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Sea power
Wednesday 28 July 2004 9.00-9.30pm

The oceans of the world, which cover approximately two thirds of the Earth's surface, represent a potentially vast source of energy. It has been estimated that, if less than 0.1 per cent of the renewable energy within the oceans could be converted into electricity, it would satisfy the present world demand for energy more than five times over.

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Quentin Cooper travels to Lynmouth in Devon and to Edinburgh to look at new British technology companies who are tapping into sea power in different ways but who are both at potentially exciting stages.

The UK, with its favourable wave climate and very strong tidal streams, is particularly well-placed to exploit sea power resources.

The DTI's Energy Technology Support Unit (ETSU) estimated that practicably (as at 1999) the energy which could be obtained from around the UK was:
  • offshore wave - 50 TWh/year
  • near shore wave (closer that 20 miles to the coast) - 2.1 TWh/year
  • tidal energy (at the 10 most promising sites only) - 36 TWh/year
The UK's annual electricity consumption is 329.9 TWh.

This represents a massive and inexhaustible energy source, which very few other nations enjoy in such abundance. It has been suggested that one reason why wave energy, in particular, has not developed more world-wide, is that, when the UK effectively abandoned its wave programme in 1982, other nations felt that, if it could not work in the British Isles' excellent wave climate, it could not work anywhere.

However, there is renewed interest and technological advances in tapping into this supply of power from the sea and Quentin Cooper talks to Martin Wright of Marine Current Turbines, who have been testing their tidal turbine off the coast from Lynmouth in North Devon. Martin says that you can think of it simply as an upside down windmill with the sails underwater, and powered not by the wind, but by the tidal stream.

Meanwhile, Ocean Power Delivery in Edinburgh have also been testing a prototype of their Pelamis wave-snake device and just been installed the full-scale model off the coast of Orkney.

Tidal Power

Tidal power can use either conventional or new technology to extract energy from a tidal stream. It is usually deployed in areas where there is a high tidal range. Typically a barrage with turbines is built across an estuary or a bay. As the tide ebbs and rises, it creates a height differential between the inner and outer walls of the barrage. Water can then flow through the turbines and drive generators. Some tidal barrages operate on both the rising and falling tide, but others, particularly estuarine barrages, are designed to operate purely on the falling tide. It is also possible to make use of the tidal flow that occurs between headlands and islands or in and out of estuaries. It is this application that is the focus of much research and development, and new products for this purpose are now being commercialised. These "in-flow" tidal turbines can be arranged singly or in arrays, allowing a range of power outputs to be produced.

Wave Power

The power of the waves is readily visible on nearly every ocean shore in the world. There has been much research to harness the power of these waves, and various machines have now been developed. These fall broadly into three categories:

  • Machines which channel waves into constricted chambers. As the waves flow in and out of the chamber, they force air in and out of the chamber. These airflows are in turn channelled through a specialised turbine, which is used to drive a generator. This type of machine is principally designed for use on or near the shore, or for incorporation into breakwaters. Commercially, this kind of machine is the most advanced and is particularly advantageous when incorporated into coastal protection.
  • Fixed or semi-fixed machines which utilise the pressure differential in the water that occurs at a submerged point as the wave passes over that point. The pressure differential is used by a variety of means to cause a fluid to flow in a circuit, which is then used to drive a turbine and generator.
  • Machines which utilise their buoyancy to cause movement in a part of the device as it moves up and down in the wave. The movement is used either directly or indirectly to drive a generator.
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