Ocean power

Ocean power encompasses several distinctly different approaches to power generation, which, if developed properly, promise potentially large amounts of clean, renewable energy. The importance of developing such alternative energy sources for a world largely dependent on fossil fuels—subject to escalating costs and exhaustibility, and creating environmental pollution—cannot be overstressed.

Tidal Flow Power

Attempts at harnessing ocean power involve the use of specialized technologies developed to exploit natural flows of energy within the marine environment. These energy flows are generated by the interaction of the ocean’s waters with the effects of the sun’s energy; the gravitational pull of other celestial bodies, such as the moon; and, to a much lesser extent, such influences as geothermal activity occurring on the sea bottom. Many engineering schemes have been devised to try to tap into these natural energy flows. These schemes recognize the fact that Earth’s surface is mostly ocean—71 percent of it is covered by the sea—and that, due to an interplay of natural processes, this immense fluid environment is always in motion.

The most ambitious—and, as of 2012, most productive—ocean power schemes have been tidal power projects. The efficacy of these projects is dependent on how well engineered they are to take advantage of the key factors involved. Such factors include the character of a tide at a particular coastal locale as determined by local bottom topography, surface coastline geography, and the orientation of the coast to the open sea. Submarine topographic influences can accentuate the rise of an incoming tide, acting like a wedge to lift the oncoming bulge of tidal water. Thus, tides can reach up to 15 meters on coasts having the right tide-enhancing topography and orientation. The maximum rise and fall of a tide experienced at a particular location is important in tidal power, as it represents the amount of usable head; “head” is a term used in hydraulic engineering to describe the difference in elevation between the level at which water can flow by gravity down from an upper to a lower level, thus making itself available to do work. Unfortunately, only one hundred coastal sites worldwide are classified as having significant head and qualify as optimal candidates for tidal power installations. Scientists conservatively estimate that the global, dissipated tidal power amounts to 3 terawatts per year, or 3 trillion watts. Of this amount, perhaps only 0.04 terawatt would ever be exploitable from feasible tidal power sites.

The French government has been the world pioneer in realizing tidal flow power engineering schemes, constructing a large, functioning tidal energy station at the estuary of the La Rance River in Brittany in northern France. At La Rance, a combination of factors produces a useful hydraulic head and has proved itself economically profitable for several decades. Twenty-four 10,000-kilowatt turbine generators operate within conduits inside the tidal dam, turning at the low speed of 94 revolutions per minute. The turbine blades are designed to operate bidirectionally, in response to either an incoming or an outgoing tide. Thus, the French plant exploits the free tidal water movement almost continuously.

Ocean Thermal Energy Conversion

Ranking alongside tidal flow power, in terms of economic feasibility and available technology, is the ocean thermal energy conversion (OTEC) approach. This method could ultimately be a large-scale global operation, as there are many more sites that can be used for OTEC than are available for tidal power. Projected, theoretical limits to this energy source are in the neighborhood of 1 terawatt per year. OTEC involves either the construction of floating, open-ocean plants or coastal, land-based plants that exploit the temperature differences existing between water masses at varying depths in the tropical seas. Only small pilot plants have been built thus far, and they have never been run for more than short periods.

The optimal conditions for the most efficient OTEC sites have been calculated to be those where an 18-degree Celsius temperature difference exists between the surface and depths in the range of 600 to 1,000 meters. Among the regions of the ocean that meet or approach these thermal conditions are Puerto Rico and the West Indies, the Gulf of Guinea, the Coral Sea, many of the Polynesian island groups, and the northwest African island groups. Only in these warm seas is there a large enough temperature difference between surface and deep sea. The actual process of converting the thermal difference of such areas involves a system in which a turbine is turned by heat from the warm, surface water layer. The heat is transferred through devices termed heat exchangers, which introduce the thermal energy into a closed system. A working fluid such as ammonia, contained in sealed pipes, propels the system through the process of controlled convection. The warmed ammonia is heated to a vapor in an evaporator unit that drives the turbine. Then the used vapor is conducted through a condenser unit, where cold water drawn from below cools it down to a liquid state for another usage cycle. The cool water is brought into contact with the system by the deployment of a very long pipe, hundreds of meters in length, which projects through the thermocline, or boundary, between the upper and lower water masses. Although the efficiency of the system is typically low, only 2 to 3 percent, the thermal reservoir is immense and constantly replenished by the sun. To make such projects attractive economically, they need only be built on a sufficiently large scale.

Marine Current Energy Conversion

Similar to OTEC in its use of the major oceanic flows of thermal energy are plans to exploit large-scale currents. One such current, the Gulf Stream—often called the Florida Current—constantly conveys many millions of gallons past a given point in the ocean. One plan is a direct approach, involving large turbines placed within the main flow of the current. Ideally, the generating site would be close to major electrical consumers, such as coastal cities.

A good case can be made for implementing marine current energy conversion along the eastern coast of Florida. The city of Miami, a very large consumer of electrical power, is within ten miles of the Florida Current. This current is estimated to carry approximately 30 million cubic meters of water per second past the city at a rate sometimes reaching 2.5 meters per second. One scheme would involve anchoring a large cluster of special, slow-speed turbines, or water windmills, to the bottom; the cluster would ride midwater in controlled buoyancy. This complex would function at a depth ranging from 30 to 130 meters and stretch some 20 kilometers across the flow of the Florida Current directly adjacent to Miami. Estimates of the power output of this array are on the order of 1,000 megawatts, provided constantly on a twenty-four-hour, year-round basis. It would extract roughly 4 percent of the usable kinetic energy of the Florida Current at this point in its flow, which is calculated to be in the range of 25,000 megawatts. Other strong currents exist worldwide that also may be good potential sites for future turbine arrays.

Ocean Wave Power

Another way to take advantage of oceanic energy is to utilize the kinetic energy of surface waves directly to power mechanical devices used to generate electricity. This approach capitalizes on the fact that waves raise and lower buoyant objects. If the floating object is also long and perpendicular to the ocean surface, with most of its mass below the waterline, it is inherently stable and less subject to damage. Such a device is embodied in several variations of a wave-powered pump that is beginning to see practical economic applications. In one form, a large, vertical cylinder floats in the waves. Inside, the lower end is open to the lifting and sinking of the water level in unison with wave motion. Because of its great length (tens of meters or more), it amplifies the wave motion in the column of air resting above the water. This motion propels air up and down through a double-flow electrical air turbine. As long as it remains in sufficiently deep water, it receives very little damage, no matter the magnitude of wave energy. A buoy-like device floats at the cylinder top, keeping out wave splash and snugly maintaining inside air pressure. The cylinder can be anchored by flexible moorings to the sea bottom, and electrical cables can feed the power output to shore.

Coastal-based variants also have been built solidly into rocky cliffs; they respond to wave-driven air pressure that enters from a conduit at its base. One example of the coastal-based type is the plant at Tostestallen in Norway. Numerous rocky coasts worldwide could be similarly utilized for wave power generation.

Salinity Gradient Energy Conversion

One form of ocean power that has been envisioned but not implemented is the use of salinity gradients within the sea. This idea is still in the theoretical stage because of the lack of key materials necessary for the effective technologies to work. A major drawback at this point is the lack of appropriately tough and efficient synthetic membranes necessary for this energy to be economically practical. Salinity gradient energy works on the principle of extracting energy from osmotic pressure by the use of semipermeable membranes. Such membranes would most likely be fabricated from some type of plastic that possesses the correct, chemical properties and would take advantage of the osmotic pressure between water masses with different percentages of dissolved salts. Influenced by osmotic pressure, less salty water will naturally flow through a semipermeable membrane to the side of greater salinity. The membrane would therefore be designed to be permeable to the fresher water but impermeable to the saltier water. Osmotic pressure would propel a controlled, one-way flow that could be employed to propel water-driven turbines for electricity.

A Promising Energy Source

Global human population growth is still explosively on the rise. Going hand in hand with population growth is the trend of increasing worldwide urbanization and industrialization. Because of these trends, consumption of energy to run manufacturing processes, transportation, food production, climate control, and communication systems has escalated dramatically. Coupled with this ravenous energy use are the by-products of mounting energy consumption: widespread pollution and a general degradation of the world environment. The world industrial society already faces the specter of diminishing energy sources and the eventual, ultimate exhaustion of all fossil fuels. New energy sources, such as the nuclear option, seem to possess, so far, many drawbacks to widespread usage.

Renewable energy sources that are clean and have a low impact on the quality of the environment are ideal long-term solutions. Some alternative energy forms fitting this description are either technically feasible today or almost so. The various forms of ocean power that have been developed or are in theoretical stages are excellent candidates for helping to alleviate some of the world’s more pressing energy-related problems. For at least some geographic areas, ocean power is not only an efficient and clean power source but is economically competitive with fossil fuels and nuclear power. Tidal flow power is an excellent example of the increasing viability of some forms of ocean power. By the end of the twentieth century, tidal power was producing less than 300 megawatts worldwide, although the largest site (producing 240 megawatts) has been in operation at La Rance River in France since 1967. As of 2011, fewer than a dozen small demonstration plants had been built. The most successful test OTEC system has been the one at Kailua-Kona, Hawaii, which has produced more than 50 kilowatts in net power. The ocean, however, remains Earth’s largest solar collector, and supporters of ocean power remain optimistic. Forms of ocean power represent a means to live in harmony with the environment without compromising the use and demand for energy.

Principal Terms

marine biomass energy conversion: the cultivation of marine plants, such as algae, for conversion of the harvest into synthetic natural gas and other end products

marine current energy conversion: power from the transfer of kinetic energy in major ocean currents into usable forms, such as electricity

ocean thermal energy conversion: power derived from taking advantage of the significant temperature differences found in some tropical seas between the surface and deeper waters

ocean wave power: the use of wind-generated ocean surface waves to propel various mechanical devices incorporated as an electrical generating system

salinity gradient energy conversion: power generated by the passage of water masses with different salinities through a special, semipermeable membrane, taking advantage of osmotic pressure to operate turbines

tidal flow power: power from turbines that are sited in coastal areas to take advantage of the tidal flow’s rise and ebb

Bibliography

Anderson, Greg M., and David A. Crerar. Thermodynamics in Geochemistry: The Equilibrium Model. New York: Oxford University Press, 1993. An exploration of geochemistry and its relationship to thermodynamics and geothermometry. A thorough but somewhat technical resource. Recommended for persons with a background in chemistry and Earth sciences.

Andrews, John, and Nick Jelley. Energy Science: Principles, Technologies, and Impact. New York: Oxford University Press, 2007. Discusses various forms of energy, and environmental and socioeconomic impacts. Covers principles of energy consumption and includes information on the generation, storage, and transmission of energy. A strong mathematics or engineering background is required for some examples. Appropriate for undergraduates and professionals.

Bascom, Willard. Waves and Beaches: The Dynamics of the Ocean Surface. Rev. ed. Garden City, N.Y.: Anchor Press/Doubleday, 1980. An introduction to the subject of oceanography, emphasizing the role of wave and beach processes. Discusses energy derived from marine sources and presents the pros and cons of ocean power. Well illustrated throughout. Suitable for readers seeking a working knowledge of physical oceanographic processes.

Boyle, Godfrey, ed. Renewable Energy. 2d ed. New York: Oxford University Press, 2004. Provides a complete overview of renewable energy resources. Discusses various forms of energy, including hydroelectric energy and tidal power, and basic physics principles, technology, and environmental impact. Includes a further reading list.

Carr, Donald E. Energy and the Earth Machine. New York: W. W. Norton, 1976. A thorough survey of the primary energy sources that power the industrial world, including fossil fuels. Discusses ocean power sources within the scope of water-derived energy sources. Suitable for high school students or general readers seeking background on the subject.

Constans, Jacques A. Marine Sources of Energy. Elmsford, N.Y.: Pergamon Press, 1980. An accessible treatment of the subject of ocean power. Rich with explanatory diagrams, tables, drawings, and maps that help to explain the concepts involved. Appropriate for all readers, high school and above, especially those interested in the technical problems of ocean engineering.

Gage, Thomas E., and Richard Merrill, eds. Energy Primer, Solar, Water, Wind, and Biofuels. 2d ed. New York: Dell Publishing, 1978. Provides a wealth of information for those interested in the “nuts and bolts” of implementing alternative energy technologies. Suitable for those with a practical interest.

Gashus, O. K., and T. J. Gray, eds. Tidal Power. New York: Plenum Press, 1972. A classic book devoted exclusively to the generation of power through tidal ocean flow. Details the design and operating problems of the large-scale facility at La Rance, France, and the then-proposed facilities at the Bay of Fundy in Nova Scotia, Canada. Suitable for readers at the college level or above.

Hamblin, Kenneth W., and Eric H. Christiansen. Earth’s Dynamic Systems. 10th ed. Upper Saddle River, N.J.: Prentice Hall, 2003. Introduces the reader to basic concepts such as Earth’s gravity, rotation, and tides. Appropriate for readers without a background in physical geology.

Letcher, Trevor M., ed. Future Energy: Improved, Sustainable, and Clean. Amsterdam: Elsevier, 2008. Discusses the future of fossil fuels and nuclear power; renewable energy supplies, such as solar, wind, hydroelectric, and geothermal sources; and potential and underutilized energy sources. Covers new concepts in energy consumption and technology.

Meador, Roy. Future Energy Alternatives. Ann Arbor, Mich.: Ann Arbor Science Publishers, 1979. Surveys the major alternatives to fossil fuels, including ocean power options. Assumes no technical background and expertly introduces general readers to each energy alternative in turn. Appropriate for high school or college students, as well as general readers.

Ross, David. Power from the Waves. New York: Oxford University Press, 1995. Considers the ocean’s potential power sources. Illustrations, maps, bibliography, and index.

Tarbuck, Edward J., Frederick K. Lutgens, and Dennis Tasa. Earth: An Introduction to Physical Geology. 10th ed. Upper Saddle River, N.J.: Prentice Hall, 2010. Provides a clear picture of Earth’s systems and processes that is appropriate for the high school or college reader. Includes illustrations, graphics, and an accompanying computer disc. Bibliography and index.

Teller, Edward. Energy from Heaven and Earth. San Francisco: W. H. Freeman, 1979. Compares the major forms of energy use in industrialized countries, and the social and economic consequences of their use. Offers extensive discussions of energy-use policies and their bearing on the development of new energy sources. Introduces the overall energy picture for students or interested laypersons.

Wilhelm, Helmut, Walter Zuern, Hans-Georg Wenzel, et al., eds. Tidal Phenomena. Berlin: Springer, 1997. Collects lectures from leaders in the fields of Earth sciences and oceanography. Examines Earth’s tides and atmospheric circulation. Complete with illustrations and bibliographical references. Appropriate for readers without a strong knowledge of Earth sciences.

Wilson, Mitchell, et al., eds. Life Science Library: Energy. New York: Time, 1963. A profusely illustrated overview of the subject of energy. Outlines the history of energy-related physics and the growth of applied technology designed to exploit energy sources. Suitable for any reader at the high school level or beyond.