Tidal energy traditionally involves erecting a dam across the opening to a tidal basin. The dam includes a sluice that is opened to allow the tide to flow into the basin; the sluice is then closed, and as the sea level drops, traditional hydropower technologies can be used to generate electricity from the elevated water in the basin. Some researchers are also trying to extract energy directly from tidal flow streams.

Tidal electricity generation involves the construction of a barrage across an estuary to block the incoming and outgoing tide. The head of water is then used to drive turbines to generate electricity, similar to hydroelectric dams. Barrages can be designed to generate electricity on the ebb side, or flood side, or both.

Tidal range may vary over a wide range (4.5-12.4 m) from site to site. A tidal range of at least 7 m is required for economical operation and for sufficient head of water for the turbines.

A 240 MWe facility has operated in France since 1966, 20 MWe in Canada since 1984, and 100 MWe in China since 1987. There have been proposals for new facilities, such as on the Severn estuary (UK), which might produce 8.2 GWe. Tidal energy schemes are characterised by low capacity factors, usually in the range of 20-35%.

There is a high capital cost for a tidal energy project, with possibly a 10-year construction period. Therefore, the electricity cost is very sensitive to the discount rate.

Tidal power generation may change the sedimentation and erosion patterns in the estuary. Pollutants discharged into the rivers upstream from the barrages may accumulate in the estuary.

Tidal Power

Tidal power utilises the twice daily variation in sea level caused primarily by the gravitational effect of the Moon and, to a lesser extent the Sun on the world's oceans. The Earth's rotation is also a factor in the production of tides. Tidal power is not a new concept and has been used since at least the 11th Century in Britain and France for the milling of grains.

Tidal Physics

An understanding of the principles which give rise to tides is essential to explain tidal power.

Whilst a thorough understanding of the interactions involved is quite complex the origin of tides can be explained in general terms by investigating the gravitational effects of the Moon and the Sun on the ocean and the effect of centrifugal forces.