Editing Ocean thermal energy conversion (section)

== Power cycle types ==

Cold seawater is an integral part of each of the three types of OTEC systems: closed-cycle, open-cycle, and hybrid.  To operate, the cold seawater must be brought to the surface. The primary approaches are active pumping and desalination. Desalinating seawater near the sea floor lowers its density, which causes it to rise to the surface.<ref name=desalination-pump-patent>{{Cite 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}}</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.<ref>{{Cite book|url=https://books.google.com/books?id=QdArDwAAQBAJ&q=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%2C+thus+reducing+pumping+volumes+and+reducing+technical+and+environmental+problems+and+lowering+costs&pg=PT1470|title=Thermal Energy: Sources, Recovery, and Applications |last=Shah|first=Yatish T.|date=2018-01-12|publisher=CRC Press|isbn=9781315305936|language=en}}</ref>

=== Closed ===
[[File:Otec Closed Diagram in English.JPG|thumb|Diagram of a closed cycle OTEC plant|220x220px]]

Closed-cycle systems use fluid with a low boiling point, such as [[ammonia]] (having a boiling point around -33&nbsp;°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 | vauthors = Trimble LC, Owens WL | year = 1980 | journal = Energy to the 21st Century; Proceedings of the Fifteenth Intersociety Energy Conversion Engineering Conference | volume = 2 | pages = 1331–1338 | bibcode = 1980iece.conf.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|Diagram of an open cycle OTEC plant|220x220px]]

Open-cycle OTEC uses warm surface water directly to make electricity. The warm seawater is first pumped into a low-pressure container, which causes it to boil. In some schemes, the expanding [[vapor]] drives a low-pressure turbine attached to an [[electrical generator]]. The vapor, 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|website=OTEC News|publisher=The GreenOcean Project|access-date=4 February 2011|archive-url=https://web.archive.org/web/20081207045655/http://www.otecnews.org/articles/vega/05_open_cycle.html|archive-date=7 December 2008|url-status=dead}}</ref>

In other schemes, the rising vapor is used in a [[gas lift]] technique of lifting water to significant heights.  Depending on the embodiment, such [[mist lift|vapor lift]] pump techniques generate power from a [[water turbine|hydroelectric turbine]] either before or after the pump is used.<ref name=jsee>{{cite journal |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 |issue=2 |pages=181 |date=May 1983 |url=http://library.greenocean.org/oteclibrary/otecdesigns/mistliftotec/vapor_mistlift_otec.pdf |bibcode=1983ATJSE.105..181L |doi=10.1115/1.3266363 |access-date=2012-06-02 |archive-date=2008-11-22 |archive-url=https://web.archive.org/web/20081122005914/http://library.greenocean.org/oteclibrary/otecdesigns/mistliftotec/vapor_mistlift_otec.pdf |url-status=dead }}</ref>

In 1984, the ''Solar Energy Research Institute'' (now known as 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 steam production would only be a few percent of the incoming water). In May 1993, an open-cycle OTEC plant at Keahole Point, Hawaii, produced close to 80&nbsp;[[kW]] 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]]).<ref>{{Cite journal|last=Vega|first=L. A.|date=2002-12-01|title=Ocean Thermal Energy Conversion Primer|journal=Marine Technology Society Journal|volume=36|issue=4|pages=25–35|doi=10.4031/002533202787908626|doi-access=free|bibcode=2002MTSJ...36d..25V }}</ref>

=== 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. [[Hydrocarbon]]s too are good candidates, but they are highly flammable; in addition, this would create competition for use of them directly as fuels. The power plant size is dependent upon the vapor pressure of the working fluid. With increasing vapor pressure, the size of the turbine and heat exchangers decreases while the wall thickness of the pipe and heat exchangers increase to endure high pressure especially on the evaporator side.