The tidal power plant on the Rance river in Brittany, France, stands as a reminder of the underexploited potential of energy generation from ocean tides, waves and warmth. Inaugurated at La Richardais on the west coast by Charles de Gaulle in 1966, the plant produces about 500 Gigawatt hours of electricity per year - enough to power 250,000 out of France's 30 million-odd households.
It remains the sole power station of its type in France and one of only two large-scale tidal plants in the world - the second largest after the Sihwa scheme inaugurated in South Korea seven years ago.
"Ocean renewable energy has huge global potential, but is a largely untapped resource," Simon Neill of Bangor University's School of Ocean Sciences in Wales told AFP.
More than two-thirds of the Earth's surface is covered by oceans and seas. These offer limitless opportunity for power generation from the kinetic energy of waves and tides, the temperature differences between surface and deeper waters, and the osmosis that occurs when salt- and freshwater meet.
Yet ocean power accounts for the smallest portion of renewable energy in the world, according to the International Energy Agency (IEA), an intergovernmental policy-advisory body.
The majority of marine energy projects are in the demonstration phase even though many decades have passed since humanity first started using water motion for electricity generation in the late 19th century.
Despite being a renewable, low-polluting energy source, hydropower, which includes ocean and river dam schemes, today accounts for less than a fifth of global electricity supply, and growth in the sector is decelerating.
Electricity generation from marine technologies increased by just three percent last year. The IEA estimates it would need to expand by 24 percent per year until 2030 to meet desired targets for renewable energy.
According to France's SHF hydrotechnic research institute, tidal energy has the potential to provide 1,250 Terrawatt hours (TWh) of energy per year, up from just one TWh today - about 0.2 percent of France's annual consumption.
It has a distinct advantage in being "entirely predictable", Neill said. As it is not dependent on weather variables such as wind or sunshine, the contribution of tidal schemes to the energy mix can be calculated far in advance.
Tidal energy is generated by installing a dam-like structure called a barrage across a bay or lagoon to create a basin which is filled on the incoming high tide and emptied through turbines as the water ebbs out again.
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