Above: Comparison of low and high tide at Alma, New Brunswick, located on the Bay of Fundy (Dylan Kereluk)
Did you know? Compared to other planets’ natural satellites, Earth’s moon is unusually large in relation to the planet it orbits.Nearly all forms of renewable energy ultimately rely on energy from the Sun. But would you believe there is a form of renewable energy that comes from the gravitational pull of the Moon? It’s tidal power, and it can be harnessed to generate electricity in several different ways.
|An example of a tidal barrage: the dam of the tidal power plant on the estuary of the Rance River in Britanny, France. Click image to enlarge (Dani 7C3)
If you’ve ever been to an ocean beach, you probably noticed how sea level changes over the course of a day. Ocean tides are caused by the gravitational pull of the Moon—and, to a lesser extent, of the Sun—on Earth’s surface. The effect is most obvious on the oceans, where the water rises and falls according to the Moon’s gravitational field and its orbit around Earth.
As you can imagine, a vast amount of energy is required to raise and lower entire oceans. And since the tides are governed almost entirely by extraterrestrial bodies (the Moon and the Sun), they are also highly predictable. Just as we can predict when the Sun and Moon will rise and set everyday, we can predict when the tides will ebb and flow (and by how much).
Did you know? The Moon is believed to have originated from a collision between Earth and a Mars-sized planet about 4.5 billion years ago.So is it possible to harness this vast and predictable source of energy? I hope you said yes, because people have been doing it for centuries! In fact, tidal energy has been used since the Middle Ages, when incoming tidewater was collected in large reservoirs. The water was then used to power grain mills as it flowed back out at low tide.
|"SeaGen", the world's first commercial tidal stream generator in Strangford Lough, Northern Ireland. A tidal farm would be made up of multiple tidal generators. Click image to enlarge (Fundy)
Since the mid 1900s, at least three ways of harnessing the tides to generate electricity have been developed:
• Tidal barrages: tried and true (but environmentally damaging)
• Tidal farms: an emerging technology, with multiple projects in the works
• Dynamic tidal power: at the theoretical stage, but very promising
Tidal barrages, one of the two types of tidal power generators currently in use, operate on the exact same principles as the tidal grain mills used in the Middle Ages. But instead of mills, tidal energy (energy from moving water) is now used to spin turbines, like those in hydroelectric dams. The first such tidal power station was built in France in the 1960s.
Did you know? The highest tidal range (difference in sea level between high and low tides) is found in the Bay of Fundy on the east coast of Canada. Tidal ranges of about 15 metres have been measured there!However, by completely enclosing large bodies of water, tidal barrages can have adverse effects on local ecosystems. They prevent the formation of mud flats, which provide fertile feeding grounds for many birds. They also prevent some species of fish from swimming upstream to their breeding grounds. Furthermore, tidal barrages are relatively expensive to build.
Tidal farms are one alternative to tidal barrages that has already been tested. Since the moving water in tidal zones possesses a large amount of kinetic energy, that energy can be used directly to power turbines and generate electricity.
|Top-down view of a T-shaped dynamic tidal power dam. Blue and dark red colors indicate low and high tides, respectively. Note that the tide is both high and low simultaneously on opposite sides of the dam. Click image to enlarge (UNguyinChina)
So just as wind turbines transform the energy of the wind, tidal generators can use energy from ocean tides. Several small-scale tidal stream generators have already been tested. And there are plans to build large tidal farms—modelled on wind farms—off the coasts of North America and the Europe.
Dynamic tidal power
Did you know? Ocean tides are actually responsible for slowing down the Earth's rotation, at a rate of 2.3 milliseconds per century. As a result, a day on Earth is now about two hours longer than it was 620 million years ago!A new twist on the technology used in tidal barrages and tidal farms, dynamic tidal power is a theoretical but promising method of generating electricity from the tides. A very long barrier would be built straight out into the ocean from the coast. As the tides ebbed and flowed, differing water levels would be created on either side of the barrier. Water flowing from the higher side to the lower side would turn turbines similar to those used in a tidal barrage.
Unlike tidal barrages, a dynamic tidal power barrier would not completely enclose large areas of water. Therefore, entire ecosystems would not be cut off and the environmental impact of the project would be minimized.
Overall, tidal power technology is still in its very early stages of development. But as a renewable energy resource it could make significant contributions to supplying the needs of coastal communities in the very near future.
General news and science websites
Earth’s Moon: Formation, Composition, and Orbit (Space.com) Marine Energy Projects Pick Up Momentum (Beth Gardiner, New York Times) Testing the Waters with Tidal Energy (Bruce Dorminey and The Daily Climate, Scientific American) UK tidal power has huge potential, say scientists (Matt McGrath, BBC News)
Our Restless Tides (National Ocean and Atmospheric Administration)
The POWER Programme (POWER Group)
Tidal Energy (Ocean Energy Council)
Bay of Fundy Tides (bayoffundy.com)
G Word: Tidal Power (Hank Green, How Stuff Works)