Wave Energy

The oceans store large amounts of kinetic (moving) energy from the wind. The wind generates waves as it blows across the sea's) surface. The larger the wave, the more energy the wave contains. Wave energy provides a continuous source of renewable, non-polluting energy that can be converted to electricity at wave power plant sites around the world.

Where the waves are

Windy coastlines around large oceans are the best places to build power plants that harness wave energy. Strong winds that blow continuously over long stretches of open water create the largest waves, which contain the most energy. Strong, steady winds that blow in Earth's major wind belts (zones of wind in a prevailing direction) generate massive waves. In the subtropical zone on either side of the equator (imaginary circle around Earth halfway between the North and South Poles), suitable wind power sites are along east-facing coastlines in the path of the westerly trade winds, such as the east coast of Florida. (Winds are named for the direction from which they blow; the trade winds are strong winds that blow from east to west in the subtropics on either side of the equator.) In mid-latitudes (imaginary lines on Earth that tell how far north or south a place is from the equator), storms along the course of Easterly jet streams, a current of fast-moving air in the upper atmosphere, produce waves that pound west-facing coastlines such as the Pacific Coast of the United States. At higher latitudes, sub-polar easterly winds produce some of the largest waves in the world, up to 100 feet (30.5 meters) high in the North Atlantic Ocean.

Harnessing wave energy

There are two types of energy technologies used to capture wave energy and generate electricity: fixed devices and floating devices. Fixed devices are attached to the shore or sea floor. Tapered channels (TAPCHANS) are fixed devices that direct large waves into raised pools on the shore. Water draining from the pools turns turbines that generate electricity. (Turbines are spinning wheels or other devices that convert the energy in falling water to mechanical and electrical energy.) A power plant on the North Coast of Norway uses a TAPCHAN to harness wave energy. TAPCHANS are relatively easy and inexpensive to maintain. Water can be stored in the reservoir (body of water) so that power can be generated when needed. An appropriate TAPCHAN site however, is difficult to find because it must have consistent waves, small tidal variations, deep water close to the shoreline, and a place to build a reservoir.

Oscillating water channels (OWCs) are fixed devices that consist of a small opening, where waves can enter and retreat, attached to a vertical closed pipe. As a wave flows into the OWC, the water forces air up the pipe, turning turbine blades as it passes. When the wave ebbs (pulls back) the air is sucked back down the pipe and turns the turbine blades again. Adequate locations for oscillating water channels are also difficult to find, in part, because the relatively new technology requires that OWCs be embedded in the shoreline, limiting OWC usage to rocky coasts.

Floating devices have several advantages over fixed devices: they have less visual impact on the shoreline, are less likely to change wave patterns and disrupt wildlife, and are quieter than TAPCHANS. Floating devices use the cyclic motion of waves to generate electricity. A Salter Duck (developed by University of Edinburgh professor Steven Salter in the 1970s) looks like a row of floating ducks that are anchored to the sea floor. As a wave passes, the duck rotates about a central turning point. This rotational motion causes fluid within the duck to move thereby generating electricity.

Japan, Denmark, Norway, England, Spain, and Portugal are all developing new technologies to use wave energy to generate power. As technologies for generating electricity from wave energy are developed, costs for these systems will decrease, and commercial wave power plants will most likely become more common.

Laurie Duncan, Ph.D. andMarcy Davis, M.S.

For More Information

Books

Penney, Terry R., and Desikan Bharathan. "Power from the Sea." Scientific American (January 1987).

Websites

"Ocean Energy." California Energy Commission.http://www.energy.ca.gov/development/oceanenergy (accessed on August 26, 2004).

"Wave Energy." U.S. Department of Energy: Energy Efficiency and Renewable Energy.http://www.eere.energy.gov/RE/ocean_wave.html (accessed on August 26, 2004).

"Wave Power." The Chartered Institution of Water and Environmental Management.http://www.ciwem.com/policy/factsheets/fs7.asp (accessed on August 26, 2004).