Tidal power has long been utilized as a vast energy resource, used early on in tidal mills to ground grain throughout the Middle Ages, among other uses. More recently, it has also been used to generate electricity, for instance, in the 240 MW tidal barrage at La Rance, France. Tidal currents, however, are still largely an untapped energy source. While several marine current turbine prototypes have been tested in the last few years, there are, to our knowledge, only a few of these prototypes deployed, which are generating electrical power to smaller communities to demonstrate their viability†.
There is currently a renewed interest in using the ocean to generate electricity, using both traditional hydropower technologies and new hydrokinetic technologies. This interest is being spurred by recent federal (U.S.) and foreign governmental legislation and initiatives promoting development of cleaner energy sources and diversification of energy supplies through use of alternative and renewable sources.
Tidal energy projects hold the promise of producing indigenous, renewable, predictable electricity without greenhouse gas emissions and with higher power densities (power output per unit area) than either solar photo-voltaic or wind projects. Tidal energy projects also may be useful in “filling in the gaps” in generation from other intermittent energy sources, such as wind projects‡.
Marine current energy conversion concerns the potential generation of electricity from kinetic energy of freely flowing water. This contrasted with traditional hydro power, (ie. a dam), where the amount of energy extracted from a river is dependent on the head between the reservoir and the water level below the dam. A similar hydro power approach can in fact be used for tidal power in areas with high tides. In that case, a barrage can be constructed in a narrow bay or estuary to utilize the head between low and high water. Another way to generate electricity from water would be to convert the kinetic energy of flowing water similar to the way a wind turbine would extract energy from the wind. In such case, a dam or reservoir is not necessary; rather, sites where the currents are strong, such as a narrow sound, straight, estuary, around a headland or in a river, would be desirable.
The term marine currents as used here includes any kind of water currents, be it tidal currents, unregulated rivers or other ocean currents driven for instance by thermal gradients or differences in salinity.
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The power P (Power output) in a flowing fluid (in our case sea water) through a given cross-section A (surface area in which the sea water comes in contacht with) can be expressed as where  is the density of the fluid (sea water is estimated to be more than 800 times more dense than air and V and is the velocity of the fluid (speed of the ocean current) through the cross-section A. Thus, as water is much denser than air, energy conversion from marine currents is possible even for relatively low velocities§.
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