There’s “a tide in the affairs of men” quite different from what the Bard meant. Scientists are trying to harness the vast energy of ocean tides, or “tidal energy”, to generate electrical power. A fraction of the energy locked up in the oceans could, in theory, meet the world’s entire electricity needs. Seawater is 832 times as dense as air, so an ocean current traveling at, say, eight knots has the kinetic energy of a wind blowing at nearly 400 km/h. This makes tides a powerful energy source: clean, inexhaustible, and free from the climatic irregularities that dog wind and solar power. Since we can predict tides hundreds of years ahead, we needn’t bother about the lack of wind or rain, or fluctuations in the El Nino or La Nina cycles.
Tides are produced by the twice-daily variations in sea level, as the Moon and — to a lesser extent — the Sun, exert their gravitational force on Earth’s oceans. Because of these gravitational pulls, oceans bulge out towards the Moon. On the opposite side, the gravitational effect is partly shielded by Earth, resulting in a smaller interaction so that oceans bulge out away from the Moon, due to centrifugal forces. Similarly, the Sun causes oceans to bulge towards and away from it on the sides of Earth facing and opposite it. The difference between low and high tides is greatest during ‘spring tide’, when the Sun and Moon are in the same plane as Earth, than at ‘neap tide’, when they are at right angles to each other.
When tides come into the shore, they can be trapped in reservoirs behind a high-capacity dam built across an estuary. At low tide, sluice gates in the dam open to release the water, which drives a turbine to generate electricity, just like in a regular hydroelectric power plant. But you need large increases in tides — of at least 16 feet — between low tide and high tide, and there are only a few places on Earth where tidal changes on such a large scale occur. Also, there must be a source of power supply to make up for the gaps in power production linked to the times of high and low tides. Still, the tide may turn once the price of oil is dictated by environmental costs rather than economics.