Ocean thermal energy conversion

In some tropical regions of the Earth, there is virtually limitless energy in the ocean for possible conversion to electric power. The efficiency of the conversion is very low, however, and the engineering problems are challenging. Development of ocean thermal energy conversion (OTEC) has been slow.

Background

In tropical oceans, the temperatures of warm and cold layers of water may differ significantly even though the layers are less than 1,000 meters apart. This phenomenon results from global circulation currents caused by the Sun. Solar energy warms water near the surface, and colder, more dense water moves to lower depths. At the same time, the rotation of the Earth causes the cold water to flow from the poles toward the tropics. As it is warmed, this cool water then rises toward the surface as its decreases, causing the warm to flow toward the polar regions, where it is cooled.

Differences of 20° to 25° Celsius over a distance of 500 to 1,000 meters are found in the Caribbean Sea and the Pacific Ocean near the Hawaiian Islands. In accordance with the second law of thermodynamics, thermal energy from the warm layer can be used as a “fuel” for a heat engine that exhausts energy to the cool layer. Typically, the warm layer has a temperature between 27° and 29° Celsius, and the cool layer is between 4° and 7°. The second law of thermodynamics indicates that the maximum efficiency of the conversion from thermal energy to mechanical energy will be very low. For example, if the warm layer is at 25° Celsius and the cold layer is at 5°, the maximum efficiency will be less than 7 percent; even this figure is between two and three times the actual efficiency that can be achieved in an energy conversion plant.

History

The concept of OTEC was first suggested in 1882 by the French physicist Jacques Arsène d’Arsonval, but it was not until 1926 that the French scientist Georges Claude made an attempt to implement the idea at Matanzas Bay, Cuba. The facility in Cuba was a small, land-based plant which was so inefficient that it required more power to operate than it produced. It ran for only a few weeks. Beginning in the 1960s, improvements in design and materials led to considerable research. Feasibility as a practical method of power generation was first demonstrated in the 1980s.

Advances in OTEC have depended on governmental support. In the mid-1970s, only the US and Japanese governments were supporting research and development. The French government later became interested, and sponsorship followed in the Netherlands, the United Kingdom, and Sweden.

Basic Designs

Broadly speaking, designs are either open cycle (OC) or closed cycle (CC). In the OC method, the incoming warm seawater is continuously sent into an evaporator operating at low pressure, where a small portion of the water “flashes” into steam. The steam in turn passes through a turbine connected to an electric power generator. The low-pressure steam leaving the turbine is then cooled and condensed in a heat exchanger by the cold seawater stream. The condensed water is fresh water, the salt of the ocean having been left behind in the evaporator. Hence, this water can be used for drinking and other household uses.

In the CC process, heat from the warm stream is transferred in a heat exchanger to a “working fluid” such as propane or ammonia. This fluid is vaporized and passed through a turbine generator in the same fashion as in the OC process. The vapor leaving the turbine is then condensed in a second heat exchanger. The condensate is recycled to the first exchanger, where it is again vaporized. Thus, the working fluid is never in direct contact with the seawater. Some hybrid plants have been designed which are combinations of OC and CC technology.

Though the first plant was a land-based unit, some plant designs involve plants located offshore, possibly floating or submerged. One of the key elements in the process is the water pipe which carries the cold water to the plant. This pipe is typically between 1 and 2 kilometers long. Originally, Claude used a corrugated steel pipe, 1.6 meters in diameter, which was fragile and not corrosion-resistant. Steel has been replaced by fiberglass-reinforced plastic or high-density polyethylene. Diameters larger than this have been considered in some studies but are not feasible owing to a lack of flexibility.

Engineering Problems

Designs for OTEC plants with power capacities on the order of 10 megawatts or more have been made, but actual plants have been much smaller, with outputs on the order of tens of kilowatts. In spite of these relatively small outputs, the equipment and the engineering problems are challenging. Both cold and warm water flow rates are large because the efficiency of the conversion process is so low. The seawater carries considerable dissolved gases, notably nitrogen and oxygen, and these gases must be vented if flash evaporation is used. The presence of noncondensable gases poses difficult problems both in the evaporator which precedes the power turbine and in the condenser which follows it. These gases not only increase the sizes of the units but also, because they are below atmospheric pressure, must be pumped out to maintain the vacuum levels in the process.

The CC method can avoid some of these problems. The operating pressures in the cycle using propane are relatively high, so a turbine of reasonable size can be used. Moreover, because the pressures are greater than atmospheric, vacuum and deaeration problems are eliminated. The CC process introduces additional problems, however, owing to the heat-transfer steps between the working medium and the hot and cold water.

Advantages

In view of the very low efficiency of OTEC, it may seem hard to imagine how the process can be profitable. However, the “fuel” is free and virtually unlimited. In addition, the OC process can produce sizable quantities of fresh water, which is often valuable in places where OTEC plants are located. Some OC plants may even be profitable on the basis of their freshwater production alone. Nevertheless, OTEC, even in the best of circumstances, poses both engineering and economic challenges that will continue to hamper its development for many years.

Bibliography

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Charlier, Roger Henri, and John R. Justus. “Current Assessment of Ocean Thermal Energy Potential.” In Ocean Energies: Environmental, Economic, and Technological Aspects of Alternative Power Sources. New York: Elsevier, 1993.

Congressional Research Service. Energy from the Ocean. Honolulu, Hawaii: University Press of the Pacific, 2002.

Goldin, Augusta. Oceans of Energy: Reservoir of Power for the Future. New York: Harcourt Brace Jovanovich, 1980.

Krock, Hans-Jurgen, ed. Ocean Energy Recovery: Proceedings of the First International Conference, ICOER ’89. New York: American Society of Civil Engineers, 1990.

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Zhao Jing, Trianshi Du, et al. "Growth of Ocean Thermal Energy Conversion Resources Under Greenhouse Warming Regulated by Oceanic Eddies." Nature Communications, 25 Nov. 2022, doi.org/10.1038/s41467-022-34835-z. Accessed 27 Dec. 2024.