Physics Principles involved in Ocean Wave Energy

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23/04/08
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Wave energy, is created by natural conversion of part of the wind energy above the oceans. Wind energy is created by natural conversion of part of solar energy. By heating air, the sun creates winds. This is because warmer air rises up while cooler air falls due to the difference in density. Hence, this creates movement in air, which is wind. Then, the winds blowing along the water surface create waves. Wind pressure pushes down wave troughs (lowest portion of the waves, as shown in the diagram below) and lifts up wave crests (the highest portion of the waves), due to Bernoulli's principle.

Bernoulli's principle states as the speed of a moving fluid increases, the pressure within the fluid decreases, and conversely, as the internal pressure increases, the speed of a fluid decreases proportionally.

(For more information and illustrated diagrams on Bernoulli's principle, please refer to http://home.earthlink.net/~mmc1919/venturi.html)

Hence, as the velocity of the wind increases, the pressure above the surface of the ocean decreases. This creates higher pressure under the ocean surface. The difference in pressure then pushes the water upwards and lifts it up. This is how wave is created. According to Bernoulli's principle, it also means that when the velocity of wind is higher, wavelength is also higher as wavelength determines wave power.

--yingshi

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29/05/08

Both air pressure differences between the upwind and the lee side of a wave crest, as well as friction on the water surface by the wind shear stress cause the growth of the waves. The wave height increases with increasing wind speed, duration since the wind started to blow, and of the fetch (the distance of open water that the wind has blown over).

Thus, in general, wave power is determined by wave height, wave speed, wavelength and the density of water.

Oscillatory motion is the highest at the surface of the ocean and hence most devices used to extract wave energy are built on the surface.

Among different types of ocean waves, wind generated waves have the highest energy concentration. Wind waves are derived from the winds as they blow across the oceans. This energy transfer provides a natural storage of wind energy in the water near the free surface. Once created, wind waves can travel thousands of kilometres with little energy losses, unless they encounter head winds.

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Ocean waves encompass two forms of energy: the kinetic energy of the water particles, that in general follow circular paths; and the potential energy of elevated water particles. On the average, the kinetic energy in a linear wave equals its potential energy. The energy flux in a wave is proportional to the square of the amplitude and to the period of the motion. The average power in long period, large amplitude waves commonly exceeds 40-50 kW per meter width of oncoming wave.

--cassandra

Edited and reviewed by the same people who wrote it:

11/06/08, 27/06/08