Definitions

extractible

Marine current power

Marine current power, is a form of power based on the harnessing of the kinetic energy of marine currents. It includes both tidal power and energy derived from ocean currents such as the Gulf stream. A report from the US government agency, the Department of the Interior in 2006 states: "It has been estimated that capturing just 1/1,000th of the available energy from the Gulf Stream, which has 21,000 times more energy than Niagara Falls in a flow of water that is 50 times the total flow of all the world’s freshwater rivers, would supply Florida with 35% of its electrical needs."

Although not yet widely used, marine current power has an important potential for future electricity generation. Marine currents are more predictable than wind energy and solar power.

Comparison with Tidal stream energy

Main article: Tidal stream generator
A lot of development is currently happening in tidal stream energy systems in places such as the United Kingdom, the Bay of Fundy and South Korea where the tides run often in excess of 7 knots in relatively shallow water. Since the power generated from any turbine such as a wind generator varies both with the density of the fluid and the cube of its speed, it's important to find places where this movement is fast enough. Water is about 840 times denser than air, so speeds of nearly one-tenth of that usable for wind is possible.

Although easier to exploit, tidal power suffers in two ways from periodicity: the direction of the current changes four times a day in most places, and the amount of energy available at spring tides is considerably greater than at neaps. Whereas the effect of the first can be minimized by using generators at different places along the coast, the second means that less power is generated for a period of a week or more.

By contrast, ocean currents flow relatively steadily throughout the year and in some cases the flow is very considerable. An example is the Straits of Florida where the Gulf Stream flows out of the Carribean Sea and into the North Atlantic on its way to northern Europe. The speed of the current is around 4 knots (2m/s) at the surface although less towards the bottom. There is a potential extractible power of 1KW/m2 near the surface.

Technical challenge

A 300 kW full scale plant has been operating at Lynmouth, Devon (UK) since May 2003. Although a prototype for tidal systems such as Seagen, MCT has also been planning deep sea marine current systems, which could be constructed in large farms and thus use economies of scale both in construction and maintenance and in the infrastructure for bringing the electricity to shore.

Another approach which has identified the potential of the Gulf stream is the Gorlov helical turbine. This is a vertical axis turbine which is being currently prototyped in South Korea.

Environmental considerations

Concerns have been expressed about the danger to fish and other marine life, both directly from the turbines and through the disturbance to the sea floor by construction, anchorages and supply cables. In general the turbines would be relatively slow moving and it is believed that fishes will detect the pressure changes and avoid being caught. This is a contrast to the turbines on barrage tidal systems where 15% mortality of fish passing through the turbines is observed. To confirm this, a monitoring programme has been established at Strangford Lough.

Damage to seabed flora is also potentially dangerous and designs are being explored which are anchored to the seabed but operate at a distance, rather than having towers built on the bed. Since there are at present no firm plans for deployment of these devices, it is difficult to evaluate whether this will be a serious problem.

See also

Ocean energy

Notes

External links

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