Tides are generated by the gravitational and rotational forces on the earth, involving the sun and the moon. Tidal energy is one of the oldest forms of energy used by humans. Indeed, tide mills, in use on the Spanish, French and British coasts, date back to the 11th century. The modern version of a tide mill is a barrage constructed over an estuary. Tidal power stations operate by capturing the tidal water, behind a barrage, and releasing the water at low tide through turbines generating electricity.
More sophisticated systems generate both on the ebb and the flood tides.
A large tidal range is essential, which only occurs in certain estuaries where the length and depth create water movement in resonance with the natural tidal periods. The equivalent to burning 26 million tonnes of coal per year. Electricity capacity of over 8,600 MW - as much electricity as 2 of the largest power stations. Electricity generated would cost 7p per kWh at commercial interest rates and excluding externalities UK Government has funded several large scale and well documented studies of all aspects of the scheme. The water turbines can be run in reverse to pump more water behind the barrage.