利用者:Shinkansen Fan/海流発電
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Marine current power, is a form of power based on the harnessing of the kinetic energy ofmarine currents. It includes both tidal power and energy derived from ocean currents such as theGulf stream. A 2006 report from US government agency Department of the Interior estimates " . . that capturing just 1/1,000th of the available energy from the Gulf Stream, which has 21,000 times more energy thanNiagara 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."[1]
Although not widely used at present, marine current power has an important potential for future electricity generation. Marine currents are more predictable than wind energy and solar power.[2]
Comparison with Tidal stream energy
[編集]Tidal stream energy systems are currently in development in places such as Norway, the United Kingdom, the Bay of Fundy and South Korea where the tides run often in excess of 7 kn (13 km/h; 8.1 mph)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. Ocean water is some 840 times denser than air,[3] so speeds of about one-tenth of that of the wind speed can generate the same power.[4]
Although tidal currents are easier to exploit, their periodicity must be taken into consideration. The tidal current strength oscillates 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 — in different phases of the tidal wave — the second means that potential power generated varies from minimum to maximum in each two-week period.
By contrast, ocean currents flow relatively steadily throughout the year and in some cases the flow is considerable. An example is the Straits of Florida where the Gulf Stream flows out of theCaribbean Sea and into the North Atlantic on its way to northern Europe. The speed of the current is around 4 kn (7.4 km/h; 4.6 mph) at the surface,[5] but decreases with depth. There is a potential extractable power of 1 kW/m2 near the surface.[1]
Technical challenge
[編集]A 300 kW full-scale plant installed by Marine Current Turbines (MCT) has been operating at Lynmouth, Devon (UK) since May 2003.[6] Although this serves as 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, a vertical-axis turbine which is being currently prototyped in South Korea.
The challenge of marine current power is quite new. If you are designing a wind turbine which operates at maximum efficiency at, say, 50 km/h (31 mph), then you have to consider what happens in a storm, when the speed might be 100–200 km/h (62–124 mph) or even higher. This places severe constraints on the approaches. If you are dealing with a marine current of 1 m/s (1.9 kn) then you can be reasonably certain it will never exceed 1.5 m/s (2.9 kn). Consequently, totally new approaches might be possible.
One of these that is already being realised in tidal systems is the Venturi effect. You can funnel a large area of water through a small aperture because you know you don't have to worry about abnormal conditions (though you may have to worry about environmental effects). Radically different turbine designs might be possible, such as the Gorlov turbine, or oscillating devices such as the Stingray. On the other hand, the cost of getting the power back to shore, dealing with the marine environment, or the cost of servicing may make the whole thing uneconomic.
Environmental considerations
[編集]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 marine life 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
[編集]Notes
[編集]- ^ a b Department of the Interior White Paper
- ^ http://science.howstuffworks.com/ocean-current4.htm How Stuff Works website, Ocean Currents p. 4
- ^ Since water density depends on temperature and amount of salinity, and since air density depends on temperature and pressure, it is not possible to accurately state the "relative density" of air and water.
- ^ Twidell, John; Weir, Anthony D.; Weir, Tony (2006), Renewable Energy Resources, Taylor & Francis, ISBN 0419253203. See pp. 442–443.
- ^ The Florida Current
- ^ Marine Current Turbines Ltd MCT Website