Tidal energy
Tidal energy exploits the natural ebb and the flow of costal tidal waves caused principally by the interaction of the gravitational fields of the Earth-Moon-Sun system.
The costal water level fluctuates twice daily, alternatively filling and emptying natural basins along the shoreline. The currents flowing in and out of these basins and 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 featured such as headlands and inlets and straits or by the shape of the seabed when water is forced through narrow channels.
Although tidal power is variable, it is reliable and predictable and can make a valuable contribution to an electrical system which has a variety of sources. The technology required to harness tidal energy is well established, however tidal power remains expensive and there are relatively few applications worldwide. It is early days for tidal and tidal stream technology. There are no projects currently contributing to electricity supplies in the UK but development work is ongoing.
Likely contribution to renewable energy targets
The large tidal range along the west coasts of England and Wales provides some of the most favorable conditions in the world for the utilization of tidal power. If all reasonable exploitable estuaries were utilized, annual generation of electricity from tidal power plants would be some 50TWh, equivalent to about 15% of current UK electricity consumption.
Future development
At least 30 suitable tidal stream locations-have been identified around the UK. The country is also at the forefront of the development of these technologies. Two of the most advanced tidal stream prototypes, currently supported under the DTI Renewable Energy Research and Development Programme, are the Stingray project and the Sea flow projects, which are expected to reach full-scale demonstration in 2004. From 2010, tidal stream has the potential to make a significant contribution to renewable energy generation.
Pollution
Tidal energy and tidal stream energy do not result in the emission of gases responsible for global warming or acid rain associated with fossil fuel generated electricity.
Ecosystems
Changing tidal flows by damming a bay or estuary could result in negative impacts on aquatic and shoreline ecosystems as well as navigation and recreation. There are concerns that the enhanced mixing of water caused by tidal barrages has the potential to stimulate the growth of the 'red tide' organism Gonyalaux excavata which causes paralysis in shellfish. Estuaries are of key importance to migratory species of fish, many of which are the foundation for commercial fisheries - barrages could act as barriers to migration and damage fish.
Tidal alterations
Experience at La Ranee barrage in France showed only minor changes to local tides and the environmental impact has been negligible, but this may not be the case for other sites.