Wave Energy

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Ocean wave motion is a clean, renewable and yet untapped energy source.  Two-thirds of Earth's surface is open ocean. Harnessing significant quantities of this energy would have global implications. It could be achieved avoiding the costly geographic or environmental impacts so often associated with large scale renewable energy projects on land. 

Hundreds of ideas have been tried to harvest this energy in a feasible way, but few are realized and hardly anyone so far is in commercial operation. Wave energy is difficult. There are challenges including storms, fouling, corrosion and costly connections to shore. Wave energy is also a dispersed kind of energy. To achieve a high yield energy must be gathered from a large area. In practice, this means deploying many units.

The easiest way to collect wave energy is to transfer the motion energy from the waves to a floating buoy. The larger the buoy, the stronger it will be affected by the wave, provided that the buoy is significantly smaller than the wave. A ten-meter diameter buoy will collect ten times as much energy as a buoy one meter in diameter. But it is impossible for a buoy to absorb all the energy in the wave. The buoy will, due to its own weight, have a delayed movement relative to the wave. Buoy thus acquires a self-movement relative to the sea surface. This causes it to create its own waves. This requires energy, which reduces the yield. In practice, it is difficult to convert more than 25% of a wave's energy into electrical power.

Wave Energy is not evenly distributed on the oceans. The map below shows an assessment of global wave energy resources made by Andrew Cornett at the Canadian Hydraulics Centre.

Andrew M. Cornett, A GLOBAL WAVE ENERGY RESOURCE ASSESSMENT, Canadian Hydraulics Centre,
National Research Council, Ottawa, Ontario, Canada. Conference Paper in Sea Technology, July 2008.

Sea waves' energy potentials are presented as annual average effect (KW) per meter wave front, KW/m. The map shows that the global wave power resources are concentrated in two zones, roughly limited by the 40th and 60th latitude of the northern and southern hemispheres. These are the west wind belts where warm air from the low latitudes meet the cold air from the polar areas and, because of Earth's rotation, the wind mainly comes from western directions.

This effect is particularly pronounced in the westerlies in the southern hemisphere where the entire zone consists of ocean. Due to smaller annual temperature fluctuations over oceans, the seasonal variation is less here than up north. There is less extreme weather in the winter and always wind and waves in the summer. This contributes to the high annual effect. In addition, the fetch-area for the waves are not restricted by landmasses as are the case in the northern hemisphere. The waves are enormous.

In the southern hemisphere, there are millions of square kilometres with an annual average wave power of over 100 KW/m, according to the map. This is two to three times as high as in the middle of the North Sea. We can make a calculation to illustrate the opportunities. Since the waves are large, with long wavelengths, the buoys can also be large, for example having a diameter of 15 meters. The buoys will therefore be affected by a 15-meter wave front with an output of 100 KW/m x 15 m = 1,500 KW.

If the power converter connected to the buoy can absorb and convert 25% of this effect, the average result will be 1,500 KW x 0.25 = 375 KW. Over an hour, such a unit will produce 375 KWh on average. As it is 8640 hours (h) in a year, the annual energy yield for this unit will be 375 KWh x 8 760 = 3,285,000 KWh = 3.3 GWh.

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