From the ocean - electricity?

WITHIN two months, a group of Canadians will trudge out to a small island in the middle of a silty estuary in the Bay of Fundy and make energy history: They will turn on the only tide-operated electric generating plant in North America.

It will be an inauspicious beginning, however. For one thing, the demonstration plant will produce a humble 20 megawatts - enough to keep only about 8,000 households aglow. For another, it will be switched on after considerable delay, partly because of an electricians' strike.

But more important, it represents the inauguration of an energy source that continues to hold more curiosity than real kilowatt-generating potential, at least for the near future.

Indeed, the decades-old dream of harnessing the tides in North America, particularly on a large scale, has been becalmed in recent years by steadying world oil prices and uncertainties about the environmental impact of some of the biggest projects. ''It has been held hostage to oil prices,'' says John Gever of the University of New Hampshire's Complex Systems Research Center.

Tapping the tides, of course, still has its advantages. Tidal power is a clean, limitless source of energy that doesn't produce radioactive residues or require dependence on Mideast sheikhdoms.

On the other hand, there are a limited number of places in the world where the tides are high enough to produce power economically - and some of them are far away from consumers. Also, the plants usually require a lot of money up front: They cost little to operate, but plenty to build.

The potential sites around the world are now fairly well known, and a few small plants are churning out electricity. The French have a small turbine on the LaRance Estuary in the north and are looking at sites for other ones along the Atlantic coast. The Soviets turned on a 400-kilowatt pilot plant in 1967 at Kislaya Guba on the Barents Sea. The British intermittently flirt with the idea of drawing power from the Severn Estuary in the southeast. South Korea has a coastline with some potential. The Chinese have many small tidal plants, although in recent years, US observers say, some have been shut down because they interfered with rice-growing.

It is in Canada's undulating Bay of Fundy, however, where some of the greatest promise - and problems - of tidal power lie. Twice a day, to the rhythm of lunar forces, the water rises and falls 54 feet - as much as anywhere else on earth - in this watery finger between Nova Scotia and New Brunswick. The small one-turbine demonstration plant at Annapolis Royal was to be the forerunner of a longstanding plan to build a huge tidal plant in the Minas Basin, an inlet at the upper end of the bay, to harness some of this power.

But concern about the impact of such great intervention in the tidal flow on wildlife and the coastal ecology of eastern Canada and New England continues to stir controversy among politicians and scientists in both countries. The most-developed idea is to span the estuary with a five-mile-long, 150-foot-high concrete wall containing 106 underwater turbines and 60 sluice gates. The system would work like conventional tidal plants: At rising tide, seawater would flood through the gates and be trapped behind the sea wall (''barrage'') to form a ''head'' lake. At low tide, the impounded water would be released through power-generating turbines in the wall.

The system would generate 12 million megawatts of electricity - enough to supply 21/2 times the energy needs of Nova Scotia today. But it would come at a high cost: anywhere from $5 billion to $17 billion to build. Therein lies one problem.

While officials at Nova Scotia's Tidal Power Corporation, the provincial body overseeing the project, argue that it is economical when compared with most conventional energy sources - particularly considering likely fuel prices in the late 1990s, when the plant might come on line - they will need huge sums of money to build it. This would require help from investors in the United States, such as New England utilities, which would probably buy some of the surplus power. But so far response from the US side has been tepid.

The reason may be the environmental uncertainty surrounding the project. Scientists have suggested that tampering with the tides on such a scale would raise the water level several inches along the Eastern Seaboard, perhaps as far south as Cape Cod. Reason: The Bay of Fundy is part of a larger tidal system that includes the whole Gulf of Maine. Some alarmists go further. They argue that altering the tidal flow and range would wipe out some of New England's best beaches and kill large populations of shellfish and certain bird species.

Whether or not these claims are true, even proponents of the probject concede that it would cause at least some property damage along the coastline and probably affect populations of two annual migrants to the area: the sandpiper and shad fish.

The Annapolis Royal plant may offer a clue as to how tidal power would affect certain species in a localized area. It is also giving Canadian officials a chance to test a new turbine technology.

Still, enough scientific, legal, and financial snags remain to keep the larger project in the idea shop for several years yet, if not permanently. ''The large-scale development around the Bay of Fundy begins to raise questions about whether the effects will spill over from one country to another,'' says James Fay, a professor of mechanical engineering at the Massachusetts Institute of Technology.

For this reason, some experts believe that if tidal power is to become part of the future energy mix, it will have to be done on a smaller scale. But even here interest has ebbed in these days of oil plenty. In Maine, the Passamaquoddy Indians' plans for a 12-megawatt plant near Eastport, once expected to be on line by 1985, has been put off indefinitely, partly because of problems getting federal permits. A tiny power plant for Vinalhaven Island, off the central Maine coast, is moving ahead slowly - but only because of a strong push form local entrepreneurs.

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