Marine

• The principle of storing water at high tide is used in tidal barrage schemes, the best known of which is the 240 MW French scheme on the river Rance in Brittany, built in the 1960s.
  

  The Rance dam has a total length of 720 m and carries a four-lane highway. More than 17,000 boats use the lock (at the left) each year, to pass between the sea and the basin. Credit: EdF.

  
  

• A similar, but much larger, barrage scheme has been under discussion for some years for the Bristol Channel in the UK. The proposed barrage across the Severn Estuary, a site with one of the largest and most impressive tidal ranges in the world, would in theory be capable of supplying 5 per cent of the UK electricity demand. A 10-mile armoured wall across the estuary would house more than 200 hydro-electric turbines and generate electricity with the ebb and flow of the tide. The project, which would cost �14 billlion ($28 billion), will now be subject to a feasibility study.
  

  Two separate schemes have been proposed to harness the Severn's tidal power. Credit: Sustainable Development Commission

  
  
• Tidal mill and barrage schemes both make use of the height difference between high and low tide, but it is also possible to use to flow of the tides to generate power. Just as wind generators use the energy of moving air to make electricity, so a marine current turbine (or MCT) can use the energy of flowing water. The best MCT sites are those with strong currents, which tend to be around headlands, between islands, or in channels, where the tidal flow is concentrated.
  

• A study of potential sites around Europe estimated that marine current turbines could generate around 12,500 MW of electricity. This is a considerable resource, which could go a long way towards reducing reliance on fossil fuels, and meeting government commitments to cut the emissions of greenhouse gases.
  

• Wave power can also be harnessed to generate power. The world's first commercial wave-power generator was launched on the island of Islay, Scotland in November 2000. As waves break against a partially submerged concrete structure called the Limpet 500, some of the water rushes into a column behind the concrete shell. The water entering the column compresses the air above it, driving it through turbine propellers which power electrical generators, producing 500 kW.
  

• A British company has developed an offshore wave energy converter called Pelamis. The sea snake-like device is made up of a series of three semi-submerged cylinders linked by hinged joints. The motion of each hinge is resisted by hydraulic rams driving generators which produce electricity. The power conversion units in each cylinder generates up to 750kW of electricity. The entire device is approximately 150 metres long and 3.5 metres in diameter.
  

  Pelamis wave converter during trials.

• The first fullscale pre-production prototype has been built, using four Pelamis machines with a combined output of 3MW, and is being tested at the European Marine Energy Centre in Orkney. A typical 30MW installation would occupy a square kilometre of ocean and provide sufficient electricity for 20,000 homes.
  

• In the first stage of a commercial wave power project, three Pelamis machines will be moored in the Atlantic, three miles off Povoa de Varzim in Northern Portugal, providing 2.5MW of electricity to the Portuguese grid, enough to provide power for 1,500 homes.
  

See Harvesting the power of the sea

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