Generating power from tidal currents

Power generation by tidal currents requires the establishment of a pressure difference across a turbine. This limits the power output to a fraction of the undisturbed energy flux through the same cross-sectional area, though the maximum possible value of this fraction is not well established. It is further shown that an array of turbines in the entrance to a bay is most effective if it is uniformly distributed across the entrance. The maximum power available for a quadratic drag law occurs when the tidal range inside the bay is reduced to 0.74 of the original amplitude, suggesting that power generation is compatible with the maintenance of good flushing and with use of the bay for other purposes. Moreover, this exploitation of tidal power through continuous operation of current turbines in the entrance is not much less productive than more conventional schemes that rely on trapping the water at high tide and releasing it during a small part of the tidal cycle.