The waves have long been used to generate energy. In the 18th century, several plants were built on Europe's coasts where the waves reached a reservoir with floodgates. These were closed when the tide was high; once it came down, the water could only escape from the reservoir by passing through a hydraulic turbine that turned to generate energy.
The same principle was applied to a power plant built in France in the 1960s. A dam was built in the estuary of the Rance River, Brittany region, with 24 machines that can function as turbines, regardless of whether the water flows in one direction or the other.
When the tide comes in, water accumulates against the dam until there is a 1.5 metre difference between the two sides. It is then passed through the electricity-generating turbines. As the tide goes down, the turbine blades turn and the water that returns to the sea generates electricity again.
The amount of electricity produced depends on the water level on both sides of the dam. The greater the difference in level, the more electricity is generated, because the water is under greater pressure and turns the turbines with greater force.
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