Wave Energy

From Thermal-FluidsPedia

As wind blows across the ocean, some of its energy is transferred to the water in the form of waves. Since wind itself is a result of non-uniform heating of the earth’s surface by the sun, waves can be considered a stored form of solar energy (Figure 1). The rougher the water, the larger the ripples and the easier it is for the wind to transfer its energy. As wind loses its energy, waves become calmer and eventually reach their steady-state oscillations. As waves get closer to the coast and enter shallower water, they grow taller, their lengths decrease, and water particles move forward. The particles closer to the bottom fall behind until the wave eventually breaks on to the shore.

Waves have form (potential energy) and motion (kinetic energy). Potential energy is that which is expended to distort a flat sea surface into the shape of the wave. Kinetic energy is the energy put into wave by water motion in and beneath the wave, causing molecules of water to rotate in a circular fashion, higher in diameter at the surface and decreasing exponentially with depth.(1) Contrary to popular belief, waves do not move along the water surface – the water simply cycles around in small circles without any mass being transported forward. These are called stationary or transverse waves and are very different than longitudinal waves such as sound waves and waves generated by compressing a spring.

Figure 1: As wind blows over large bodies of water it continuously supplies energy to waves.

Surface Waves

Surface waves result from the superposition of many waves of varying wavelengths and heights. The wavelengths are, however, limited to about twice the average depth of the ocean. Ocean floors are, at maximum, 4 kilometers deep, so waves can be as long as 8 kilometers. Longer surface waves travel faster and further than shorter ones, but the energy content of a wave is related solely to its height. The surface currents flow clockwise in the northern and counter-clockwise in the southern hemispheres.

Wave power is measured in kW/m of wave crest (the highest point on a wave). The maximum power a wave can carry is about 100 kW per meter of wave front in the high seas, about 30-60 kW/m around the Atlantic coasts of Europe, and 10 kW/m off the western coast of the United States (See box “Power of a Wave”). Due to their greatest potential, the west coast of Europe is the most suitable for developing wave energy schemes (Table 1).(2) For example, it is estimated that waves in the British Isles could be tapped for more than 20% of British and 75% of Irish electricity needs.(3) At this time, most efforts remain in the research stage, although the few demonstration projects promise to play a significant role for wave technology in the near future. The European Union projects that by 2010 up to 1,000 megawatts of electricity could come from ocean waves.

Table 1: Potential for wave energy for major European countries

Underwater (Marine) Currents

Just as wind is affected by local terrain, coastal waves are affected by underwater topography. Narrow channels between islands and around the coastal edges of oceans are best suited for power generation and can provide as much as one megawatt of power per square kilometer of seabed. To increase the velocity of the water and thus its kinetic energy, some propose building barriers across channels and narrow straights. The best locations are near the shore at depths of between 20 to 30 meters, where wave velocities are approximately 2-3 meters per second.

References

(1) Mayo, N., “Ocean Waves -- Their Energy and Power,” Physics Teacher, Vol. 35, September 1997.

(2) Thorpe, T. W., “A Brief Review of Wave Energy,” Report no. ETSU-R120, The UK Department of Trade and Industry, May 1999.

(3) Edwards, R.“The Big Break,” New Scientist, pp. 30-34, October 1998.

Further Reading

Bose, N. and Brooke, J., Wave Energy Conversion, Elsevier, 2003.

Ross, D., Energy from the Waves, Oxford University Press, 1995.

Cruz, J., Ocean Wave Energy: Current Status and Future Perspectives, Springer Series in Green Energy and Technology, Springer-Verlag, Berlin, 2008

International Journal of Wave Motion, Elsevier Science Publishing Company.

International Journal of Renewable Energy, Elsevier Science Publishing Company.

External Links

National Oceanic and Atmospheric Administration (NOAA) Coastal Services Center (http://www.csc.noaa.gov).

European Commission on Tidal Energy (http://europa.eu.int/comm/energy_transport/atlas/htmlu/tidal.html).

OTEC, U.S. DoE, Energy Efficiency and Renewable Energy (http://www.eere.energy.gov/RE/ocean.html).

Wave Energy Council: Survey of Energy Resources (http://www.worldenergy.org/wec-geis/publications/reports/ser/wave/wave.asp).