Tides

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.