How tides are used to produce electricity?

The tides have been used to provide power for hundreds of years. In the 18th century, the coast of Europe was dotted with tidal mills, which let the incoming tide into a reservoir through open sluices. At high tide the sluices were closed and the only way the water could escape as the tide fell was by passing through and propelling a waterwheel, so providing turning power.

The same principle was used in a power station built in France in the 1960s. a dam was built across the estuary of the River Rance at St Malo in Brittany, with 24 machines that could be used as turbines in either direction.

As the tide comes in, it is allowed to build up against the dam until there is a difference of 5ft (1.5m) between one side and the other. Then it is allowed to pass through the turbines, driving them and generating electricity. When the tide begins to fall, the turbine blades are reversed, and the water generates electricity again.

The amount of electricity generated depends on the ‘head’ of water – the difference in the level of the water between one side of the dam and the other. The larger the head, the greater the amount of electricity that will be generated, because the water is under greater pressure and so turns the turbines with more force.

At high tide the sluices are shut and extra water is pumped from the sea into the estuary. The water level in the estuary is raised above high tide, so when the sea falls back to low tide the difference in levels has been accentuated.

Once all the water has been allowed to flow into the sea – driving the turbines as it does so – extra water is pumped out to make the level in the estuary artificially low.

When it is high tide again, the turbines are reversed, water flows back into the estuary, and the cycle starts once more. Of course, pumping consumes electricity, but the additional heads produce considerably more electricity than the pumps use.

The scheme at La Rance generates 240 megawatts at peak output – sufficient for a medium-sized city such as Rennes or Caen, but it has had few followers. The immense cost of building the dams and the lack of suitable sites have discouraged everybody except the Russians and Canadians.

The Bay of Fundy in Nova Scotia has the biggest tidal range in the world, with up to 59ft (18m) height difference between tides.

A successful pilot plant was opened across an inlet of the bay at Annapolis Royal in 1984. If the power of the tides across the whole bay could be harnessed it would produce ten times more energy than could be used locally. The surplus electricity could be used in New England and New York Experts believe that it is just a matter of time before the project goes ahead.


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