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By Elizabeth Blunt
BBC World Service correspondent, Canada
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The world's highest tides are a source of electricity for Canada
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The one thing that every schoolchild learns about the Bay of Fundy is that it has the highest tides in the world.
The North Atlantic waters are funnelled up this deep inlet between the Canadian provinces of New Brunswick and Nova Scotia, swirling through the narrowest parts of the bay with impressive speed and power.
So when in the 1980s the Canadian government wanted to explore the feasibility of tidal power, this was a natural site for an experimental station.
The site chosen at Annapolis Royal had already been closed off by a causeway, built to control tidal flow further up the Annapolis River.
The idea was that water flowing through the sluices could also pass through a turbine and generate power on its way down the Bay of Fundy and out into the sea.
Now there is a neat box-like structure in the middle of the causeway, one of only a handful of operational tidal power plants in the world.
Plant engineering
The Bay of Fundy funnels the power of the Atlantic
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Twice every day, as the tide rises, the sluice gates are opened to let water flow up into the lower part of the Annapolis river, which now serves as the headpond for the power station.
Just before high tide, the gates are closed, leaving only a narrow passage for fish to pass. Now all the operators have to do is wait for the tide to turn and the water level on the seaward side to drop.
When there is enough difference between the water levels on the two sides, they begin to let water flow through the giant turbine, slowly at first to get it turning, and then at full strength.
Once the huge 25-metre diameter wheel is up to its operating speed of 50 revolutions per minute, the station starts to generate. At peak power it supplies a very respectable 20 megawatts to the Canadian grid.
Once the tide has gone out and the water level is equalised, the turbine slows to a stop, before the whole cycle begins again.
Stuart MacDonald of Nova Scotia Power remembers the thrill of seeing the plant in action for the first time. He loves the elegant simplicity of its engineering and its reliability, but accepts that tidal power has its limitations.
Built for purpose
It may be a predictable source of electricity, but only while the tide is going out.
Water flows through a giant turbine at Annapolis Royal
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And it is not the sort of thing that can be bought off the shelf; machinery has to be purpose-built for the site, which makes it expensive for a power station which is always going to be idle for at least 12 hours a day.
But at least the fuel is free; and although the plant was designed for a 70 year lifespan, Stuart MacDonald believes that with proper maintenance it could last 200 years.
The technology, though, has yet to take off in a significant way.
The biggest tidal station in world is in France, near St Malo, 12 times larger than Annapolis Royal; Russia has a much smaller plant, while China has constructed several small facilities.
With fossil fuels getting scarce and expensive, and concern rising about the damage they are causing to the atmosphere, interest in tidal power is growing; because these plants produce no greenhouse gas emissions.
Other projects are planned, and designers are now working on plans involving individual turbines moored in the middle of the tidal flow, easier to build and less environmentally controversial than the fixed barrages used by the existing stations.
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