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[[File:Temperaturunterschiede Ozeane.png|thumb|right|350px|Temperature differences between the surface and 1000m depth in the oceans]]
'''Ocean thermal energy conversion''' ('''OTEC''') uses the temperature difference between cooler deep and warmer shallow or surface [[ocean]] waters to run a [[heat engine]] and produce useful work, usually in the form of electricity.  OTEC is a base load electricity generation system, i.e. 24hrs/day all year long. However, the temperature differential is small and this impacts the economic feasibility of ocean thermal energy for electricity generation.

Systems may be either closed-cycle or open-cycle.  Closed-cycle engines use working fluids that are typically thought of as [[refrigerant]]s such as [[ammonia]] or [[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]] using a low-pressure turbine. Open-cycle engines use vapour from the [[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]] distilled from the sea.<ref name="PopSci95"/>

OTEC theory was first developed in the 1880s and the first bench size demonstration model was constructed in 1926. Currently the world's only operating OTEC plant is in Japan, overseen by Saga University.

[[File:Otec produkty-2 (English).png|thumb|right|350px|OTEC diagram and applications]]

==History==
Attempts to develop and refine OTEC technology started in the 1880s. In 1881, [[Jacques Arsene d'Arsonval]], a French [[Physics|physicist]], proposed tapping the thermal energy of the ocean. D'Arsonval's student, [[Georges Claude]], built the first OTEC plant, in Matanzas, [[Cuba]] in 1930.<ref>{{Cite journal
  | last = Chiles
  | first = James
  | author-link =
  | title = The Other Renewable Energy
  | journal = Invention and Technology
  | volume = 23
  | issue = 4
  | pages = 24–35
  | date = Winter 2009
  | year =
  | url =
  | doi =
  | id = }}
</ref><ref>[http://books.google.com/books?id=qOIDAAAAMBAJ&pg=PA881&dq=Popular+Science+1930+plane+%22Popular+Mechanics%22&hl=en&ei=_7BlTsWeBYTWgQf9mIiLCg&sa=X&oi=book_result&ct=result&resnum=10&sqi=2&ved=0CE8Q6AEwCQ#v=onepage&q&f=true "Power from the Sea" ''Popular Mechanics'', December 1930, pp 881-882] detail article and photos of Cuban power plant</ref> The system generated 22 [[Kilowatt|kW]] of [[electricity]] with a low-[[pressure]] [[turbine]].<ref name=dow-book>{{Cite book | title = Deep Ocean Water as Our Next Natural Resource | last = Takahashi | first = Masayuki Mac | coauthors = Translated by: Kitazawa, Kazuhiro and Snowden, Paul | year = 2000 | publisher = Terra Scientific Publishing Company | location = Tokyo, Japan | isbn = 4-88704-125-X | url = http://www.terrapub.co.jp/e-library/dow/index.html | origyear = 1991}}</ref> The plant was later destroyed in a storm.<ref>Avery, William H. and Chih Wu. Renewable Energy From the Ocean: A Guide to OTEC.	New York: Oxford University Press. 1994.</ref>

In 1935, Claude constructed a plant aboard a 10,000-[[ton]] cargo vessel moored off the coast of Brazil. Weather and waves destroyed it before it could generate net power.<ref name=dow-book/> (Net power is the amount of power generated after subtracting power needed to run the system).

[[File:OTEC in Hawaii.jpg|thumb|200px|right|View of a land based OTEC facility at [[Keahole Point]] on the [[Kona District, Hawaii|Kona]] coast of [[Hawaii]] ([[United States Department of Energy]])]]

In 1956, French scientists designed a 3 [[Megawatt|MW]] plant for [[Abidjan]], [[Côte d'Ivoire]]. The plant was never completed, because new finds of large amounts of cheap petroleum made it uneconomical.<ref name=dow-book/>

In 1962, [[J. Hilbert Anderson]] and [[James H. Anderson, Jr.]] focused on increasing component efficiency. They patented their new "closed cycle" design in 1967.<ref name=closed-cycle-patent>{{Ref patent | country = US | number = 3312054 | status = patent | title = Sea Water Power Plant | gdate = 1967-04-04 | fdate = 1966-09-27 | invent1 = J.H. Anderson}}</ref> 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.<ref>Avery, William H. and Chih Wu. Renewable Energy From the Ocean: A Guide to OTEC.	New York: Oxford University Press. 1994.</ref>

Japan is a major contributor to the development of OTEC technology.<ref name=osti-assessment>{{Cite journal | title = An Assessment of Research and Development Leadership in Ocean Energy Technologies |date=April 1994| last = Bruch | first = Vicki L. | publisher = Sandia National Laboratories: Energy Policy and Planning Department | volume = SAND93-3946 | url =http://www.osti.gov/bridge/servlets/purl/10154003-z9rVWD/native/10154003.PDF|format=PDF}}</ref> Beginning in 1970 the [[Tokyo Electric Power Company]] successfully built and deployed a 100&nbsp;kW closed-cycle OTEC plant on the island of [[Nauru]].<ref name=osti-assessment/>  The plant became operational on 14 October 1981, producing about 120&nbsp;kW of electricity; 90&nbsp;kW was used to power the plant and the remaining electricity was used to power a school and other places.<ref name=dow-book/> 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.<ref name=nauru-outline>{{Cite journal | title =  Outline of the 100 kW OTEC Pilot Plant in the Republic of Nauru | last = Mitsui | first = T. |coauthors = Ito, F.; Seya, Y.; Nakamoto, Y. |date=September 1983 | journal = IEEE Transactions on Power Apparatus and Systems | volume = PAS-102 | issue = 9 | pages = 3167–3171 |url = http://library.greenocean.org/oteclibrary/otecdesigns/OTEC-nauru.pdf/view | doi =  10.1109/TPAS.1983.318124}}</ref> 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%.<ref>Finney, Karen Anne. “Ocean Thermal Energy Conversion”. Guelph Engineering Journal. 2008.</ref> 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 electricy from OTEC systems by 1999.<ref>{{cite news|last=Daly|first=John|title=Hawaii About to Crack Ocean Thermal Energy Conversion Roadblocks?|url=http://oilprice.com/Alternative-Energy/Renewable-Energy/Hawaii-About-To-Crack-Ocean-Thermal-Energy-Conversion-Roadblocks.html|accessdate=28 March 2013|newspaper=OilPrice.com|date=December 5, 2011}}</ref>

In 1974, The U.S. established the [[Natural Energy Laboratory of Hawaii]] Authority (NELHA) at [[Keahole Point]] on the [[Kona District, Hawaii|Kona coast]] of [[Hawaii (island)|Hawaii]]. 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.<ref name=elec-costs>{{cite web | title =  Average Retail Price of Electricity to Ultimate Customers by End-Use Sector, by State | publisher = [[Energy Information Administration]] |date = September 2007| url = http://www.eia.doe.gov/cneaf/electricity/epm/table5_6_a.html}}</ref> 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"<ref name="lockheed1">{{cite web|last=L. Meyer, D. Cooper, R. Varley|title=Are We There Yet? A Developer's Roadmap to OTEC Commercialization|url=http://hinmrec.hnei.hawaii.edu/wp-content/uploads/2010/01/OTEC-Road-to-Commercialization-September-2011-_-LM.pdf|work=Hawaii National Marine Renewable Energy Center|accessdate=28 March 2013}}</ref> For three months in 1979, a small amount of electricity was generated.

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.<ref name=":0">Avery, William H. and Chih Wu. Renewable Energy From the Ocean: A Guide to OTEC. New York: Oxford UniversityPress. 1994.</ref> I[[Government of India|ts government]] continues to sponsor research.<ref>{{cite web | url = http://www.makai.com/p-otec.htm | title = Deep Pipelines for Ocean Thermal Energy Conversion | accessdate=2009-02-16}}</ref>

In 1993, a 250kW was designed by Dr. Luis Vega and constructed by the National Energy Laboratory of Hawaii (then called NELH) and the Pacific International Center for High Technology Research. The plant operated for six years and produced a record amount of 255kW of energy.<ref name=":0" />

In 2006, Makai Ocean Engineering was awarded a contract from the U.S. [[Office of Naval Research]] (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<ref name="lockheed1" />

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).<ref>{{cite web|last=otecfoundation|title=Makai Ocean Engineering's Heat Exchanger Test Facility opened|url=http://www.otecnews.org/2011/11/makai-ocean-engineerings-heat-exchanger-test-facility-opened/|work=www.otecnews.org|accessdate=28 March 2013}}</ref> 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.<ref>{{cite web|last=Staff|title=Makai Ocean Engineering working with Navy on Big Island OTEC project|url=http://www.bizjournals.com/pacific/news/2013/03/19/makai-ocean-engineering-working-with.html|accessdate=28 March 2013}}</ref><ref>{{cite web|last=IDEA|title=Makai Ocean Engineering to add 100kW turbine generator to Kona, Hawaii OTEC test facility|url=http://www.districtenergy.org/blog/2013/03/19/makai-ocean-engineering-to-add-100kw-turbine-generator-to-kona-hawaii-otec-test-facility/?utm_source=rss&utm_medium=rss&utm_campaign=makai-ocean-engineering-to-add-100kw-turbine-generator-to-kona-hawaii-otec-test-facility|work=International District Energy Association}}</ref>

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 largest SDC plant which was planned for a resorted in The Bahamas, which will use cold deep seawater as a method of air-conditioning.<ref name=":1">“Projects”. Ocean Thermal EnergyPLC. Web. 24 June 2013. available at: http://www.oteplc.com/strategic_development.html</ref> Unfortunately, this project was postponed due to schedule delays.<ref>http://news.caribseek.com/index.php/caribbean-islands-news/bahamas-news/the-nassau-guardian-news/item/20288-eia-puts-otecproject-on-rocks</ref>

In March 2013, Saga University with various Japanese industries completed the installation of a new OTEC plant.<ref>http://www3.nhk.or.jp/nhkworld/english/radio/asx/201307301430.asx</ref> Currently, this is the only operating OTEC plant in the world.

==Thermodynamic efficiency==
A [[heat engine]] gives greater efficiency when run with a large [[temperature]] difference. In the oceans the temperature difference between surface and deep water is greatest in the [[tropics]], although still a modest 20 to 25&nbsp;°C. It is therefore in the tropics that OTEC offers the greatest possibilities.<ref name="PopSci95"/> 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]]{{Citation needed|date=December 2010}}. OTEC plants can operate continuously providing a [[base load]] supply for an electrical power generation system.<ref name="PopSci95">{{cite journal|last=DiChristina|first=Mariette|date=May 1995|title=Sea Power|journal=Popular Science|pages=70–73|url=http://books.google.co.uk/books?id=4jETasQk7aoC&pg=PA73&dq=otec&hl=en&ei=qFXATtD1GIamhAfNs53IBA&sa=X&oi=book_result&ct=result&resnum=1&ved=0CDgQ6AEwAA#v=onepage&q=otec&f=false|accessdate=Nov 2011}}</ref>

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]]. Early OTEC systems were 1 to 3 percent [[thermal efficiency|thermally efficient]], well below the theoretical maximum 6 and 7 percent for this temperature difference.<ref name=microbial-countermeasures/>  Modern designs allow performance approaching the theoretical maximum [[Carnot efficiency]] and the largest built in 1999 by the USA generated 250&nbsp;kW.

==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.<ref name=desalination-pump-patent>{{Ref patent | country = US | number = 4311012 | status = patent | title = Method and apparatus for transferring cold seawater upward from the lower depths of the ocean to improve the efficiency of ocean thermal energy conversion systems | gdate = 1982-01-19 | fdate = 1980-02-19 | invent1 = Warren T. Finley | class = F03G 704}}</ref>

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.{{Citation needed|date=December 2010}}

===Closed===
[[File:Otec Closed Diagram in English.JPG|right|thumb|350px|Diagram of a closed cycle OTEC plant]]
Closed-cycle systems use fluid with a low boiling point, such as [[ammonia]] (having a boiling point around -33°C at atmospheric pressure), to power a [[turbine]] to generate electricity. Warm surface [[seawater]] is pumped through a [[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.<ref name=mini-otec>{{Cite journal | title = Review of mini-OTEC performance | first = L.C. | last = Trimble | coauthors = Owens, W.L. | year = 1980 | journal = Energy to the 21st century; Proceedings of the Fifteenth Intersociety Energy Conversion Engineering Conference | volume = 2 | pages = 1331–1338 | bibcode = 1980iece....2.1331T}}</ref> The mini OTEC vessel was moored {{convert|1.5|mi|km}} off the Hawaiian coast and produced enough net electricity to illuminate the ship's light bulbs and run its computers and television.

===Open===
[[File:Otec Open Diagram in English.JPG|thumb|right|350px|Diagram of an open cycle OTEC plant]]
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 [[steam]] drives a low-pressure turbine attached to an [[electrical generator]]. The steam, which has left its [[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 [[desalinization|desalinized]] fresh water, suitable for [[drinking water]], [[irrigation]] or [[aquaculture]].<ref name="vega05">{{cite web|url=http://www.otecnews.org/articles/vega/05_open_cycle.html|title=Open Cycle OTEC|last=Vega|first=L.A.|year=1999|work=OTEC News|publisher=The GreenOcean Project|accessdate=4 February 2011}}</ref>

In other schemes, the rising steam is used in a [[gas lift]] technique of lifting water to significant heights.  Depending on the embodiment, such [[mist lift|steam lift]] pump techniques generate power from a [[water turbine|hydroelectric turbine]] either before or after the pump is used.<ref name=jsee>{{citation|journal=Journal of Solar Energy Engineering |last1=Lee |first1= C. K. B. |last2= Ridgway |first2= Stuart |title=Vapor/Droplet Coupling and the Mist Flow (OTEC) Cycle |volume=105 |date= May, 1983 |url=http://library.greenocean.org/oteclibrary/otecdesigns/mistliftotec/vapor_mistlift_otec.pdf}}</ref>

In 1984, the ''Solar Energy Research Institute'' (now the [[National Renewable Energy Laboratory]]) developed a vertical-spout evaporator to convert warm seawater into low-pressure steam for open-cycle plants. Conversion efficiencies were as high as 97% for seawater-to-steam conversion (overall efficiency using a vertical-spout evaporator would still only be a few per cent). In May 1993, an open-cycle OTEC plant at Keahole Point, Hawaii, produced 50,000 [[watt]]s of electricity during a net power-producing experiment.<ref name=nrel-otec-achievements>{{cite web | title = Achievements in OTEC Technology | publisher = [[National Renewable Energy Laboratory]] | url = http://www.nrel.gov/otec/achievements.html}}</ref> This broke the record of 40&nbsp;kW set by a Japanese system in 1982.<ref name=nrel-otec-achievements/>

===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]] 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 [[Desalination|desalinated water]] (see [[heat pipe]]).{{Citation needed|date=December 2010}}

===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 [[Chlorofluorocarbon|CFC]]s and [[HCFC]]s are not toxic or flammable, but they contribute to ozone layer depletion. Hydrocarbons 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]], could produce enough hydrogen to completely replace all projected global fossil fuel consumption.{{Citation needed|date=January 2009}}  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.

[[File:otec2.jpg|thumb|right|200px|Left: Pipes used for OTEC. <br /> Right: Floating OTEC plant constructed in [[India]] in 2000]]

===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]] 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]]. 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.<ref name="Land Based Design and Location">{{cite web|title=Design and Location|url=http://www.nrel.gov/otec/design_location.html|work=What is Ocean Thermal Energy Conversion?|publisher=National Renewable Energy Laboratory|accessdate=22 January 2012}}</ref>

===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 {{convert|100|m|ft|sp=us}}. 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.<ref name="Land Based Design and Location" /> {{Citation needed|date=December 2010}}

===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.<ref name="Land Based Design and Location" /> {{Citation needed|date=December 2010}}

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.<ref name="Land Based Design and Location" /> {{Citation needed|date=December 2010}}

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 {{convert|2000|m|ft|sp=us}}. Even at shallower depths, the cost of mooring may be prohibitive.{{Citation needed|date=December 2010}}

==Some proposed projects==
OTEC projects under consideration include a small plant for the [[U.S. Navy]] base on the [[United Kingdom|British]] overseas territory island of [[Diego Garcia]] in the [[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{{clarify|date=March 2012}}<!--USgal or Imp gal?--> per day of potable water.<!-- are our figures messed up, or is the current plant not fully utilized?[[User:Pakaran|Pak]][[User talk:Pakaran|aran]] 21:04, 18 Jun 2005 (UTC)--> This project is currently waiting for changes in US military contract policies. OTE has proposed building a 10-MW OTEC plant on [[Guam]].

===Hawaii===
[[Lockheed Martin]]'s Alternative Energy Development team has partnered with Makai Ocean Engineering<ref>{{cite news
|title=Lockheed Martin awarded another $4.4M for OTEC work in Hawaii
|date=November 22, 2010
|url=http://www.bizjournals.com/pacific/news/2010/11/22/lockheed-martin-awarded-another-44m.html
|accessdate=December, 2010}}</ref>
to complete the final design phase of a 10-MW closed cycle OTEC pilot system which will become operational in [[Hawaii]] in the 2012-2013 time frame. This system is being designed to expand to 100-MW commercial systems in the near future. In November, 2010 the U.S. [[Naval Facilities Engineering Command]] (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 [[United States Department of Energy|Department of Energy]] grants totaling over $1 million in 2008 and March 2010.<ref>{{cite news
|title=More funds for Hawaii's Ocean Thermal Energy Conversion plant
|first=Ben |last=Coxworth
|date=November 26, 2010
|url=http://www.gizmag.com/lockheed-martin-otec-hawaii/17081/
|accessdate=December, 2010}}</ref>
This effort was canceled when the Navy determined that the system was not viable.

===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]] island.<ref name=E&E5113>{{cite news|title=CLEAN TECHNOLOGY: U.S.-designed no-emission power plant will debut off China's coast|url=http://www.eenews.net/public/climatewire/2013/05/01/1|accessdate=May 2, 2013|newspaper=ClimateWire E&E Publishing|date=May 1, 2013|author=Daniel Cusick}}</ref> A plant of that size would power several thousand homes.<ref name=Reuters41613>{{cite news|title=Lockheed to build 10-megawatt thermal power plant off southern China|url=http://www.reuters.com/article/2013/04/16/us-usa-energy-lockheed-idUSBRE93F0IH20130416|accessdate=April 17, 2013|newspaper=Reuters|date=April 16, 2013|author=David Alexander}}</ref><ref name=LMNews>{{cite web|title=Tapping Into the Ocean's Power: Lockheed Martin signs agreement for largest ever OTEC plant|url=http://www.lockheedmartin.com/us/mst/features/2013/130416-tapping-into-the-oceans-power.html|publisher=Lockheed Martin|accessdate=April 17, 2013}}</ref> The Reignwood Group acquired Opus Offshore in 2011 which forms its Reignwood Ocean Engineering division which also is engaged in development of [[deepwater drilling]].<ref name=Opus>{{cite web|title=Reignwood Ocean Engineering|url=http://www.reignwood.com/OceanEngineering.asp|publisher=Reignwood Group|accessdate=April 17, 2013}}</ref>

===Japan===
Okinawa Prefecture has announced the start of the OTEC operation test at Kume Island on April 15, 2013. The main aim is to examine the expected fluctuation of electricity supply caused by changes in weather, season, and sea temperature. The testing and research will be conducted with the support of Saga University until the end of 2014. IHI Plant Construction Co. Ltd, Yokogawa Electric Corporation, and Xenesys Inc were entrusted to construct the 50 kilowattplant in the territory of the Okinawa Prefectural Deep Sea Water Research Center. The plant installation was finished in March and the first trial run was held on the 30th of March. The location was specifically chosen in order to utilize the existing intake pipe developed by the research center. The pipe had been used in the past for the intake of deep sea water for fishery and agricultural use.<ref>http://www.otecnews.org/2013/05/otec-testing-in-okinawa/</ref>



==Related activities==
OTEC has uses other than power production.

===Desalination===
Desalinated water can be produced in open- or hybrid-cycle plants using [[surface condenser]]s to turn evaporated seawater into potable water. System analysis indicates that a 2-megawatt plant could produce about {{convert|4300|m3}} of desalinated water each day.<ref>Block and Lalenzuela 1985</ref> Another system patented by Richard Bailey creates condensate water by regulating deep ocean water flow through surface condensers correlating with fluctuating dew-point temperatures.<ref>{{patent|US|7726138}}</ref> This condensation system uses no incremental energy and has no moving parts.

===Air conditioning===
The {{convert|41|F}} 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 {{convert|1|ft|m}} in diameter can deliver 4,700 gallons of water per minute. Water at {{convert|43|F}} 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.<ref>[[United States Department of Energy|U.S. Department of Energy]], 1989</ref>

The [[InterContinental]] Resort and Thalasso-Spa on the island of [[Bora Bora]] uses an OTEC system to air-condition its buildings.<ref>{{cite web|url=http://www.youtube.com/watch?v=zTGvPrrkVAA|title=YouTube video on the OTEC air-conditioning system used at the InterContinental Resort and Thalasso-Spa on the island of Bora Bora|accessdate=2007-05-28}}</ref>  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.<ref>Green Tech. “Copenhagen’s SeawaterCooling Delivers Energy And Carbon Savings”. 24 October 2012. Forbes.  </ref>  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]]s to be grown in the [[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).<ref>{{patent|us|7069689}}</ref> 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.{{Citation needed|date=December 2010}}

===Aquaculture===
[[Aquaculture]] 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 delicacies, such as [[salmon]] and [[lobster]], thrive in this nutrient-rich, deep, seawater. [[Microalgae]] such as ''[[Spirulina (dietary supplement)|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]], [[trout]], [[oyster]]s, and [[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.<ref>Ponia, Ben. “Aquaculture Updates inthe Northern Pacific: Hawaii, Federated Sates of Mirconesia, Palau and Saipan”. SPCFisheries Newsletter. July 2006. Web. 25 June 2013. available at: http://www.spc.int/DigitalLibrary/Doc/FAME/InfoBull/FishNews/118/FishNews11 8_58_Ponia.pdf.</ref> 

The NELHA plant established in 1993 produced anaverage 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.<ref>Thomas, Daniel. “A Brief History of OTEC Research at NELHA”.  NELHA. August 1999. Web. 25 June 2013. available at: http://library.greenocean.org/oteclibrary/otecpapers/OTEC%20History.pdf</ref>  

===Hydrogen production===
[[Hydrogen]] can be produced via [[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]]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]]. With OTEC plants supplying water, the only cost is for extraction.{{Citation needed|date=December 2010}}
The Japanese investigated the possibility of extracting [[uranium]] and found developments in other technologies (especially materials sciences) were improving the prospects.{{Citation needed|date=December 2010}}

==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]] treaty (UNCLOS). This treaty grants coastal nations 3-, 12-, and {{convert|200|mi|km|adj=on}} zones of varying legal authority from land, creating potential conflicts and regulatory barriers. OTEC plants and similar structures would be considered [[artificial islands]] under the treaty, giving them no independent legal status. OTEC plants could be perceived as either a threat or potential partner to [[fisheries management|fisheries]] or to seabed mining operations controlled by the [[International Seabed Authority]].

==Cost and economics==
For OTEC to be viable as a power source, the technology must have tax and subsidy treatment similar to competing energy sources. Because OTEC systems have not yet been widely deployed, cost estimates are uncertain. One study estimates power generation costs as low as US $0.07 per kilowatt-hour, compared with $0.05 - $0.07 for subsidized wind systems.<ref>[http://www.pichtr.org/Luis_Vega_OTEC_Summary.pdf ]{{dead link|date=June 2012}}</ref>

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,{{Citation needed|date=August 2008}} (often within 20° of the equator)<ref>{{cite web|url=http://www.nrel.gov/otec/markets.html |title=NREL: Ocean Thermal Energy Conversion - Markets for OTEC |publisher=Nrel.gov |date= |accessdate=2012-06-12}}</ref> the geopolitical effects of [[petroleum]] dependence, compatibility with alternate forms of ocean power such as wave energy, tidal energy and [[methane hydrate]]s, and supplemental uses for the seawater.<ref>{{cite web|url=http://www.nrel.gov/otec/ |title=NREL: Ocean Thermal Energy Conversion Home Page |publisher=Nrel.gov |date= |accessdate=2012-06-12}}</ref>

== Thermodynamics ==
A rigorous treatment of OTEC reveals that a 20&nbsp;°C temperature difference will provide as much energy as a hydroelectric plant with 34&nbsp;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 metric tonnes) per minute.<ref>{{citation|title=Challenge And Promise Of OTEC  |first1=Duke |last1 =Hartman | journal = Ocean News |date = October 2011| accessdate= June 2012| url= http://virtual.ocean-news.com/display_article.php?id=864488 }}</ref>  For comparison, pumps must move a mass of water greater than the weight of the [[German battleship Bismarck|Battleship Bismark]], which weighed 41,700 metric tons, 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.<!--Da Rosa, P.147 -->  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,<ref name=" Da Rosa">{{cite book|last= Da Rosa|first= Aldo Vieira|title=Fundamentals of renewable energy processes|url=http://books.google.co.uk/books?id=vDuec62ube8C&pg=PA146&dq=otec&hl=en&ei=SFjATqf3OsrDhAe76qDABA&sa=X&oi=book_result&ct=result&resnum=4&ved=0CEEQ6AEwAw#v=onepage&q=otec&f=false|year=2009|publisher=Academic Press |isbn=0-12-374639-6|pages=139 to 152 |chapter=Chapter 4:Ocean Thermal Energy Converters}}</ref> 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.<ref>{{citation|contribution=Comparison Of Aluminum Alloys And Manufacturing Processes Based On Corrosion Performance For Use In OTEC Heat Exchangers|last1=Eldred|first1=M. last2=Landherr |first2= A. |last3=Chen |first3=I.C. |doi=10.4043/20702-MS | url=http://www.onepetro.org/mslib/servlet/onepetropreview?id=OTC-20702-MS | title= Offshore Technology Conference 2010 (OTC 2010) |publisher= Curran Associates, Inc.  | isbn=9781617384264 | date= July 2010 |accessdate= May 28, 2010}}</ref>

===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: {{math|5.45&times;10<sup>18</sup> MJ/yr &times; 0.7 &times; 0.5 &times; 0.15 {{=}} 2.87&times;10<sup>17</sup> MJ/yr}}

We can use [[Lambert's law]] to quantify the solar energy absorption by water,

:<math>-\frac{dI(y)}{dy}=\mu I</math>

where, ''y'' is the depth of water, ''I'' is intensity and ''μ'' is the absorption coefficient.
Solving the above [[differential equation]],

:<math> I(y)=I_{0}\exp(-\mu y) \,</math>

The absorption coefficient ''μ''  may range from 0.05 m<sup>&minus;1</sup> for very clear fresh water to 0.5 m<sup>−1</sup> 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 {{convert|25|C}}, while at {{convert|1|km|mi|sp=us}}, the temperature is about {{convert|5|-|10|C|F}}. 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.{{Clarify|date=January 2013}}

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 {{convert|27|C}} enters an evaporator at pressure slightly below the [[saturation pressure]]s causing it to vaporize.

:<math> H_{1}=H_{f} \,</math>

Where ''H{{sub|f}}'' is [[enthalpy]] of liquid water at the inlet temperature, ''T''{{sub|1}}.

[[File:otec oc t-s dia.jpg]]

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''{{sub|2}}.

:<math>H_{2}=H_{1}=H_{f}+x_{2}H_{fg} \,</math>

Here, ''x''{{sub|2}} is the fraction of water by mass that vaporizes. The warm water mass flow rate per unit [[turbine]] mass flow rate is 1/''x''{{sub|2}}.

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.

:<math>H_{3}=H_{g} \,</math>

Here, ''H{{sub|g}}'' corresponds to ''T''{{sub|2}}. For an ideal [[isentropic process|isentropic]] ([[reversible adiabatic]]) turbine,

:<math>s_{5,s}=s_{3}=s_{f}+x_{5,s}s_{fg} \, </math>

The above equation corresponds to the temperature at the exhaust of the turbine, ''T''{{sub|5}}. ''x''{{sub|5,''s''}} is the mass fraction of vapor at state 5.

The enthalpy at ''T''{{sub|5}} is,

:<math> H_{5,s}=H_{f}+x_{5,s}H_{fg} \,</math>

This enthalpy is lower. The adiabatic reversible turbine work = ''H''{{sub|3}}-''H''{{sub|5,''s'' }}.

Actual turbine work {{Math|1=''W''{{sub|T}} = (''H''{{sub|3}}-''H''{{sub|5,''s''}}) x ''polytropic efficiency''}}

:<math>H_{5}=H_{3}-\ \mathrm{actual}\ \mathrm{work} </math>

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''{{sub|6}}=''H{{sub|f}}'', at ''T''{{sub|5}}. ''T''{{sub|7}} is the temperature of the exhaust mixed with cold sea water, as the vapour content now is negligible,

:<math>H_{7}\approx H_{f}\,\ at\ T_{7} \,</math>

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|irreversibilities]] that reduce the overall temperature difference.

The cold water flow rate ''per'' unit turbine mass flow rate,

:<math>\dot{m_{c}=\frac{H_{5}-\ H_{6}}{H_{6}-\ H_{7}}} \,</math>

Turbine mass flow rate, <math>\dot{M_{T}}=\frac{\mathrm{turbine}\ \mathrm{work}\ \mathrm{required}}{W_{T}} </math>

Warm water mass flow rate, <math> \dot{M_{w}}=\dot{M_{T}\dot{m_{w}}} \,</math>

Cold water mass flow rate <math>\dot{\dot{M_{c}}=\dot{M_{T}m_{C}}} \,</math>
'''

===Closed Anderson cycle===
Developed starting in the 1960s by [[J. Hilbert Anderson]] of Sea Solar Power, Inc. In this cycle, ''Q{{sub|H}}'' 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{{sub|T}}''. 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{{sub|C}}'', to the cold sea water. The condensate is then compressed to the highest pressure in the cycle, requiring condensate pump work, ''W{{sub|C}}''. 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 viscous 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{{sub|C}}'', 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{{sub|CT}}'', and the warm water pump work, ''W{{sub|HT}}''. Denoting all other parasitic energy requirements by ''W{{sub|A}}'', the net work from the OTEC plant, ''W{{sub|NP}}'' is

:<math> W_{NP}=W_{T}+W_{C}+W_{CT}+W_{HT}+W_{A} \,</math>

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

:<math> W_{N}=Q_{H}+Q_{C} \,</math>

where {{math|1=''W{{sub|N}}'' = ''W{{sub|T}}'' + ''W{{sub|C}}''}} 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,

:<math> Q_{H}=\int_{H}T_{H}ds \,</math>

and

:<math> Q_{C}=\int_{C}T_{C}ds \,</math>

so that the net thermodynamic cycle work becomes

:<math> W_{N}=\int_{H}T_{H}ds+\int_{C}T_{C}ds \,</math>

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. {{citation needed|date=September 2012}}

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. {{citation needed|date=September 2012}}

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.<ref name="bioplume">{{cite web|last=Grandelli|first=Pat|title=Modeling the Physical and Biochemical Influence of Ocean Thermal Energy Conversion Plant Discharges into their Adjacent Waters|url=http://www.osti.gov/bridge/servlets/purl/1055480/1055480.pdf|work=US Department of Energy - Office of Scientific and Technical Information|accessdate=27 March 2013}}</ref> 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 Work===
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.
<ref name="hydroplume">{{cite journal|last=Rocheleau|first=Greg|coauthors=Pat Grandelli|title=Physical and biological modeling of a 100 megawatt Ocean Thermal Energy Conversion discharge plume|journal=Institute of Electrical and Electronics Engineers|date=22 September 2011|page=3|url=http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=6107077&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D6107077|accessdate=27 March 2013}}</ref>

===Biological Modeling Work===
The nutrients from an OTEC discharge could potentially cause increased biological activity if they accumulate in large quantities in the [[photic zone]].<ref name="hydroplume" /> 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.<ref name="bioplume" /> 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.<ref name="bioplume" />

===Environmental Impact Studies===
A previous Final Environmental Impact Statement (EIS) for the United States' NOAA from 1981 is available,<ref>{{cite web|title=Final Environmental Impact Statement for Commercial Ocean Thermal Energy Conversion (OTEC) Licensing|url=http://hinmrec.hnei.hawaii.edu/wp-content/uploads/2010/01/OTEC-Programmatic-EIS-NOAA-1981.pdf|work=U.S. Dept of Commerce, National Oceanic and Atmospheric Administration|accessdate=27 March 2013}}</ref> 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.<ref>{{cite web|last=L.Vega, C.Comfort|title=Environmental Assessment of Ocean Thermal Energy Conversion in Hawaii|url=http://hinmrec.hnei.hawaii.edu/wp-content/uploads/2010/01/Environmental-Assessment-of-OTEC-in-Hawaii1.pdf|work=Hawaii National Marine Renewable Energy Center|accessdate=27 March 2013}}</ref> 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.<ref>{{cite web|title=Ocean Thermal Energy Conversion: Assessing Potential Physical, Chemical, and Biological Impacts and Risks|url=http://coastalmanagement.noaa.gov/otec/docs/otecjun10wkshp.pdf|work=National Oceanic and Atmospheric Administration, Office of Ocean and Coastal Resource Management|accessdate=27 March 2013}}</ref>
<ref>{{cite web|title=Ocean Thermal Energy Conversion: Information Needs Assessment|url=http://coastalmanagement.noaa.gov/otec/docs/otecassessment.pdf|work=National Oceanic and Atmospheric Administration (NOAA) Office of Response and Restoration (ORR) and the Environmental Research Group at the University of New Hampshire (UNH)|accessdate=27 March 2013}}</ref>

==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 {{convert|8.5|m|ft|sp=us}} of the tube. The trade-off between [[wikt:dearation|pre-dearation]]<ref>[http://m-w.com/cgi-bin/dictionary?va=deaerate deaeration]</ref> 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 {{convert|25|to|50|μm}} can degrade heat exchanger performance by as much as 50%.<ref name=microbial-countermeasures/> 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.<ref name=microbial-fouling>{{cite journal |author=Aftring RP, Taylor BF |title=Assessment of Microbial Fouling in an Ocean Thermal Energy Conversion Experiment |journal=Appl. Environ. Microbiol. |volume=38 |issue=4 |pages=734–739 |date=October 1979 |pmid=16345450 |pmc=243568 |doi= |url=http://aem.asm.org/cgi/pmidlookup?view=long&pmid=16345450}}</ref> 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.<ref name=microbial-fouling/>

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.<ref name=microbial-countermeasures>{{cite journal |author=Berger LR, Berger JA |title=Countermeasures to Microbiofouling in Simulated Ocean Thermal Energy Conversion Heat Exchangers with Surface and Deep Ocean Waters in Hawaii |journal=Appl. Environ. Microbiol. |volume=51 |issue=6 |pages=1186–1198 |date=June 1986 |pmid=16347076 |pmc=239043 |doi= |url=http://aem.asm.org/cgi/pmidlookup?view=long&pmid=16347076}}</ref> 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.<ref name=microbial-brush-cleaning>{{cite journal |author=Nickels JS, Bobbie RJ, Lott DF, Martz RF, Benson PH, White DC |title=Effect of Manual Brush Cleaning on Biomass and Community Structure of Microfouling Film Formed on Aluminum and Titanium Surfaces Exposed to Rapidly Flowing Seawater |journal=Appl. Environ. Microbiol. |volume=41 |issue=6 |pages=1442–1453 |date=June 1981 |pmid=16345798 |pmc=243937 |doi= |url=http://aem.asm.org/cgi/pmidlookup?view=long&pmid=16345798}}</ref> The increased growth rate after subsequent cleanings appears to result from selection pressure on the microbial colony.<ref name=microbial-brush-cleaning/>

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&nbsp;mg per liter for 1 hour per day may prove effective for long term operation of a plant.<ref name=microbial-countermeasures/>  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.<ref name=microbial-countermeasures/>

Besides water temperature, microbial fouling also depends on nutrient levels, with growth occurring faster in nutrient rich water.<ref name=biofilm-dynamics>{{Cite journal | title =  Dynamics of Biofilm Processes | last = Trulear | first = MG | coauthors = Characklis, WG |date=September 1982 | journal =  Journal of the Water Pollution Control Federation | volume = 54 | issue = 9 | pages = 1288–1301 | url = http://md1.csa.com/partners/viewrecord.php?collection=ENV&recid=8301080&uid=792056511&q=&uid=792056511&q=&uid=792056511&setcookie=yes}}</ref> 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.<ref name=microbial-brush-cleaning/>  In contrast, [[titanium]] tubing allows biofouling to occur faster but cleaning is more effective than with aluminium.<ref name=microbial-brush-cleaning/>

===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, seawater can be 22 °C (72 °F) warmer than ambient air temperature. 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 {{convert|-0.5|C|F}} and its non-solubility in water.<ref>{{cite news| url=http://www.time.com/time/magazine/article/0,9171,751867,00.html | work=Time | title=Science: Cold Power | date=1929-04-22}}</ref> Assuming a level of efficiency of realistic 4%, calculations show that the amount of energy generated with one cubic meter water at a temperature of {{convert|2|C|F}} in a place with an air temperature of {{convert|-22|C|F}} equals the amount of energy generated by letting this cubic meter water run through a hydroelectric plant of 4000 feet (1,200 m) height.<ref>{{cite web|url=http://www.buch-der-synergie.de/c_neu_html/c_06_10_wasser_temperaturgradient.htm |title=Achmed Khammas - Das Buch der Synergie - Teil C - Temperaturgradient |publisher=Buch-der-synergie.de |date=2007-10-25 |accessdate=2012-06-12}}</ref>

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&nbsp;km away from Glasgow, detects monthly mean temperatures below {{convert|-15|C|F}} during 6 winter months in the year.<ref>{{cite web|url=http://www.globalbioclimatics.org/station/de-myggb.htm |title=Denmark - Myggbuka |publisher=Globalbioclimatics.org |date= |accessdate=2012-06-12}}</ref>

==See also==
{{Portal|Energy|Sustainable development}}
*[[Deep lake water cooling]]
*[[Heat engine]]
*[[Offshore construction|Ocean engineering]]
*[[Osmotic power]]
*[[Seawater air conditioning]]

==Footnotes==
{{Reflist|columns=2}}

==References==
*http://otecfoundation.org/
*http://otecnews.com/
*http://www.ioes.saga-u.ac.jp/en/about_lab.html (Saga University OTEC Reseach Facility)
*http://www.OTEC.ws
*http://www.lockheedmartin.com/us/products/otec.html
*http://www.makai.com/e-otec.htm
*http://www.bluerise.nl/
*http://www.ocees.com


==Sources==
*''Renewable Energy From The Ocean - A Guide To OTEC'',  William H. Avery, Chih Wu, Oxford University Press, 1994. Covers the OTEC work done at the Johns Hopkins Applied Physics Laboratory from 1970–1985 in conjunction with the Department of Energy and other firms.

==External links==
*[http://www.otecnews.org/ OTEC News - OTEC News website]
*[http://oceanexplorer.noaa.gov/edu/learning/player/lesson11/l11la1.html Educational material about OTEC by the NOAA Ocean Exploration program]
*[http://www.oceanenergycouncil.com/index.php/Ocean-Thermal-OTEC/OTEC.html Ocean Energy Council: How does OTEC work?]
*[http://www.nrel.gov/otec/what.html nrel.gov - what is OTEC?]
*[http://www.eere.energy.gov/consumerinfo/factsheets/nb1.html US Department of Energy, Information Resources]
*[[Wired Magazine]]'s [http://www.wired.com/wired/archive/13.06/craven.html interview with John Piña Craven on the future of OTEC]
*[http://www.worldenergy.org/publications/survey_of_energy_resources_2007/default.asp 2007 edition of the Survey of Energy Resources produced by the World Energy Council]
*[http://library.greenocean.org/oteclibrary/ The Green Ocean Project - OTEC Library]
* [http://www.newscientist.com/article/mg20026836.000-plumbing-the-oceans-could-bring-limitless-clean-energy.html Plumbing the oceans could bring limitless clean energy ]
* [http://www.engineeringtoolbox.com/steel-pipes-flow-capacities-d_640.html] Maximum water flow capacity of steel pipes - dimensions ranging 2 - 24&nbsp;inches
The Engineering Toolbox. for other types of ocean energy go to ((tidal energy))
* [http://www.power-technology.com/projects/hainan-ocean-thermal-energy-conversion-otec-power-plant/ Hainan Ocean Thermal Energy Conversion (OTEC) Power Plant], China

{{Ocean energy|state=expanded}}
{{physical oceanography|expanded=none}}

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[[Category:Energy conversion]]
[[Category:Geothermal energy]]
[[Category:Ocean energy]]
[[Category:Power station technology]]

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