HOME

TheInfoList




Ocean Thermal Energy Conversion (OTEC) uses the
ocean thermal gradient A thermocline (also known as the thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water column, water, as in an ocean or lake; or air, e.g. an atmosphere) in which temperature changes more dras ...
between cooler deep and warmer shallow or surface
seawater Seawater, or salt water, is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for al ...

seawater
s to run a
heat engine In thermodynamics and engineering, a heat engine is a system that converts heat to mechanical energy, which can then be used to do work (physics), mechanical work. It does this by bringing a working substance from a higher state temperature to ...

heat engine
and produce useful
work Work may refer to: * Work (human activity), intentional activity people perform to support themselves, others, or the community ** Manual labour, physical work done by humans ** House work, housework, or homemaking * Work (physics), the product of ...

work
, usually in the form of
electricity Electricity is the set of physical Physical may refer to: *Physical examination, a regular overall check-up with a doctor *Physical (album), ''Physical'' (album), a 1981 album by Olivia Newton-John **Physical (Olivia Newton-John song), "Physi ...

electricity
. OTEC can operate with a very high
capacity factor The net capacity factor is the unitless ratio of an actual electrical energy output over a given period of time to the maximum possible electrical energy output over that period. The capacity factor is defined for any electricity producing install ...
and so can operate in
base load The baseload (also base load) on a grid is the minimum level of demand on an electrical grid over a span of time, for example, one week. This demand can be met by unvarying power plants, dispatchable generation, or by a collection of smaller in ...
mode. The denser cold water masses, formed by ocean surface water interaction with cold atmosphere in quite specific areas of the North Atlantic and the
Southern Ocean The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the World Ocean, generally taken to be south of 60° S latitude and encircling Antarctica. As such, it is regarded as the second-smallest of t ...

Southern Ocean
, sink into the deep sea basins and spread in entire deep ocean by the
thermohaline circulation Thermohaline circulation (THC) is a part of the large-scale Ocean current, ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective ''thermohaline'' derives from ''wikt:thermo-, t ...

thermohaline circulation
.
Upwelling Upwelling is an oceanographic phenomenon that involves wind Wind is the flow of gases on a large scale. On the surface of the Earth, wind consists of the bulk movement of air. Winds are commonly classified by their scale (spatial), spatial ...

Upwelling
of cold water from the deep ocean is replenished by the
downwelling Downwelling is the process of accumulation and sinking of higher density material beneath lower density material, such as cold or saline water beneath warmer or fresher water or cold air beneath warm air. It is the ''sinking'' limb of a convectio ...

downwelling
of cold surface sea water. Among ocean energy sources, OTEC is one of the continuously available
renewable energy resource Renewable energy is useful energy that is collected from renewable resources, which are naturally replenished on a Orders of magnitude (time), human timescale, including carbon neutral sources like sunlight, wind power, wind, rain, tidal power ...
s that could contribute to base-load power supply.Lewis, Anthony, et al. IPCC: Special Report on Renewable Energy Sources and Climate Change Mitigation, 2011 The resource potential for OTEC is considered to be much larger than for other ocean energy forms. Up to 88,000
TWh The kilowatt-hour ( SI symbol: kW⋅h or kW h; commonly written as kWh) is a unit Unit may refer to: Arts and entertainment * UNIT Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fictio ...
/yr of power could be generated from OTEC without affecting the ocean's thermal structure. Systems may be either closed-cycle or open-cycle. Closed-cycle OTEC uses working fluids that are typically thought of as
refrigerant A refrigerant is a working fluid For fluid power, a working fluid is a gas or liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an app ...
s such as
ammonia Ammonia is a chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula NH3. A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct ch ...

ammonia
or
R-134a 1,1,1,2-Tetrafluoroethane (also known as norflurane (INN Inns are generally establishments or buildings where travelers can seek lodging, and usually, food and drink. Inns are typically located in the country or along a highway; before th ...

R-134a
. These fluids have low boiling points, and are therefore suitable for powering the system's generator to generate electricity. The most commonly used heat cycle for OTEC to date is the
Rankine cycle The Rankine cycle is an idealized thermodynamic cycle A thermodynamic cycle consists of a linked sequence of thermodynamic processes that involve transfer of heat and work into and out of the system, while varying pressure, temperature, and othe ...
, using a low-pressure turbine. Open-cycle engines use vapor from the
seawater Seawater, or salt water, is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for al ...

seawater
itself as the working fluid. OTEC can also supply quantities of cold water as a by-product. This can be used for air conditioning and refrigeration and the nutrient-rich deep ocean water can feed biological technologies. Another by-product is
fresh water Fresh water or freshwater is any naturally occurring liquid or frozen water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in ...

fresh water
distilled from the sea. OTEC theory was first developed in the 1880s and the first bench size demonstration model was constructed in 1926. Currently operating pilot-scale OTEC plants are located in Japan, overseen by
Saga University ; abbreviated as or , is a national universityA national university is generally a university created or managed by a government, but which may at the same time operate autonomously without direct control by the state. National University may re ...
, and Makai in Hawaii.


History

Attempts to develop and refine OTEC technology started in the 1880s. In 1881,
Jacques Arsene d'Arsonval Ancient and noble French family names, Jacq, Jacques, or James are believed to originate from the Middle Ages in the historic northwest Brittany Brittany (; french: link=no, Bretagne ; br, Breizh, or ; Gallo language, Gallo: ''Bertaèyn'' ) ...
, a French
physicist A physicist is a scientist A scientist is a person who conducts scientific research The scientific method is an Empirical evidence, empirical method of acquiring knowledge that has characterized the development of science since at leas ...

physicist
, proposed tapping the thermal energy of the ocean. D'Arsonval's student,
Georges Claude Georges Claude (24 September 187023 May 1960) was a French engineer and inventor. He is noted for his early work on the industrial liquefaction of air, for the invention and commercialization of neon lighting , London, 1962 Neon lighting cons ...
, built the first OTEC plant, in Matanzas, Cuba in 1930. The system generated 22 of
electricity Electricity is the set of physical Physical may refer to: *Physical examination, a regular overall check-up with a doctor *Physical (album), ''Physical'' (album), a 1981 album by Olivia Newton-John **Physical (Olivia Newton-John song), "Physi ...

electricity
with a low-
pressure Pressure (symbol: ''p'' or ''P'') is the force In physics, a force is an influence that can change the motion (physics), motion of an Physical object, object. A force can cause an object with mass to change its velocity (e.g. moving fr ...

pressure
turbine A turbine ( or ) (from the Greek , ''tyrbē'', or Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', ...

turbine
. The plant was later destroyed in a storm.Avery, William H. and Chih Wu. Renewable Energy From the Ocean: A Guide to OTEC. New York: Oxford University Press. 1994. In 1935, Claude constructed a plant aboard a 10,000-
ton The ton is a unit of measure A unit of measurement is a definite magnitude of a quantity Quantity is a property that can exist as a multitude or magnitude, which illustrate discontinuity and continuity. Quantities can be compared ...
cargo vessel moored off the coast of Brazil. Weather and waves destroyed it before it could generate net power. (Net power is the amount of power generated after subtracting power needed to run the system). In 1956, French scientists designed a 3 MW plant for
Abidjan Abidjan ( , ; N'Ko script, N’ko: ߊߓߌߖߊ߲߬) is the economic capital of Ivory Coast, Côte d'Ivoire and one of the most populous French-speaking cities in Africa. According to the 2014 census, Abidjan's population was 4.7 million, whi ...

Abidjan
,
Ivory Coast Ivory Coast, also known as Côte d'Ivoire, officially the Republic of Côte d'Ivoire, is a country located on the south coast of West Africa. Côte d'Ivoire's political capital is Yamoussoukro in the centre of the country, while its largest ...
. The plant was never completed, because new finds of large amounts of cheap petroleum made it uneconomical. In 1962, J. Hilbert Anderson and James H. Anderson, Jr. focused on increasing component efficiency. They patented their new "closed cycle" design in 1967. This design improved upon the original closed-cycle Rankine system, and included this in an outline for a plant that would produce power at lower cost than oil or coal. At the time, however, their research garnered little attention since coal and nuclear were considered the future of energy. Japan is a major contributor to the development of OTEC technology. Beginning in 1970 the
Tokyo Electric Power Company , also known as or TEPCO, is a Japanese electric utility holding company servicing Japan's Kantō region, Yamanashi Prefecture, and the eastern portion of Shizuoka Prefecture. This area includes Tokyo. Its headquarters are located in Uchisai ...
successfully built and deployed a 100 kW closed-cycle OTEC plant on the island of
Nauru Nauru ( or ; na, Naoero), officially the Republic of Nauru ( na, Repubrikin Naoero) and formerly known as Pleasant Island, is an island country An island country or an island nation is a country A country is a distinct territory, t ...

Nauru
. The plant became operational on 14 October 1981, producing about 120 kW of electricity; 90 kW was used to power the plant and the remaining electricity was used to power a school and other places. This set a world record for power output from an OTEC system where the power was sent to a real (as opposed to an experimental) power grid. 1981 also saw a major development in OTEC technology when Russian engineer, Dr. Alexander Kalina, used a mixture of ammonia and water to produce electricity. This new ammonia-water mixture greatly improved the efficiency of the power cycle. In 1994 Saga University designed and constructed a 4.5 kW plant for the purpose of testing a newly invented Uehara cycle, also named after its inventor Haruo Uehara. This cycle included absorption and extraction processes that allow this system to outperform the Kalina cycle by 1-2%. Currently, the Institute of Ocean Energy, Saga University, is the leader in OTEC power plant research and also focuses on many of the technology's secondary benefits. The 1970s saw an uptick in OTEC research and development during the post 1973 Arab-Israeli War, which caused oil prices to triple. The U.S. federal government poured $260 million into OTEC research after President Carter signed a law that committed the US to a production goal of 10,000 MW of electricity from OTEC systems by 1999. In 1974, The U.S. established the
Natural Energy Laboratory of Hawaii Authority The Natural Energy Laboratory of Hawaii Authority (''NELHA'') administers the Hawaii Ocean Science and Technology Park (HOST Park). ''NELHA'' was founded in 1974. At , HOST Park is perhaps the largest single green economic development project in t ...
(NELHA) at Keahole Point on the Kona coast of
Hawaii Hawaii ( ; haw, Hawaii or ) is a state State may refer to: Arts, entertainment, and media Literature * ''State Magazine'', a monthly magazine published by the U.S. Department of State * The State (newspaper), ''The State'' (newspape ...
. Hawaii is the best US OTEC location, due to its warm surface water, access to very deep, very cold water, and high electricity costs. The laboratory has become a leading test facility for OTEC technology. In the same year, Lockheed received a grant from the U.S. National Science Foundation to study OTEC. This eventually led to an effort by Lockheed, the US Navy, Makai Ocean Engineering, Dillingham Construction, and other firms to build the world's first and only net-power producing OTEC plant, dubbed "Mini-OTEC" For three months in 1979, a small amount of electricity was generated. A European initiative EUROCEAN - a privately funded joint venture of 9 European companies already active in offshore engineering - was active in promoting OTEC from 1979 to 1983. Initially a large scale offshore facility was studied. Later a 100 kW land based installation was studied combining land based OTEC with Desalination and Aquaculture nicknamed ODA. This was based on the results from a small scale aquaculture facility at the island of St Croix that used a deepwater supply line to feed the aquaculture basins. Also a shore based open cycle plant was investigated. The location of the case of study was the Dutch Kingdom related island Curaçao. Research related to making open-cycle OTEC a reality began earnestly in 1979 at the Solar Energy Research Institute (SERI) with funding from the US Department of Energy. Evaporators and suitably configured direct-contact condensers were developed and patented by SERI (see). An original design for a power-producing experiment, then called the 165-kW experiment was described by
Kreith Mutters is a municipality in the Innsbruck-Land The Bezirk Innsbruck-Land is an administrative district (''Bezirk The German term ''Bezirk'' (plural ''Bezirke'', derived from la, circulus, "circle") translated as "district A district is a ty ...
and Bharathan (, and) as the
Max Jakob Memorial AwardThe Max Jakob Memorial Award recognizes an 'eminent scholarly achievement and distinguished leadership' in the field of heat transfer. Awarded annually to a scholar by the American Society of Mechanical Engineers (ASME) and the American Institute of ...
Lecture. The initial design used two parallel axial turbines, using last stage rotors taken from large steam turbines. Later, a team led by Dr. Bharathan at the National Renewable Energy Laboratory (NREL) developed the initial conceptual design for up-dated 210 kW open-cycle OTEC experiment (). This design integrated all components of the cycle, namely, the evaporator, condenser and the turbine into one single vacuum vessel, with the turbine mounted on top to prevent any potential for water to reach it. The vessel was made of concrete as the first process vacuum vessel of its kind. Attempts to make all components using low-cost plastic material could not be fully achieved, as some conservatism was required for the turbine and the vacuum pumps developed as the first of their kind. Later Dr. Bharathan worked with a team of engineers at the Pacific Institute for High Technology Research (PICHTR) to further pursue this design through preliminary and final stages. It was renamed the Net Power Producing Experiment (NPPE) and was constructed at the Natural Energy Laboratory of Hawaii (NELH) by PICHTR by a team led by Chief Engineer Don Evans and the project was managed by Dr. Luis Vega. In 2002, India tested a 1 MW floating OTEC pilot plant near Tamil Nadu. The plant was ultimately unsuccessful due to a failure of the deep sea cold water pipe. Its government continues to sponsor research. In 2006, Makai Ocean Engineering was awarded a contract from the U.S.
Office of Naval Research The Office of Naval Research (ONR) is an organization within the United States Department of the Navy responsible for the science and technology programs of the U.S. Navy and United States Marine Corps, Marine Corps. Established by Congress in 19 ...
(ONR) to investigate the potential for OTEC to produce nationally significant quantities of hydrogen in at-sea floating plants located in warm, tropical waters. Realizing the need for larger partners to actually commercialize OTEC, Makai approached Lockheed Martin to renew their previous relationship and determine if the time was ready for OTEC. And so in 2007, Lockheed Martin resumed work in OTEC and became a subcontractor to Makai to support their SBIR, which was followed by other subsequent collaborations In March 2011, Ocean Thermal Energy Corporation signed an Energy Services Agreement (ESA) with the Baha Mar resort, Nassau, Bahamas, for the world's first and largest seawater air conditioning (SWAC) system. In June 2015, the project was put on pause while the resort resolved financial and ownership issues. In August 2016, it was announced that the issues had been resolved and that the resort would open in March 2017.http://otecorporation.com/2016/08/30/ocean-thermal-energy-corporation-reports-announcement-bahamian-government-remobilization-completion-opening-baha-mar-beach-resort/ It is expected that the SWAC system's construction will resume at that time. In July 2011, Makai Ocean Engineering completed the design and construction of an OTEC Heat Exchanger Test Facility at the Natural Energy Laboratory of Hawaii. The purpose of the facility is to arrive at an optimal design for OTEC heat exchangers, increasing performance and useful life while reducing cost (heat exchangers being the #1 cost driver for an OTEC plant). And in March 2013, Makai announced an award to install and operate a 100 kilowatt turbine on the OTEC Heat Exchanger Test Facility, and once again connect OTEC power to the grid. In July 2016, the Virgin Islands Public Services Commission approved Ocean Thermal Energy Corporation's application to become a Qualified Facility. The company is thus permitted to begin negotiations with the Virgin Islands Water and Power Authority (WAPA) for a Power Purchase Agreement (PPA) pertaining to an Ocean Thermal Energy Conversion (OTEC) plant on the island of St. Croix. This would be the world's first commercial OTEC plant.


Currently operating OTEC plants

In March 2013, Saga University with various Japanese industries completed the installation of a new OTEC plant. Okinawa Prefecture announced the start of the OTEC operation testing at Kume Island on April 15, 2013. The main aim is to prove the validity of computer models and demonstrate OTEC to the public. The testing and research will be conducted with the support of Saga University until the end of FY 2016. IHI Plant Construction Co. Ltd, Yokogawa Electric Corporation, and Xenesys Inc were entrusted with constructing the 100 kilowatt class plant within the grounds of the Okinawa Prefecture Deep Sea Water Research Center. The location was specifically chosen in order to utilize existing deep seawater and surface seawater intake pipes installed for the research center in 2000. The pipe is used for the intake of deep sea water for research, fishery, and agricultural use.The plant consists of two 50 kW units in double Rankine configuration. The OTEC facility and deep seawater research center are open to free public tours by appointment in English and Japanese. Currently, this is one of only two fully operational OTEC plants in the world. This plant operates continuously when specific tests are not underway. In 2011, Makai Ocean Engineering completed a heat exchanger test facility at NELHA. Used to test a variety of heat exchange technologies for use in OTEC, Makai has received funding to install a 105 kW turbine. Installation will make this facility the largest operational OTEC facility, though the record for largest power will remain with the Open Cycle plant also developed in Hawaii. In July 2014, DCNS group partnered with Akuo Energy announced NER 300 funding for their NEMO project. If successful, the 16MW gross 10MW net offshore plant will be the largest OTEC facility to date. DCNS plans to have NEMO operational by 2020. An ocean thermal energy conversion power plant built by Makai Ocean Engineering went operational in Hawaii in August 2015 . The governor of Hawaii,
David Ige David Yutaka Ige (; , ''Ige Yutaka'', born January 15, 1957) is an American politician serving as the eighth governor of Hawaii. A Democratic Party (United States), Democrat, he previously served in the Hawaii State Senate. In the 2014 Hawaii gub ...

David Ige
, "flipped the switch" to activate the plant. This is the first true closed-cycle ocean Thermal Energy Conversion (OTEC) plant to be connected to a U.S. electrical grid . It is a demo plant capable of generating 105 kilowatts, enough to power about 120 homes.


Thermodynamic efficiency

A
heat engine In thermodynamics and engineering, a heat engine is a system that converts heat to mechanical energy, which can then be used to do work (physics), mechanical work. It does this by bringing a working substance from a higher state temperature to ...

heat engine
gives greater efficiency when run with a large
temperature Temperature ( ) is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concept ...

temperature
difference. In the oceans the temperature difference between surface and deep water is greatest in the
tropics The tropics are the region of Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% ...

tropics
, although still a modest 20 to 25 °C. It is therefore in the tropics that OTEC offers the greatest possibilities. OTEC has the potential to offer global amounts of energy that are 10 to 100 times greater than other ocean energy options such as
wave power Wave power is the capture of energy of wind wave In fluid dynamics In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdiscipline ...

wave power
. OTEC plants can operate continuously providing a
base load The baseload (also base load) on a grid is the minimum level of demand on an electrical grid over a span of time, for example, one week. This demand can be met by unvarying power plants, dispatchable generation, or by a collection of smaller in ...
supply for an electrical power generation system. The main technical challenge of OTEC is to generate significant amounts of power efficiently from small temperature differences. It is still considered an
emerging technology ''Emerging'' is the title of the only album by the Phil Keaggy Band, released in 1977 on NewSong Records NewSong is an American contemporary Christian music group that was founded in 1981, at Morningside Baptist Church in Valdosta, Georgia. ...
. Early OTEC systems were 1 to 3 percent thermally efficient, well below the theoretical maximum 6 and 7 percent for this temperature difference. Modern designs allow performance approaching the theoretical maximum
Carnot efficiency Carnot's theorem, developed in 1824 by Nicolas Léonard Sadi Carnot, also called Carnot's rule, is a principle that specifies limits on the maximum efficiency any heat engine can obtain. The efficiency of a Carnot engine depends solely on the tempe ...

Carnot efficiency
.


Power cycle types

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. The primary approaches are active pumping and desalination. Desalinating seawater near the sea floor lowers its density, which causes it to rise to the surface. The alternative to costly pipes to bring condensing cold water to the surface is to pump vaporized low boiling point fluid into the depths to be condensed, thus reducing pumping volumes and reducing technical and environmental problems and lowering costs.


Closed

Closed-cycle systems use fluid with a low boiling point, such as
ammonia Ammonia is a chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula NH3. A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct ch ...

ammonia
(having a boiling point around -33 °C at atmospheric pressure), to power a
turbine A turbine ( or ) (from the Greek , ''tyrbē'', or Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', ...

turbine
to generate electricity. Warm surface
seawater Seawater, or salt water, is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for al ...

seawater
is pumped through a
heat exchanger A heat exchanger is a system used to transfer heat between two or more fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. Fluids are a Phase (m ...

heat exchanger
to vaporize the fluid. The expanding vapor turns the turbo-generator. Cold water, pumped through a second heat exchanger, condenses the vapor 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 off the Hawaiian coast and produced enough net electricity to illuminate the ship's light bulbs and run its computers and television.


Open

Open-cycle OTEC uses warm surface water directly to make electricity. The warm seawater is first pumped into a low-pressure container, which causes it to boil. In some schemes, the expanding
vapor In physics, a vapor (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of variety (linguistics), varieties of the English language native to the United St ...
drives a low-pressure turbine attached to an
electrical generator In electricity generation Electricity generation is the process of generating electric power from sources of primary energy. For electric utility, utilities in the electric power industry, it is the stage prior to its Electricity delivery, deliv ...
. The vapor, which has left its
salt Salt is a mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure fo ...

salt
and other contaminants in the low-pressure container, is pure fresh water. It is condensed into a liquid by exposure to cold temperatures from deep-ocean water. This method produces desalinized fresh water, suitable for
drinking water Drinking water, also known as potable water, is water that is safe to drinking, drink or use for food preparation. The amount of drinking water required to maintain good health varies, and depends on physical activity level, age, health-related ...

drinking water
,
irrigation Irrigation is the agricultural Agriculture is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary Image:Family watching television 1958.jpg, Exercise trends, Increases in seden ...

irrigation
or
aquaculture Aquaculture (less commonly spelled aquiculture), also known as aquafarming, is the farming of fish Fish are , , -bearing animals that lack with . Included in this definition are the living , s, and and as well as various extinct rel ...
. In other schemes, the rising vapor is used in a
gas lift Gas lift or bubble pumps use the artificial liftArtificial lift refers to the use of artificial means to increase the flow of liquids, such as crude oil or water, from a production well. Generally this is achieved by the use of a mechanical device ...
technique of lifting water to significant heights. Depending on the embodiment, such vapor lift pump techniques generate power from a either before or after the pump is used. In 1984, the ''Solar Energy Research Institute'' (now known as the
National Renewable Energy Laboratory The National Renewable Energy Laboratory (NREL) in the US specializes in the research and development Research and development (R&D, R+D), known in Europe as research and technological development (RTD), is the set of innovative activi ...

National Renewable Energy Laboratory
) developed a vertical-spout evaporator to convert warm seawater into low-pressure steam for open-cycle plants. Conversion efficiencies were as high as 97% for seawater-to-steam conversion (overall steam production would only be a few percent of the incoming water). In May 1993, an open-cycle OTEC plant at Keahole Point, Hawaii, produced close to 80 of electricity during a net power-producing experiment. This broke the record of 40 kW set by a Japanese system in 1982.


Hybrid

A hybrid cycle combines the features of the closed- and open-cycle systems. In a hybrid, warm seawater enters a vacuum chamber and is flash-evaporated, similar to the open-cycle evaporation process. The steam vaporizes the
ammonia Ammonia is a chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula NH3. A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct ch ...

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 Desalination is a process that takes away mineral components from saline water. More generally, desalination refers to the removal of salts and minerals from a target substance, as in Soil salinity control, soil desalination, which is an issue f ...
(see
heat pipe A heat pipe is a heat-transfer device that combines the principles of both thermal conductivity The thermal conductivity of a material is a measure of its ability to heat conduction, conduct heat. It is commonly denoted by k, \lambda, or \kappa. ...

heat pipe
).


Working fluids

A popular choice of working fluid is ammonia, which has superior transport properties, easy availability, and low cost. Ammonia, however, is toxic and flammable. Fluorinated carbons such as
CFC CFC, cfc, or Cfc may stand for: Science and technology * Chlorofluorocarbon, a class of chemical compounds * Cardiofaciocutaneous Syndrome, a rare and serious genetic disorder * Subpolar oceanic climate (''Cfc'' in the Köppen climate classifica ...
s and
HCFC Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are fully or partly halogenated paraffinParaffin may refer to: Substances * Paraffin wax, a white or colorless soft solid that is used as a lubricant and for other applications * L ...
s are not toxic or flammable, but they contribute to ozone layer depletion.
Hydrocarbon In organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, prop ...
s too are good candidates, but they are highly flammable; in addition, this would create competition for use of them directly as fuels. The power plant size is dependent upon the vapor pressure of the working fluid. With increasing vapor pressure, the size of the turbine and heat exchangers decreases while the wall thickness of the pipe and heat exchangers increase to endure high pressure especially on the evaporator side.


Land, shelf and floating sites

OTEC has the potential to produce gigawatts of electrical power, and in conjunction with
electrolysis In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in ...

electrolysis
, could produce enough hydrogen to completely replace all projected global fossil fuel consumption. Reducing costs remains an unsolved challenge, however. OTEC plants require a long, large diameter intake pipe, which is submerged a kilometer or more into the ocean's depths, to bring cold water to the surface.


Land-based

Land-based and near-shore facilities offer three main advantages over those located in deep water. Plants constructed on or near land do not require sophisticated mooring, lengthy power cables, or the more extensive maintenance associated with open-ocean environments. They can be installed in sheltered areas so that they are relatively safe from storms and heavy seas. Electricity, desalinated water, and cold, nutrient-rich seawater could be transmitted from near-shore facilities via trestle bridges or causeways. In addition, land-based or near-shore sites allow plants to operate with related industries such as
mariculture Fish cages containing salmon in Sound of Arisaig, Loch Ailort, Scotland. Mariculture is a specialized branch of aquaculture (which includes freshwater aquaculture) involving the cultivation of marine organisms for food and other products in the op ...
or those that require desalinated water. Favored locations include those with narrow shelves (volcanic islands), steep (15-20 degrees) offshore slopes, and relatively smooth sea floors. These sites minimize the length of the intake pipe. A land-based plant could be built well inland from the shore, offering more protection from storms, or on the beach, where the pipes would be shorter. In either case, easy access for construction and operation helps lower costs. Land-based or near-shore sites can also support mariculture or chilled water agriculture. Tanks or lagoons built on shore allow workers to monitor and control miniature marine environments. Mariculture products can be delivered to market via standard transport. One disadvantage of land-based facilities arises from the turbulent wave action in the
surf zone As ocean surface waves come closer to shore they breaking wave, break, forming the foamy, bubbly surface called Surfing, surf. The region of breaking waves defines the surf zone. After breaking in the surf zone, the waves (now reduced in height) co ...
. OTEC discharge pipes should be placed in protective trenches to prevent subjecting them to extreme stress during storms and prolonged periods of heavy seas. Also, the mixed discharge of cold and warm seawater may need to be carried several hundred meters offshore to reach the proper depth before it is released, requiring additional expense in construction and maintenance. One way that OTEC systems can avoid some of the problems and expenses of operating in a surf zone is by building them just offshore in waters ranging from 10 to 30 meters deep (Ocean Thermal Corporation 1984). This type of plant would use shorter (and therefore less costly) intake and discharge pipes, which would avoid the dangers of turbulent surf. The plant itself, however, would require protection from the marine environment, such as breakwaters and erosion-resistant foundations, and the plant output would need to be transmitted to shore.


Shelf based

To avoid the turbulent surf zone as well as to move closer to the cold-water resource, OTEC plants can be mounted to the continental shelf at depths up to . A shelf-mounted plant could be towed to the site and affixed to the sea bottom. This type of construction is already used for offshore oil rigs. The complexities of operating an OTEC plant in deeper water may make them more expensive than land-based approaches. Problems include the stress of open-ocean conditions and more difficult product delivery. Addressing strong ocean currents and large waves adds engineering and construction expense. Platforms require extensive pilings to maintain a stable base. Power delivery can require long underwater cables to reach land. For these reasons, shelf-mounted plants are less attractive.


Floating

Floating OTEC facilities operate off-shore. Although potentially optimal for large systems, floating facilities present several difficulties. The difficulty of mooring plants in very deep water complicates power delivery. Cables attached to floating platforms are more susceptible to damage, especially during storms. Cables at depths greater than 1000 meters are difficult to maintain and repair. Riser cables, which connect the sea bed and the plant, need to be constructed to resist entanglement. As with shelf-mounted plants, floating plants need a stable base for continuous operation. Major storms and heavy seas can break the vertically suspended cold-water pipe and interrupt warm water intake as well. To help prevent these problems, pipes can be made of flexible polyethylene attached to the bottom of the platform and gimballed with joints or collars. Pipes may need to be uncoupled from the plant to prevent storm damage. As an alternative to a warm-water pipe, surface water can be drawn directly into the platform; however, it is necessary to prevent the intake flow from being damaged or interrupted during violent motions caused by heavy seas. Connecting a floating plant to power delivery cables requires the plant to remain relatively stationary. Mooring is an acceptable method, but current mooring technology is limited to depths of about . Even at shallower depths, the cost of mooring may be prohibitive.


Political concerns

Because OTEC facilities are more-or-less stationary surface platforms, their exact location and legal status may be affected by the
United Nations Convention on the Law of the Sea The United Nations Convention on the Law of the Sea (UNCLOS), also called the Law of the Sea Convention or the Law of the Sea Treaty, is an international agreement A treaty is a formal legally binding written agreement between actors in intern ...

United Nations Convention on the Law of the Sea
treaty (UNCLOS). This treaty grants coastal nations 12- and zones of varying legal authority from land, creating potential conflicts and regulatory barriers. OTEC plants and similar structures would be considered
artificial islands An artificial island or man-made island is an island upright=1.15, Great_Britain.html"_;"title="Ireland_(left)_and_Great_Britain">Ireland_(left)_and_Great_Britain_(right),_are_large_islands_of_north-west_Europe image:Small_Island_in_Lower ...
under the treaty, giving them no independent legal status. OTEC plants could be perceived as either a threat or potential partner to
fisheries Fishery is the enterprise of raising or harvesting fish and other aquatic life. Commercial fisheries include wild fisheries and Fish farming, fish farms, both in fresh water (about 10% of all catch) and the oceans (about 90%). About 500 million pe ...
or to seabed mining operations controlled by the
International Seabed Authority The International Seabed Authority (ISA) (french: Autorité internationale des fonds marins) is an intergovernmental body based in Kingston, Jamaica, that was established to organize, regulate and control all mineral-related activities in the ...
.


Cost and economics

Because OTEC systems have not yet been widely deployed, cost estimates are uncertain. A 2010 study by University of Hawaii estimated the
cost of electricity Electricity pricing (also referred to as electricity tariffs or the price of electricity) can vary widely by country or by locality within a country. Electricity prices are dependent on many factors, such as the price of power generation, governme ...

cost of electricity
for OTEC to 94.0 cent (US) per
kilowatt hour The kilowatt-hour ( SI symbol: kW⋅h or kW h; commonly written as kWh) is a unit Unit may refer to: Arts and entertainment * UNIT Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fictio ...
(kWh) for a 1.4 MW plant, 44.0 cent per kWh for a 10 MW plant and 18.0 cent per kWh for a 100 MW plant. A 2015 report by the organization Ocean Energy Systems under the
International Energy Agency The International Energy Agency (IEA; french: Agence internationale de l'énergie) is a Paris Paris () is the Capital city, capital and List of communes in France with over 20,000 inhabitants, most populous city of France, with an estima ...
gives an estimate of about 20 cent per kWh for 100 MW plants. Another study estimates power generation costs as low as 7 cent per kWh. Comparing to other energy sources, a 2019 study by Lazard estimated the unsubsidized cost of electricity to 3.2 to 4.2 cent per kWh for
Solar PV A photovoltaic system, also PV system or solar power system, is a power system designed to supply usable solar power Solar power is the energy transformation, conversion of energy from sunlight into electricity, either directly using photov ...
at utility scale and 2.8 to 5.4 cent per kWh for
wind power Wind power or wind energy is the use of wind turbine A wind turbine is a device that converts Religious conversion is the adoption of a set of beliefs identified with one particular religious denomination A religious denomination ...

wind power
. A report published by IRENA in 2014 claimed that commercial use of OTEC technology can be scaled in a variety of ways. “...small-scale OTEC plants can be made to accommodate the electricity production of small communities (5 000-50 000 residents), but would require the production of valuable by-products – like fresh water or cooling – to be economically viable”. Larger scaled OTEC plants would have a much higher overhead and installation costs. Beneficial factors that should be taken into account include OTEC's lack of waste products and fuel consumption, the area in which it is available (often within 20° of the equator), the geopolitical effects of
petroleum Petroleum, also known as crude oil and oil, is a naturally occurring, yellowish-black liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isoc ...

petroleum
dependence, compatibility with alternate forms of ocean power such as wave energy, tidal energy and
methane hydrate . Methane clathrate (CH4·5.75H2O) or (4CH4·23H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound A clathrate is a chemical substance A chemical substance i ...

methane hydrate
s, and supplemental uses for the seawater.


Some proposed projects

OTEC projects under consideration include a small plant for the
U.S. Navy ), (unofficial)."''Non sibi sed patriae''" ( en, "Not for self but for country") (unofficial). , colors = Blue and gold  , colors_label = Colors , march = " Anchors Aweigh" ...
base on the British overseas territory island of
Diego Garcia Diego Garcia is an island of the British Indian Ocean Territory, an overseas territory of the United Kingdom. It is a militarised atoll An atoll (), sometimes known as a coral atoll, is a ring-shaped coral reef A coral reef is an und ...
in the
Indian Ocean The Indian Ocean is the third-largest of the world's five ocean The ocean (also the or the world ocean) is the body of that covers approximately 70.8% of the surface of and contains 97% of . Another definition is "any of the large ...

Indian Ocean
. Ocean Thermal Energy Corporation (formerly OCEES International, Inc.) is working with the U.S. Navy on a design for a proposed 13-MW OTEC plant, to replace the current diesel generators. The OTEC plant would also provide 1.25 million gallons per day of potable water. This project is currently waiting for changes in US military contract policies. OTE has proposed building a 10-MW OTEC plant on
Guam Guam (; ch, Guåhan ) is an organized, unincorporated territory of the United States in the Micronesia Micronesia (, ; from grc, μικρός ''mikrós'' "small" and ''nêsos'' "island") is a subregion of Oceania, consisting of thousa ...

Guam
.


Bahamas


Ocean Thermal Energy Corporation (OTE)
currently has plans to install two 10 MW OTEC plants in the US Virgin Islands and a 5-10 MW OTEC facility in The Bahamas. OTE has also designed the world's larges
Seawater Air Conditioning (SWAC)
plant for a resort in The Bahamas, which will use cold deep seawater as a method of air-conditioning. In mid-2015, the 95%-complete project was temporarily put on hold while the resort resolved financial and ownership issues. On August 22, 2016, the government of the Bahamas announced that a new agreement had been signed under which the Baha Mar resort will be completed. On September 27, 2016, Bahamian Prime Minister Perry Christie announced that construction had resumed on Baha Mar, and that the resort was slated to open in March 2017. This is on hold, and may never resume.


Hawaii

Lockheed Martin Lockheed Martin Corporation is an American aerospace Aerospace is a term used to collectively refer to the and . Aerospace activity is very diverse, with a multitude of commercial, industrial and military applications. consists of and . ...

Lockheed Martin
's Alternative Energy Development team has partnered with Makai Ocean Engineering to complete the final design phase of a 10-MW closed cycle OTEC pilot system which planned to become operational in
Hawaii Hawaii ( ; haw, Hawaii or ) is a state State may refer to: Arts, entertainment, and media Literature * ''State Magazine'', a monthly magazine published by the U.S. Department of State * The State (newspaper), ''The State'' (newspape ...

Hawaii
in the 2012-2013 time frame. This system was designed to expand to 100-MW commercial systems in the near future. In November, 2010 the U.S.
Naval Facilities Engineering Command The Naval Facilities Engineering Systems Command (NAVFAC) is the United States Navy ), (unofficial)."''Non sibi sed patriae''" ( en, "Not for self but for country") (unofficial). , colors = Blue and gold  , color ...
(NAVFAC) awarded Lockheed Martin a US$4.4 million contract modification to develop critical system components and designs for the plant, adding to the 2009 $8.1 million contract and two
Department of EnergyA Ministry of Energy or Department of Energy is a government department in some countries that typically oversees the production of fuel and electricity; in the United States, however, it manages nuclear weapons development and conducts energy-relate ...
grants totaling over $1 million in 2008 and March 2010. A small but operational ocean thermal energy conversion (OTEC) plant was inaugurated in Hawaii in August 2015. The opening of the research and development 100-kilowatt facility marked the first time a closed-cycle OTEC plant was connected to the U.S. grid.


Hainan

On April 13, 2013, Lockheed contracted with the Reignwood Group to build a 10 megawatt plant off the coast of southern China to provide power for a planned resort on
Hainan Hainan (, ; ) is the smallest and southernmost province A province is almost always an administrative division Administrative division, administrative unitArticle 3(1). , country subdivision, administrative region, subnational enti ...

Hainan
island. A plant of that size would power several thousand homes. The Reignwood Group acquired Opus Offshore in 2011 which forms its Reignwood Ocean Engineering division which also is engaged in development of
deepwater drilling Deepwater drilling, or Deep well drilling, is the process of creating holes by drilling rig for oil mining in the deep sea. There are approximately 3400 deepwater wells in the Gulf of Mexico The Gulf of Mexico ( es, Golfo de México) is an oc ...
.


Japan

Currently the only continuously operating OTEC system is located in Okinawa Prefecture, Japan. The Governmental support, local community support, and advanced research carried out by Saga University were key for the contractors, IHI Plant Construction Co. Ltd, Yokogawa Electric Corporation, and Xenesys Inc, to succeed with this project. Work is being conducted to develop a 1MW facility on Kume Island requiring new pipelines. In July 2014, more than 50 members formed the Global Ocean reSource and Energy Association
GOSEA
an international organization formed to promote the development of the Kumejima Model and work towards the installation of larger deep seawater pipelines and a 1MW OTEC Facility. The companies involved in the current OTEC projects, along with other interested parties have developed plans for offshore OTEC systems as well. - For more details, see "Currently Operating OTEC Plants" above.


United States Virgin Islands

On March 5, 2014, Ocean Thermal Energy Corporation (OTEC) and the 30th Legislature of the United States Virgin Islands (USVI) signed a Memorandum of Understanding to move forward with a study to evaluate the feasibility and potential benefits to the USVI of installing on-shore Ocean Thermal Energy Conversion (OTEC) renewable energy power plants and Seawater Air Conditioning (SWAC) facilities. The benefits to be assessed in the USVI study include both the baseload (24/7) clean electricity generated by OTEC, as well as the various related products associated with OTEC and SWAC, including abundant fresh drinking water, energy-saving air conditioning, sustainable aquaculture and mariculture, and agricultural enhancement projects for the Islands of St Thomas and St Croix. On July 18, 2016, OTE's application to be a Qualifying Facility was approved by the Virgin Islands Public Services Commission. OTE also received permission to begin negotiating contracts associated with this project.


Kiribati

South Korea's Research Institute of Ships and Ocean Engineering (KRISO) received Approval in Principal from Bureau Veritas for their 1MW offshore OTEC design. No timeline was given for the project which will be located 6 km offshore of the Republic of Kiribati.


Martinique

Akuo Energy and DCNS were awarded NER300 funding on July 8, 2014 for their NEMO (New Energy for Martinique and Overseas) project which is expected to be a 10.7MW-net offshore facility completed in 2020. The award to help with development totaled 72 million Euro.


Maldives

On February 16, 2018, Global OTEC Resources announced plans to build a 150 kW plant in the Maldives, designed bespoke for hotels and resorts. "All these resorts draw their power from diesel generators. Moreover, some individual resorts consume 7,000 litres of diesel a day to meet demands which equates to over 6,000 tonnes of CO2 annually," said Director Dan Grech. The EU awarded a grant and Global OTEC resources launched a crowdfunding campaign for the rest.


Related activities

OTEC has uses other than power production.


Desalination

Desalinated water can be produced in open- or hybrid-cycle plants using
surface condenser A surface condenser is a commonly used term for a water-cooled shell and tube heat exchanger installed to condense exhaust steam from a steam turbine in thermal power stations. These Condenser (heat transfer), condensers are heat exchangers which c ...

surface condenser
s to turn evaporated seawater into potable water. System analysis indicates that a 2-megawatt plant could produce about of desalinated water each day. Another system patented by Richard Bailey creates condensate water by regulating deep ocean water flow through surface condensers correlating with fluctuating dew-point temperatures. This condensation system uses no incremental energy and has no moving parts. On March 22, 2015, Saga University opened a Flash-type desalination demonstration facility on Kumejima. This satellite of their Institute of Ocean Energy uses post-OTEC deep seawater from the Okinawa OTEC Demonstration Facility and raw surface seawater to produce desalinated water. Air is extracted from the closed system with a vacuum pump. When raw sea water is pumped into the flash chamber it boils, allowing pure steam to rise and the salt and remaining seawater to be removed. The steam is returned to liquid in a heat exchanger with cold post-OTEC deep seawater. The desalinated water can be used in hydrogen production or drinking water (if minerals are added). The NELHA plant established in 1993 produced an average of 7,000 gallons of freshwater per day. KOYO USA was established in 2002 to capitalize on this new economic opportunity. KOYO bottles the water produced by the NELHA plant in Hawaii. With the capacity to produce one million bottles of water every day, KOYO is now Hawaii's biggest exporter with $140 million in sales.


Air conditioning

The cold seawater made available by an OTEC system creates an opportunity to provide large amounts of cooling to industries and homes near the plant. The water can be used in chilled-water coils to provide air-conditioning for buildings. It is estimated that a pipe in diameter can deliver 4,700 gallons of water per minute. Water at could provide more than enough air-conditioning for a large building. Operating 8,000 hours per year in lieu of electrical conditioning selling for 5-10¢ per kilowatt-hour, it would save $200,000-$400,000 in energy bills annually. The
InterContinental InterContinental Hotels & Resorts is a British luxury hotel brand created in 1946 by Pan American World Airways, Pan Am's founder Juan Trippe. It has been part of IHG Hotels & Resorts, InterContinental Hotels Group since 1998. As of November 202 ...
Resort and Thalasso-Spa on the island of
Bora Bora Bora Bora (French: ''Bora-Bora''; Tahitian: ''Pora Pora'') is an island group in the Leeward Islands The Leeward Islands are a group of islands situated where the northeastern meets the western Atlantic Ocean. Starting with the east of , ...

Bora Bora
uses an SWAC system to air-condition its buildings. The system passes seawater through a heat exchanger where it cools freshwater in a closed loop system. This freshwater is then pumped to buildings and directly cools the air. In 2010, Copenhagen Energy opened a district cooling plant in Copenhagen, Denmark. The plant delivers cold seawater to commercial and industrial buildings, and has reduced electricity consumption by 80 percent. Ocean Thermal Energy Corporation (OTE) has designed a 9800-ton SDC system for a vacation resort in The Bahamas.


Chilled-soil agriculture

OTEC technology supports chilled-soil agriculture. When cold seawater flows through underground pipes, it chills the surrounding soil. The temperature difference between roots in the cool soil and leaves in the warm air allows plants that evolved in
temperate climate In geography Geography (from Ancient Greek, Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and phenomena of the Earth and Solar System, plan ...
s to be grown in the
subtropics The subtropical zones or subtropics are geographical zone, geographical and Köppen climate classification, climate zones located to the north and south of the tropics, Torrid Zone. Geographically part of the Geographical zone#Temperate zones, ...

subtropics
. Dr. John P. Craven, Dr. Jack Davidson and Richard Bailey patented this process and demonstrated it at a research facility at the Natural Energy Laboratory of Hawaii Authority (NELHA). The research facility demonstrated that more than 100 different crops can be grown using this system. Many normally could not survive in Hawaii or at Keahole Point. Japan has also been researching agricultural uses of Deep Sea Water since 2000 at the Okinawa Deep Sea Water Research Institute on Kume Island. The Kume Island facilities use regular water cooled by Deep Sea Water in a heat exchanger run through pipes in the ground to cool soil. Their techniques have developed an important resource for the island community as they now produce spinach, a winter vegetable, commercially year round. An expansion of the deep seawater agriculture facility was completed by Kumejima Town next to the OTEC Demonstration Facility in 2014. The new facility is for researching the economic practicality of chilled-soil agriculture on a larger scale.


Aquaculture

Aquaculture Aquaculture (less commonly spelled aquiculture), also known as aquafarming, is the farming of fish Fish are , , -bearing animals that lack with . Included in this definition are the living , s, and and as well as various extinct rel ...
is the best-known byproduct, because it reduces 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 sea animals, such as
salmon Salmon is the common name for several species of ray-finned fish Actinopterygii ( New Latin ('having rays') + Greek ( 'wing, fins')), members of which are known as ray-finned fishes, is a clade A clade (; from grc, , ''klados'', ...

salmon
and
lobster Lobsters are a family In , family (from la, familia) is a of people related either by (by recognized birth) or (by marriage or other relationship). The purpose of families is to maintain the well-being of its members and of society ...

lobster
, thrive in this nutrient-rich, deep, seawater.
Microalgae Microalgae or microphytes are microscopic The microscopic scale (from , ''mikrós'', "small" and σκοπέω, ''skopéō'' "look") is the scale of objects and events smaller than those that can easily be seen by the naked eye, requiring a len ...
such as '' Spirulina'', a health food supplement, also can be cultivated. Deep-ocean water can be combined with surface water to deliver water at an optimal temperature. Non-native species such as salmon, lobster,
abalone Abalone ( or ; via Spanish ', from Rumsen ''aulón'') is a common name Common may refer to: Places * Common, a townland in County Tyrone, Northern Ireland * Boston Common Boston Common (also known as the Common) is a central public park ...

abalone
,
trout Trout are species of freshwater fish belonging to the genera ''Oncorhynchus'', ''Salmo'' and ''Salvelinus'', all of the subfamily (biology), subfamily Salmoninae of the family Salmonidae. The word ''trout'' is also used as part of the name o ...

trout
,
oyster Oyster is the common name Common may refer to: Places * Common, a townland in County Tyrone, Northern Ireland * Boston Common Boston Common (also known as the Common) is a central public park in downtown Boston, Massachusetts. It is someti ...

oyster
s, and
clam Clam is a common name Common may refer to: Places * Common, a townland in County Tyrone, Northern Ireland * Boston Common Boston Common (also known as the Common) is a central public park in downtown Boston, Massachusetts. It is sometime ...

clam
s can be raised in pools supplied by OTEC-pumped water. This extends the variety of fresh seafood products available for nearby markets. Such low-cost refrigeration can be used to maintain the quality of harvested fish, which deteriorate quickly in warm tropical regions. In Kona, Hawaii, aquaculture companies working with NELHA generate about $40 million annually, a significant portion of Hawaii's GDP. The NELHA plant established in 1993 produced an average of 7,000 gallons of freshwater per day. KOYO USA was established in 2002 to capitalize on this new economic opportunity. KOYO bottles the water produced by the NELHA plant in Hawaii. With the capacity to produce one million bottles of water every day, KOYO is now Hawaii's biggest exporter with $140 million in sales.


Hydrogen production

Hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

Hydrogen
can be produced via
electrolysis In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in ...

electrolysis
using OTEC electricity. Generated steam with electrolyte compounds added to improve efficiency is a relatively pure medium for hydrogen production. OTEC can be scaled to generate large quantities of hydrogen. The main challenge is cost relative to other energy sources and fuels.


Mineral extraction

The ocean contains 57
trace element __NOTOC__ A trace element, also called minor element, is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and beha ...
s in salts and other forms and dissolved in solution. In the past, most economic analyses concluded that mining the ocean for trace elements would be unprofitable, in part because of the energy required to pump the water. Mining generally targets minerals that occur in high concentrations, and can be extracted easily, such as
magnesium Magnesium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

magnesium
. With OTEC plants supplying water, the only cost is for extraction. The Japanese investigated the possibility of extracting
uranium Uranium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

uranium
and found developments in other technologies (especially materials sciences) were improving the prospects.


Climate control

Ocean thermal gradient A thermocline (also known as the thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water column, water, as in an ocean or lake; or air, e.g. an atmosphere) in which temperature changes more dras ...
can be used to enhance rainfall and moderate the high ambient summer temperatures in tropics to benefit enormously the mankind and the
flora and fauna In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), taxonomy into groups such as Multic ...

flora and fauna
. When
sea surface temperature Sea surface temperature (SST), or ocean surface temperature, is the water temperature close to the ocean's surface. The exact meaning of ''surface'' varies according to the measurement method used, but it is between and below the sea surfac ...
s are relatively high on an area, lower atmospheric pressure area is formed compared to atmospheric pressure prevailing on the nearby land mass inducing winds from the landmass towards the ocean. Oceanward winds are dry and warm which would not contribute to good rainfall on the landmass compared to landward moist winds. For adequate rainfall and comfortable summer ambient temperatures (below 35 °C) on the landmass, it is preferred to have landward moist winds from the ocean. Creating high pressure zones by artificial
upwelling Upwelling is an oceanographic phenomenon that involves wind Wind is the flow of gases on a large scale. On the surface of the Earth, wind consists of the bulk movement of air. Winds are commonly classified by their scale (spatial), spatial ...

upwelling
on sea area selectively can also be used to deflect / guide the normal monsoon global winds towards the landmass. Artificial upwelling of nutrient-rich deep ocean water to the surface also enhances fisheries growth in areas with tropical and temperate weather. It would also lead to enhanced
carbon sequestration Carbon sequestration or carbon dioxide removal File:Tree planting closeup.jpg, Planting trees is a means of carbon dioxide removal. Carbon dioxide removal (CDR), also known as greenhouse gas removal, is a process in which carbon dioxide g ...

carbon sequestration
by the oceans from improved
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Co ...

algae
growth and from the extra snow fall mitigating
sea level rise Tide gauge measurements show that the current global sea level rise began at the start of the 20th century. Between 1900 and 2017, the globally averaged sea level Mean sea level (MSL) (often shortened to sea level) is an average In colloqu ...

sea level rise
or
global warming Contemporary climate change includes both the global warming caused by humans, and its impacts on Earth's weather patterns. There have been previous periods of climate change, but the current changes are more rapid than any known even ...

global warming
process.
Tropical cyclones A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation Atmospheric circulation is the large-scale movement of Atmosphere of Earth, air and together with oc ...
also do not pass through the high pressure zones as they intensify by gaining energy from the warm surface waters of the sea. The cold deep sea water (<10 °C) is pumped to the sea surface area to suppress the sea surface temperature (>26 °C) by artificial means using electricity produced by mega scale
floating wind turbine A floating wind turbine is an mounted on a floating structure that allows the turbine to generate in water depths where fixed-foundation turbines are not feasible. Floating wind farms have the potential to significantly increase the sea area a ...
plants on the deep sea. The lower sea water surface temperature would enhance the local ambient pressure so that atmospheric landward winds are created. For
upwelling Upwelling is an oceanographic phenomenon that involves wind Wind is the flow of gases on a large scale. On the surface of the Earth, wind consists of the bulk movement of air. Winds are commonly classified by their scale (spatial), spatial ...

upwelling
the cold sea water, a stationary hydraulically driven propeller (≈50 m diameter) is located on the
deep sea The deep sea or deep layer is the lowest layer in the ocean The ocean (also the sea The sea, connected as the world ocean or simply the ocean The ocean (also the sea or the world ocean) is the body of salt water which co ...

deep sea
floor at 500 to 1000 m depth with a flexible
draft tube Draft Tube is a diverging tube fitted at the exit of runner of turbine and used to utilize the kinetic energy available with water at the exit of runner. '. This draft tube at the end of the turbine increases the pressure of the exiting flui ...

draft tube
extending up to the sea surface. The draft tube is anchored to the sea bed at its bottom side and top side to floating Float (nautical), pontoons at the sea surface. The flexible draft tube would not collapse as its inside pressure is more compared to outside pressure when the colder water is pumped to the sea surface. Middle east, north east Africa, Indian subcontinent and Australia can get relief from hot and dry weather in summer season, also prone to erratic rainfall, by pumping deep sea water to the sea surface from the Persian gulf, Red sea, Indian Ocean and Pacific Ocean respectively.


Thermodynamics

A rigorous treatment of OTEC reveals that a 20 °C temperature difference will provide as much energy as a hydroelectric plant with 34 m head for the same volume of water flow. The low temperature difference means that water volumes must be very large to extract useful amounts of heat. A 100MW power plant would be expected to pump on the order of 12 million gallons (44,400 tonnes) per minute. For comparison, pumps must move a mass of water greater than the weight of the ''German battleship Bismarck, battleship Bismarck'', which weighed 41,700 tonnes, every minute. This makes pumping a substantial parasitic loss, parasitic drain on energy production in OTEC systems, with one Lockheed design consuming 19.55 MW in pumping costs for every 49.8 MW net electricity generated. For OTEC schemes using heat exchangers, to handle this volume of water the exchangers need to be enormous compared to those used in conventional thermal power generation plants, making them one of the most critical components due to their impact on overall efficiency. A 100 MW OTEC power plant would require 200 exchangers each larger than a 20-foot shipping container making them the single most expensive component.


Variation of ocean temperature with depth

The total insolation received by the oceans (covering 70% of the earth's surface, with clearness index of 0.5 and average energy retention of 15%) is: We can use Beer–Lambert law, Beer–Lambert–Bouguer's law to quantify the solar energy absorption by water, :-\frac=\mu I where, ''y'' is the depth of water, ''I'' is intensity and ''μ'' is the absorption coefficient. Solving the above differential equation, : I(y)=I_\exp(-\mu y) \, The absorption coefficient ''μ'' may range from 0.05 m−1 for very clear fresh water to 0.5 m−1 for very salty water. Since the intensity exponential decay, falls exponentially with depth ''y'', heat absorption is concentrated at the top layers. Typically in the tropics, surface temperature values are in excess of , while at , the temperature is about . The warmer (and hence lighter) waters at the surface means there are no convection current, thermal convection currents. Due to the small temperature gradients, heat transfer by heat conduction, conduction is too low to equalize the temperatures. The ocean is thus both a practically infinite heat source and a practically infinite heat sink. This temperature difference varies with latitude and season, with the maximum in tropical, subtropical and Equatorial climate, equatorial waters. Hence the tropics are generally the best OTEC locations.


Open/Claude cycle

In this scheme, warm surface water at around enters an evaporator at pressure slightly below the saturation pressures causing it to vaporize. : H_=H_ \, Where ''H'' is enthalpy of liquid water at the inlet temperature, ''T''. This temporarily superheating, superheated water undergoes volume boiling as opposed to pool boiling in conventional boilers where the heating surface is in contact. Thus the water partially flashes to steam with two-phase equilibrium prevailing. Suppose that the pressure inside the evaporator is maintained at the saturation pressure, ''T''. :H_=H_=H_+x_H_ \, Here, ''x'' is the fraction of water by mass that vaporizes. The warm water mass flow rate per unit
turbine A turbine ( or ) (from the Greek , ''tyrbē'', or Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', ...

turbine
mass flow rate is 1/''x''. The low pressure in the evaporator is maintained by a vacuum pump that also removes the dissolved non-condensable gases from the evaporator. The evaporator now contains a mixture of water and steam of very low vapor quality (steam content). The steam is separated from the water as saturated vapor. The remaining water is saturated and is discharged to the ocean in the open cycle. The steam is a low pressure/high specific volume working fluid. It expands in a special low pressure turbine. :H_=H_ \, Here, ''H'' corresponds to ''T''. For an ideal isentropic process, isentropic (reversible adiabatic) turbine, :s_=s_=s_+x_s_ \, The above equation corresponds to the temperature at the exhaust of the turbine, ''T''. ''x'' is the mass fraction of vapor at state 5. The enthalpy at ''T'' is, : H_=H_+x_H_ \, This enthalpy is lower. The adiabatic reversible turbine work = ''H''-''H''. Actual turbine work :H_=H_-\ \mathrm\ \mathrm The condenser temperature and pressure are lower. Since the turbine exhaust is to be discharged back into the ocean, a direct contact condenser is used to mix the exhaust with cold water, which results in a near-saturated water. That water is now discharged back to the ocean. ''H''=''H'', at ''T''. ''T'' is the temperature of the exhaust mixed with cold sea water, as the vapor content now is negligible, :H_\approx H_\,\ at\ T_ \, The temperature differences between stages include that between warm surface water and working steam, that between exhaust steam and cooling water, and that between cooling water reaching the condenser and deep water. These represent external Reversible process (thermodynamics), irreversibilities that reduce the overall temperature difference. The cold water flow rate per unit turbine mass flow rate, :\dot \, Turbine mass flow rate, \dot=\frac Warm water mass flow rate, \dot=\dot \, Cold water mass flow rate \dot \,


Closed Anderson cycle

As developed starting in the 1960s by J. Hilbert Anderson of Sea Solar Power, Inc., in this cycle, ''Q'' is the heat transferred in the evaporator from the warm sea water to the working fluid. The working fluid exits the evaporator as a gas near its dew point. The high-pressure, high-temperature gas then is expanded in the turbine to yield turbine work, ''W''. The working fluid is slightly superheated at the turbine exit and the turbine typically has an efficiency of 90% based on reversible, adiabatic expansion. From the turbine exit, the working fluid enters the condenser where it rejects heat, ''-Q'', to the cold sea water. The condensate is then compressed to the highest pressure in the cycle, requiring condensate pump work, ''W''. Thus, the Anderson closed cycle is a Rankine-type cycle similar to the conventional power plant steam cycle except that in the Anderson cycle the working fluid is never superheated more than a few degrees Fahrenheit. Owing to viscosity effects, working fluid pressure drops in both the evaporator and the condenser. This pressure drop, which depends on the types of heat exchangers used, must be considered in final design calculations but is ignored here to simplify the analysis. Thus, the parasitic condensate pump work, ''W'', computed here will be lower than if the heat exchanger pressure drop was included. The major additional parasitic energy requirements in the OTEC plant are the cold water pump work, ''W'', and the warm water pump work, ''W''. Denoting all other parasitic energy requirements by ''W'', the net work from the OTEC plant, ''W'' is : W_=W_-W_-W_-W_-W_ \, The thermodynamic cycle undergone by the working fluid can be analyzed without detailed consideration of the parasitic energy requirements. From the first law of thermodynamics, the energy balance for the working fluid as the system is : W_=Q_-Q_ \, where is the net work for the thermodynamic cycle. For the idealized case in which there is no working fluid pressure drop in the heat exchangers, : Q_=\int_T_ds \, and : Q_=\int_T_ds \, so that the net thermodynamic cycle work becomes : W_=\int_T_ds-\int_T_ds \, Subcooled liquid enters the evaporator. Due to the heat exchange with warm sea water, evaporation takes place and usually superheated vapor leaves the evaporator. This vapor drives the turbine and the 2-phase mixture enters the condenser. Usually, the subcooled liquid leaves the condenser and finally, this liquid is pumped to the evaporator completing a cycle.


Environmental impact

Carbon dioxide dissolved in deep cold and high pressure layers is brought up to the surface and released as the water warms. Mixing of deep ocean water with shallower water brings up nutrients and makes them available to shallow water life. This may be an advantage for aquaculture of commercially important species, but may also unbalance the ecological system around the power plant. OTEC plants use very large flows of warm surface seawater and cold deep seawater to generate constant renewable power. The deep seawater is oxygen deficient and generally 20-40 times more nutrient rich (in nitrate and nitrite) than shallow seawater. When these plumes are mixed, they are slightly denser than the ambient seawater. Though no large scale physical environmental testing of OTEC has been done, computer models have been developed to simulate the effect of OTEC plants.


Hydrodynamic modeling

In 2010, a computer model was developed to simulate the physical oceanographic effects of one or several 100 megawatt OTEC plant(s). The model suggests that OTEC plants can be configured such that the plant can conduct continuous operations, with resulting temperature and nutrient variations that are within naturally occurring levels. Studies to date suggest that by discharging the OTEC flows downwards at a depth below 70 meters, the dilution is adequate and nutrient enrichment is small enough so that 100-megawatt OTEC plants could be operated in a sustainable manner on a continuous basis.


Biological modeling

The nutrients from an OTEC discharge could potentially cause increased biological activity if they accumulate in large quantities in the photic zone. In 2011 a biological component was added to the hydrodynamic computer model to simulate the biological response to plumes from 100 megawatt OTEC plants. In all cases modeled (discharge at 70 meters depth or more), no unnatural variations occurs in the upper 40 meters of the ocean's surface. The picoplankton response in the 110 - 70 meter depth layer is approximately a 10-25% increase, which is well within naturally occurring variability. The nanoplankton response is negligible. The enhanced productivity of diatoms (microplankton) is small. The subtle phytoplankton increase of the baseline OTEC plant suggests that higher-order biochemical effects will be very small.


Studies

A previous Final Environmental Impact Statement (EIS) for the United States' NOAA from 1981 is available, but needs to be brought up to current oceanographic and engineering standards. Studies have been done to propose the best environmental baseline monitoring practices, focusing on a set of ten chemical oceanographic parameters relevant to OTEC. Most recently, NOAA held an OTEC Workshop in 2010 and 2012 seeking to assess the physical, chemical, and biological impacts and risks, and identify information gaps or needs. The Tethys (database), Tethys database provides access to scientific literature and general information on the potential environmental effects of OTEC.


Technical difficulties


Dissolved gases

The performance of direct contact heat exchangers operating at typical OTEC boundary conditions is important to the Claude cycle. Many early Claude cycle designs used a surface condenser since their performance was well understood. However, direct contact condensers offer significant disadvantages. As cold water rises in the intake pipe, the pressure decreases to the point where gas begins to evolve. If a significant amount of gas comes out of solution, placing a gas trap before the direct contact heat exchangers may be justified. Experiments simulating conditions in the warm water intake pipe indicated about 30% of the dissolved gas evolves in the top of the tube. The trade-off between wikt:dearation, pre-dearation of the seawater and expulsion of non-condensable gases from the condenser is dependent on the gas evolution dynamics, deaerator efficiency, head loss, vent compressor efficiency and parasitic power. Experimental results indicate vertical spout condensers perform some 30% better than falling jet types.


Microbial fouling

Because raw seawater must pass through the heat exchanger, care must be taken to maintain good thermal conductivity. Biofouling layers as thin as can degrade heat exchanger performance by as much as 50%. A 1977 study in which mock heat exchangers were exposed to seawater for ten weeks concluded that although the level of microbial fouling was low, the thermal conductivity of the system was significantly impaired. The apparent discrepancy between the level of fouling and the heat transfer impairment is the result of a thin layer of water trapped by the microbial growth on the surface of the heat exchanger. Another study concluded that fouling degrades performance over time, and determined that although regular brushing was able to remove most of the microbial layer, over time a tougher layer formed that could not be removed through simple brushing. The study passed sponge rubber balls through the system. It concluded that although the ball treatment decreased the fouling rate it was not enough to completely halt growth and brushing was occasionally necessary to restore capacity. The microbes regrew more quickly later in the experiment (i.e. brushing became necessary more often) replicating the results of a previous study. The increased growth rate after subsequent cleanings appears to result from selection pressure on the microbial colony. Continuous use of 1 hour per day and intermittent periods of free fouling and then Water chlorination, chlorination periods (again 1 hour per day) were studied. Chlorination slowed but did not stop microbial growth; however chlorination levels of .1 mg per liter for 1 hour per day may prove effective for long term operation of a plant. The study concluded that although microbial fouling was an issue for the warm surface water heat exchanger, the cold water heat exchanger suffered little or no biofouling and only minimal inorganic fouling. Besides water temperature, microbial fouling also depends on nutrient levels, with growth occurring faster in nutrient rich water. The fouling rate also depends on the material used to construct the heat exchanger. Aluminium tubing slows the growth of microbial life, although the aluminium oxide, oxide layer which forms on the inside of the pipes complicates cleaning and leads to larger efficiency losses. In contrast, titanium tubing allows biofouling to occur faster but cleaning is more effective than with aluminium.


Sealing

The evaporator, turbine, and condenser operate in partial vacuum ranging from 3% to 1% of atmospheric pressure. The system must be carefully sealed to prevent in-leakage of atmospheric air that can degrade or shut down operation. In closed-cycle OTEC, the specific volume of low-pressure steam is very large compared to that of the pressurized working fluid. Components must have large flow areas to ensure steam velocities do not attain excessively high values.


Parasitic power consumption by exhaust compressor

An approach for reducing the exhaust compressor Parasitic loss, parasitic power loss is as follows. After most of the steam has been condensed by spout condensers, the non-condensible gas steam mixture is passed through a counter current region which increases the gas-steam reaction by a factor of five. The result is an 80% reduction in the exhaust pumping power requirements.


Cold air/warm water conversion

In winter in coastal Arctic locations, the temperature difference between the seawater and ambient air can be as high as 40 °C (72 °F). Closed-cycle systems could exploit the air-water temperature difference. Eliminating seawater extraction pipes might make a system based on this concept less expensive than OTEC. This technology is due to H. Barjot, who suggested butane as cryogen, because of its boiling point of and its non-solubility in water. Assuming a realistic level of efficiency of 4%, calculations show that the amount of energy generated with one cubic meter water at a temperature of in a place with an air temperature of equals the amount of energy generated by letting this cubic meter water run through a hydroelectric plant of 4000 feet (1,200 m) height. Barjot Polar Power Plants could be located on islands in the polar region or designed as swimming barges or platforms attached to the ice cap. The weather station Myggbuka at Greenlands east coast for example, which is only 2,100 km away from Glasgow, detects monthly mean temperatures below during 6 winter months in the year. This technology can also be used to create artificial ice caps or glaciers on Antarctica valleys located near the sea coast. Thus sea level rise due to carbon emissions can be mitigated and also the generated electricity is used for Bitcoin network, Bitcoin currency mining and the heat liberated in the process is utilized for HVAC, space heating requirements. In this article "'
Proposal_of_an_aeraulic_thermosiphon
, it is envisaged different possibilities to generate electric energy from the difference of temperatures between the ambient atmospheric air and the surface sea water. Finally, it is proposed a machine based on an aeraulic thermosiphon, installed between the seashore and a high relief (300 m at least). This electric generation would be done without insoluble constraints on the refrigerant, but, unfortunately, at high cost.


Application of the thermoelectric effect

In 1979 SERI proposed using the Seebeck effect to produce power with a total conversion efficiency of 2%. In 2014 Liping Liu, Associate Professor at Rutgers University, envisioned an OTEC system that utilises the solid state thermoelectric effect rather than the fluid cycles traditionally used.


See also

* Deep water source cooling * Heat engine * Floating wind turbine * Offshore construction, Ocean engineering * Osmotic power * Seawater air conditioning * Thermogalvanic cell


References


Sources

*


External links

*http://www.otecorporation.com *http://www.bluerise.nl/
OTEC News - OTEC News website
*[https://web.archive.org/web/20090130004257/http://www.oceanenergycouncil.com/index.php/Ocean-Thermal-OTEC/OTEC.html Ocean Energy Council: How does OTEC work?]
nrel.gov - what is OTEC?
*Wired Magazine'
interview with John Piña Craven on the future of OTEC2007 edition of the Survey of Energy Resources produced by the World Energy CouncilThe Green Ocean Project - OTEC Library




* [http://www.power-technology.com/projects/hainan-ocean-thermal-energy-conversion-otec-power-plant/ Hainan Ocean Thermal Energy Conversion (OTEC) Power Plant], China
20,000 megawatts under the sea: Oceanic steam engines. New Scientist, March 1, 2014. Preview only.
*http://otecfoundation.org/ *http://otecnews.com/ *https://web.archive.org/web/20140321052029/http://www.ioes.saga-u.ac.jp/en/about_lab.html (Saga University OTEC Research Facility) *http://www.OTEC.ws *http://www.lockheedmartin.com/us/products/otec.html *http://www.makai.com/e-otec.htm *http://www.ocees.com *http://www.otecokinawa.com (Okinawa OTEC Project) {{DEFAULTSORT:Ocean Thermal Energy Conversion Energy conversion Renewable energy technology Marine energy Power station technology Oceanographical terminology