The Physics of Tidal Energy 

By Matthew L. Wald | New York Times:

As my colleague Jess Bidgood reported in Friday’s paper, a tidal energy project is moving ahead in Maine, with high costs but high hopes too. But the 180-kilowatt unit that the Ocean Renewable Power Company hopes to put under water next week is really just a first step. The big question is, how well will it withstand the force of the rushing water?

The region, the Bay of Fundy, is famous for strong tides, but the company has picked a spot called Cobscook Bay, where the current is relatively slow, an average of 5.8 knots, or 6.7 miles an hour. That is the speed at which the hardware will produce 180 kilowatts.

If the equipment performs well there, the next step is for the company to put a similar piece of hardware in water moving at 8 knots, or 9.2 miles an hour. In that water, the same equipment would produce 498 kilowatts, or 2.8 times as much.

If the water speed is greater by only about one-third, why does power nearly triple? Because the energy in the passing water rises with the cube of its speed; that is, the ratio of the energy in a 5.8 knot current to an 8 knot current is the ratio of 5.8 x 5.8 x 5.8 versus 8 x 8 x 8.

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