In the oceans, enormous masses of water move around but at very low speeds (10 to 20 km/h). They form marine currents close to the coasts, principally caused by the tides. These so-called tidal streams have a major advantage compared to other forms of renewable energy, such as wind or solar energy, in that they are totally predictable, years in advance. They form marine currents, which are in part caused by tides. Underwater "windmills" can capture this energy. The mills currently being studied and tested have large blades (like their big land-based sisters), or batteries of underwater turbines, either fixed on the seabed at depths of 20 to 40 m, or floating underwater, or operating attached to a long stake fixed into the seabed.
The principal problems of the water turbines:
- Very heavy costs, due principally to heavy maintenance operations;
- Corrosion of the equipment by the sea water:
- The opposition of trawler fishermen, since the turbines effectively prevent access to the zones where they are installed (a water turbine farm = a fish sanctuary!).
A 20 m diameter wind turbine has been
tested
at a depth of 75m since January 2004, in the Kvalsund area, in the north of Norway. Projects are
being studied
in the United States (exploitation of the Gulf Stream), in Italy (for the Straits of Messina) and in Great Britain.
Several projects have been undergoing tests since 2002. They all attempt to reduce maintenance costs, in other words, the numerous repairs that have to be regularly carried out since the devices operate in a very aggressive, stormy and corrosive milieu.
- The Seagen Project of the MCT Company (United Kingdom) : 2 turbines, each 600 kW and 16m in diameter, attached to a stake driven into the sea floor (along which the turbine blades can be brought to the surface for maintenance) ;
- The Marénergie and Lunar RTT Projects set up by Hydrohélix (France) and Lunar Energy (United Kingdom) : batteries of small turbines are fixed to a pile on the seabed. For maintenance operations, it suffices to unhook the central « cassette » of turbine blades from its base and bring it back up to the surface ;
- Blue Concept Project of the company Hammerfest Strom (Norway): a three-blade turbine, 20 m in diameter, is attached to a tripod weighted down with a load of 120 tonnes which stabilises it on the seabed. The tripod is brought to the surface for maintenance operations;
- The Tidel Project operated by the company SMD Hydrovision (United Kingdom): twin turbines are maintained underwater by a floating device anchored to the sea floor by a system of cables. Remounting the turbines for maintenance is achieved simply by lengthening the cables;
- The Stingray Project developed by the company The Engineering Business (United Kingdom): an oscillating wing, 15m wide, is fixed to the end of an articulated arm. The entire 20m-high device is positioned on the sea floor;
- The B2 and the Enermar Projects designed by Blue Energy (Canada) and Ponte di Archimede (Italy): they are based on systems of vertical-shaft turbines which, of course, turn in a horizontal direction . The difficulty with this system is to keep the blade in the tidal stream. In the Italian project, this problem is addressed by fixing the electrical generator to a floating circular pontoon anchored to the seabed by cables.
It is not yet known whether either or both of these projects will be successful in the future. Certainly, it will be the one(s) which prove(s) to be the most robust and the least costly!
The exploitable potential of tidal currents throughout the world could reach a level of 150 GW (including 6 GW in France, almost the equivalent of 5 standard 1300-MW nuclear power plants.)
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