales

Tidal stream energy is a reliable and predictable low-carbon energy source. Over the next few years the first arrays of multiple tidal stream turbines will be deployed mostly in UK, French and Canadian waters. The potential environmental effects should first be examined, in order to scale and site them appropriately. Large theoretical arrays of tidal stream turbines were designed for Scottish Waters (UK), to quantify the available power and the ocean response to its extraction. A comprehensive assessment of the tidal energy resource realistically available for electricity generation and the study of the potential environmental impacts associated with its extraction in Scottish Waters, can then lead the way to further development in different countries with potential tidal stream energy sources. In order to examine both local (100 km) spatial scales, the Scottish Shelf Model, an unstructured grid three-dimensional FVCOM (Finite Volume Community Ocean Model) implementation, is a useful tool, since it covers the entire NW European Shelf, but with a high resolution where the tidal stream energy is extracted. The arrays of tidal stream turbines were implemented in the model using the momentum sink approach, in which a momentum sink term represents the loss of momentum due to tidal energy extraction. A typical annual cycle of the NW European Shelf hydrodynamics was reproduced by the SSM model and compared with the same period perturbed by tidal stream energy extraction. The power extracted by the tidal stream arrays was estimated, taking into account the tidal stream energy extraction feedbacks on the flow and considering the realistic operation of a generic tidal stream turbine, which is limited to operate in a range of flow velocities due to technological constraints. The ocean response to tidal stream power extraction was then analysed at the temporal scale of a spring-neap tidal cycle and, for the first time, on longer term seasonal timescales. It is shown that the very large tidal stream arrays can introduce detectable changes to the tidal elevation, marine currents and ocean stratification patterns. Tidal elevation mainly increases upstream of the tidal farms locations (considering the direction of propagation of the tidal wave), while a decrease in the mean spring tidal range is observed downstream, along the UK East Coast and also in the Irish Sea. Marine currents, both tidal and residual flows, are also affected. They can slow down due to the turbines' action or speed up due to flow diversion and blockage processes, on both a local and regional scale. The strongest signal in tidal velocities is an overall reduction, which can in turn decrease the energy of tidal mixing and perturb the seasonal stratification on the NW European Shelf. Although the strength of summer stratification has been found to slightly increase, the extent of the stratified region does not greatly change, thus suggesting the enhanced biological and pelagic biodiversity hotspots, e.g. tidal mixing front locations, are not displaced. Such large-scale tidal stream energy extraction is unlikely to occur in the near future, since very large numbers of devices are required, but such potential changes should be considered when planning future tidal energy exploitation. It is likely that large scale developments around the NW European shelf will interact and could, for example, intensify or weaken the changes predicted here, or even be used as mitigation measures (e.g. coastal defence) for other changes, e.g. effects of climate change.

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