Ocean thermal energy conversion (OTEC or OTE) 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. 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%). 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.
View of a land based OTEC facility at Keahole Point on the Kona coast of Hawaii (United States Department of Energy)
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
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.
In 1931, Nikola Tesla released "On Future Motive Power" which covered an ocean thermal energy conversion system. 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. (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.
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 of the problems in Claude's design they patented their new "closed cycle" design in 1967.
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.
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. Beginning in 1970 the Tokyo Electric Power Company successfully built and deployed a 100 kW closed-cycle OTEC plant on the island of Nauru. 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. This set a world record for power output from an OTEC system where the power was sent to a real power grid.
India piloted a 1 MW floating OTEC plant near Tamil Nadu. Its government continues to sponsor various research in developing floating OTEC facilities.
Work principles
Some energy experts believe that if it could become cost-competitive with conventional power technologies, OTEC could produce gigawatts of electrical power, and in conjunction with electrolysis, could produce enough hydrogen to completely replace all projected global fossil fuel consumption. Managing costs is still a huge challenge, however. All OTEC plants require an expensive, large diameter intake pipe, which is submerged a kilometre or more into the ocean's depths, to bring very cold water to the surface.
1) Depending on the location
Land based plant
Shelf based plant
Floating plant
2) Depending on the cycle used
Open cycle
Closed cycle
Hybrid cycle
Closed cycle
This cold seawater is an integral part of each of the three types of OTEC systems: closed-cycle, open-cycle, and hybrid. To operate, the cold seawater must be brought to the surface. This can be accomplished through direct pumping. A second method is to desalinate the seawater near the sea floor; this lowers its density, which will cause it to "float" up through a pipe to the surface.
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.
In 1979, the Natural Energy Laboratory and several private-sector partners developed the mini OTEC experiment, which achieved the first successful at-sea production of net electrical power from closed-cycle OTEC. The mini OTEC vessel was moored 1.5 miles (2.4 km) off the Hawaiian coast and produced enough net electricity to illuminate the ship's light bulbs, and run its computers and televisions.
Then, the Natural Energy Laboratory in 1999 tested a 250 kW pilot closed-cycle plant, the largest of its kind ever put into operation. Since then, there have been no tests of OTEC technology in the United States, largely because the economics of energy production today have delayed the financing of a permanent, continuously operating plant.
Outside the United States, the government of India has taken an active interest in OTEC technology. India has built and plans to test a 1 MW, closed-cycle, floating OTEC plant.
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.
In 1984, the Solar Energy Research Institute (now the National Renewable Energy Laboratory) developed a vertical-spout evaporator to convert warm seawater into low-pressure steam for open-cycle plants. Energy conversion efficiencies as high as 97% were achieved for the seawater to steam conversion process (overall efficiency of an OTEC system using a vertical-spout evaporator would still only be a few per cent). In May 1993, an open-cycle OTEC plant at Keahole Point, Hawaii, produced 50,000 watts of electricity during a net power-producing experiment. This broke the record of 40,000 watts set by a Japanese system in 1982.
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)
The electricity produced by the system can be delivered to a utility grid or used to manufacture methanol, hydrogen, refined metals, ammonia, and similar products.
Some proposed projects
OTEC projects on the drawing board include a small plant for the U.S. Navy base on the British-occupied island of Diego Garcia in the Indian Ocean. OCEES International, Inc. is working with the U.S. Navy on a design for a proposed 13 MW OTEC plant, which would replace the current power plant running diesel generators. The OTEC plant would also provide 1.25 MGD of potable water to the base. A private U.S. company also has proposed building at 10 MW OTEC plant on Guam.
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