The capture of energy from the highly predictable tides has intrigued people for hundreds of years. 105 As a result, there have been development projects since the 18th century. Tide times can be predicted with astronomical accuracy and the height can be predicted to within 0.1 meter except when the weather is unusually violent. These characteristics could lead to a power source of great reliability. The largest of the current tidal power plants is a 50-year-old facility located in the Ranee Estuary in northern France. It produces 240 megawatts; it is coupled to the commercial grid.
The Bay of Fundy, between Nova Scotia and New Brunswick on the East coast of Canada, has very high tides, in places more than 10 meters. This great height is the result of a match between the natural period required to fill and empty the bay and the ebb and flow of the tides. A dam on the Cobequid Bay, a small bay at the east end of the Bay of Fundy, could provide a potential site for a tidal power plant. Preliminary studies indicate that the dam would be 5 to 10 kilometers long and its cost would make the power uneconomical. A shorter dam would be cheaper, but would produce too little power to be valuable. 106
Passamaquoddy Bay is located at the west end of the Bay of Fundy, on the border between the State of Maine in the United States and the Province of New Brunswick in Canada. During World War II, extensive studies were made of the requirements and costs for a tidal power plant at this bay. Passamaquoddy is separated from Fundy by two channels on either side of Deer Island. The total length of the dams for closing is less than 3 kilometers. The tidal rise and fall is not as great at •Passamaquoddy Bay as it is at other locations on the Bay of Fundy but it is adequate for power production. This project is close enough to being economically feasible that it is still being discussed. However, uncertainty about long-term success has seriously inhibited implementation. One of the major concerns for these plants stems from the affect they will have on the ocean shoreline ecosystems. Changing the rhythm of the tidal flow in the bay will change the rhythm of all life forms that live within its bounds. These effects are likely to be undesirable.
There is a 400-kilowatt tidal energy plant near Murmansk, in Russia. It was built by the Soviet Union. This plant is about the same age as the French plant in the Ranee estuary. Russia has announced plans to build a 40-megawatt plant at Mezen Bay on the Kola Peninsula and a 300-megawatt facility at Lumbovka on the White Sea. China has experimented with several small plants and brought a 10-
106 Greenberg, David A., "Modeling Tidal Power", Scientific American, Vol. 257, No. 5, November 1987, Page 128
megawatt unit on-line in 1986. Canada installed an 18-megawatt plant in conjunction with an existing flood control dam. It is located at Annapolis Royal, Nova Scotia; it began operation in 1984. Several problems prevent large-scale exploitation of this energy source.
The potential energy in the tides is very difficult to harvest. 107 Even under the best conditions, the tidal rise is no more than about 10 meters. In most places, the difference between high and low tide is only 1 to 2 meters. A height difference of only two meters produces little water pressure. Since the tides are cyclic, the maximum pressure is only available for a short period when water stored at high tide is allowed to flow out at low tide. The remainder of the time, the average pressure is about half the highest. It is difficult to build turbines that operate efficiently at very low pressures. A very large flow of water is needed to extract significant power from a low-pressure flow. These characteristics define the type of facility required to extract tidal energy.
The first requirement is for a very large shallow reservoir that can be rapidly filled when the tide is high. The second requirement is for inlet gates to control and trap the water at high tide. Since there is little driving pressure to fill the reservoir, the inlet gates must be very wide. Finally, the reservoir must be equipped with large turbines capable of extracting energy from a low-pressure head with large volume flow. The cost of the very large facilities required for the harvesting of tidal energy has thus far inhibited wide scale use of this type of facility.
Like other renewable energy sources, tidal energy is limited to a few places where the juxtaposition of natural conditions makes them practical. The total amount of energy available is small compared to the requirements. Ecological effects may negate the advantage of the use of this renewable source. 108 Tidal energy can be used for the production of electric power. Like most other renewables, it provides no storable portable medium to match the output of the generator to the needs of the users or to provide a storable fuel.
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