Ocean Wave Energy Conversion

Michael E. McCormick

• Publisher: Dover Publications
• Year: 2013
• ISBN:
  9780486318165
• Topic: Physical Sciences
• Subtopic: Energy

This volume will prove of vital interest to those studying the use of renewable resources. Scientists, engineers, and inventors will find it a valuable review of ocean wave mechanics as well as an introduction to wave energy conversion. It presents physical and mathematical descriptions of the nine generic wave energy conversion techniques, along with their uses and performance characteristics.
Author Michael E. McCormick is the Corbin A. McNeill Professor of Naval Engineering at the U.S. Naval Academy. In addition to his timely and significant coverage of possible environmental effects associated with wave energy conversion, he provides a separate treatment of several electro-mechanical energy conversion techniques. Many worked examples throughout the book will be particularly useful to readers with a limited mathematical background. Those interested in research and development will benefit from the extensive bibliography.

1

Introduction

The most conspicuous form of ocean energy is the surface wave. Waves are simply energy in transition, that is, the energy being carried away from its origin. The sources of wave energy are the following four phenomena: (1) bodies moving on or near the surface causing relatively low period waves of low energy; (2) winds generating seas and swells; (3) seismic disturbances causing the (misnamed) “tidal wave” or “tsunami”; (4) the lunar and solar gravitational fields causing the largest waves, the tides (the actual “tidal wave”). The relative energies of these waves have been estimated by Munk et al. (1957) and are shown in Figure 1.1. The tides are predictable, and wind-generated waves are also predictable if the nature of the wind is known. From the results in Figure 1.1 we see that these two wave types also have the highest relative wave energies. In this book our attention is focused on the conversion of wind-wave energy into more usable forms. Tidal energy conversion has been discussed in a number of publications, the more notable of which is the book edited by Gray and Gashus (1972) and the report by Wayne (1977).

Wind waves are actually a form of solar energy since the primary source of wind energy is the sun. Solar radiation is collected by both land and water masses, and the water is the more efficient collector of the two. The air above a warmed water mass is then heated. The warm air rises into the higher elevations replacing the cooler, more dense air which, in turn, descends. Thus thermal air currents are generated. In addition to these vertically oriented currents, wind circulation patterns are established in which the warm air above the equatorial waters rises and moves toward the polar regions where the air is cooled, descends, and again flows toward the equator. These wind circulation patterns are modified by both the presence of land masses and the rotation of the earth. The resulting global circulation patterns of the wind are sketched in Figure 1.2. For more thorough discussions of the meteorological aspects of wind generation, the reader is referred to the books of Voss (1972) and Dietrich (1963).

It is interesting to note that in each transformation of the solar energy the power intensity (power per unit area normal to the direction of energy transmission) increases. For example, the average solar insolation at a latitude of 15° N is 0.170 kW/m². The wind velocity at this latitude in the mid-Pacific ocean may be approximately 20 knots (10 m/sec) within the Northeast Trades and thus have a power intensity of 0.580 kW/m². The average wave generated by this wind has a power intensity of 8.42 kW/m². The averaged power densities, on a global scale, however, are reversed. Estimates of the total powers of the five ocean energy forms are given by Wick and Isaacs (1976) and are shown in Figure 1.3.

Figure 1.1 Relative energy spectrum of naturally created water waves. After Munk (1957).

Figure 1.2 Solar-induced wind circulation patterns.

Figure 1.3 Power or energy flux for various sources of ocean energy sources. After Wick and Isaacs (1976).

Since wave energy is so conspicuous, many inventive individuals have been inspired to devise methods of converting the energy of ocean waves into more usable forms. For example, on March 1, 1898 P. Wright patented the devise shown in Figure 1.4. An operating system was also constructed by Bouchaux-Praceique in France in the early part of the twentieth century and is described by Palme (1920). A sketch of this system, taken from the paper by McCormick (1976), is shown in Figure 1.5. Referring to Figure 1.5, we see that the rise and fall of the water surface excites the air column above it which, in turn, drives the turbogenerator. The system was said to supply all of Bouchaux-Praceique’s electrical power requirements. The schemes shown in Figures 1.4 and 1.5 are two of many wave energy conversion ideas. In fact, there are over 1000 patented wave energy conversion techniques in Japan, North America, Western Europe, and the United Kingdom. Most of these patented ideas are variations of a few very basic ideas described in Chapter 4 of this book. Appendix B contains a partial list of patent numbers referring to wave energy conversion schemes.

Figure 1.4 Sketch of the patented “wave motor” of P. Wright, U.S. Patent Number 599, 756, March 1, 1898. The outrigger configuration.

Figure 1.5 Bouchaux-Praceique wave energy convenor. After Palme (1920).

The economics of wave energy conversion depend on three considerations: (1) the magnitude and dependability of the wave resource, (2) the cost of construction and maintenance of the conversion system, and (3) the energy transmission from the site to the user. McCormick (1976) calculated the wave power per crest length along the coasts of the continental United States on the basis of observed wave height and period data presented in the U.S. Army’s Shore Protection Manual (1973). The results of these calculations are presented in Figure 1.6, where the seasonal variation of the monthly averaged wave power per crest length is shown. The results in Figure 1.6 suggest that a significant resource exists only along the Northwest coastline. Pierson and Salfi (1976) show that the magnitude of the resource in deep waters well away from the Northwest coast are an order of magnitude greater than the values shown in Figure 1.6. Dispersive and shoaling processes, described in Chapter 3, reduce the power per crest length of waves (generated in deep water) by the time these waves reach the coast. On the basis of the resource estimates, therefore, one can conclude that wave energy conversion is more feasible in deep waters than in coastal waters. The problem arising in deep water wave energy conversion is in finding an economical method of energy transmission from the conversion site to the energy market. This problem also exists in ocean thermal energy conversion (OTEC), where the resource is primarily in equatorial waters, that is, between 20° N latitude and 20° S latitude. In connection with OTEC, energy transmission has been thoroughly studied, and the reader is referred to the publication by Konopka et al. (1977) for more information on this subject. Direct electrical transmission is possible up to 80 miles or 128 km. Furthermore, for energy conversion sites located more than 80 miles (128 km) from shore, the manufacturing of energy-intensive products, such as aluminum, is possible. Thus deep water wave energy conversion is not only feasible but is probably cost effective.

Figure 1.6 Wave power variations over 1 year. Data from coastal waters, but calculations based on deep water assumptions. After McCormick (1976).

All of the topics mentioned in this introductory chapter are more thoroughly discussed in the chapters that follow. The reader is also encouraged to consult the listed references at the end of each chapter for more detailed discussions of the various topics.

References

Dietrich, G. (1963),General Oceanography, Wiley-Interscience, New York.

Gray, T. J., and Gashus, O. K., Eds. (1972),Tidal Power, Plenum, New York.

Konopka, A., Talib, A., Yudow, B., Blazek, C., and Biederman (1977), “Alternate Energy Transmission Systems from OTEC Plants,” U.S. Department of Energy Report No. DSE/2426-20.

McCormick M. E. (1976). “Salinity Gradients, Tides and Waves as Energy Sources,” Energy from the Oceans Conference, North Carolina State University, Raleigh, UNC-SG-76-04, January, pp. 33-41.

Munk, W., Tucker, M., and Snodgrass, F. (1957), “Remarks on the Ocean Wave Spectrum,” National Academy of Sciences, Publication No. 515, Washington, D.C., pp. 45-60.

Palme, A. (1920), “Wave Motion Turbine,”Power, Vol. 52, No. 18, pp. 200-201.

Pierson, W. J., and Salfi, R. E. (1976), “The Temporal and Spacial Variability of Power from Ocean Waves Along the West Coast of North America,” Wave and Salinity Gradient Workship Proceedings, University of Delaware, Paper E.

U.S. Army (1973),Shore Protection Manual, U.S. Government Printing Office, Stock No. 0822-00077, Washington, D.C.

Voss, G. L. (1972),Oceanography, Golden Press, New York.

Wayne, W. W. (1977), “Tidal Power Study for the United States Energy Research and Development Administration,” Stone and Webster Engineering Corporation, Boston, Mass.

Wick, G. L., and Isaacs, J. D. (1976), “Utilization of the Energy from Salinity Gradients,” Wave and Salinity Gradient Energy Workshop, University of Delaware, Paper A.

2

Wave Properties

The two measurable properties of water waves are the height and the period. These properties are measured by using an assortment of wave gauges or, in some situations, estimated by sight. Much of the recorded wave data actually result from visual observations. Researchers fo...