In looking for alternative energy sources to meet future needs, some common physical phenomena are obvious candidates. One of these is tidal power. Twice each day on every coastline in the world, bodies of water are pulled onto and off of the shore as a result of gravitational forces exerted by the moon and sun. Only on ocean coasts is this change large enough to notice, however, and therefore, to take advantage of as an energy source.
The potential of tidal power as an energy source is clearly demonstrated. Pieces of wood are carried onto a beach and then off again every time the tide comes in or goes out. In theory, the energy that moves this wood could also push against a turbine blade and turn a generator.
In fact, the number of places on the earth where tides are strong enough to spin a turbine is relatively small. The simple back-and-forth movement seen on any shoreline does not contain enough energy by itself. Geographical conditions must concentrate and focus tidal action in a limited area. In such places, tides do not move in and out at a leisurely pace, but rush in and out with the force of a small river.
One of the few commercial tidal power stations in operation is located at the mouth of 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 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.
Hence the plant produces electricity only four times each day, during each of two high tides and each of two low tides. Although it generates only 250 megawatts daily, the plant's 25% efficiency rate is about equal to that of a power plant operating on fossil fuels .
One area where tides are high enough to make a power plant feasible is on Canada's Bay of Fundy . Experiments suggest that plants with capacities of 1–20 megawatts could be located at various places along the bay. So far, however, the cost of building such plants is significantly greater than the cost of building conventional power plants of similar capacity. Still, optimists suggest that serious development of tidal power could provide up to a third of the electrical energy now obtained from hydropower worldwide at some time in the future.
Despite the drawbacks and expense of tidal power, China is one country that has used it extensively. Since energy needs are often modest in China, low capacity plants are more feasible. As of the late 1980s, therefore, the Chinese had built more than 120 tidal power plants to provide electricity for small local regions.
[David E. Newton ]
Greenburg, D. A. "Modeling Tidal Power." Scientific American 257 (November 1987): 128–135.
Holloway, T. "Tidal Power." Sea Frontiers 35 (March-April 1989): 114–49.
"The Potential of Tidal Power—Still All At Sea." New Scientist 126 (May 19, 1990): 52.
Webb, J. "Tide of Optimism Ebbs Over Underwater Windmill." New Scientist 138 (April 24, 1993): 10.