TIDAL STREAMS

Tidal streams are cyclical flows set up by the gravitational attraction of bodies in our solar system, particularly the moon and the sun. They vary in amplitude and direction roughly on a daily cycle and a monthly cycle but are very predictable and reliable in their occurrence.

Recent studies have considered the UK’s tidal stream resource and produced the following predictions.

The reports by Black & Veatch (B&V) were prepared for the Carbon Trust under the Marine Energy Challenge programme. This report built on research carried out by Robert Gordon University (RGU) into the energy that can be extracted from a site without destroying the flow.

The B&V report gives lower levels of extractable energy than the other studies (see Table above). The extractable energy values quoted represents electrical energy after allowance for the overall conversion efficiency. More recent theories as to the energy in tidal stream flows using the impedance method (Salter) and shallow water wave theory (MacKay) have produced much larger estimates of the extractable resource. The 2010 Offshore Valuation Group report uses the simplified power density approach but arrives at an extractible power level for UK sites similar to Salter and MacKay.

The B&V report categorises the available tidal stream energy according to water depth. Nearly 80% of the available energy is in water depths greater than 40 metres.

The worldwide resource has not been assessed to the same level of breakdown as the UK's but is conservatively estimated to be in the order of 200 to 400TWh/y.

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