Earth’s gravity creates tidal forces on the moon that keep it locked with one side always facing Earth. This is why we always see the same face of the moon and never see the backside.

By contrast, Earth’s surface is slippery since it consists of more than 70 percent water. The moon’s gravity creates tidal forces that generate a bulge of water on both sides of Earth. Earth’s rotation carries the bulge along a little ahead of the moon’s overhead position so the high tides occur a few minutes earlier than the moon’s zenith.

Several complications affect the timing and height of the tide at a given location, but the local topography of land and sea has the most influence.

A typical open ocean tide has a height of only 1 foot, but a constriction can magnify the height of the bulge several times.

The greatest tidal range of all is in the Bay of Fundy, where it can reach 50 feet. The bay, in southeastern Canada between New Brunswick and Nova Scotia, is just to the south of Maine.

The amount of water that flows in and out of the bay is enormous. In each 12-hour, 25-minute tide cycle, more than 41 trillion (that is 41 with 12 zeros) gallons flow in and out.

The peak speed exceeds 11 miles per hour, which is two times faster than the flow of the Colorado River rapids and half as much as the Devil’s Hole at Niagara Falls.

One would think that this represents the potential for a valuable and sustainable source of electrical power. The U.S. Department of Energy identifies nationwide tidal energy potential up to 1,400 terawatt- hours per year. This is enough electricity to power 120 million homes.

All that is needed is a big water wheel connected to a generator! In principle this is true but in practice is not so simple.

There are many problems associated with capturing tidal power. Here is a list from tidalpower.co.uk, submitted without further elaboration to give an idea of the difficulty of making tidal energy generation facilities a reality.

>> They change the movement of water into and out of estuaries.

>> They can prevent the movement of fish and other wildlife into and out of estuaries.

>> They can kill wildlife that tries to swim through the turbines.

>> They can increase silt deposits, which can block out sunlight and prevent plants from growing.

>> They can change the structure of the coast and lead to erosion of dunes and other critical habitat.

>> They can prevent access to rivers and cut off shipping.

>> They are costly to build and maintain.

>> Locations are highly limited.

>> They can interfere with shipping channels.

>> Lifetime of structures is unknown.

There have been many attempts to transform Earth’s rotation into energy by harnessing the tides. Yet, in its current state of development, tidal power is not feasible as a large-scale energy solution despite the first plant having opened in 1966.

The largest generator currently in use is the Sihwa Lake Tidal Power Station in South Korea, which generates only 254MW of energy.

The incredible potential of tidal power makes it attractive, but there are significant obstacles to be overcome before it becomes a major player in the human energy budget.

Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.