Tidal energy or tidal power refers to the use of the oceans’ tides to generate electricity. Tides can be utilized as energy sources through the currents they cause or through the associated variations in ocean level. The most effective way of taking advantage of these vertical water displacements is through impoundment. Power stations that are producing electricity from tides are usually dams, converting tidal energy into electricity.

How Does Tidal Energy Work?

The gravity of the moon exerts a pull on the Earth every day. Since the oceans’ waters are fluid, so as the moon’s gravity pulls on them, they bulge outward. These bulges, which place along an axis (an imaginary line) that points toward the moon, are called lunar tides; on the other side of the Earth, the side away from the moon, the waters bulge out away from the gravitational pull of the center of the Earth. While the moon does most of this work, the sun helps out, but to a lesser extent. Although the sun is much bigger than the moon, the moon is much closer to the Earth, so it exerts a greater gravitational pull. Nonetheless, the sun’s gravitational pull also creates tides, called solar tides. Tidal energy is produced form both of these tides.

Tidal Energy Facts

When the Earth, moon, and sun are aligned in a straight line during a full or new moon, both the sun and moon are pulling together in the same direction. During a full moon the pull is greatest, creating large tides called spring tides. During half-moon periods, when the moon and sun are at right angles, or 90 degrees, to each other, the tides created, called neap tides, are lower. During these times the coasts have two low and two high tides over a period of less than twenty-four hours. At the same time, the Earth rotates beneath these bulges, passing under each one during a twenty-four-hour period. The result is that tides rise and fall rhythmically along the world’s coastlines approximately twice each day in predictable patterns. These flows of water are very like the flows of rivers, and their energy can be harnessed in much the same way that a river’s energy is by a hydroelectric dam.

There are two ways to harness energy in tidal power-generating stations: the tidal barrage and tidal streams. A tidal barrage, also called an ebb generating system, is very similar to a dam. The barrage is constructed at the mouth of a bay or estuary (a water passage where the tide meets the lower end of a river). A barrage works, when the difference in water elevation between low tide and high tide is at least 16 feet (5 meters).

Tidal Energy Companies

Currently, only one major tidal power station is in operation throughout the world. This station is located on the estuary of the La Rance River in France. Construction of the barrage began in 1960 and was completed in 1966. The barrage is almost 1,100 feet (330 meters) long with a 13.7-square-mile (22-square-kilometer) basin. Twenty-four turbines, each of 17.7 feet (5.4 meters) diameter are being used by the station. Each turbine is engineered to produce about 10 megawatts of power, so the station can produce a maximum of 240 megawatts.

Also, the other nations have explored the possibility of harnessing tidal power. Australia’s Renewable Energy Commercialisation Program awarded a grant to develop a 50-megawatt plant in the Derby region of Australia. Scotland, too, has explored tidal energy, and proposals have been made for the construction of a tidal station on Solway Firth in southwest Scotland; in the 1970s Scotland built a 15-kilowatt experimental tidal turbine on Loch Linnhe. In England, the Severn River has been identified as a promising site for a tidal power station. The most promising site for tidal power plant in the world is the Bay of Fundy in Canada, which, at up to 56 feet (17 meters), has the highest tides in the world.

Advantages of Tidal Energy

Using tides for energy have some benefits. The chief benefits of tidal energy, as of most forms of alternative energy, are that it is clean, renewable, and does not consume resources such as coal or oil. In addition, it does not discharge pollutants into the water or atmosphere, so it does not contribute to acid rain or global warming. Further, the energy source is free. There is a secondary benefit of tidal power barrages. They can function as bridges linking communities on opposite sides of an estuary, making travel quicker.

Tidal Energy Pros and Cons

The chief disadvantage of tidal power stations is their expense. It has been estimated, for example, that construction of a tidal power station on the Severn River in England would cost about $15 billion. A second drawback is that not every coastal region is suitable for tidal power. Generally, a difference between high and low tides of about 16 feet (5 meters) is necessary for a tidal power station to be cost-effective. Only about forty such sites in the world have been identified. Another drawback is that the tides are in motion only about ten hours per day. This means that tidal energy cannot be provided consistently throughout the day and would have to be supplemented with other forms of energy.