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I think you'll find current developments are aimed at much lower tidal flow sites than the super-sites you mention. And I think your overall conjecture is wrong. There is no reason why these kind of turbines should cost too much more than wind turbines, in fact quite feasibly less. The big point is that in a moving liquid a turbine is vastly more efficient than air so the turbines can be much smaller. The other point of course is that tidal flow is totally reliable and predictable so this is a big factor in driving research to harvest the lower flows that are universal on all coastlines. Just like there is massive energy potential in winds, the trick will be to get the capital cost, and efficient lower-energy capture right. Don't discount it.
Posted by rhodez