Giles Sparrow
1476

Ocean Thermal Energy Conversion

Ocean thermal energy conversion, or OTEC, is one of the least publicized methods for extracting energy from the world's seas, and yet it is also one of the most practical and economically viable. OTEC makes use of the temperature difference between the sea's surface and its depths to generate clean, "green", electrical power. Although it is generally more expensive than traditional sources of energy, it costs less than wave or tidal power schemes and is particularly effective on small islands, where importing traditional fuels is expensive, and the geography of the surrounding seabed lends itself particularly well to OTEC.

How OTEC Works

OTEC uses the temperature difference between warm surface water and cold deep waters to boil a "working fluid." The vapor produced by this working fluid is used to spin electricity turbines, and the fluid is then recondensed. In practice, there are two main types of OTEC: closed and open-cycle.

Closed-cycle OTEC: In this method, the working fluid is a liquid, typically ammonia, with a low boiling point. Warm water is collected from the sea surface and passes through heat exchangers inside the plant (a heat exchanger is a network of narrow pipes with a large surface area, which allows heat to be transferred efficiently between a fluid inside the exchanger and another fluid surrounding it). These heat exchangers are used to warm the working fluid so that it boils, and the expanding vapor is used to turn the turbines. The working fluid then passes over another set of heat exchangers, which are filled with cold water pumped from the deep ocean. The temperature drop causes the fluid to recondense, and the cycle can repeat.

Open-cycle OTEC: In this variation, the warm seawater actually becomes the working fluid. After it enters the plant, it is pumped into a vacuum chamber where the low air pressure causes it to boil at a temperature lower than 100°C (212°F). This produces steam, which spins the turbines and is then passed over heat exchangers filled with cold deep sea water. The steam condenses and is pumped back into the sea, or is used as a supply of desalinated water. The pressure drop created as the steam recondenses is used to create the partial vacuum needed to evaporate more warm water at the start of the cycle.

OTEC In Practice

The most important practical limitation on the use of OTEC comes from the need for a sufficient temperature difference between the warm and cold water used in the system. Experiments have shown that a temperature difference of at least 20°C (36°F) is needed, and that OTEC is only practical if the cold water can be pumped from depths of less than around 1 kilometer (3000 feet). This makes OTEC practical in those regions where the sea surface receives the most heat from the sun, largely within a band 20 to 30° north and south of the equator. Within this region, tropical islands offer the best prospects for inexpensive development of OTEC; they frequently have steep ocean shelves allowing the deep sea cold water close to the shore. Islands are also often reliant on expensive fuel imports, which makes OTEC even more economically sensible.

Uses of OTEC

As well as generating electricity, OTEC has a number of useful by-products. Desalination is one of the most important and is particularly useful on isolated islands. Either type of OTEC cycle can produce desalinated water. The open-cycle process is described above, but in a closed-cycle plant, the cold water used to condense the working fluid can be passed on through another set of heat exchangers to condense smaller amounts of water out of the humid tropical atmosphere (a third variation, called the hybrid cycle, evaporates and recondenses the warm sea water to produce desalinated water, but uses the steam to boil a separate closed-cycle working fluid).

Even after it has passed through the complete OTEC cycle, deep sea water remains very cold and rich in nutrients. Rather than simply returning it to the ocean floor, it can be useful in a variety of applications. In mariculture, for example, cold water from an OTEC plant provides an ideal breeding ground for plankton and algae as a source of food for farmed fish. The cold water can even be used directly to provide an environment for rearing salmon, trout, and other fish not normally found in the tropics.

Cold OTEC water can also be used for refrigeration and air-conditioning plants. Although it contains nutrients and microorganisms, experiments have shown that introducing just small, environmentally safe amounts of chlorine into the water will prevent the bacteria in it from breeding and clogging pipes.

Another long-term goal for OTEC plants is to chemically extract minerals from seawater. At present, "mining" of seawater is uneconomical, but this can change if the seawater is already being brought to the surface for another purpose. In Japan, for instance, experiments for extracting uranium from seawater are already underway.

History of OTEC

OTEC has a surprisingly long history—the technique was invented by French engineer Jacques d'Arsonval in 1881. D'Arsonval developed the closed-cycle system, but was unable to put it into practice. However, his student Georges Claude took things a step further, devising the open cycle, and building two prototype plants - one in Cuba, and one on board a cargo ship off the coast of Brazil. But both plants were destroyed by the weather before Claude could demonstrate their ability to produce more energy than was needed to run them. Another French project, begun in the 1950s on the west coast of Africa, failed because of competition from cheaper hydroelectric power.

In the early 1980s the prospects for OTEC began to improve. Japan demonstrated a working plant capable of generating around 30 kilowatts of electricity on the Pacific island of Nauru, and experiments proved that aluminum could be used to replace the expensive titanium previously used in OTEC heat exchangers. Improvements in the heat exchanger design also meant that it was possible to extract more energy from a less expensive plant. From 1979 onwards, the U.S. Natural Energy Laboratory of Hawaii (NELHA) began to build a series of test facilities to demonstrate and improve OTEC technology. NELHA has also demonstrated many of the useful by-products of OTEC, ranging from desalination to mariculture. The current record for electricity generation by an OTEC plant is held by NELHA's open-cycle plant at Keahole Point, Hawaii, which generated 50 kilowatts in continuous tests in May 1993. At present, most OTEC research is directed towards improving turbine designs to make the electricity generation process even more efficient.


Future Prospects

Today, there are still no commercial OTEC plants in existence. The major obstacle to their development is the high initial cost of building such a plant, but improvements in design and cost-effectiveness mean that the possibility of economically viable OTEC is getting closer.

Because the oceans absorb so much solar energy every day, there is potentially a huge resource waiting to be tapped, offering billions of watts of clean, renewable power. Over the next few years, OTEC plants are likely to be built in small island groups such as those of the Pacific Ocean. These would only have to generate around 1 megawatt of power, and when coupled to desalination plants they could be economically viable, because of the high costs of fuel imports. As fossil fuel prices continue to rise, larger OTEC plants will become a more feasible energy source for larger island groups, such as Hawaii. Surveys have shown that, in the long term, more than 70 developing nations are situated in regions where they could take advantage of OTEC.

More ambitious plans for the future would involve using large "plantships" that could float off the continental shelves of the Americas or Australia. The power these plantships could generate would either be sent back to shore by submarine cables, or used in onboard factories for energy-intensive manufacturing processes such as the making of hydrogen, ammonia, or methanol.


Further Reading

Avery, William H. and Chih Wu. Renewable Energy from the Ocean. New York: Oxford University Press, 1994.

Cohen, R. (1982). "Energy from the Ocean." Philosophical Transactions of the Royal Society of London; Series A: Mathematical and Physical Sciences; Vol. 307, No. 1499, pp.405-437.

Dunbar, L.E. (1981). "Potential For Ocean Thermal Energy Conversion as a Renewable Energy Source For Developing Nations." Science Applications International Corporation.

Johnson, F.A. (1988). "Energy from the Oceans: A Small Land-Based Ocean Thermal Energy Plant." Proceedings, EEZ Resources Technology Assessment; January 1988, Honolulu, Hawaii.

Marine Board-National Research Council. (1982). Ocean Engineering for Ocean Thermal Energy Conversion. Washington, D.C.: National Academy Press.

Penney, T.R.; Bharathan, D. (January 1987). "Power From the Sea." Scientific American; Vol. 256, No. 1.

Seymour, Richard J. (Ed). Ocean Energy Recovery: The State of the Art. American Society of Civil Engineers, 1992.