The ocean's tides contain a tremendous amount of energy that goes virtually untapped. It is estimated that the US alone has about 2100 terawatt-hours of energy tied up on its coasts. About one-eighth of that power, about 260 terawatt-hours, has been quoted as being able to be fully developed with minimal environmental impact. This is equivalent to the output of all of the US's hydroelectric dams.
Tidal generators have been in limited use in some locations for decades. For example, the La Rance Estuary in Northern France is basically an advanced, specialized dam, called a "barrage", built across a river mouth. When the tides comes in or out, water flows through tunnels in the barrage, driving electrical turbines. The La Rance barrage is able to generate 240 megawatts of energy.
The main disadvantage of this kind of tidal generator is that it can generate power only when the tide is coming in or out, or for only about 10 hours a day. Environmental concerns also exist in placing a large dam at the mouth of any river. However, the huge advantage of tidal power is that once the initial facility is built, the power is free, dependable, and renewable.
Tech Level: 11
A more efficient way of harnessing tidal power is with tidal turbines. Resembling upside-down wind turbines, these are buoyed well off shore and submerged in at least several meters depth of water. Under the water, currents constantly flow from tidal forces, providing the turbines with a near-inexhaustible source of power twenty four hours a day.
The constant flow of currents causes propeller-like turbines to turn, generating power. Because the water currents are much denser than air, tidal turbines can be made smaller than wind turbines and still deliver as much energy. A single turbine in a "proof of concept" demonstration offshore of Britain produced 300 kilowatts worth of power. Turbines would be deployed in large arrays called "farms" to generate many megawatts of energy for onshore consumers.
Most of the tidal devices work well in a 4-5 knot current and anything less would be uneconomical in energy production terms. Anything more than that would result in stressing the turbine fans and reducing the optimal lifespan of the turbine.
There are also vertical-axis tidal turbine designs, resembling oversized eggbeaters or enclosed turnstiles.
CIRCULAR TIDAL BARRAGE
Tech Level: 11
In order to further utilize the surface-barrage concept while greatly minimizing their impact from blocking the mouths of rivers, it has been proposed to build a circular tidal barrage "reef" just off-shore of river deltas. These reefs would not interfere with the flow of the river or with the migration of wildlife like the La Rance facility, but would still take advantage of the more intense surface flow caused by tidal currents. During high tide the water would flow into the large circular area cordoned off by the barrage reef, and during low tide the water would flow out.
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