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The waves on the surface of the sea are created by wind. The quantity of energy generated is very small (1 W/m²/year, that is to say, 200 times less than that of direct solar energy). But as the waves move in a very economical manner, we can hope to recover almost all of the energy created on the vast marine surfaces, by installing energy-harnessing devices along the coast. The theoretical power that can be recovered is estimated at 50 kW/m of coast. The problem is that the energy tends to dissipate the nearer we get to the coast: a figure of 50 kW/m at 20 km from the coast can drop to only 20 kW/m at 1 km away. It is therefore necessary to find a compromise between distance from the coast (costs increase the further away we are) and the energy that can be recovered (which diminishes as we approach the coast). Wave energy is not the same everywhere and it varies according to the season. It is very significant in North-West Europe, in particular along the British coasts.
4 main types of design exist for the recovery of energy from the waves:
Underwater buoys in movement (immersed columns of oscillating water), which rise, fall and sway at the whim of the waves. Anchored on the sea floor, their movement drives a piston, that sucks the sea water into a turbine,
or compresses air or oil and turns a motor;
Columns of oscillating water positioned on the coast: at the end of their journey, the waves enter a box where they compress air contained inside. This compressed air is used to drive a turbine;
Overflows in a channel: the waves rush into a channel that gets narrower and narrower. The level rises and water overflows the embankment into a reservoir that slowly fills. The water from the reservoir goes back to the sea, driving a turbine on the way. The reservoir can be situated on the coast, but there is also a project for a floating version of this type of system (what is known as a slack-moored overtopping platform);
Floating caissons linked together by articulated hinges. The waves move the caissons in all directions. Energy is recovered at the moving articulations between the caissons, by means of pistons driving pressured-oil pumps. |
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| Diagram of the operation of a wave engine installation. |
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Several projects exist for “wave motor” power plants. These projects are under test or still on paper and it is not known which will prove their worth in the future - certainly the most resistant to storms or with the longest expected life:
The Pelamis Project using the technology of articulated segments – 750 kW, in the Isles of Orkney, under test since 2004; 3 x 750 kW off the coast of Portugal, at the stage of a commercial pilot project since 2006.
In July 2004, a “sea snake wave energy converter” was launched in Scotland. Four floating boxes are linked to each other by articulated hinges. The waves move the boxes in all directions. The energy is recovered at the level of the moving joints between the boxes, by means of pistons that drive pressurised oil pumps. Christened Pelamis (after a mythological serpent), the monster is 150 m long and weighs 750 tons. All being well, a herd of 30 to 40 Pelamis could soon form an electricity power plant capable of supplying 20000 households with electricity with a capacity of 20 MW/sq km of ocean.
The Wave Dragon Project using the technology of a slack-moored overtopping platform – 7 MW, in Wales, at prototype stage in 2007
The waves go across an upwardly-inclined plane and fill a reservoir, which subsequently empties, driving a turbine. A mooring device allows the installation to orient itself perpendicular to the direction of the waves. The size of this sea dragon is impressive: a span of more than 200m and a weight of some 30 000 tonnes. In the event of a heavy storm, there is a lot of tension in the mooring cables : beware of the structure escaping! It could become a dangerous drifting hasard for navigation! It is for this reason that long-term trials are essential.
Project Limpet using the technology of columns of oscillating water positioned on the coast – 500 kW, on the Isle of Islay in Scotland, prototype under test since 2000
These coastal projects have a major disadvantage : they disfigure the coast where they are installed. A possible solution: hide them in the nooks of an artificially created cliff.
Several projects with columns of oscillating water or oscillating buoys are under trial, or at the prototype stage (AWS in Portugal – 2 MW ; Powerbuoy at Santona in Spain – 125 kW ; IST from Lisbon, to the north-west of the Azores – 400 kW ; MRC 1000 off the coast of Brest – 2 MW ; Manchester Bobber in England)
Project Searev with an original technology based on an internal balance wheel, a 500 kW prototype of which will go on trial in 2008.
Sealed in a watertight shell, an enormous concrete cylinder (400 tonnes) with its top half empty oscillates in the waves like a pendulum. The concentrated mass of the half cylinder amplifies the movement of the waves. The energy of this mega-pendulum is transformed into electricity by means of hydraulic pumps. A Searev is 24m long and 14m high and will weigh 1000 tonnes. It will be able to deliver 20 to 30 MW/sq km of ocean. |
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| Waves formed on the surface of the seas are an energy source to be explored. |
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