The world’s first large-scale electrical power-generating plant using the energy of ocean tides began operation in France.
The ebb and flow of ocean tides has captured the imagination of writers and poets since time immemorial. The thought of harnessing some of the enormous energy of the tides has challenged technologically minded humans for centuries. Water-powered mills operating from tidal motion were used in England as early as the twelfth century, and by the eighteenth century this technology had been imported to New England. A tidal-powered sewage pump was used in Hamburg, Germany, until 1880, and a tidal water pump installed in 1580 under the London Bridge operated successfully for two and one-half centuries. These systems were eventually replaced by more economical and convenient electric pumps.
During the 1950’s, the search for environmentally acceptable methods of producing electric power stimulated renewed interest in tidal energy. Although there is no other source of energy that is environmentally more benign, tidal energy is difficult to harness and is often only marginally economical.
Many nations had for decades considered plans for converting the flow of the tides into low-cost electricity, but it was the French who first solved the technical problems and built the world’s premier large-scale tidal power-generating plant. This plant, located on Brittany’s narrow Rance River, has a production capacity of 240,000 kilowatts and was built for less money than a comparable hydroelectric plant. The Rance River estuary was chosen because of its well-known tidal fluctuation, which averaged 29 feet, and because, by blocking a half-mile-wide inlet, water could be trapped in a 9-square-mile estuary.
The site consisted of a 2,400-foot dam across the Rance River, located 2 miles upstream from the place where the river’s mouth separates the resort towns of Dinard and St. Malo. The structure includes spillways and a lock system for boats. The power-producing units, specifically designed to solve the problem of irregular power production, are located in the central part of the dam.
It is possible that the engineers of Électricité de France
As the tide comes in, the flow of water is stopped by a dam and is directed through a set of turbines. The inflowing water turns the turbines that drive the generators to produce electricity. At the same time, the tidal water fills the reservoir behind the dam. After the tide has ebbed, the trapped water is released, flowing back through the turbines and again producing power. The two daily high and low tides do not produce their peak output power when people most want to use it. Furthermore, the time of high and low tide changes each day.
To offset these difficulties, engineers devised bulb-shaped power units, 45 feet long and 18 feet in diameter, each housing a turbine and a generator, sunk into the dam below the level of the lowest tide. The passing tidal water rotates the blades, which can run in either direction to accommodate both incoming and outgoing tides. The advantage is that the turbines can also be used as pumps, whether the basin is being emptied or filled. This makes it possible to use surplus electricity to fill the reservoir behind the dam to a greater depth than the top level of the tides. Later, this extra water can be released to turn the turbines at a time when power is most needed, rather than when the ocean recedes. Thus the Rance plant achieves optimum operation by striving to maximize the value, rather than the amount, of energy extracted from the tides.
Preliminary work on the estuary site started in July, 1960, and full-scale construction began in January, 1961. The left bank, including a lock system, was constructed first, so that river traffic would not be obstructed during the main construction phase. The sluice gate structure on the right bank was second. Two separate cofferdams, upstream and downstream, were needed to block the inlet so that the main power station could be constructed in a dry enclosure. Because of the stronger currents and the high tides in the narrow estuary, the standard caisson construction, with precast modular units, was judged to be unsuitable.
An alternative design by French engineer Albert Irenee Caquot used separate components of light construction. These units became progressively more stable against capsizing as they were filled with material dredged from the riverbed. The hourglass-shaped sand-filled cylinders, with a 60-foot diameter at the ends and a 30-foot diameter in the center, were connected by two curved pilings forming large circular cells between the caissons and were immediately filled with sand. The final closure of the estuary from the sea occurred on July 20, 1963. The upstream cofferdam was then completed by early November, and the resulting space between the cofferdams, 600 feet at the widest point, was completely dry and ready for dam construction to commence by the year’s end. By November 26, 1966, the power station was in full operation.
The Rance River Tidal Power Generating Station.
The bulb machine has several advantages. Dimensions are smaller, and hydraulic losses are reduced. Savings in the installation were possible because of the simple foundations and superstructure of the power station. Because of the simple hydraulic system, a reversible- flow unit could be realized. A compact model that could operate as a pump as well as a generator was commissioned in 1959.
A compelling factor in the decision to build the Rance plant was the fear of power shortages caused by the Suez Canal crisis. Western Europe was particularly aware of its dependence upon imported fuel oil; thus, utilization of renewable sources of energy had obvious attractions. The Rance plant was considered a necessary preliminary step—a full-scale pilot study—to gain valuable experience for eventually building more and larger tidal power plants elsewhere.
In the decades after the generators first began to whirl at the Rance estuary, this unique tidal generating system proved to be remarkably trouble free and provided electrical energy to the French citizens on the coast of Brittany with no cost for fuel.
The Rance River Tidal Power Generating Station has proven that economical generation of energy by means of the tides is a practical reality. Important indirect benefits accrued from the Rance plant, such as the development of large bulb-set turbines and corrosion control techniques, as well as a number of social benefits.
Perfecting the technology of bulb turbines has exerted a profound influence on the economics of low-head river sites for hydroelectric plants. Larger versions (twenty to forty megawatts) of the Rance turbines have been produced and profitably installed in plants around the world that otherwise would not have been cost-effective. Because corrosion of equipment by seawater and salt-laden air was an important problem addressed in the design, major advances have been made in the knowledge of corrosion control for concrete, metals, paint, and cathodic protection. Improved understanding of seawater corrosion is quite useful because large thermal or nuclear plants often use seawater for cooling.
The roadway over the barrage and power station reduces the road distance from St. Malo to Dinard from 25 miles to 4 miles. This unique project also has fostered France’s tourism: Tens of thousands of people visit the site and its associated museum annually.
The greatest impact of the Rance project, however, is the efficient and environment-friendly production of electricity. On a global scale, the maximum potential tidal power from all the world’s best sites is 64,000 megawatts. Assuming that only 20 percent of this power is recoverable, the approximate possible magnitude of the world’s tidal electric power is thirteen thousand megawatts. Although this is only 1 percent of the world’s potential water power and an even smaller fraction of the world’s power needs, it is capable of being developed in many favorable locations. Tidal power has the advantage of neither consuming nonrenewable resources nor producing noxious air pollutants. There is no slag or fly ash to dispose of, and there are no long-life radioactive waste products to be stored.
Tidal power stations have a useful life of seventy-five years, compared to thirty years for a fossil fuel plant, and twenty-five years for nuclear plants. Tidal plants, using multiple smaller turbines, are more reliable than conventional plants, which use a few large turbines. Less reserve equipment is needed, and the tides are extremely predictable.
Tidal power stations result in only a minimal disturbance to the ecology and virtually no aesthetic pollution. Thus, there are many social advantages and few disadvantages to utilizing tidal power wherever the tides and topographical factors combine to make this feasible.
Although the major factor in the decision to proceed with the Rance project was the fear of future power shortages, the Rance plant was a necessary preliminary step toward achieving energy independence for France. It was a full-scale pilot study that provided valuable experience in tidal power generation, facilitating the construction of larger projects in the future.
Whatever the doubts surrounding the future of tidal power in the world, many valuable lessons have been learned from the Rance scheme. There have been profound technological advances, important social benefits, and a verification that tidal power plants have an extremely low impact on the environment, producing no air or thermal pollution. The tides, like wind, water, and the sun, are constantly being renewed.
There is an urgent need to develop and to use renewable sources of energy because the supplies of nonrenewable fuels are rapidly declining. The Rance Tidal Power Plant has proven that tidal energy is not only a promising new source of electrical energy for the future but also a source that should be developed and utilized as a valuable complement to dwindling resources.
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Boyle, Godfrey, ed. Renewable Energy. 2d ed. New York: Oxford University Press, 2004. An excellent resource on renewable energy, including tidal power. Chapter 6 discusses technical, environmental, and economic factors; the potentials of tidal energy; tidal streams; tidal current turbines; the future of tidal power, and more. -
Charlier, Roger Henri. Tidal Energy. New York: Van Nostrand Reinhold, 1982. This encyclopedic reference is a valuable survey of the entire range of issues pertaining to tidal power. One complete chapter is devoted to a detailed discussion of the Rance Tidal Power Station. -
Clancy, Edward P. The Tides: Pulse of the Earth. Garden City, N.Y.: Doubleday, 1968. See especially chapters 6 and 8. A comprehensive examination of the tides, including considerations of the world’s most feasible sites for future development of tidal power stations. -
Gray, Thomas J., and O. K. Gashus. Tidal Power. New York: Plenum Press, 1972. A valuable compendium of individual papers presented at an international conference on tidal power. One paper details the Rance estuary project, and other papers discuss additional possible sites where tidal power could be developed profitably. -
Hubbert, M. King. “Tidal Power.” In Resources and Man. Washington, D.C.: National Academy of Sciences, 1969. A detailed analysis of the world’s potential for tidal power and the practically realizable limits. -
Pielou, E. C. The Energy of Nature. Chicago: University of Chicago Press, 2001. A recommended resource for those interested in the inherent energy of the natural world. Includes discussion of the power of water, tides, and ocean waves. Also addresses the question, “What Is Energy? Some Preliminary Physics.” -
Thirring, Hans. “Tidal Power.” In Energy for Man: From Windmills to Nuclear Power. Bloomington: Indiana University Press, 1976. A compact but comprehensive account of the history, technology, and possible future prospects of tidal power production. Discussion of the Kaplan turbine and power-capacity computations is included.
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