Tidal stream power is a very attractive, renewable energy source and has many measurable benefits:



Most of the necessary superstructure is below the sea surface Environmentally unobtrusive
The superstructure can be suitably screened or caged Completely harmless to all forms of marine life and ship   navigation
It involves negligible carbon emissions or other pollutants Environmentally friendly
The accessible tidal stream power resource is considerable.For example, taken as a cyclic time average, it is 90GW   (800TWh/y) globally based on a 3% estimation of the total power in the   world’s tidal streams This is sufficient to satisfy 5% of the present world   electricity consumption, which is 1875GW (16,424TWh/y)
The energy density stored in a tidal current is 800 times that   in a normal wind for a given speed of current/wind Tidal stream structures can be made much smaller than wind   turbine structures
Tidal streams are highly directional Neighbouring turbines in tidal stream farms (also called   arrays) can be placed closer together than wind turbines, saving on   construction, cabling and maintenance costs
Tidal stream generators produce completely predictable   variations in power over a 12-hour cycle Unlike wind turbines they can be used as a sole source of   power over larger regions.

The main commercial consideration with tidal stream generators, which at present remains an emerging technology, is the expected costs of lifetime maintenance in hostile marine environments.  Theoretical estimates of €203,000 per MW/year and €47,250 per MW/year having been reported for difficult and easy sites respectively compared with ~ €30,000 per MW/year for typical wind turbine sites. Underwater rotating components in the generator drive train will be subject to random and periodic wave loading involving stress cycles of much larger amplitude than arises with wind turbines, as well loading from marine fouling. The replacement of failed components is relatively expensive because of the underwater environment, especially if catastrophic failure is involved through the breakdown of one component in the generator drive train causing collateral damage in other components in the drive train.

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