Tidal energy exploits the natural ebb and flow of coastal tidal waters caused principally by the interaction of the gravitational fields of the earth, moon and sun. The coastal water level fluctuates twice daily, alternatively filling and emptying natural basins along the shoreline. The currents flowing in and out of these basins can be exploited to turn mechanical devices to produce electricity.

A variant of tidal energy is tidal stream (or marine current) technology. Tidal streams are fast sea currents created by the tides, often magnified by topographical features, such as headlands, inlets and straits, or by the shape of the seabed when water is forced through narrow channels.

How it works

The technology required to convert tidal energy into electricity is very similar to that used in traditional hydroelectric power plants. Gates and turbines are installed along a dam or ‘barrage’ that goes across a tidal bay or estuary. When there is an adequate difference in the height of water on either side of the dam, the gates are opened and the ‘hydrostatic head’ that is created causes water to flow through the turbines, turning a generator to produce electricity.

Electricity can be generated by water flowing both into and out of a bay. As there are two high and two low tides each day, electrical generation from tidal power plants is characterised by periods of maximum generation every six hours. Alternatively, the turbines can be used as pumps to pump extra water into the basin behind the barrage during periods of low electricity demand. This water can then be released when demand on the system is at its greatest. This allows the tidal plant to function with some of the characteristics of a . In order to produce practical amounts of electricity, a difference between high and low tides of at least 5 metres is required.

The technology used for tidal streams is slightly different to that used in tidal barrages, and is still in its infancy. Tidal stream devices are similar to submerged wind turbines and are used to exploit the kinetic energy in tidal currents.

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