Tidal energy

DEFINITION: Power generated during the rise and fall of the tides

The energy generated during the rise and fall of the tides may be cleanly and safely converted into electrical power, but large-scale tidal power installations can have severe consequences for the environment, including decimation of fisheries; destruction of the feeding grounds of migrating birds; damage to shellfish populations; interference with ship travel, port facilities, and recreational boating; and disruption of the tidal cycle over a wide area.

Tidal power can be an important source of local electricity generation because such projects produce energy that is free, clean, and renewable; they produce neither air pollution nor thermal pollution, and they do not consume exhaustible natural resources. Only a limited number of places in the world offer the potential for such power installations, however, because a vertical tidal rise of 5 meters (16.4 feet) or more is required. Installations must also be near major centers so that transmission requirements are minimized, and a natural bay or river estuary is typically required to store a large amount of water with a minimum of expense for construction. The seawater impounded behind the dam at high tide produces a hydrostatic head so that electricity is generated as the water passes through the dam’s turbines when sea level falls. If the turbines in the dam are reversible, power can be generated on both incoming and outgoing tides.

Tidal power plants have been constructed on the Rance River near St. Malo, France (240 megawatts of power), on the Annapolis River in Nova Scotia, Canada (20 megawatts), on the Yalu River in the People’s Republic of China (3.2 megawatts), in Kislaya Guba, Russia (1.7 megawatts), and on Strangford Lough in Northern Ireland (1.2 megawatts). In the first decades of the twenty-first century, South Korea constructed a plant on Sihwa Lake (254 megawatts) and made plans to build several more around the country. As of 2024, the Sihwa Lake plant is the world's largest tidal power plant. The Rance River plant, which has been in continuous operation since November, 1966, was for many years the world’s largest tidal power installation. It bridges the estuary with a dam nearly 0.8 kilometer (0.5 mile) long and provides power for 300,000 people.

The environmental impacts of the Rance River dam have generally been limited to the modification of fish species distributions, the disappearance of some sandbanks, and the creation of high-speed currents near the sluices and the powerhouse. Tidal patterns have also changed, with the maximum average rise reduced from about 13.4 meters to 12.8 meters (44 feet to 42 feet) and a corresponding increase in the height of the mean low-tide level.

The environmental impacts of the smaller Annapolis River plant in Nova Scotia, which became operational in 1984, reportedly have included the generation of silt, which destroyed clam beds in the basin behind the dam, and increased erosion of the river’s banks. The Nova Scotia Power Corporation reached a settlement with one nearby landowner whose house suffered a cracked foundation and shifted toward the river as a result of erosion.

Several tidal power projects were proposed for the United States during the early and mid-twentieth century but were never built because of environmental concerns. A proposed tidal power plant on the upper Saint John River in Maine was halted, for example, because damming the river would have destroyed a unique stand of a rare wildflower. The flower was later found growing elsewhere. Objections cited for other projects included possible effects on historic and archaeological sites, as well as presumed economic and social impacts on Native American communities such as the Passamaquoddy.

Shortly after the dramatic jump in world oil prices during the 1970’s, the Tidal Power Corporation, a venture owned by the Nova Scotia government, proposed building a major tidal power project in the Bay of Fundy, which lies between Nova Scotia and New Brunswick in eastern Canada. This plant would have been the world’s largest tidal power installation, producing 4,560 megawatts of power—nearly twenty times the output of the Rance River plant and more than three times the output of Hoover Dam on the Colorado River in the United States. A major feature of the project was to be a dam 8.5 kilometers (5.3 miles) long across the Bay of Fundy, which has the largest tidal range in the world, averaging more than 15 meters (50 feet). The enormous scope of the project forced scientists to pay close attention to its anticipated environmental consequences, and these appeared to be so severe that the project was never begun.

Disrupted bird migrations were predicted after the dam’s completion because of the submersion of tidal mudflats where large numbers of semipalmated sandpipers and other shorebirds annually gorge on mud shrimp before beginning their fall migrations to wintering grounds in South America and the Caribbean. Damage to fish stocks was also predicted because of repeated passage of the fish through the dam’s turbines as the tides rose and fell. Particularly affected would have been the American shad, a member of the herring family, which migrates to the Bay of Fundy each year from as far away as Florida in order to fatten itself on mysid shrimp living on the tidal mudflats. Oceanographers also used computer modeling to show that dam construction would alter tidal patterns over a broad area, resulting in tidal levels 10 percent higher and lower as far south as Cape Cod, Massachusetts, 400 kilometers (250 miles) away. They predicted that these tidal changes would flood coastal lands and threaten roads, bridges, waterfront homes, water wells, systems, salt areas, harbors, and docking areas along the entire coast.

Bibliography

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Fairley, Peter. "Tidal Power Makes a Surprising Comeback." IEEE Spectrum, June 19, 2013.

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"Hydropower Explained." US Energy Information Administration, 24 May 2024, www.eia.gov/energyexplained/hydropower/tidal-power.php. Accessed 24 July 2024.

McKinney, Michael L., Robert M. Schoch, and Logan Yonavjak. Environmental Science: Systems and Solutions. 6th ed. Sudbury, Mass.: Jones and Bartlett, 2016

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