Tidal power uses the differential between low and high tides to generate electricity. A precursor in this field, EDF has been harnessing this energy since 1966 at its plant built across the estuary of the La Rance River in Brittany, France. In a world first for this source of renewable energy, it produces around 500 GWh/year.
Tidal Energy: renewable and low environmental impact
Tidal energy seeks to harness the differential between low and high tides. There are many advantages in using tides to generate electricity: tides are a perfectly predicable phenomenon (unique for a renewable energy source), they are inexhaustible and carbon-free, and they have low environmental impact.
The principle involves building a barrage to create an artificial reservoir and so a differential in water levels to drive the turbines and alternators that will generate electricity. A tidal power plant uses the rising and falling movement of tides to create the level differential needed to produce energy.
La Rance tidal power plant: a world first
EDF was a precursor with this technology when, in 1966, it built the tidal power plant on the mouth of the La Rance River in Brittany, France. It is one of just two such plants in the world along with Sihwa in South Korea. The La Rance plant has an installed capacity of 240 MW distributed between 24 bulb-type turbine generators, each with a capacity of 10 MW. For almost 50 years, it has been producing around 500 GWh/year, equivalent to the consumption of a city the size of Rennes, France.
The La Rance plant is one of just two such plants in the world along with Sihwa in South Korea.
The La Rance plant operates according to two different modes: ebb only generation or flood and ebb generation.
ebb generation: the barrage’s six valves are opened until high tide is reached to fill the reservoir. As the tide recedes, the differential created between the impounded water and the sea is sufficient to drive the turbines and alternators to generate electricity
flood and ebb generation: during spring tides, electricity is also produced on the incoming as well as on the outgoing tides. The differential between the low tide and the level upstream from the barrage is sufficient to drive the bulb-type turbines and generate electricity on the flood tide. The turbines and alternators were specifically designed to be able to operate in both directions.
Lastly, the La Rance plant’s turbines can also pump water. At the end of the incoming tide, the generator operates in reverse mode and pumps seawater in order to further raise the level behind the barrage before slack water is reached (the short period when there is no movement either way in the tidal stream). This enables production to start up sooner and so maximise output.
A precursor in tidal power, EDF is currently involved in detailed research on mechanisms to harness marine energy. The Group is working with partners like IFREMER (French Research Institute for Exploitation of the Sea) to expand its expertise in this field.