La Rance tidal plant starting 10-year renovation

The La Rance tidal power plant in Brittany, France, is celebrating its 30th birthday and starting on the first program to renovate all of its turbines. The 10-year project will cost (US)$78 million and create 30 new jobs. In the past 30 years, the plant has generated 16 billion kWh in 160,000 hours of operation.

La Rance is the oldest and most powerful existing tidal plant in the world, at 240 MW, with the next largest less than one-tenth the size. Electricity is generated by taking advantage of the difference in water levels between high and low tides. As the tide comes in, the sea water is channeled through 24 turbines, creating electric power. When the tide is in, the turbine channels are closed, waiting for low tide.

Once the tide is completely out, the water held behind the barrier is released through the same turbines. Tidal power from La Rance costs (US)3.7 cents/kWh to generate. France`s nuclear plants average (US)3.8 cents/kWh and thermal plants (US)10.5 cents/kWh. Only hydroelectric plants at (US)3.2 cents/kWh are more economically efficient.

Turbine repair

The turbines being replaced are based on a horizontal rather than vertical axis and have a double effect–producing power as the tide comes in and goes out. The facility is also used as a pump to store water behind the barrier or lower the water level in the bay during high tide.

With the renovation of the turbines, the plant will be able to continue generating electricity in the coming decades, while maintaining its 90 percent availability record. Three teams of 15 people will each work and repair one turbine each year in the next decade.

Tide technology

The technology for such facilities was proven long ago, but the limitation in building more plants is finding suitable sites. A tidal power plant must be placed where there is a significant difference in water height between high and low tides.

At La Rance the difference can reach more than 44 feet during equinoxes. Also, it must be possible to build the closure needed to take advantage of the tide changes. Finally, an interconnected electrical network must exist because tidal power is not continuous.