Tidal current power plants

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Most of the currently known renewable energy technologies, such as hydro power, wind, solar, wave power or biomass use either directly or indirectly solar radiation and convert it into electrical energy. There is however another energy source which does not rely on the sun. The gravitational forces of the moon and the rotation of the moon around the earth are the origin of tidal flows, which can also be transformed into electrical energy.
Since the 60s tidal power plants have been in operation, impounding the water masses which flow periodically into a bay at high tide and release them at low tide to drive a conventional hydro turbine.
This technology requires a large dam in the sea. Therefore, Voith Siemens Hydro is developing new damfree tidal power plants, which do not extract the head of the retained water masses but instead, similar to a wind turbine, utilize the kinetic energy of the in and out flowing water. These power plants are therefore called "Tidal Current Power Plants".
To extract the kinetic energy more efficiently, preferred locations for tidal current power plants are concentrated in straits or locations between islands where the water has to "squeeze through" and thereby becomes accelerated.
Tidal current power plants could for example be turbine nacelles which are mounted on a monopile, extracting the kinetic energy of the in and out flowing water masses and transforming that into electrical energy. These innovative power plants could be operated fully submerged and hence produce renewable electricity without any visual impact ­a large advantage for the application of this technology in tourist regions, such as Norwegian fjords.
Due to the high density of water (in comparison to air), tidal power plants are significantly smaller than wind turbines, although the velocity of tidal flows is at approximately 24 meters per second significantly smaller. For comparison: While a one megawatt wind turbine has a rotor diameter of approximately 50 meters a tidal current turbine only requires 15 meters.
The biggest challenge of this new technology is the difficult accessibility ­especially if the plants are fully submerged. Special lifting equipment and maintenances ships that are frequently used in this harsh environment are very expensive and downtimes of the plant can reduce the economics tremendously.