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 ...

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 ...

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 ...

, 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 ...

. 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 ...

, 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 ...

.
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 ...

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 ...

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 ...

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 ...

. 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 ...

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 ...

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 ...

, 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 ...

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 ...

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'', ...

. 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 ...

,
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 ...

## 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 ...

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 ...

## 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 ...

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 ...

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 ...

. 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 ...

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 ...

. 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 ...

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 ...

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 ...

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 ...

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 ...

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 ...

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 ...

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 ...

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, :$I\left(y\right)=I_\exp\left(-\mu y\right) \,$ 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'', ...

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.

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

# Sources

*

*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