Wave and Tidal Power

Generating technologies for deriving electrical power from the ocean include tidal power and wave power, ocean thermal energy conversion, ocean currents, ocean winds and salinity gradients. Of these, the two most developed technologies are tidal power and wave power.

Tidal power requires large tidal differences. The principle of harnessing the energy of the tides dates back to eleventh century England when tides were used to turn waterwheels, producing mechanical power. More recently, rising and falling tides have been used to generate electricity. The technology required to convert tidal energy into electricity is very similar to the technology used in traditional hydroelectric power plants. The first requirement is a dam across a tidal bay or estuary. At certain points along the dam, gates and turbines are installed. When there is an adequate difference in the elevation of the water on the different sides of the barrage, the gates are opened. This hydrostatic head that is created causes water to flow through the turbines, turning an electric generator to produce electricity.
The biggest scheme to date is found in La Rance, France, and has been successfully working for almost 40 years, with a commercial capacity of 240 MW. Tidal power has many advantages: it is cost efficient, it does not pollute the atmosphere, and electricity production is highly predictable. Much like hydropower, however, the impact on the local environment, most notably the natural streams and the fish habitat, and the high initial investment have prevented much commercial development.

New tidal technologies, known as "free flow" turbines, do not involve the construction of a dam, are much more flexible in terms of size, and have a very limited impact on the environment. Still in the development stage, they might be the next generation of small tidal power facilities.

The other promising area is wave energy. Waves form a potentially large world-wide resource estimated at more than 2 TW. Until recently the commercial use of wave power has been limited to small systems of tens to hundreds of watts, like ocean buoys. As a buoy heaves up and down in waves, an oscillating water column in the centre pipe of the buoys hull acts like a piston, alternately pushing air out of the top of the pipe and drawing it in.
This pneumatic power can be converted to light by spinning a turbine generator. Similar systems can be built onshore, producing small but constant amounts of energy.