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Solar Energy from the Tropical Oceans

Long neglected, ocean thermal energy conversion deserves renewed attention and federal support.

The recent climate change conference in Kyoto underscored once again how profoundly the world needs new energy sources that do not produce carbon dioxide or create other environmental problems. Yet little attention is being paid to one completely untapped resource with the potential to become an enormous energy store. It is ocean thermal energy conversion (OTEC), an option largely neglected since the energy crisis of the 1970s.

OTEC is an application of solar energy that exploits the heat that the ocean captures from the sun's rays. It is particularly appealing because the energy it generates can produce enormous quantities of nonpolluting fuels (such as hydrogen and ammonia) for transportation and also furnish energy for other applications that are now dependent on fossil fuels. It thus has environmental advantages over fossil fuels and nuclear power; avoids land-use problems associated with renewable energy technologies such as solar, wind, biomass, and hydroelectric power; and has the potential to produce far more useful and affordable energy than could be obtained from other renewable sources.

OTEC is a technology for converting some of the energy that the tropical oceans absorb from the sun, first into electricity and then into fuels. During an average day, the 60 million square kilometers of surface waters of the tropical oceans (located approximately 10 degrees north to 10 degrees south of the equator) absorb one quadrillion megajoules of solar energy-equivalent to the energy that would be released by the combustion of 170 billion barrels of oil per day. The surface waters are a warm-water reservoir 35 to 100 meters deep that is maintained night and day at a temperature of 25 to 28 degrees celsius (°C). Below about 800 meters, an enormous source of ice-cold water, which is fed by currents flowing along the ocean bottom from the northern and southern polar regions, is maintained at about 4°C.

OTEC uses this temperature difference to generate electric power. In principle, it is not complicated. Warm water is drawn from the surface layer into a heat exchanger (boiler) to vaporize a liquid with a boiling point of about -30°C (liquid propane, liquid ammonia, and several fluorocarbons are examples). The vapor drives a turbine attached to an electric generator. Exhaust vapor from the turbine is subsequently condensed in a second heat exchanger, which is cooled by water pumped from the cold water source below. The condensed vapor is then returned to the boiler to complete a cycle that will generate electricity 24 hours a day throughout the year (with a few weeks of down time for plant maintenance).

Analysis of the OTEC cycle indicates that equatorial OTEC plant ships slowly "grazing" on warm surface water at 1/2 knot could continuously generate more than 5 megawatts-electric (MWe) of net electric power per square kilometer of tropical ocean. The electricity generated would be converted to chemical energy on board the plant ship by electrolyzing water into hydrogen and oxygen. For some uses, such as furnishing fuel for the space shuttle, these chemicals can be liquefied and stored for periodic transfer to shore. However, to provide products that can be handled more easily for delivery to world ports, the hydrogen would be combined on shipboard with nitrogen (extracted from the air via liquefaction) to synthesize ammonia. Methanol fuel may also be produced with a supply of carbon, which coal colliers could bring to the plant ship. Engineering studies indicate that OTEC plant ships designed to produce 100 to 400 MWe (net) of electricity (which is between 10 and 40 percent of the output of a large conventional power plant) would be the optimum size for commercial operation.

The U.S. Department of Energy (DOE) sponsored engineering designs that were developed between 1975 and 1982 by industrial teams under the technical direction of the Johns Hopkins University Applied Physics Laboratory (APL). Designs are available for a 46-MWe pilot OTEC plant ship that would produce 15 metric tons per day of liquid hydrogen (or 140 metric tons per day of liquid ammonia) in a conventional chemical plant installed on the OTEC vessel. It would use the same synthesis process that produces ammonia on land but would eliminate the costly methane-reforming step of that process.

An APL conceptual design is available for a 365-MWe commercial OTEC plant ship that would produce 1,100 metric tons per day of liquid ammonia. Used as a motor vehicle fuel, this could replace approximately 150,000 gallons of gasoline. If operating experience confirms the utility of this conceptual design, 2,000 OTEC ammonia plant ships could supply enough ammonia fuel per day to match the total daily mileage of all the automobiles presently in the United States. If these plant ships were distributed uniformly over the tropical ocean, an area of about 60 million square kilometers, they would be spaced 175 kilometers apart.

A history of success

OTEC's potential for providing the United States with an alternative to imported oil was recognized in l974 after the Organization of Petroleum Exporting Countries imposed its oil embargo. Between l975 and l982, DOE spent approximately $260 million on OTEC R&D in a detailed analysis of OTEC technical feasibility. Foreign studies also contributed to our information about OTEC. The findings included:

Technical feasibility. Tests and demonstrations at reasonable scale validated the power cycle performance; the cold water pipe design, construction, and deployment; the OTEC plant ship's ability to withstand l00-year storms (storms of an intensity that occurs, on average, once in 100 years); the durability of its materials; and methods for controlling biofouling of the heat exchangers.

Successful at-sea tests of a complete OTEC system (Mini-OTEC), including a 2,200-foot cold water pipe, were conducted with private funding near Kailea-Kona, Hawaii, in 1979. The program employed a Navy scow as a platform and used off-the-shelf components supplied by industrial partners in the venture. In four months of operation, Mini-OTEC generated 50 kilowatts-electric of gross power, which confirmed the engineering predictions. It demonstrated total system feasibility at reduced scale and was the first demonstration of OTEC net power generation.

A heat-exchanger test vessel, OTEC-1, was deployed with DOE funding in l980 and satisfactorily demonstrated projected heat-exchanger performance, water-ducting, and biofouling control at a 1-MWe scale. These results provided the scientific justification for the planned next step-a 40-MWe pilot plant demonstration.

Environmental effects. Effects of the environment on OTEC plant ship operations and effects of OTEC on ocean ecology were studied and analyzed. Hurricanes do not occur near the equator where OTEC plant ships will be deployed. Small-scale water-tunnel tests indicated that the pilot plant ship and cold water pipe can withstand equatorial 100-year-storm conditions with a good safety margin. A commercial 365-MWe OTEC ammonia plant ship would be about the size of a large oil tanker and would be less affected by waves and current than was the pilot plant.

OTEC uses large volumes of warm and cold water that pass through fish barriers to the heat exchangers and are mixed and discharged at the bottom of the ship. The discharged waters are denser than the surface ocean waters, so they descend to a depth of about 500 meters, there spreading laterally to form a disk where the density of the discharged plume matches that of the ambient ocean water. Diffusion of heat from this layer to the surface is negligible for one plant ship. But effects on the surface layer could become detectable and possibly significant if large numbers of plant ships were deployed close together, or if the cold nutrient-rich water discharged were deliberately mixed into the surface layer. This option could lead to a substantial increase in marine life, like to that occurring off Peru where upwelling brings nutrient-rich cold water to the surface.

Plant ship spacing would have to be chosen on the basis of an acceptable tradeoff between total power delivery and environmental impact. If one-tenth of one percent of the incident solar energy were converted to electricity, one square kilometer of ocean would generate 0.2 MWe of net electric power. Roughly 1,800 square kilometers could supply solar heat for continuous operation of a 365-MWe OTEC plant. This would mean an average spacing between ships of 45 kilometers, and the fuel produced would be equivalent to 14 times the total U.S. gasoline energy use in l996.

OTEC ammonia fuel commercial development. Tests of ammonia fuel (shown in bench tests to have an octane number of 130) in a four-cylinder Toyota engine have demonstrated performance at an optimum fuel-air ratio, in accord with theoretical predictions. Early work indicated that some hydrogen, which could be supplied by partial dissociation of the ammonia entering the engine or by other means, would be needed in an ammonia-fueled internal combustion engine to achieve adequate performance over the desired operating range. The tests show that operation at slightly fuel-rich conditions reduces nitrogen oxide emissions to one-tenth the concentration observed in today's automobiles.

Further work to develop OTEC fuels could lead to significant reductions in carbon emissions, air pollution, and oil imports.

The physical properties of ammonia are nearly the same as those of liquid propane, so the current procedures established for gas-tight safe handling and storage of liquid propane in automobiles and filling stations are applicable to ammonia. Ammonia can become a practical motor vehicle fuel, but much more engine R&D and storage and delivery design work will be necessary to define the total system requirements and costs for widespread ammonia car operations.

Ammonia is a major industrial chemical presently made by partial oxidation (reforming) of natural gas. Liquid ammonia is produced and distributed safely worldwide by tankers, pipelines, and trucks in quantities exceeding 100 million metric tons per year. Commercial experience in producing, storing, and transporting liquid ammonia (and hydrogen) suggest that adherence to existing regulations will ensure safe operations. Most ammonia is used as fertilizer and is commonly sprayed directly on the soil by individual farmers. It has a penetrating odor and is toxic in high concentrations but does not burn or explode at atmospheric pressure. No serious health-related problems or explosive hazards have been experienced in its use.

Competitiveness and financing. OTEC systems are "low" technology. Operating temperatures and pressures are the same as those in household air conditioners. About two-thirds of the required OTEC system components and subsystems are commercially available. Another 10 to 15 percent need to be scaled up and optimized for OTEC use, which adds some cost unpredictability. Only the cold water pipe construction, platform attachment, and deployment will require new types of equipment and procedures. If we assign l00 percent cost uncertainty to these items, the overall investment uncertainty of the OTEC system is around 15 to 25 percent. This relatively low uncertainty permits cost estimates to be made with reasonable confidence.

The ultimate sales price of fuel from OTEC plant ships depends on the cost to amortize plant investment (including construction costs) over plant life, plus operation and maintenance costs, including shipping to consumers. For a range of scenarios, the cost of OTEC-ammonia delivered to U.S. ports is estimated to vary from $0.30 to $0.60 per gallon (in 1995 dollars). Adjusting for the lower mileage per gallon of ammonia, this would be equivalent to gasoline costing $0.80 to $1.60 per gallon. These estimates are strongly dependent on assumed interest rates, amortization times, and whether tax credits and other subsidies that are available to gasoline users would be available to ammonia producers as well.

In the future, gasoline prices are expected to increase because of resource depletion. But with improved technology and expanded production, prices for ammonia produced by OTEC should decrease.

Finishing the job

The seven-year DOE R&D program provided positive answers to doubts about OTEC. It demonstrated at a reasonable scale that the OTEC concept for ocean energy production is technically feasible. The next step was to have been construction of a 40-MWe (nominal) pilot plant that would provide firm cost and engineering data for the design of full-scale OTEC plant ships. Planned funding for this step was canceled in 1982 when the Reagan administration, with different energy priorities from those of the Carter administration, took office. Since l982, government support of OTEC development has been undercut further by the drop in oil prices that has reduced public fears of an oil shortage and its economic consequences, as well as by the opposition of vested interests that are committed to conventional energy resources.

Lack of support for OTEC research is part of a general lack of interest in energy alternatives designed to address fundamental problems that will not become critical for several decades. If and when the need for measures to forestall energy shortages and/or severe environmental effects from present energy sources becomes evident, the long lead times needed for the costly transition from fossil fuels to sustainable energy resources may prevent action from being taken in time to be effective. It is prudent to renew OTEC R&D now.

There are questions about OTEC that cannot be answered without further development and testing: the effects of scale-ups on projected costs, required spacing of a network of OTEC plant ships to satisfy environmental restrictions, logistical problems associated with the widespread use of ammonia as a transportation fuel, and requirements for good performance in automobile and other combustion systems.

The date by which OTEC might be expected to become commercially viable is too far off to attract entrepreneurs. In view of the substantial capital cost of plant ship construction, government support is essential. At this stage, we cannot promise that OTEC will be commercially viable. But it is certainly promising enough to justify a substantial federal research investment.

We believe that further work to develop OTEC fuels could lead, by the middle of the next century, to significant reductions in carbon emissions, air pollution, and oil imports. The nation should be willing to make a small investment in designing, building, and evaluating a 45-MWe OTEC plant ship to demonstrate the feasibility and economics of the concept on a scale large enough to permit confident construction and operation of full-scale commercial OTEC plant ships.

The United States should begin a program that includes the following features:

  • Identify potential suppliers of OTEC systems and components, and bring up to date the predictions of their costs and performance. The survey should include new high-performance OTEC heat-exchanger options that have been demonstrated in R&D programs.
  • Conduct systems engineering studies to define a program with a long-range goal of OTEC ammonia plant ship development and commercialization that could attract government and industry support. We recommend an introductory program, lasting a few years, that will test attractive options with cost-shared funding. It would include analysis and experimental programs to provide firm data for heat-exchanger optimization (including designs, materials, and costs) and the hydrodynamics of OTEC water inlet and exhaust trajectories, including their interactions with surface and subsurface water flows and temperatures.
  • Define potential roles of government and industry in initiating and conducting the development program.
  • Determine a schedule and funding plan for OTEC development that would make it possible to have significant OTEC commercial operation by the year 2050.

William H. Avery is the coauthor (with Chih Wu) of Renewable Energy from the Oceans (Oxford University Press, 1994). He was director of ocean energy programs and assistant director for R&D at the Johns Hopkins University Applied Physics Laboratory. Walter G. Berl was a member of the principal staff at APL.