Tidal Power.

The basic idea is to build a dam across an estuary to trap water at high tide. Sometimes called 'sea dams'. The water is then released, on the falling tide, through  turbine's to generate electricity. The Seven and  Mersey estuaries are possible sites for UK. One of the few tidal power systems in the world is to be found in the Rance Estuary in France. It was constructed in 1966.

The barrage is 750 metres long, from Brebis point in the west to Briantais point in the east. It is located south of Dinard and Saint Malo, at the mouth of the river. The power plant portion of the dam is 332.5 metres long. The tidal basin measures 22.5 kmē. The plant's turbines utilise the energy of the tides and the river current in turn; and as such are bi-directional.

In spite of the high cost of the project, the plant's costs have now been recovered, and electricity production costs are lower than for nuclear power generation. (1.8c per kWh, versus 2.5c per kWh for nuclear).