Ocean Thermal Energy Conversion (OTEC)

Ocean thermal energy conversion (OTEC) is a method for generating electricity which uses the temperature difference that exists between deep and shallow waters to run a heat engine. As with any heat engine, the greatest efficiency and power is produced with the largest temperature difference. This temperature difference generally increases with decreasing latitude, i.e. near the equator, in the tropics. However, evaporation prevents the surface temperature from exceeding 27 deg.C (80 deg.F). Also the subsurface water rarely falls below 5 deg.C. Historically, the main technical challenge of OTEC was to generate significant amounts of power, efficiently, from this very small temperature ratio. Changes in efficiency of heat exchange in modern designs allow performance approaching the theoretical maximum efficiency.

The Earth's oceans are continually heated by the sun and cover nearly 70% of the Earth's surface, this temperature difference contains a vast amount of solar energy which can potentially be harnessed for human use. If this extraction could be made cost effective on a large scale, it could provide a source of renewable energy needed to deal with energy shortages, and other energy problems. The total energy available is one or two orders of magnitude higher than other ocean energy options such as wave power, but the small magnitude of the temperature difference makes energy extraction comparatively difficult and expensive, due to low thermal efficiency. Earlier OTEC systems had an overall efficiency of only 1 to 3% (the theoretical maximum efficiency lies between 6 and 7%^[1]). Current designs under review will operate closer to the theoretical maximum efficiency. The energy carrier, seawater, is free, although it has an access cost associated with the pumping materials and pump energy costs. Although an OTEC plant operates at a low overall efficiency, it can be configured to operate continuously as a Base load power generation system. Any thorough Cost-benefit analysis should include these factors to provide an accurate assessment of performance, efficiency, operational and construction costs and returns on investment.

The concept of a heat engine is very common in thermodynamics engineering, and much of the energy used by humans passes through a heat engine. A heat engine is a thermodynamic device placed between a high temperature reservoir and a low temperature reservoir. As heat flows from one to the other, the engine converts some of the heat energy to work energy. This principle is used in steam turbines and internal combustion engines, while refrigerators reverse the direction of flow of both the heat and work energy. Rather than using heat energy from the burning of fuel, OTEC power draws on temperature differences caused by the sun's warming of the ocean surface.

The only heat cycle suitable for OTEC, is the Rankine cycle, using a low-pressure turbine. Systems may be either closed-cycle or open-cycle. Closed-cycle engines use working fluids that are typically thought of as refrigerants such as ammonia or R-134a. Open-cycle engines use the water heat source as the working fluid.

History of OTEC

Even though an OTEC system is technologically advanced, the concept has a long history of development. There have been many periodic attempts to develop and refine the technology starting in the 1800s. In 1881, Jacques Arsene d'Arsonval, a French physicist, proposed tapping the thermal energy of the ocean. It was d'Arsonval's student, Georges Claude who actually built the first OTEC plant, in Cuba in 1930. The system generated 22 kW of electricity with a low-pressure turbine.^[2]

In 1931, Nikola Tesla released "On Future Motive Power" which covered an ocean thermal energy conversion system.^[3] Although initially excited about the idea, Tesla ultimately came to the conclusion that the scale of engineering required for the project made it impractical for large scale development.

In 1935, Claude constructed another plant, this time aboard a 10,000-ton cargo vessel moored off the coast of Brazil. Weather and waves destroyed both plants before they could become net power generators.^[2] (Net power is the amount of power generated after subtracting power needed to run the system.)

In 1956, French scientists designed a 3MW plant for Abidjan, Côte d'Ivoire. The plant was never completed, however, because large amounts of cheap oil became available in the 1950s making oil fired power plants more economical.^[2]

In 1962, J. Hilbert Anderson and James H. Anderson, Jr. started designing a cycle to accomplish what Claude had not; they focused on developing new, more efficient component designs. After working through some some of the problems in Claude's design they patented their new "closed cycle" design in 1967.^[4]

The United States became involved in OTEC research in 1974, when the Natural Energy Laboratory of Hawaii Authority was established at Keahole Point on the Kona coast of Hawaii. The laboratory has become one of the world's leading test facilities for OTEC technology. Hawaii is often said to be the best location in the US for OTEC, due to the warm surface water, excellent access to very deep, very cold water, and because Hawaii has the highest electricity costs in the US.^[5]

Although Japan has no potential OTEC sites it has been a major contributor to the development of the technology, primarily for export to other countries.^[6] Beginning in 1970 the Tokyo Electric Power Company successfully built and deployed a 100 kW closed-cycle OTEC plant on the island of Nauru.^[6] The plant, which became operational 1981-10-14, produced about 120 kW of electricity; 90 kW was used to power the plant itself and the remaining electricity was used to power a school and several other places in Nauru.^[2] This set a world record for power output from an OTEC system where the power was sent to a real power grid.^[7]

India piloted a 1 MW floating OTEC plant near Tamil Nadu. Its government continues to sponsor various research in developing floating OTEC facilities.

How OTEC works

Closed-cycle

Closed-cycle systems use fluid with a low boiling point, such as ammonia, to rotate a turbine to generate electricity. Warm surface seawater is pumped through a heat exchanger where the low-boiling-point fluid is vaporized. The expanding vapor turns the turbo-generator. Then, cold, deep seawater—pumped through a second heat exchanger—condenses the vapor back into a liquid, which is then recycled through the system.

Open-cycle

Open-cycle OTEC uses the tropical oceans' warm surface water to make electricity. When warm seawater is placed in a low-pressure container, it boils. The expanding steam drives a low-pressure turbine attached to an electrical generator. The steam, which has left its salt and contaminants behind in the low-pressure container, is pure fresh water. It is condensed back into a liquid by exposure to cold temperatures from deep-ocean water. This method has the advantage of producing desalinized fresh water, suitable for drinking water or irrigation.

Hybrid

A hybrid cycle combines the features of both the closed-cycle and open-cycle systems. In a hybrid OTEC system, warm seawater enters a vacuum chamber where it is flash-evaporated into steam, similar to the open-cycle evaporation process. The steam vaporizes the ammonia working fluid of a closed-cycle loop on the other side of an ammonia vaporizer. The vaporized fluid then drives a turbine to produce electricity. The steam condenses within the heat exchanger and provides desalinated water. (see heat pipe)

Other related technologies

Air conditioning

The cold (5°C, 41°F) seawater made available by an OTEC system creates an opportunity to provide large amounts of cooling to operations that are related to or close to the plant. The cold seawater delivered to an OTEC plant can be used in chilled-water coils to provide air-conditioning for buildings. It is estimated that a pipe 0.3-meters in diameter can deliver 0.08 cubic meters of water per second. If 6°C water is received through such a pipe, it could provide more than enough air-conditioning for a large building. If this system operates 8000 hours per year and local electricity sells for 5¢-10¢ per kilowatt-hour, it would save $200,000-$400,000 in energy bills annually (U.S. Department of Energy, 1989). The InterContinental Resort and Thalasso-Spa on the island of Bora Bora uses an OTEC system to air-condition its buildings.^[11] The system accomplishes this by passing cold seawater through a heat exchanger where is cools freshwater in a closed loop system. This cool freshwater is then pumped to buildings and is used for cooling directly (no conversion to electricity takes place).

Chilled-soil agriculture

OTEC technology also supports chilled-soil agriculture. When cold seawater flows through underground pipes, it chills the surrounding soil. The temperature difference between plant roots in the cool soil and plant leaves in the warm air allows many plants that evolved in temperate climates to be grown in the subtropics. The Common Heritage Corporation, a former tenant at the Natural Energy Laboratory, and the holder of the patent on this process, maintained a demonstration garden with more than 100 different fruits and vegetables, many of which would not normally survive in Hawaii. No chilled soil agriculture is presently being undertaken at the Natural Energy Laboratory.

Aquaculture

Aquaculture is the most well-known byproduct of OTEC. It is widely considered to be one of the most important ways to reduce the financial and energy costs of pumping large volumes of water from the deep ocean. Deep ocean water contains high concentrations of essential nutrients that are depleted in surface waters due to biological consumption. This "artificial upwelling" mimics the natural upwellings that are responsible for fertilizing and supporting the world's largest marine ecosystems, and the largest densities of life on the planet.

Cold-water delicacies, such as salmon and lobster, thrive in the nutrient-rich, deep, seawater from the OTEC process. Microalgae such as Spirulina, a health food supplement, also can be cultivated in the nutrient rich water. Because the OTEC process uses cold, deep-ocean water and warm ocean water from the surface, it can be combined in various ratios to deliver sea water of a specific temperature conducive to maintaining an optimal environment for aquaculture. For example, Maine lobster could be grown in a tropical island environment in a temperature controlled mixture of cold and warm sea water.

Seafood not indigenous to tropical waters, can also be raised in pools created by OTEC-pumped water, such as Salmon, lobster, abalone, trout, oysters, and clams. This extends the variety of fresh seafood products available for nearby markets. Likewise, the low-cost refrigeration provided by the cold seawater can be used to upgrade or maintain the quality of indigenous fish, which tend to deteriorate quickly in warm tropical regions.

Desalination

Desalinated water can be produced in open- or hybrid-cycle plants using surface condensers. In a surface condenser, the spent steam is condensed by indirect contact with the cold seawater. This condensate is relatively free of impurities and can be collected and dispensed to local communities where supplies of natural freshwater for agriculture or drinking are limited. System analysis indicates that a 2-megawatt (electric) (net) plant could produce about 4300 cubic meters of desalinated water each day (Block and Lalenzuela 1985).

Hydrogen Production

Hydrogen can be produced via electrolysis using electricity generated by the OTEC process. The steam generated can be used as a relatively pure medium for electrolysis with electrolyte compounds added to improve the overall efficiency. OTEC technology can be scaled to generate large quantities of hydrogen which can supply the burgeoning global marketplace. OTEC installations on islands, platforms, barges and ships have the potential for large scale, global hydrogen generation with supply to major ports via tanker ships. For example, this is the method of delivery currently used to transport hydrogen to the Kennedy Space Center for use by NASA. The main challenges include the high costs of production, transportation, and distribution, relative to other energy sources and fuels. Considering the increasing price of petroleum products on world markets, costs for large scale Hydrogen production and distribution could be subject to change in a relatively small amount of time.

Mineral extraction

Another undeveloped opportunity, is the potential to mine ocean water for its 57 elements contained in salts and other forms and dissolved in solution. In the past, most economic analyses concluded that mining the ocean for trace elements dissolved in solution would be unprofitable, in part because much energy is required to pump the large volume of water needed. More significantly, it is often very expensive to separate the minerals from seawater. Generally this method is limited to minerals that occur in high concentrations, and can be extracted easily, such as magnesium.

However, with OTEC plants supplying the pumped water, the remaining problem is the cost of the extraction process. The Japanese recently began investigating the concept of combining the extraction of uranium dissolved in seawater with wave-energy technology. They found developments in other technologies (especially materials sciences) were improving the viability of mineral extraction processes that employ ocean energy.

References

^ ^a ^b ^c ^d ^e Berger, LR; Berger, JA (June 1986). "Countermeasures to Microbiofouling in Simulated Ocean Thermal Energy Conversion Heat Exchangers with Surface and Deep Ocean Waters in Hawaii". Applied and Environmental Microbiology 51 (6): 1186 - 1198.
^ ^a ^b ^c ^d Takahashi, Masayuki Mac; Translated by: Kitazawa, Kazuhiro and Snowden, Paul [1991] (2000). Deep Ocean Water as Our Next Natural Resource. Tokyo, Japan: Terra Scientific Publishing Company. ISBN 4-88704-125-x.
^ Tesla, Nikola (December 1931). "On Future Motive Power". Everyday Science and Mechanics: 230 - 236.
^ US patent 3312054, "Sea Water Power Plant", granted 1967-04-04
^ Average Retail Price of Electricity to Ultimate Customers by End-Use Sector, by State. Energy Information Administration (September 2007).
^ ^a ^b Bruch, Vicki L. (April 1994). "An Assessment of Research and Development Leadership in Ocean Energy Technologies" SAND93-3946. Sandia National Laboratories: Energy Policy and Planning Department.
^ Mitsui, T.; Ito, F.; Seya, Y.; Nakamoto, Y. (September 1983). "Outline of the 100 kw OTEC Pilot Plant in the Republic of Nauru". IEEE Transactions on Power Apparatus and Systems PAS-102 (9): 3167 - 3171.
^ US patent 4311012, "Method and apparatus for transferring cold seawater upward from the lower depths of the ocean to improve the efficiency of ocean thermal energy conversion systems", granted 1982-01-19
^ Trimble, L.C.; Owens, W.L. (1980). "Review of mini-OTEC performance". Energy to the 21st century; Proceedings of the Fifteenth Intersociety Energy Conversion Engineering Conference 2: 1331 - 1338.
^ ^a ^b Achievements in OTEC Technology. National Renewable Energy Laboratory.
^ YouTube video on the OTEC air-conditioning system used at the InterContinental Resort and Thalasso-Spa on the island of Bora Bora. Retrieved on 2007-05-28.
^ ^a ^b Aftring, RP; Taylor, BF (October 1979). "Assessment of Microbial Fouling in an Ocean Thermal Energy Conversion Experiment". Applied and Environmental Microbiology 38 (4): 734 - 739.
^ ^a ^b ^c ^d Nickels, JS; Bobbie, RJ: Lott, DF; Martz, RF; Benson, PH; White, DC (June 1981). "Effect of Manual Brush Cleaning on Biomass and Community Structure of Microfouling Film Formed on Aluminum and Titanium Surfaces Exposed to Rapidly Flowing Seawater". Applied and Environmental Microbiology 41 (6): 1442 - 1453.
^ Trulear, MG; Characklis, WG (September 1982). "Dynamics of Biofilm Processes". Journal of the Water Pollution Control Federation 54 (9): 1288 - 1301.