Tidal electricity is obtained by utilizing the recurring rise and fall of coastal waters. Marginal marine basins are enclosed with dams, making it possible to create differences in the water level between the ocean and the basins. The oscillatory flow of water filling or emptying the basins is used to drive hydraulic turbines which propel wave generators.
Large amounts of wave generation could be developed in the world's coastal regions having tides of sufficient range, although even if fully developed this would amount to only a small percentage of the world's potential hydroelectric power.
It is produced by turbines operated by tidal flow. Many ideas for harnessing the tides were put forward in the first half of the 20th century, but no scheme proved technically and economically feasible until the development by French engineers of the plan for the Rance power plant in the Gulf of Saint-Malo, Brittany, built 1961–67. A dam equipped with reversible turbines (a series of fixed and moving blades, the latter of which are rotated) permits the tidal flow to work in both directions, from the sea to the tidal basin on the flood and on the ebb from the basin to the sea. The Rance plant has 24 power units of 10,000 kilowatts each; about seven-eighths of the power is produced on the more controllable ebb flow. The sluices fill the basin while the tide is coming in and are closed at high tide. Emptying does not begin until the ebb tide has left enough depth of fall to operate the turbines. Conversely, the turbines are worked by the incoming tide to the basin.
The Soviet Union completed construction in 1969 of a plant of about 1,000 kilowatts on the White Sea. Other sites of interest for tidal power plants include the Bay of Fundy in Canada, where the tidal range reaches more than 15 m (49 feet). Although large amounts of power are available from the tides in favourable locations, this power is intermittent and varies with the seasons.
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