ocean thermal energy conversion
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What is ocean thermal energy conversion (OTEC)?
Ocean thermal energy conversion (OTEC) is a technology that uses the temperature difference between warm surface ocean water and cold deep ocean water to generate power in a conventional heat engine. For OTEC to be economically practical, the temperature difference should be at least 20 °C (36 °F) in the first 1,000 meters (about 3,300 feet) below the surface.
The OTEC concept, first proposed in the early 1880s, uses a closed-cycle system where heat from warm surface water vaporizes a working fluid, like ammonia, which then drives a turbine to produce electricity. Cold water cools the vapor, turning it back into a fluid, and the cycle repeats. There are also open-cycle systems that use water vapor as the working fluid and hybrid systems that combine aspects of both.
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How does OTEC use temperature differences in the ocean to produce energy?
Ocean thermal energy conversion (OTEC) uses the temperature difference between warm surface waters and cold, deep waters to generate power. The warm water vaporizes a working fluid like ammonia, which then turns a turbine to produce electricity. Cold water pumped from the ocean depths condenses the vapor, which is then reused in a closed system.
Open-cycle systems use water vapor as the working fluid, while hybrid systems combine both approaches. OTEC offers additional benefits such as air conditioning, chilled-soil agriculture, seawater desalination, and hydrogen production.
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What are the main parts of an OTEC system?
An ocean thermal energy conversion (OTEC) system uses the ocean's warm surface water and cold, deep water to generate power. There are three types of OTEC systems:
* Closed-cycle systems use a working fluid, such as ammonia, that vaporizes and turns a turbine to generate electricity. Cold water then condenses the vapor back into a liquid.
* Open-cycle systems use water vapor as the working fluid. Warm water is vaporized in a vacuum, and the steam turns a turbine. Cold water condenses the steam, and a vacuum pump maintains pressure.
* Hybrid systems combine elements of closed-cycle and open-cycle systems.1 Britannica SourceCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
What types of OTEC technologies exist (like closed-cycle and open-cycle)?
Ocean thermal energy conversion (OTEC) uses the temperature difference between warm surface waters and cold, deep waters to generate power. There are three main types of OTEC systems:
- Closed-cycle systems use a working fluid, such as ammonia, that vaporizes by heat transferred from warm surface water. The vapor then turns a turbine to generate electricity before being condensed by cold seawater.
- Open-cycle systems use water vapor as the working fluid. Warm surface seawater is partially vaporized in a vacuum, and the resulting steam drives a low-pressure steam turbogenerator. Cold seawater condenses the steam.
- Hybrid systems combine elements of closed-cycle and open-cycle systems. Steam from warm water vaporizes a secondary working fluid, which then drives a turbine.
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What are some benefits and challenges of using OTEC technology?
Ocean thermal energy conversion (OTEC) uses the temperature difference between warm surface waters and cold, deep waters to generate power. OTEC offers benefits such as the production of clean power and useful by-products, including air conditioning, chilled-soil agriculture, seawater desalination, and access to trace elements in deep-ocean seawater. Also, hydrogen can be extracted from water through electrolysis for use in fuel cells.
However, OTEC is a relatively expensive technology due to the high costs of constructing OTEC plants and infrastructure. Floating facilities may be more feasible than land-based ones because the number of land-based sites with access to deep water in the tropics is limited.
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How can OTEC impact coastal communities and the environment?
Ocean thermal energy conversion (OTEC) can significantly impact coastal communities and the environment. OTEC plants offer clean power and useful by-products such as cool water for air conditioning and chilled-soil agriculture, seawater desalination, and access to trace elements from deep-ocean water. Hydrogen can also be extracted from water through electrolysis for use in fuel cells.
While OTEC is relatively expensive to implement, it may be possible to generate relatively inexpensive electricity once facilities are operational. Floating facilities may also be more feasible than land-based ones, because the number of land-based sites with access to deep water in the tropics is limited.
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ocean thermal energy conversion (OTEC), form of energy conversion that makes use of the temperature differential between the warm surface waters of the oceans, heated by solar radiation, and the deeper cold waters to generate power in a conventional heat engine. The difference in temperature between the surface and the lower water layer can be as large as 50 °C (90 °F) over vertical distances of as little as 90 metres (about 300 feet) in some ocean areas. To be economically practical, the temperature differential should be at least 20 °C (36 °F) in the first 1,000 metres (about 3,300 feet) below the surface. In the first decade of the 21st century, the technology was still considered to be experimental, and thus far no commercial OTEC plants have been constructed.
The OTEC concept was first proposed in the early 1880s by the French engineer Jacques-Arsène d’Arsonval. His idea called for a closed-cycle system, a design that has been adapted for most present-day OTEC pilot plants. Such a system employs a secondary working fluid (a refrigerant) such as ammonia. Heat transferred from the warm surface ocean water causes the working fluid to vaporize through a heat exchanger. The vapour then expands under moderate pressures, turning a turbine connected to a generator and thereby producing electricity. Cold seawater pumped up from the ocean depths to a second heat exchanger provides a surface cool enough to cause the vapour to condense. The working fluid remains within the closed system, vaporizing and reliquefying continuously.
Some researchers have centred their attention on an open-cycle OTEC system that employs water vapour as the working fluid and dispenses with the use of a refrigerant. In this kind of system, warm surface seawater is partially vaporized as it is injected into a near vacuum. The resultant steam is expanded through a low-pressure steam turbogenerator to produce electric power. Cold seawater is used to condense the steam, and a vacuum pump maintains the proper system pressure. Hybrid systems, which combine elements of closed-cycle and open-cycle systems, also exist. In these systems, steam produced by warm water passing through a vacuum chamber is used to vaporize a secondary working fluid that drives a turbine.
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During the 1970s and ’80s the United States, Japan, and several other countries began experimenting with OTEC systems in an effort to develop a viable source of renewable energy. In 1979 American researchers put into operation the first OTEC plant able to generate usable amounts of electric power—about 15 kilowatts of net power. This unit, called Mini-OTEC, was a closed-cycle system mounted on a U.S. Navy barge a few kilometres off the coast of Hawaii. In 1981–82 Japanese companies tested another experimental closed-cycle OTEC plant. Located in the Pacific island republic of Nauru, this facility produced 35 kilowatts of net power. Since that time researchers have continued developmental work to improve heat exchangers and to devise ways of reducing corrosion of system hardware by seawater. By 1999 the Natural Energy Laboratory of Hawaii Authority (NELHA) had created and tested a 250-kilowatt plant.
The prospects for commercial application of OTEC technology seem bright, particularly on islands and in developing countries in the tropical regions where conditions are most favourable for OTEC plant operation. It has been estimated that the tropical ocean waters absorb solar radiation equivalent in heat content to that of about 250 billion barrels of oil each day. Removal of this much heat from the ocean would not significantly alter its temperature, but it would permit the generation of tens of millions of megawatts of electricity on a continuous basis.
Beyond the production of clean power, the OTEC process also provides several useful by-products. The delivery of cool water to the surface has been used in air-conditioning systems and in chilled-soil agriculture (which allows for the cultivation of temperate-zone plants in tropical environments). Open-cycle and hybrid processes have been used in seawater desalination, and OTEC infrastructure allows access to trace elements present in deep-ocean seawater. In addition, hydrogen can be extracted from water through electrolysis for use in fuel cells.
OTEC is a relatively expensive technology, since the construction of costly OTEC plants and infrastructure is necessary before power can be generated. However, once facilities are made operational, it may be possible to generate relatively inexpensive electricity. Floating facilities may be more feasible than land-based ones, because the number of land-based sites with access to deep water in the tropics is limited. Few cost analyses exist; however, one study, which was performed in 2005, placed the cost of electricity produced by OTEC at 7 cents per kilowatt-hour. Although this figure was based on the assumption of a 100-megawatt OTEC facility located some 10 km (6 miles) off the coast of Hawaii, it is comparable to the cost of energy derived from fossil fuels. (The cost of coal-generated electricity is estimated at 4–8 cents per kilowatt-hour.)
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