Tidal power is similar to the more familiar water power of streams except that the flow in streams is unidirectional, whereas that of the tides reverses four times per day as tidal basins are alternately emptied and filled. Although large tidal projects such as that proposed for Passama — quoddy Bay on the Maine-New Brunswick boundary have been under consideration for nearly half a century, the first such plant, that in the La Ranсe estuary in France, with a planned capacity of 320 megawatts, be­gan operation in 1966.

In the USSR, a small plant of 400 kilowatts in the Kislaya Inlet, 80 kilometers northeast of Murmansk, began operation in 1968, and much larger installation of 320 megawatts is planned for the Lombovska River on the northeast coast of the Kola Peninsula.

Large-size tidal-power plants are possible only in a limited number of favorable localities around the world. These require a combination of a large tidal range and a bay or estuary capable of being enclosed by dams. A summary of such localities, and the tidal power potentially obtainable is given in the tabulation below (sources: Bemshtein, 1961; 1965, Table
5-5, p. 173; Trendholm, 1961). The total capacity of all of the sites amounts to about 64,000 megawatts. This is about the same magnitude as the world’s geothermal-power capacity, but only about 2 per cent of the world’s potential water-power capacity, and an even smaller fraction of the world’s power needs. Tidal power, nevertheless, is capable in favora­ble localities of being developed in large units, and it has the further ad­vantage of producing a minimum disturbance to the scenic and ecological environment. Hence, there are many social advantages, and few disadvan­tages, in developing tidal power wherever practicable.

Average Potential