Harnessing ocean energy
France Bequette
The tidal power station at La Rance (France).
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The winds of heaven and the waters of the sea are sufficient to move me; even if we do not see the sunrise the journey will not have been in vain. Liu T’ieh-yün |
Ocean tides constitute a clean and inexhaustible energy source, free from the climatic irregularities which are a constraint on wind and solar power. But places suitable for exploiting them are few and far between, for if a tidal power plant is to operate efficiently certain conditions must be met. A river estuary where the difference between high and low tide is at least five metres is necessary. It must be possible to construct a dam, and there must be a nearby source of electricity supply to make up for the intermittence of power production linked to the times of high and low tides.
One of the world’s most suitable sites is the estuary of the river Rance, in western France, where the difference between high and low tides averages 8.17 metres, peaking at 13.5 metres during the equinoxes. The world’s first tidal power plant began operating there in 1966. It is still the biggest, with a capacity of 240 megawatts (MW).
Tidal power plants consist of a high-capacity dam built across an estuary to hold back the water at high tide. At low tide sluice gates in the dam are opened to release a cascade of water that drives a turbine to generate electricity.
In China, says Singh, “eight plants with a total capacity of 6,210 kilowatts exploit tidal energy.” There is a 20MW plant at Annapolis in Canada. But although many other suitable sites exist, construction costs are considered too high, especially as hydroelectric power is plentiful and cheaper. Tidal power plants are planned on Russia’s White Sea and on the Severn and Mersey estuaries in the United Kingdom.
Putting the waves to work
In 1945, Japan became the first country to consider using sea waves as an energy source, followed by Norway and the UK. The first power plant to use wave power, OSPREY (Ocean Swell Powered Renewable Energy), began operating in northern Scotland at the beginning of August 1995. A 2MW facility, OSPREY was designed along the following lines: waves entering a kind of submerged chamber open at the base pushed air into turbines to generate electricity sent via an underwater cable to the shore about 300 metres away. Unfortunately, the plant was damaged by the waves and then destroyed by a storm. The engineers who designed it did not give up, however, and a cheaper and more efficient version is being developed to supply small islands with much-needed electricity and to power a seawater desalinization plant.
Quite different technology is needed to turn the sea’s thermal energy into electricity. The sea’s surface temperature in the tropics ranges from 27 to 31°C all the year round and that of deep water ranges from 6°C at a depth of 1,000 metres to almost 0°C at 4,000 metres. This temperature difference can be used to power a motor based on the same principle as that of the heat pump. A liquid is turned into gas in an evaporator, and then drives a turbine which generates electricity, before passing through a condenser, where it is turned back into liquid.
The snag is that the process requires huge turbines. The first practical application of this method was by the French engineer Georges Claude, who in 1930 loaded pipes and turbines onto a ship off the coast of Cuba. American engineers are working on the idea of siting floating power plants off the southern coast of the U.S. and one facility has been operating in Hawaii since 1981. Such plants could supply floating cities of the future with electricity, air-conditioning and fresh water, while the unpolluted, nutrient-rich cold water brought up from the depths could be used to breed fish and shellfish and grow edible seaweed.
But according to Scottish engineer S. H. Salter, the most promising device is a “tapering channel”, invented by a Norwegian, Even Mehlum. As Singh describes it, “The waves are funnelled into a tapered natural or artificial channel. The water level rises and the water eventually spills into an elevated reservoir behind the narrow end of the channel. The water then flows back to the sea through a turbine, generating electricity in the process.” This reliable, low-cost system is already operating in Norway and Java.
Salter believes that if we use our imagination we will find an endless variety of ways of harnessing the sea’s energy. One of them would be to use the difference between sea levels. In Egypt, water running through an underground canal linking the Mediterranean to the El-Qattara depression could be used to generate electricity. In Israel, the same principle could be used in a canal between the Mediterranean and the Dead Sea which would gradually descend 400 metres, although the estimated billion-dollar price tag is a deterrent.
Mr. Singh concludes that “clean energy from the sea will be especially popular when the price of oil is no longer governed by economics but by environmental costs. Cutting carbon dioxide emissions will help save the planet.”
