Tides are caused by the gravitational pull of the moon and sun, and the rotation of the earth. Near the shore, water levels can vary up to 40 feet as a result of tides. The movement of water as a result of tidal forces can be used to produce energy.
Tidal power is more predictable than wind energy and solar power. A tidal range of 10 feet is needed to produce tidal energy economically.
A simple system for tidal energy plants uses a structure similar to a dam called a barrage. The barrage can be located across an inlet of an ocean bay or lagoon that forms a tidal basin. Sluice gates (gates commonly used to control water levels and flow rates) on the barrage allow the tidal basin to fill on the incoming high tides and allows the tidal basin to empty through the turbine system on the outgoing ebb tide. There are two-way systems that generate electricity from both the incoming and outgoing tides.
A potential disadvantage of tidal power is the negative effect a tidal station can have on plants and animals in estuaries of the tidal basin. Tidal barrages can change the tidal level in the basin and increase turbidity (the amount of matter in suspension in the water). They can also affect navigation and recreation.
There are currently six tidal power barrages operating in the world. The largest is the Sihwa Lake Tidal Power Station in South Korea with a total power output capacity of 254 Megawatts (MW). The second-largest and oldest is in La Rance, France with 240 MW capacity. The next largest is in Annapolis Royal in Nova Scotia, Canada at 20 MW, and it is followed by the 3.7 MW Jiangxia Tidal Power Station in China, the 1.7 MW kilowatt tidal barrage in Kislaya Guba, Russia, and the 1 MW Uldolmok Tidal Power Station in South Korea.
The United States does not have any tidal power plants, and it only has a few sites where tidal energy could be produced economically. France, England, Canada, and Russia have much more potential to use tidal power.
Tidal fences can also harness the energy of tides. A tidal fence has vertical axis turbines mounted in a fence. All the water that passes is forced through the turbines. Tidal fences can be used in areas between two landmasses like channels. Tidal fences are cheaper to install than tidal barrages, and they have less impact on the environment than tidal barrages. However, tidal fences can disrupt the movement of large marine animals. A tidal fence is planned for the San Bernardino Strait in the Philippines.
Tidal turbines are basically wind turbines in the water that can be located anywhere there is strong tidal flow. Because water is about 800 times denser than air, tidal turbines have to be much sturdier than wind turbines. Tidal turbines are heavier and more expensive to build but capture more energy. There is an operating 1.5 MW tidal turbine in Strangford Lough, Scotland, and at Uldolmok, South Korea.