Where Does Tidal Energy Originate?: Hydropower: Eco20/20

Where Does Tidal Energy Originate?

To understand tidal energy, it is important to first understand a few astronomy basics. The tides are caused by gravity and inertia. Gravity is the attraction between two masses. The force of gravity is relative to the size of the two masses and the distance between them. The Sun and Moon both have gravitational pulls on the Earth. As the Earth rotates on its axis, it creates a force called inertia, which Sir Isaac Newton created a law about, stating that an object in motion remains in motion until acted upon by an equal and opposite force.

When gravity and inertia are acting against each other, a small tidal swell is created. Because water is liquid, it's easily moved. The small tidal swell moves towards the Earth when the gravitational pull is greater than the inertial force. The small tidal swell moves away from the Earth when the inertial force is greater than the gravitational force. When gravity and inertia act together, meaning the gravitational pull and the inertial force are moving in the same direction, a large tidal swell is created. There are always two tidal swells on the planet Earth at a time, one closest to the Moon and one the greatest distance away from the Moon. As the Earth and Moon rotate and orbit, the tidal swells follow the movement. These movements create the actual tide. There are four tides in a day. There are two high tides and two low tides a day. The tides are fairly well predictable due to the consistency of the movements of the Earth, the sun and the moon.

Oceanic tides can also create tidal bores or walls. This occurs when the ocean water is flowing into a river. The shoreline pulls the tide in, creating high waves, walls, or tidal bores. These walls are extremely efficient at producing high velocity currents as well as high wave crests. These bores occur when high tide is first setting in. These are good for several types of hydropower, such as tidal turbines, barrages and dams.

There are other causes of tides than just celestial beings, though. For example, shifting tectonic plates in the Earth's crust can cause tsunamis. These shifting plates may be volcanic eruptions, earthquakes or even underwater explosions. Tsunamis are known to create larger-than-average tidal wave crests.

Tide sizes vary due to many dependent factors other than gravity and inertia, however. The shore line can play an important part in the size of tides. For example, if the ocean lets off into a river, the narrow inlet shore line will cause the tides to swell and break at higher levels. Wind and weather patterns can also influence the size of tides. Strong winds and violent weather patterns can force tides to rise, and have at times been known to nearly double the level of high tide in certain areas.

The size of the tide crests and the velocity of the bores both play major parts in the efficiency of tidal energy. The more stable the velocity, the greater the efficiency. This is because the turbines don't have to continually start and stop turning, which increases friction, which in turn causes loss of energy.