The gravitational pull of the moon and sun along with the rotation of the earth create tides in the oceans. In some places, tides cause water levels near the shore to rise and fall up to 40 feet. People in Europe harnessed this movement of water to operate grain mills more than a 1,000 years ago. Today, tidal energy systems generate electricity. Producing tidal energy economically requires a tidal range of at least 10 feet.

The United States does not have any commercially operating tidal energy power plants, although several demonstrations projects are in various stages of development. Two places in the United States with potential for tidal power are the Cook Inlet of Alaska, which has the second-highest tidal range in North America, and several places in Maine.

Tidal barrages

One type of tidal energy system uses a structure similar to a dam called a barrage. The barrage is installed across an inlet of an ocean bay or lagoon that forms a tidal basin. Sluice gates on the barrage control water levels and flow rates to allow the tidal basin to fill on the incoming high tides and to empty through an electricity turbine system on the outgoing ebb tide. A two-way tidal power system generates electricity from both the incoming and outgoing tides.

A potential disadvantage of tidal power is the 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.

Several tidal power plants operate around the world. The largest is the Sihwa Lake Tidal Power Station in South Korea, at 254 megawatts of electricity-generation capacity. The oldest and second-largest operating tidal power plant is in La Rance, France, with 240 MW of electricity-generation capacity. Smaller tidal power plant are in Canada, China, Russia, and South Korea.

Tidal turbines

Tidal turbines are similar to wind turbines because they both have blades that turn a rotor to power a generator. They can be placed on the sea floor where the tidal flow is strong. Because water is about 800 times denser than air, tidal turbines have to be much sturdier and heavier than wind turbines. Tidal turbines are more expensive to build than wind turbines but can capture more energy with the same size blades.

The United States does not have any commercially operating tidal power plants, but several demonstration projects have been implemented or are in various stages of development:

• Roosevelt Island Tidal Energy (RITE) Project Pilot in the East River of New York
• Western Passage Tidal Energy Project in Maine
• Cobscook Bay Tidal Energy Project in Maine

Tidal fences

A tidal fence has vertical-axis turbines mounted on the sea bed in a fence or row, similar to tidal turbines. Water passing through the turbines generates electricity. In 2023, no tidal fence projects were operating in the United States.

Last updated: May 24, 2024.