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Tides

Tidal Power Plants

Engineers have long recognized that the movement of tidal currents might be an inexpensive, efficient, and environmentally safe source of power for human use. In general, the plan would be to construct a dam across the entrance of an estuary through which tidal currents flow with significant speed. Then, as tidal currents flow into and out of the estuary twice each day, they could be used to drive turbines which, in turn, could be used to operate electrical generators.

One of the few commercial tidal power stations in operation is located at the mouth of the La Rance River in France. Tides at this location reach a maximum of 44 ft (13.5 m). Each time the tide comes in, a dam at the La Rance station holds water back until it reaches its maximum depth. At that point, gates in the dam are opened and water is forced to flow into the La Rance River, driving a turbine and generator in the process. Gates in the dam are then closed, trapping the water inside the dam. At low tide, the gates open once again, allowing water to flow out of the river, back into the ocean. Again the power of moving water is used to drive a turbine and generator.

The plant is able to produce electricity only four times each day, during each of two high tides and each of two low tides. It generates a modest 250 megawatts in this way with an efficiency about equal to that of a fossil-fuel plant, 25%. With present technology, few other sites exist where tidal power generation is currently considered economically feasible.

Resources

Periodicals

Kasahara, Junzo. "Tides, Earthquakes, and Volcanoes." Science 297 (July 19, 2002): 348-349.

Wunsch, Carl. "Moon, Tides and Climate." Nature 405 (June 15, 2000): 743-744.

Other

National Oceanic and Atmospheric Administration, U.S. Department of Commerce. "Our Restless Tides: A Brief Explanation of the Basic Astronomical Factors which Produce Tides and Tidal Currents." Center for Operational Oceanographic Products and Services. February 1998 [December 29, 2002]. <http://www.co-ops.nos.noaa.gov/restles1.html>.

K. Lee Lerner
Larry Gilman
David E. Newton

KEY TERMS

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Diurnal

—Occurring once per day.

Ebb tide

—The period when the water level is falling; the period after high tide and before low tide.

Flood tide

—The period when the water level is rising; the period after low tide and before high tide.

High tide

—The event corresponding to the largest increase in water level in an area that is induced by tidal forces.

Low tide

—The event corresponding to the largest decrease in water level in an area that is induced by tidal forces.

Neap tides

—Period of minimum tidal range that occurs about every two weeks when the Moon and Sun are at 90° to each other, that is, at the first and third quarter moons.

Period

—The interval of time between two recurring events, such as the high tides in an area.

Semidiurnal

—Occuring twice per day.

Slack tide

—Period during which the water level is neither rising nor falling.

Spring tides

—Period of maximum tidal range; occurs about every two weeks, when the Moon and Sun are in line with each other, i.e., at the new and full moons.

Tidal current

—Horizontal movement of water due to tidal forces.

Tidal range

—Vertical distance between high tide and low tide during a single tidal cycle.

Additional topics

Science EncyclopediaScience & Philosophy: Thallophyta to ToxicologyTides - History, Theories Of Tidal Action, Variables Affecting Tidal Forces, Tide Tables, Semidiurnal And Diurnal Tides