Tidal Power Overview
- Energy Source: Tides
- Energy Type Converted: Kinetic and potential
- U.S. Theoretical Energy Potential: 445 TWh/year
- Power Density: 2 – 50 W/m2 (varies with tidal current speed)
- Device Types: Tidal stream generators, barrages, and tidal lagoons
To understand tidal power, we must first understand tides. Tides are periodic changes in the height of the ocean surface at particular places on the planet caused by the gravitational force of celestial bodies and the inertia of our planet. It may be helpful to think of tides as a wave (not the tidal wave you are probably thinking of), that can have a wavelength almost half the circumference of the earth.
Lunar gravity causes our planet’s oceans to bulge in the direction of the moon and simultaneously on the opposite side of the planet inertia causes a similar bulge, or high tide. The troughs between these bulges are the low tides that we experience. Tides happen like clockwork, this makes them extremely reliable and easy to predict. To get an idea of how extreme the difference between high and low tide can be, check out this time lapse of the tide at the Bay of Fundy.
When the tidal flow is restricted by natural features it will increase the velocity of the fluid, kind of like when you cover the opening of a hose with your finger to get a more forceful stream. For wind and water the power available is directly proportional with the cube of the velocity of the fluid, so at these locations where there is an increase in velocity we can expect a significant increase in available power.
There are three accepted methods to generate tidal power: tidal streams, barrages, and tidal lagoons.
Tidal stream generation is akin to the traditional wind turbine you often see on shore, and like wind turbines they rely on the kinetic energy of the fluid to generate power. There are several different flavors of tidal stream generators including axial turbines, crossflow turbines, and even kite systems.
A tidal barrages is just a fancy type of dam. These devices utilize potential energy that is created by the difference in height between high and low tide. An incoming tide is used to a fill a reservoir behind a dam. Once the tide drops to low the water is released, passing through turbines in the dam that are connected to generators. These systems are quite large and often span entire rivers, straits, or estuaries. A good example of this system is the Rance Tidal Power Station in France which was the world’s first tidal power plant and was built in the 1960’s.
A tidal lagoon is pretty much what it sounds like: a reservoir that is partially enclosed by manmade or natural barriers. This method of tidal power is similar in operating principle to a barrage, except these lagoons are typically developed in areas that don’t have pre-existing naturally advantageous geographic features. The Swansea Bay Tidal Lagoon will be the world’s first tidal lagoon power station and it is being construction along the coast line of South West Wales.