Tides are twice-daily rises and falls of water level relative to land. Ocean tides can produce strong currents (a steady flow of ocean waters in a prevailing direction) along some coastlines. Humans have sought to harness the kinetic (motion-induced) energy of the tides for hundreds of years.
Residents of coastal England and France have used tidal energy to turn water wheels and generate mechanical energy for grain mills since the eleventh century. In modern day, tidal currents are used to generate electricity.
Tidal energy is a non-polluting, renewable energy source. Modern day technologies for exploiting tidal energy are, however, relatively expensive and are limited to a few coastlines with extremely high and low tides. Tidal energy may, in the future, become more widely used and economically practical.
The power in tides
Tides result from the gravitational pulls of the Moon and Sun on the surface of the spinning Earth. Gravity is the force of attraction between all masses. The shape of the shore and adjacent seafloor affects the tidal range (difference between high and low tides) along specific coastlines.
Some places, like the English Channel between France and England, and the Bay of Fundy in Nova Scotia, Canada, experience very high and low tides. The tides protected Medieval monasteries in the English Channel since the eighth century.
Mont-St.-Michel in western France and Lindisfarne (Holy Island) in northern England are churches built on small islands surrounded by miles of tidal flats (a broad, flat area of coastline alternately covered and exposed by the tides). Today, they are connected to the mainland by roadways but in Medieval times, only devout pilgrims rushed to make the hurried trip across miles (kilometers) of shifting sand between roaring tidal pulses.
For tidal energy to be a practical source for electricity generation, the tidal range in a coastal area must be at least 16.5 feet (5 meters). The greater an area’s tidal range, the more electricity will be produced. Although tidal energy is reliable and plentiful, only a handful of suitable tidal power station locations have been proposed worldwide.
Two large tidal power plants are in operation today at La Rance in Brittany, France, and in the Canadian town of Annapolis Royal, Nova Scotia. In the United States, tidal energy as a power source is realistic only in Alaska and Maine.
Exploiting tidal energy
A barrage is a shore-based, dam-like structure that is built across a narrow-mouthed estuary (the part of a river where it nears the sea and fresh and salt water mix). As the tide ebbs and flows (moves in and out) through tunnels in the barrage, the water turns large fan-like turbines and generates electricity.
Barrages are expensive to build and can harm estuarine life by restricting water flow over the tidal flats. Electricity produced by tidal energy has no harmful wastes or emissions such as greenhouse gases. Once built, the barrage is easy to maintain and inexpensive to run.
Ocean-based systems include tidal fences and offshore turbines. Tidal fences are like giant subway turnstiles built across the sea floor between the mainland and an island or between two islands.
When a tidal fence is built across an open body of water, water is forced to pass through the vertical turbine gates and electricity is generated. Tidal fences may restrict tidal flow and the ability of wildlife to pass through. Offshore turbines are like giant propellers placed on large posts that are set in a line across the sea floor.
Ocean currents flow past the turbines and cause the blades to spin and generate electricity. Offshore turbines are like giant propellers placed on large posts that are set in a line across the sea floor. Ocean currents flow past the turbines and cause the blades to spin and generate electricity.
Tidal fence turbines have a much lower initial cost when compared to barrages and are much less harmful to the environment because they do not restrict tidal ebb and flow. Tidal turbines also allow wildlife and small boats to pass through the area.
Offshore turbine blades are smaller and less protected than those housed in barrages or tidal fences and so they are more prone to damage from strong tidal currents. Ocean-based systems are much less expensive to install than barrages, but are more expensive to maintain due to their remote location.