Editing Refrigeration (section)

==Methods of refrigeration==
Methods of refrigeration can be classified as ''non-cyclic'', ''cyclic'', ''thermoelectric'' and ''magnetic''.

===Non-cyclic refrigeration===
{{Main|Ice trade}}
This refrigeration method cools a contained area by melting ice, or by sublimating [[dry ice]].<ref>{{cite web
|url= https://www.brighthubengineering.com/hvac/20353-methods-of-refrigeration-ice-refrigeration-and-dry-ice-refrigeration/
|title= Methods of Refrigeration: Ice Refrigeration, Dry Ice Refrigeration
|website= Brighthub Engineering
|access-date= 2016-02-29
|date= 2008-12-22
}}</ref> Perhaps the simplest example of this is a portable cooler, where items are put in it, then ice is poured over the top. Regular ice can maintain temperatures near, but not below the freezing point, unless salt is used to cool the ice down further (as in a [[ice cream maker#Hand-cranked machines|traditional ice-cream maker]]). Dry ice can reliably bring the temperature well below water freezing point.

===Cyclic refrigeration===
{{Main|Heat pump and refrigeration cycle}}
This consists of a refrigeration cycle, where heat is removed from a low-temperature space or source and rejected to a high-temperature sink with the help of external work, and its inverse, the [[thermodynamic power cycle]]. In the power cycle, heat is supplied from a high-temperature source to the engine, part of the heat being used to produce work and the rest being rejected to a low-temperature sink. This satisfies the [[second law of thermodynamics]].

A ''refrigeration cycle'' describes the changes that take place in the refrigerant as it alternately absorbs and rejects heat as it circulates through a [[refrigerator]]. It is also applied to heating, ventilation, and air conditioning [[HVACR]] work, when describing the "process" of refrigerant flow through an HVACR unit, whether it is a packaged or split system.

Heat naturally flows from hot to cold. [[Mechanical work|Work]] is applied to cool a living space or storage volume by pumping heat from a lower temperature heat source into a higher temperature heat sink. [[Thermal insulation|Insulation]] is used to reduce the work and [[energy]] needed to achieve and maintain a lower temperature in the cooled space. The operating principle of the refrigeration cycle was described mathematically by [[Nicolas Léonard Sadi Carnot|Sadi Carnot]] in 1824 as a [[Carnot heat engine|heat engine]].

The most common types of refrigeration systems use the reverse-Rankine [[vapor-compression refrigeration]] cycle, although [[absorption heat pump]]s are used in a minority of applications.

Cyclic refrigeration can be classified as:
#Vapor cycle, and
#Gas cycle

Vapor cycle refrigeration can further be classified as:
#[[Vapor-compression refrigeration]]
#Sorption Refrigeration
##[[Absorption refrigerator|Vapor-absorption refrigeration]]
##[[Adsorption refrigeration]]

====Vapor-compression cycle====
{{See also|Vapor-compression refrigeration}}
[[File:Refrigeration.png|frame|right|Figure 1: Vapor compression refrigeration]]
[[File:RefrigerationTS.png|frame|right|Figure 2: Temperature–Entropy diagram]]
The vapor-compression cycle is used in most household refrigerators as well as in many large commercial and [[Industrial refrigerator|industrial refrigeration]] systems. Figure 1 provides a schematic diagram of the components of a typical vapor-compression refrigeration system.

The [[thermodynamics]] of the cycle can be analyzed on a diagram<ref>[http://web.me.unr.edu/me372/Spring2001/Vapor%20Compression%20Refrigeration%20Cycles.pdf The Ideal Vapor-Compression Cycle] {{webarchive|url=https://web.archive.org/web/20070226113352/http://web.me.unr.edu/me372/Spring2001/Vapor%20Compression%20Refrigeration%20Cycles.pdf |date=2007-02-26}}</ref> as shown in Figure 2. In this cycle, a circulating refrigerant such as a low boiling hydrocarbon or [[hydrofluorocarbons]] enters the [[gas compressor|compressor]] as a vapour. From point 1 to point 2, the vapor is compressed at constant [[entropy]] and exits the compressor as a vapor at a higher temperature, but still below the [[vapor pressure]] at that temperature. From point 2 to point 3 and on to point 4, the vapor travels through the [[condenser (heat transfer)|condenser]] which cools the vapour until it starts condensing, and then condenses the vapor into a liquid by removing additional heat at constant pressure and temperature. Between points 4 and 5, the liquid refrigerant goes through the [[thermal expansion valve|expansion valve]] (also called a throttle valve) where its pressure abruptly decreases, causing [[flash evaporation]] and auto-refrigeration of, typically, less than half of the liquid.

That results in a mixture of liquid and vapour at a lower temperature and pressure as shown at point 5. The cold liquid-vapor mixture then travels through the evaporator coil or tubes and is completely vaporized by cooling the warm air (from the space being refrigerated) being blown by a fan across the evaporator coil or tubes. The resulting refrigerant vapour returns to the compressor inlet at point 1 to complete the thermodynamic cycle.

The above discussion is based on the ideal vapour-compression refrigeration cycle, and does not take into account real-world effects like frictional pressure drop in the system, slight [[thermodynamic reversibility|thermodynamic irreversibility]] during the compression of the refrigerant vapor, or [[ideal gas|non-ideal gas]] behavior, if any. Vapor compression refrigerators can be arranged in two stages in [[cascade refrigeration]] systems, with the second stage cooling the condenser of the first stage. This can be used for achieving very low temperatures.

More information about the design and performance of vapor-compression refrigeration systems is available in the classic ''[[Perry's Chemical Engineers' Handbook]]''.<ref>{{cite book|author1=Perry, R.H. |author2=Green, D.W. |name-list-style=amp |title=Perry's Chemical Engineers' Handbook|edition=6th |publisher=McGraw Hill, Inc.|year=1984|isbn=978-0-07-049479-4|title-link=Perry's Chemical Engineers' Handbook}} (see pp. 12-27 through 12-38)</ref>

====Sorption cycle====
{{unreferenced section|date=February 2020}}

=====Absorption cycle=====
{{Main|Absorption refrigerator}}
In the early years of the twentieth century, the vapor absorption cycle using water-ammonia systems or [[lithium bromide|LiBr]]-water was popular and widely used. After the development of the vapor compression cycle, the vapor absorption cycle lost much of its importance because of its low [[coefficient of performance]] (about one fifth of that of the vapor compression cycle). Today, the vapor absorption cycle is used mainly where fuel for heating is available but electricity is not, such as in [[recreational vehicles]] that carry [[liquefied petroleum gas|LP gas]]. It is also used in industrial environments where plentiful waste heat overcomes its inefficiency.

The absorption cycle is similar to the compression cycle, except for the method of raising the pressure of the refrigerant vapor. In the absorption system, the compressor is replaced by an absorber which dissolves the refrigerant in a suitable liquid, a liquid pump which raises the pressure and a generator which, on heat addition, drives off the refrigerant vapor from the high-pressure liquid. Some work is needed by the liquid pump but, for a given quantity of refrigerant, it is much smaller than needed by the compressor in the vapor compression cycle. In an absorption refrigerator, a suitable combination of refrigerant and absorbent is used. The most common combinations are ammonia (refrigerant) with water (absorbent), and water (refrigerant) with lithium bromide (absorbent).

=====Adsorption cycle=====
{{Main|Adsorption refrigeration}}
The main difference with absorption cycle, is that in adsorption cycle, the refrigerant (adsorbate) could be ammonia, water, [[methanol]], etc., while the adsorbent is a solid, such as [[silica gel]], [[activated carbon]], or [[zeolite]], unlike in the absorption cycle where absorbent is liquid.

The reason adsorption refrigeration technology has been extensively researched in recent 30 years lies in that the operation of an adsorption refrigeration system is often noiseless, non-corrosive and environment friendly.<ref>{{Cite journal|last1=Goyal|first1=Parash|last2=Baredar|first2=Prashant|last3=Mittal|first3=Arvind|last4=Siddiqui|first4=Ameenur. R.|date=2016-01-01|title=Adsorption refrigeration technology – An overview of theory and its solar energy applications|journal=Renewable and Sustainable Energy Reviews|language=en|volume=53|pages=1389–1410|doi=10.1016/j.rser.2015.09.027|bibcode=2016RSERv..53.1389G |issn=1364-0321}}</ref>

====Gas cycle====
{{unreferenced section|date=February 2020}}
When the [[working fluid]] is a gas that is compressed and expanded but does not change phase, the refrigeration cycle is called a ''gas cycle''. [[Air]] is most often this working fluid. As there is no condensation and evaporation intended in a gas cycle, components corresponding to the condenser and evaporator in a vapor compression cycle are the hot and cold gas-to-gas [[heat exchanger]]s in gas cycles.

The gas cycle is less efficient than the vapor compression cycle because the gas cycle works on the reverse [[Brayton cycle]] instead of the reverse [[Rankine cycle]]. As such, the working fluid does not receive and reject heat at constant temperature. In the gas cycle, the refrigeration effect is equal to the product of the specific heat of the gas and the rise in temperature of the gas in the low temperature side. Therefore, for the same cooling load, a gas refrigeration cycle needs a large mass flow rate and is bulky.

Because of their lower efficiency and larger bulk, ''air cycle'' coolers are not often used nowadays in terrestrial cooling devices. However, the [[air cycle machine]] is very common on [[gas turbine]]-powered jet [[aircraft]] as cooling and ventilation units, because compressed air is readily available from the engines' compressor sections. Such units also serve the purpose of pressurizing the aircraft.

===Thermoelectric refrigeration===
[[Thermoelectric cooling]] uses the [[Peltier effect]] to create a heat [[flux]] between the junction of two types of material.<ref name=":2">{{Cite book |last=Lundgaard |first=Christian |title=Design of segmented thermoelectric Peltier coolers by topology optimization |publisher=OXFORD: Elsevier Ltd |year=2019 |pages=1 |language=English}}</ref> This effect is commonly used in camping and portable coolers and for cooling electronic components<ref>Fylladitakis, E. (September 26, 2016) [http://www.anandtech.com/show/10695/the-phononic-hex-2-0-tec-cpu-cooler-review The Phononic HEX 2.0 TEC CPU Cooler Review]. Anandtech.com. Retrieved on 2018-10-31.</ref> and small instruments. Peltier coolers are often used where a traditional vapor-compression cycle refrigerator would be impractical or take up too much space, and in cooled image sensors as an easy, compact and lightweight, if inefficient, way to achieve very low temperatures, using two or more stage peltier coolers arranged in a [[cascade refrigeration]] configuration, meaning that two or more Peltier elements are stacked on top of each other, with each stage being larger than the one before it,<ref>{{Cite book|url=https://books.google.com/books?id=rKy-DwAAQBAJ&dq=cascade+peltier+cooler&pg=PA95|title=Conductors, Semiconductors, Superconductors: An Introduction to Solid-State Physics|first=Rudolf P.|last=Huebener|date=November 16, 2019|publisher=Springer Nature|isbn=9783030314200 |via=Google Books}}</ref><ref>{{Cite book|url=https://books.google.com/books?id=n4O1DwAAQBAJ&dq=peltier+stages&pg=PA625|title=CRC Handbook of Thermoelectrics|first=D. M.|last=Rowe|date=December 7, 2018|publisher=CRC Press|isbn=9780429956676 |via=Google Books}}</ref><ref>{{Cite book|url=https://books.google.com/books?id=Zut8CAAAQBAJ&dq=peltier+stages&pg=PA9|title=Thermoelectric Bi2Te3 Nanomaterials|first1=Oliver|last1=Eibl|first2=Kornelius|last2=Nielsch|first3=Nicola|last3=Peranio|first4=Friedemann|last4=Völklein|date=April 21, 2015|publisher=John Wiley & Sons|isbn=9783527672639 |via=Google Books}}</ref> in order to extract more heat and waste heat generated by the previous stages. Peltier cooling has a low COP (efficiency) when compared with that of the vapor-compression cycle, so it emits more waste heat (heat generated by the Peltier element or cooling mechanism) and consumes more power for a given cooling capacity.<ref name=PNL>{{cite web |last=Brown |first=D. R.| title=The Prospects of Alternatives to Vapor Compression Technology for Space Cooling and Food Refrigeration Applications |url=http://www.pnl.gov/main/publications/external/technical_reports/pnnl-19259.pdf |work=Pacific Northwest National Laboratory (PNL) |publisher=U.S. Department of Energy |access-date=16 March 2013 |author2=N. Fernandez |author3=J. A. Dirks |author4=T. B. Stout |date=March 2010}}</ref>

===Magnetic refrigeration===
{{Main|Magnetic refrigeration}}
{{unreferenced section|date=February 2020}}
Magnetic refrigeration, or [[adiabatic]] [[demagnetization]], is a cooling technology based on the magnetocaloric effect, an [[intrinsic and extrinsic properties|intrinsic property]] of magnetic solids. The refrigerant is often a [[paramagnetism|paramagnetic]] [[salt (chemistry)|salt]], such as [[cerium]] [[magnesium]] [[nitrate]]. The active [[magnetic field|magnetic]] [[dipole]]s in this case are those of the [[electron shell]]s of the paramagnetic atoms.

A strong magnetic field is applied to the refrigerant, forcing its various magnetic dipoles to align and putting these degrees of freedom of the refrigerant into a state of lowered [[entropy]]. A heat sink then absorbs the heat released by the refrigerant due to its loss of entropy. Thermal contact with the heat sink is then broken so that the system is insulated, and the magnetic field is switched off. This increases the heat capacity of the refrigerant, thus decreasing its temperature below the temperature of the heat sink.

Because few materials exhibit the needed properties at room temperature, applications have so far been limited to [[cryogenics]] and research.

===Other methods===
{{unreferenced section|date=February 2020}}
Other methods of refrigeration include the [[air cycle machine]] used in aircraft; the [[vortex tube]] used for spot cooling, when compressed air is available; and [[thermoacoustic refrigeration]] using sound waves in a pressurized gas to drive heat transfer and heat exchange; [[steam jet cooling]] popular in the early 1930s for air conditioning large buildings; thermoelastic cooling using a smart metal alloy stretching and relaxing. Many [[Stirling cycle]] heat engines can be run backwards to act as a refrigerator, and therefore these engines have a niche use in [[cryogenics]]. In addition, there are other types of [[cryocoolers]] such as Gifford-McMahon coolers, Joule-Thomson coolers, [[Pulse tube refrigerator|pulse-tube refrigerators]] and, for temperatures between 2 mK and 500 mK, [[dilution refrigerator]]s.

===Elastocaloric refrigeration===
Another potential solid-state refrigeration technique and a relatively new area of study comes from a special property of [[pseudoelasticity|super elastic]] materials. These materials undergo a temperature change when experiencing an applied mechanical [[stress (mechanics)|stress]] (called the elastocaloric effect). Since super elastic materials deform reversibly at high [[strain (mechanics)|strains]], the material experiences a flattened [[elasticity (physics)|elastic]] region in its [[stress-strain curve]] caused by a resulting phase transformation from an [[austenite|austenitic]] to a [[martensite|martensitic]] crystal phase.

When a super elastic material experiences a stress in the austenitic phase, it undergoes an [[exothermic]] [[phase transition|phase transformation]] to the martensitic phase, which causes the material to heat up. Removing the stress reverses the process, restores the material to its austenitic phase, and [[endothermic process|absorbs heat]] from the surroundings cooling down the material.

The most appealing part of this research is how potentially energy efficient and environmentally friendly this cooling technology is. The different materials used, commonly [[shape-memory alloy]]s, provide a non-toxic source of emission free refrigeration. The most commonly studied materials studied are shape-memory alloys, like [[nitinol]] and Cu-Zn-Al. Nitinol is of the more promising alloys with output heat at about 66 J/cm<sup>3</sup> and a temperature change of about 16–20 K.<ref>{{cite journal|last1=Tušek|first1=J.|last2=Engelbrecht|first2=K.|last3=Mikkelsen|first3=L.P.|last4=Pryds|first4=N.|title=Elastocaloric effect of Ni-Ti wire for application in a cooling device|journal=Journal of Applied Physics|date=February 2015|volume=117|issue=12|pages=124901|doi=10.1063/1.4913878|bibcode=2015JAP...117l4901T|s2cid=54708904 }}</ref> Due to the difficulty in manufacturing some of the shape memory alloys, alternative materials like [[natural rubber]] have been studied. Even though rubber may not give off as much heat per volume (12 J/cm<sup>3</sup> ) as the shape memory alloys, it still generates a comparable temperature change of about 12 K and operates at a suitable temperature range, low stresses, and low cost.<ref>{{cite journal|last1=Xie|first1=Zhongjian|last2=Sebald|first2=Gael|last3=Guyomar|first3=Daniel|title=Temperature dependence of the elastocaloric effect in natural rubber|journal=Physics Letters A|date=21 February 2017|volume=381|issue=25–26|pages=2112–2116|doi=10.1016/j.physleta.2017.02.014|arxiv=1604.02686|bibcode=2017PhLA..381.2112X|s2cid=119218238}}</ref>

The main challenge however comes from potential energy losses in the form of [[hysteresis]], often associated with this process. Since most of these losses comes from incompatibilities between the two phases, proper alloy tuning is necessary to reduce losses and increase reversibility and [[energy conversion efficiency|efficiency]]. Balancing the transformation strain of the material with the energy losses enables a large elastocaloric effect to occur and potentially a new alternative for refrigeration.<ref>{{cite journal|last1=Lu|first1=Benfeng|last2=Liu|first2=Jian|title=Elastocaloric effect and superelastic stability in Ni–Mn–In–Co polycrystalline Heusler alloys: hysteresis and strain-rate effects|journal=Scientific Reports|date=18 May 2017|volume=7|issue=1|pages=2084|doi=10.1038/s41598-017-02300-3|pmid=28522819|pmc=5437036|bibcode=2017NatSR...7.2084L}}</ref>

===Fridge Gate===
The Fridge Gate method is a theoretical application of using a single logic gate to drive a refrigerator in the most energy efficient way possible without violating the laws of thermodynamics. It operates on the fact that there are two energy states in which a particle can exist: the ground state and the excited state. The excited state carries a little more energy than the ground state, small enough so that the transition occurs with high probability. There are three components or particle types associated with the fridge gate. The first is on the interior of the refrigerator, the second on the outside and the third is connected to a power supply which heats up every so often that it can reach the E state and replenish the source. In the cooling step on the inside of the refrigerator, the g state particle absorbs energy from ambient particles, cooling them, and itself jumping to the e state. In the second step, on the outside of the refrigerator where the particles are also at an e state, the particle falls to the g state, releasing energy and heating the outside particles. In the third and final step, the power supply moves a particle at the e state, and when it falls to the g state it induces an energy-neutral swap where the interior e particle is replaced by a new g particle, restarting the cycle.<ref>{{cite journal|author=Renato Renner|author-link=Renato Renner|date=9 February 2012|title=Thermodynamics: The fridge gate|journal=Nature|volume=482|issue=7384|pages=164–165|bibcode=2012Natur.482..164R|doi=10.1038/482164a|pmid=22318595|doi-access=free|s2cid=4416925}}</ref>

===Passive systems===
When combining a [[passive daytime radiative cooling]] system with [[thermal insulation]] and [[Evaporative cooler|evaporative cooling]], one study found a 300% increase in ambient cooling power when compared to a stand-alone radiative cooling surface, which could extend the [[shelf life]] of food by 40% in [[Humidity|humid climates]] and 200% in [[Desert climate|desert climates]] without refrigeration. The system's evaporative cooling layer would require water "re-charges" every 10 days to a month in humid areas and every 4 days in hot and dry areas.<ref>{{Cite journal |last1=Lu |first1=Zhengmao |last2=Leroy |first2=Arny |last3=Zhang |first3=Lenan |last4=Patel |first4=Jatin J. |last5=Wang |first5=Evelyn N. |last6=Grossman |first6=Jeffrey C. |date=September 2022 |title=Significantly enhanced sub-ambient passive cooling enabled by evaporation, radiation, and insulation |journal=Cell Reports Physical Science |volume=3 |issue=10 |page=101068 |doi=10.1016/j.xcrp.2022.101068 |bibcode=2022CRPS....301068L |s2cid=252411940 |doi-access=free |hdl=1721.1/146578 |hdl-access=free }}</ref>