Our celestial neighbor, the moon, uses its gravitational pull to create tides in the world's oceans. Tides occur twice within a 24-hour period: a high tide when the water rises and low tide when the water recedes. Although this movement of water can generate electricity, only those areas where the difference between high and low tide is at least 16 feet (4.9 meters) can generate electricity efficiently. Worldwide, only about 40 sites fit the bill.
A barrage dam can harness moving water. When there is a sufficient difference in the level of water on either side of the dam, sluice gates open and allow water to flow across turbines that turn a generator, which produces electricity. It is also possible to tap into tidal currents, which pass close to shore at depths of about 65 to 100 feet (20 to 30 meters). Turbines, like those used on wind farms, allow the currents' surge to spin underwater rotors, which turn a generator.
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