Tidal currents represent a largely ignored renewable energy resource. Most early attention to harnessing the energy of the tides concentrated on tidal barrage systems, which aim to extract the energy available from the rise and fall of the water level in locations with a high tidal range. The La Rance tidal power station developed in the 1960’s in France and continued proposals for a barrage on the Severn Estuary in the UK are examples of this method of extracting energy from tides. Most recent attempts to develop the tidal energy resource have, however, been directed towards harnessing the rapid currents encountered in many coastal regions of the world. In the UK the inter island channels of Orkney and Shetland which frequently experience spring tidal currents in excess of 3 m/s appear to offer some intriguing opportunities for development. The energy flux in a 3 m/s tidal current is considerable at 14 kW/m2.
The harnessing of energy in a tidal flow requires the conversion of kinetic energy in a moving fluid, in this case sea-water, into the motion of a mechanical system, which can then drive a generator. It is not too surprising therefore, that many developers propose using technology that mirrors that which has been successfully utilized to harness the wind, which is also a moving fluid. Of course, there are obvious differences between the environment the extraction devices are exposed to and the details of the technology, but the majority of existing proposals follow the format of ‘windmills in the sea’.