Tidal Energy

Tidal Energy is a largely untapped, renewable ENERGY source based on lunar gravitation rather than solar radiation. The potential of tidal HYDROELECTRICITY has long been recognized. However, compared to river dams, tidal-power projects are very expensive, since massive structures must be built in a difficult saltwater environment. The relatively low head of water above the turbines restricts the capacity of individual generators to about 25-50 megawatts (MW = 10⁶ watts); therefore, many machines are needed to produce a significant block of power. The machinery also has to withstand the rigours of saltwater operation.

For all this investment, the average electric power output is severely limited by the twice-daily ebb and flow of tides: average output of tidal electricity is less than 40% of the installed generating capacity; production of power from river dams typically averages 70-100% of installed capacity. Finally, the lunar cycle of 24 hours 50 minutes means the raw production of tidal energy moves in and out of phase with the normal, solar-oriented daily pattern of electrical consumption. Unlike the energy from river dams, the daily, monthly and annual availability of tidal energy is fully predictable, but it must be either stored or integrated with other sources of generation that can be adjusted to accommodate the fluctuations of tidal generation.

There are relatively few coastal locations in the world where the tidal range (ie, the difference between high and low tides) is large enough to justify exploitation of the available tidal energy. Not only must there exist a sufficiently high tidal range (at least 5 m) for construction of an economically feasible plant, but the site should also include a natural bay which can store a large volume of seawater at high tide and be so situated within the estuary that the operation of the plant will not significantly change the tidal resonant system (see TIDE).

The world's most powerful tides occur in the upper reaches of the Bay of FUNDY, where tidal ranges up to 17 m are not uncommon. UNGAVA BAY and estuaries along the coast of BC also exhibit fairly high tides. The coasts of Argentina, NW Australia, Brazil, France, India, Korea, the UK, the USSR and the American states of California, Maine and Alaska possess coastal configurations and sufficiently large tidal ranges to provide sites at which potentially large sources of tidal energy may be exploited. The aggregate total capacity of all potential tidal-power sites in the world is currently estimated at about one billion kilowatts, with an expected electrical-energy output of 2-3 trillion kilowatt-hours annually, ie, 10 times Canada's present combined electrical output.

The idea of exploiting the energy of the tides is not new. Tidal mills were built in Britain, France and Spain as early as the 12th century. A mill powered partially by tidal energy was built at PORT-ROYAL, NS, in 1607. These early mills extracted only the equivalent of 20-75 kW, or less than the power available in modern compact cars. A few of the early mills are now preserved as historic sites.

Canadian Installations
Detailed studies of the Bay of Fundy tidal-power resource concluded that the most efficient scheme of development would be one that would generate power for a period of about 5 hours, twice daily, on the ebb tide. The most cost-effective project was found to be a site in MINAS BASIN, at the mouth of Cobequid Bay in the upper reaches of the Bay of Fundy. This development would have a capacity in excess of 5300 MW, an amount equal to the entire 1980 installed generating capacity of the Maritime power systems. The development has been estimated to cost about $7 billion.

A more modest alternative project, at a site in Cumberland Basin through which the NB-NS border passes, has been projected at approximately 1400 MW, about one-quarter the capacity of the larger site, but one-third the capital cost. While it is forecast that the local Maritime power systems would be able to absorb its output, the huge capital investment required for even this smaller project makes the financial considerations discouraging. However, a small, 20 MW, single-unit power station was constructed and commissioned in Aug 1984 by the NS Tidal Power Corp on a tidal reach of the Annapolis R, near ANNAPOLIS ROYAL, NS. This project was undertaken primarily to demonstrate the application of a particular type of turbine generator (trade-name Straflo) for tidal and other low-head hydro applications. The Straflo machine differs from the conventional hydroelectric turbine-generator installation in that the turbine and generator are integral rather than separate units. The average tidal range in Annapolis Basin is only about 6.4 m, but the plant, in which only one large turbine (7.6 m diameter) is installed, will produce about 50 million kW-hours annually for the electrical-utility system of NS.

Technology
The most practical scheme for harnessing tidal energy is still the old tidal-mill concept, closing off an estuary or tidal basin from the sea with a structure composed of a powerhouse, a sluiceway section and a solid embankment section. The sluiceways are closed at high tide and the ebbing sea level causes a head differential between the basin level and the sea. When this differential becomes large enough, flow is permitted through the turbo-generators until the difference in levels becomes too low to drive the turbines. When the tide begins to turn and the sea level to rise, the sluiceways are then opened once more, so that the tidal basin can be filled for the next cycle of electrical generation. These cycles take place about twice a day (2 flood tides and 2 ebb tides during each lunar day). Using new technology, turbines can be built to generate electricity in both directions and also to operate as pumps in both directions.

Modern large power systems can readily absorb the intermittent output of a tidal plant. The output from conventional fossil-fueled generating stations can be reduced when the tidal plant begins generation and it can be brought back into the system during the few hours that a tidal plant must remain idle. In this way, large savings in coal and oil can result, and pollutants from such fuels can be substantially reduced.

The NS government expects that tidal power will, in the long run, provide useful energy for the province and for export. However, because of uncertainties about the costs and the export markets for this power, only the Annapolis Royal pilot project is included in the NS electrical generation expansion program of the foreseeable future. Tidal power has also been considered by the NB and BC governments, but is not included in either province's planning. The National Energy Board did not include tidal power in its 1981 estimates of Canadian energy supply and demand for the years 1980-2000 because it considered that the case for early development of tidal power in Canada has not yet been established.

Author R.H. CLARK

Suggested Reading
R.H. Charlier, Tidal Energy (1982).

Links to Other Sites
Ocean Energy
This illustrated online brochure describes how "green power" can be harnessed from the ocean waves and tides. Also, check this site for news updates about related projects. From the Ocean Renewable Energy Group. A PDF file.

Re-Energy.ca
Re-Energy.ca is a hands-on renewable energy learning experience. Building working models of renewable energy technologies allows students in grades 7 through 12 to discover the fundamental principles of biology, chemistry and physics, and to explore the application of science and technology to social and environmental issues.