Over two-thirds of the Earth’s surface is covered by oceans, and the theoretical energy available from that source is greater than could ever be used. The diffuse nature of the resource, and its distance from where it is needed, reduce its potential usefulness; however, it remains a valuable prospect in many locations. The sources of energy that may be considered include tidal currents, wave energy, ocean thermal energy, and salt gradient energy. Ocean thermal energy could be a prolific source of energy, since it exploits the temperature difference between warm surface water and colder, deep water. However, to date only tidal energy has seen limited commercial development, although serious work in the area has been going on for only a little over a decade. This remains a potential energy source that requires a lot of research.
Until recently, tidal energy was obtained from one type of plant — a tidal barrage. This is essentially a large dam across a river estuary. As the tide comes in, the sluice gates are opened to allow the water to flow into the estuary. As the tide recedes, the flood control gates are closed to prevent the water from flowing back except through the turbine system. It is also possible to have a barrage that generates electricity on both incoming and outgoing tides.
The difficulty with tidal barrages is that they require high tides, so there are few locations (about 20) in the world which are suitable. Coupled with the high capital costs required to build the barrages, there are few operating tidal barrages in existence. One is located in Annapolis Royal, Nova Scotia on the Bay of Fundy.