Sailors and surfers can attest to the power of the sea. Tides rise and fall from the interplay between Earth’s rotation and the gravitational forces of the sun and moon. Waves develop from the pressure and friction of winds blowing over the open seas.
Humans have a long history of harnessing tidal and wave power. The ruins of a tide mill, used for grinding grain on Strangford Lough in Northern Ireland, date to A.D. 619. On a larger scale, a tidal power plant on the Rance River in northwest France has generated 600 gigawatts (GW) annually since 1967. The basic design of tidal plants has not changed in over a millennium: The power plant lowers gates when the tide is rising, allowing water to enter, and raises the gates as the tide begins to recede, trapping the water behind them. At low tide, this water escapes through a turbine that turns the millstones or an electrical generator. A wave power plant couples the rising and falling of a float with the turning of a generator.
Despite this long history, tidal and wave power are the least common forms of renewable energy. To date, only three tidal and no wave power plants are in commercial operation. Costs for these sources of electricity are not yet competitive with others because of the expense of equipment that works reliably under the constant buffeting of the sea and that withstands saltwater corrosion.
Nonetheless, plans for new tidal and wave facilities abound. In September 2008, for instance, Ocean Power Technologies deployed a wave powered buoy, which generates 1.4 megawatts (MW), 5 km off the coast of Spain. The advantage of these types of power plants is that a good site for tidal or wave power may generate nearly 50 kilowatts (kW) per meter of shoreline  with negligible emissions of greenhouse gases or other pollutants. The disadvantages are their relatively high costs and the possibility of their interfering with fish and marine mammals.
 Sims, R. E. H., R. N. Schock, A. Adegbululgbe, J. Fenhann, I. Konstantinaviciute, W. Moomaw, H. B. Nimir, B. Schlamadinger, J. Torres-Martínez, C. Turner, Y. Uchiyama, S. J. V. Vuori, N. Wamukonya, and X. Zhang (2007) Energy supply. In: Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Metz, B., O. R. Davidson, P. R. Bosch, R. Dave, and L. A. Meyer, eds. Cambridge University Press, New York. pp. 251-322.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
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