Generating electricity from tides is a relatively new concept although tidal mills have been around for hundreds of years. There are a few different designs of tidal power plants that will be covered in this article along with some of the benefits and drawbacks of using tidal energy to generate electricity.
A tidal barrage is a device or structure used for harnessing the energy of rising and falling tides. Usually a large, dam-like structure that blocks off a water-filled basin or reservoir, it stores water at high tide and then releases it back into the main body through turbines at low tide.
A barrage has several gates through which water is allowed to flow as the tide rises, filling up the basin with water. Once maximum high tide is reached, the gates are closed and the high water level is trapped inside until low tide. At this time, the water is released back into the larger body of water through turbines. As the tide rises again, the basin can be refilled through the gates or through the turbines. Filling through the turbines generates power, but does so less efficiently than during the emptying cycle.
Tidal barrages, while generally expensive to construct, do have a distinct advantage: they are able to temporarily store excess energy from the power grid. The turbine generators can be operated in reverse as large electric motors, pumping extra water into the basin using grid power, and then releasing it later during hours of peak electricity demand.
Tidal Stream Generators
Tidal stream generators, commonly called tidal turbines, are also intended to harness tidal power, but do so by harvesting the energy of currents in flowing masses of water created by tides. They can be thought of as underwater wind turbines, and indeed very closely resemble their above-ground counterparts and operate on essentially the same principles.
Tidal turbines are often capable of producing much greater power for their size than wind turbines. This is because water has a much higher density than air. Therefore, a given volume of water moving at the same speed as an equal volume of air has a much higher mass, and therefore carries much more energy. Tidal turbines can function effectively at relatively low current speeds compared to wind turbines.
These generators are often built in “bottleneck” areas where water is forced to flow during the transition from high to low tides, and vice versa. Estuarial regions are the most common locations for them to be built.
Dynamic Tidal Power
One of the least proven (and indeed untested to date) tidal power generation technology is called dynamic tidal power, or simply DTP. It consists of a massive dam-like structure twenty to forty miles long, extending from the coast of a large land mass. It does not completely enclose or block off any body of water, but it is large and long enough to create a difference in water level between the sides during changes in tides.
Such a dam would be constructed in a “T” shape, with the bottom of the T connecting to the coast and the rest extending perpendicularly away from land into the ocean. Because tidal ocean currents often run parallel to the coast, they would run into the dam and the water would get “backed up” on one side and sit at a higher level. This difference could be exploited by allowing the water to drain through turbines to the other side, generating power. When the tide reversed later in the day, it could be passed through the turbines in the opposite direction, providing more power.
Dynamic tidal power has never been directly tested, and due to its design, cannot be effectively tested without building a full-scale working prototype. Because of its unenclosed design, too much water escapes around the sides of miniature versions of the “T” dam for them to produce useful power. Power output increases with the square of the length of the dam. Only a dam twenty miles or longer, it is estimated, would be economically sound.
Tidal Power Plants Summary
When most people think of tides, massive transfers of energy are probably not the first things that come to mind. However, that is exactly what takes place as the tides shift on Earth. The mass of the Earth’s oceans is unimaginably large, and the potential energy stored by raising such a prodigious mass even a few feet is vast. If even tiny fractions of that energy are harvested, useful amounts of grid power can be produced.
However, building large structures underwater, especially ones able to withstand the immense mechanical forces involved in maintaining a difference in water level between two large bodies, is difficult and expensive. It often requires complex special underwater construction techniques, as well as vast amounts of manpower and construction material.
Another consideration is the environmental effects of tidal power systems. DTP could doubtlessly disrupt coastal tides in the vicinity, possibly affecting the breeding and feeding habits of coastal sea creatures and crustaceans. Furthermore, fish inevitably pass through turbines, often with unacceptable mortality rates, and designing ways around this is a perplexing and difficult problem.
Tidal power is a technology that shows both promise and difficulty. Only time and innovation will tell if its potential benefits can outweigh difficulties of scalability and ecology.Sponsors: