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| Fig. 1: Labeled diagram of a water wave. (Source: Wikimedia Commons) |
The ocean and atmosphere form a coupled system that is constantly exchanging energy at the air-sea interface. The flow of energy from the atmosphere to the ocean results in an "aerodynamically rough surface that can comprise dynamic, unsteady, very high and steep surface waves". [1] Famous beaches like Mavericks in San Francisco and Banzai Pipeline in Hawaii have waves up to 50 feet tall. These waves hold an immense amount of energy that is released when they crash. Harnessing this energy could provide a sustainable source of power. Facing the environmental damage and limited supply of carbon-intensive fuels, it is essential that we find renewable energy sources. Wave power is environmentally-friendly, renewable, and takes advantage of a resource that covers 71 percent of our Earth. [2]
Energy from crashing waves originates from the atmosphere. Waves are primarily caused by wind, specifically the friction between the wind and water. [3] When wind blows on the surface of the ocean water, it transfers energy and pushes the surface into forming peaks, called white caps. The water around white caps is choppy and chaotic, with water moving in random directions without a pattern. This churning water creates more surface area which allows the wind to have an even stronger effect. [1]
The continual disturbance will eventually cause the water to organize into swells of orbital progressive waves. As a swell travels, the water molecules move in circular orbits. As depicted in Fig. 1, The orbit starts at the waves trough, follows the rise to the crest, and then returns to the trough. These molecules do not travel horizontally, but merely transfer energy. Although only the top of the orbit is visible, the wave does extend to the ocean floor. As the swells travel, they collide and through constructive interference, they combine. [1]
As the energy travels towards the shore, the water depth decreases, causing the orbits diameter to decrease and the wave to slow. Because the back of the wave is in deeper water, it moves faster. This causes the wave to grow, becoming taller and steeper. The drag on the ocean floor increases and the wave beings to tilt. Near the shore, it will be shallow enough for the back of the wave to topple over the front, resulting in a break. The shape and size of this break depends on how long and hard the wind blew. Another factor is the fetch, the surface area of water over which the wind blows. When the fetch is large, more energy builds up and the waves are larger. For example, waves are taller during storms because of the strong, constant wind. The topography of the ocean floor also affects the shape and location of breaks. Ridges on the ocean floor give the impression of shallow water and can cause waves to break before reaching the shore. [1]
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| Fig. 1: Large breaking wave (Source: Wikimedia Commons). |
Harnessing crashing wave energy is a relatively nascent venture. Most wave technology focuses on waves before they crash. However, a crashing wave, as shown in Fig. 2, holds great potential power and convenience. Devices can be on land, avoiding issues like sinking buoys or scuba diver maintenance. The challenge comes from developing a technology that can survive the harsh environment without disrupting the ecosystem. Yam Pro Energy, an Israeli wave power company, has built a wave energy plant on the coast of Ghana, near Accra, that benefits more than 10,000 households. Their strategy builds hydraulic pressure from the crashing waves and converts it into electricity. [2] They placed wave breakers along the coastline and connected "floaters" that bob up and down as the waves crash. If the wave conditions become too rough, the floaters are temporarily removed.
According to their CEO, their wave power technology generates 65% of the available energy, unlike solar panels and wind turbines which generate less than 24%. [2] Like other renewable energy sources, wave power can be harnessed without emitting pollution and once set up, the running and maintenance costs are low, without any requirement of oil or gas. However, wave power has an advantage because waves are more predictable than other sources. The plant can generate up to 150 megawatts, meaning it could generate 6% of the 2500 megawatts needed to power the country of Ghana. [2] If three more similar plants were built along the coast, wave power would generate almost a fourth of the countrys power. Wave power has the potential to revolutionize power for coastal countries.
© Sophia Kazmierowicz. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] S. R. Massel, Ocean Surface Waves: Their Physics and Prediction (World Scientific, 2013).
[2] B. Britton, "Could Waves Become the Next Big Renewable Energy Source?," CNN, 3 Jan 17.
[3] P. A. E. M. Janssen and P. Viterbo, "Ocean Waves and the Atmospheric Climate," J. Climate 9, 1269 (1996).
