Editing Heat pump (section)

==Operation==
{{see also|Vapor-compression refrigeration}}
{{More citations needed section|date=May 2021}}
[[File:RefrigerationTS.png|frame|right|Figure 2: [[Temperature-entropy diagram|Temperature–entropy diagram]] of the vapor-compression cycle]]
[[File:Ecodan outdoor unit Internal view.jpg|thumb|upright|An internal view of the outdoor unit of an Ecodan air source heat pump]]
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Vapor-compression uses a circulating [[refrigerant]] as the medium which absorbs heat from one space, compresses it thereby increasing its temperature before releasing it in another space. The system normally has eight main components: a [[gas compressor|compressor]], a reservoir, a [[reversing valve]] which selects between heating and cooling mode, two [[thermal expansion valve]]s (one used when in heating mode and the other when used in cooling mode) and two heat exchangers, one associated with the external heat source/sink and the other with the interior. In heating mode the external heat exchanger is the evaporator and the internal one being the condenser; in cooling mode the roles are reversed.

Circulating refrigerant enters the compressor in the thermodynamic state known as a [[boiling point#Saturation temperature and pressure|saturated vapor]]<ref>Saturated vapors and saturated liquids are vapors and liquids at their [[saturation temperature]] and [[saturation pressure]]. A superheated vapor is at a temperature higher than the saturation temperature corresponding to its pressure.</ref> and is compressed to a higher pressure, resulting in a higher temperature as well. The hot, compressed vapor is then in the thermodynamic state known as a superheated vapor and it is at a temperature and pressure at which it can be [[condensation|condensed]] with either cooling water or cooling air flowing across the coil or tubes. In heating mode this heat is used to heat the building using the internal heat exchanger, and in cooling mode this heat is rejected via the external heat exchanger.

The condensed, liquid refrigerant, in the thermodynamic state known as a [[boiling point#Saturation temperature and pressure|saturated liquid]], is next routed through an expansion valve where it undergoes an abrupt reduction in pressure. That pressure reduction results in the adiabatic [[flash evaporation]] of a part of the liquid refrigerant. The auto-refrigeration effect of the adiabatic flash evaporation lowers the temperature of the liquid and-vapor refrigerant mixture to where it is colder than the temperature of the enclosed space to be refrigerated.

The cold mixture is then routed through the coil or tubes in the evaporator. A fan circulates the warm air in the enclosed space across the coil or tubes carrying the cold refrigerant liquid and vapor mixture. That warm air [[evaporates]] the liquid part of the cold refrigerant mixture. At the same time, the circulating air is cooled and thus lowers the temperature of the enclosed space to the desired temperature. The evaporator is where the circulating refrigerant absorbs and removes heat which is subsequently rejected in the condenser and transferred elsewhere by the water or air used in the condenser.

To complete the [[refrigeration cycle]], the refrigerant vapor from the evaporator is again a saturated vapor and is routed back into the compressor.

Over time, the evaporator may collect ice or water from ambient [[humidity]]. The ice is melted through [[auto-defrost|defrosting]] cycle. An internal heat exchanger is either used to heat/cool the interior air directly or to heat water that is then circulated through radiators or underfloor heating circuit to either heat or cool the buildings.

===Improvement of coefficient of performance by subcooling===
{{main|Subcooling}}
Heat input can be improved if the [[refrigerant]] enters the evaporator with a lower vapor content. This can be achieved by cooling the liquid refrigerant after condensation. The gaseous refrigerant condenses on the heat exchange surface of the condenser. To achieve a heat flow from the gaseous flow center to the wall of the condenser, the temperature of the liquid refrigerant must be lower than the condensation temperature.

Additional [[subcooling]] can be achieved by heat exchange between relatively warm liquid refrigerant leaving the condenser and the cooler refrigerant vapor emerging from the evaporator. The [[enthalpy]] difference required for the subcooling leads to the superheating of the vapor drawn into the compressor. When the increase in cooling achieved by subcooling is greater that the compressor drive input required to overcome the additional pressure losses, such a heat exchange improves the coefficient of performance.<ref>{{Cite book |title=Heat Pump Technology |last=Ludwig von Cube |first=Hans |publisher=Butterworths |year=1981 |isbn=0-408-00497-5 |pages=22–23 |url=https://books.google.com/books?id=YNH8BAAAQBAJ&dq=heat+pump+principle&pg=PP1 |access-date=2023-01-02 |archive-date=2023-04-03 |archive-url=https://web.archive.org/web/20230403214400/https://books.google.com/books?id=YNH8BAAAQBAJ&dq=heat+pump+principle&pg=PP1 |url-status=live }}</ref>

One disadvantage of the subcooling of liquids is that the difference between the condensing temperature and the heat-sink temperature must be larger. This leads to a moderately high pressure difference between condensing and evaporating pressure, whereby the compressor energy increases.{{Cn|date=January 2025}}

===Refrigerant choice===
{{main|Refrigerant}}
Pure refrigerants can be divided into organic substances ([[hydrocarbons]] (HCs), [[chlorofluorocarbons]] (CFCs), [[hydrochlorofluorocarbons]] (HCFCs), [[hydrofluorocarbons]] (HFCs), [[hydrofluoroolefins]] (HFOs), and HCFOs), and inorganic substances ([[ammonia]] ({{chem|NH|3}}), [[carbon dioxide]] ({{chem|CO|2}}), and [[water]] ({{chem|H|2|O}})<ref>{{Cite journal |last1=Chamoun |first1=Marwan |last2=Rulliere |first2=Romuald |last3=Haberschill |first3=Philippe |last4=Berail |first4=Jean Francois |date=2012-06-01 |title=Dynamic model of an industrial heat pump using water as refrigerant |url=https://www.sciencedirect.com/science/article/pii/S0140700711003082 |journal=International Journal of Refrigeration |volume=35 |issue=4 |pages=1080–1091 |doi=10.1016/j.ijrefrig.2011.12.007 |issn=0140-7007}}</ref>).<ref>{{cite journal |last1=Wu |first1=Di |title=Vapor compression heat pumps with pure Low-GWP refrigerants |url=https://doi.org/10.1016/j.rser.2020.110571 |journal=Renewable and Sustainable Energy Reviews |date=2021 |volume=138 |page=110571 |doi=10.1016/j.rser.2020.110571 |bibcode=2021RSERv.13810571W |s2cid=229455137 |issn=1364-0321 |access-date=2022-11-17 |archive-date=2023-09-24 |archive-url=https://web.archive.org/web/20230924115906/https://www.sciencedirect.com/science/article/abs/pii/S136403212030856X?via%3Dihub |url-status=live }}</ref> Their boiling points are usually below −25&nbsp;°C.<ref>{{Cite web |title=Everything you need to know about the wild world of heat pumps |url=https://www.technologyreview.com/2023/02/14/1068582/everything-you-need-to-know-about-heat-pumps/ |access-date=2023-09-19 |website=MIT Technology Review |language=en |archive-date=2023-08-01 |archive-url=https://web.archive.org/web/20230801104352/https://www.technologyreview.com/2023/02/14/1068582/everything-you-need-to-know-about-heat-pumps/ |url-status=live }}</ref>

In the past 200 years, the standards and requirements for new refrigerants have changed. Nowadays low [[global warming potential]] (GWP) is required, in addition to all the previous requirements for safety, practicality, material compatibility, appropriate atmospheric life,{{Clarify|date=September 2023|reason=what does "appropriate atmospheric life" mean?}} and compatibility with high-efficiency products. By 2022, devices using refrigerants with a very low GWP still have a small market share but are expected to play an increasing role due to enforced regulations,<ref>{{cite web |last1=Miara |first1=Marek |title=Heat Pumps with Climate-Friendly Refrigerant Developed for Indoor Installation |url=https://www.ise.fraunhofer.de/en/press-media/press-releases/2019/heat-pumps-with-climate-friendly-refrigerant-developed-for-indoor-installation.html |publisher=Fraunhofer ISE |date=2019-10-22 |access-date=2022-02-21 |archive-date=2022-02-20 |archive-url=https://web.archive.org/web/20220220224636/https://www.ise.fraunhofer.de/en/press-media/press-releases/2019/heat-pumps-with-climate-friendly-refrigerant-developed-for-indoor-installation.html |url-status=live }}</ref> as most countries have now ratified the [[Kigali Amendment]] to ban HFCs.<ref>{{Cite web |last=Rabe |first=Barry G. |date=2022-09-23 |title=Pivoting from global climate laggard to leader: Kigali and American HFC policy |url=https://www.brookings.edu/blog/fixgov/2022/09/23/pivoting-from-global-climate-laggard-to-leader-kigali-and-american-hfc-policy/ |access-date=2022-10-04 |website=Brookings |language=en-US |archive-date=2022-10-04 |archive-url=https://web.archive.org/web/20221004135503/https://www.brookings.edu/blog/fixgov/2022/09/23/pivoting-from-global-climate-laggard-to-leader-kigali-and-american-hfc-policy/ |url-status=live }}</ref> [[Isobutane|Isobutane (R600A)]] and [[propane|propane (R290)]] are far less harmful to the environment than conventional hydrofluorocarbons (HFC) and are already being used in [[air source heat pump|air-source heat pump]]s.<ref>{{Cite book |url=https://books.google.com/books?id=LpStalb-tF4C&q=R600A+isobutane+ozone+environment&pg=PA77|title=Green Electricity and Global Warming|last=Itteilag|first=Richard L.|date=2012-08-09|publisher=AuthorHouse|isbn=9781477217405 |pages=77|language=en |access-date=2020-11-01|archive-date=2021-11-23|archive-url=https://web.archive.org/web/20211123223807/https://books.google.com/books?id=LpStalb-tF4C&q=R600A+isobutane+ozone+environment&pg=PA77|url-status=live}}</ref> Propane may be the most suitable for high temperature heat pumps.<ref name=":2" /> Ammonia (R717) and carbon dioxide ([[Carbon dioxide#Refrigerant|R-744]]) also have a low GWP. {{As of|2023}} smaller {{Chem|CO|2}} heat pumps are not widely available and research and development of them continues.<ref>{{Cite web |title=Smart CO2 Heat Pump |url=https://www.dti.dk/co2-heat-pump-20-200-kw/44672 |access-date=2023-09-17 |website=www.dti.dk |archive-date=2023-01-30 |archive-url=https://web.archive.org/web/20230130083759/https://www.dti.dk/co2-heat-pump-20-200-kw/44672 |url-status=live }}</ref> A 2024 report said that refrigerants with GWP are vulnerable to further international restrictions.<ref>{{Cite web |title=Annex 53 Advanced Cooling/Refrigeration Technologies 2 page summary |url=https://heatpumpingtechnologies.org/publications/annex-53-advanced-cooling-refrigeration-technologies-2-page-summary/ |access-date=2024-02-19 |website=HPT – Heat Pumping Technologies |language=en}}</ref>

Until the 1990s, heat pumps, along with fridges and other related products used [[chlorofluorocarbon]]s (CFCs) as refrigerants, which caused major damage to the [[ozone layer]] when released into the [[Earth's atmosphere|atmosphere]]. Use of these chemicals was banned or severely restricted by the [[Montreal Protocol]] of August 1987.<ref>{{cite web |year=2007 |title=Handbook for the Montreal Protocol on Substances that Deplete the Ozone Layer – 7th Edition |url=http://ozone.unep.org/Publications/MP_Handbook/Section_1.2_Control_measures/Annex_A_-_Group_I.shtml |url-status=dead |archive-url=https://web.archive.org/web/20160530171422/http://ozone.unep.org/Publications/MP_Handbook/Section_1.2_Control_measures/Annex_A_-_Group_I.shtml |archive-date=2016-05-30 |access-date=2016-12-18 |publisher=United Nations Environment Programme – Ozone Secretariat}}</ref>

Replacements, including [[R-134a]] and [[R-410A]], are hydrofluorocarbons (HFC) with similar thermodynamic properties with insignificant [[ozone depletion potential]] (ODP) but had problematic GWP.<ref>{{cite web |title=Refrigerants – Environmental Properties |url=http://www.engineeringtoolbox.com/Refrigerants-Environment-Properties-d_1220.html |url-status=live |archive-url=https://web.archive.org/web/20130314143622/http://www.engineeringtoolbox.com/Refrigerants-Environment-Properties-d_1220.html |archive-date=2013-03-14 |access-date=2016-09-12 |website=The Engineering ToolBox}}</ref> HFCs are powerful greenhouse gases which contribute to climate change.<ref name="R-410A">[[R-410A#Environmental effects]].</ref><ref>{{cite web|last1=Ecometrica.com|title=Calculation of green house gas potential of R-410A|date=27 June 2012| url=http://ecometrica.com/article/calculating-the-global-warming-potential-of-refrigerant-gas-mixes|access-date=2015-07-13 |ref=Calculation of green house gas potential of R-410A|archive-date=2015-07-13|archive-url=https://web.archive.org/web/20150713232735/http://ecometrica.com/article/calculating-the-global-warming-potential-of-refrigerant-gas-mixes|url-status=live}}</ref> [[Dimethyl ether]] (DME) also gained in popularity as a refrigerant in combination with R404a.<ref name="mecanica-dme">{{Cite web|url=http://www.mecanica.pub.ro/frigo-eco/R404A_DME.pdf |archive-url=https://web.archive.org/web/20120314211640/http://www.mecanica.pub.ro/frigo-eco/R404A_DME.pdf |title=R404 and DME Refrigerant blend as a new solution to limit global warming potential|url-status=dead|archive-date=March 14, 2012|date=March 14, 2012}}</ref> More recent refrigerants include [[Difluoromethane|difluoromethane (R32)]] with a lower GWP, but still over 600.

{| class="wikitable sortable"
|-
! refrigerant !! 20-year GWP !! 100-year GWP
|-
|[[R-290 (refrigerant)|R-290]] propane<ref name="ar6">{{Harvnb|IPCC_AR6_WG1_Ch7|2021|loc=7SM-26}}</ref> 
|0.072
|0.02
|-
|[[R-600a]] isobutane|| ||3<ref>{{Cite web |last=LearnMetrics |date=2023-05-12 |title=List of Low GWP Refrigerants: 69 Refrigerants Below 500 GWP |url=https://learnmetrics.com/low-gwp-refrigerants/ |access-date=2023-09-13 |website=LearnMetrics |language=en-GB |archive-date=2023-06-10 |archive-url=https://web.archive.org/web/20230610114014/https://learnmetrics.com/low-gwp-refrigerants/ |url-status=live }}</ref>
|-
|[[Difluoromethane|R-32]]<ref name="ar6"/>||491||136
|-
|[[R-410a]]<ref name=":1">{{Cite web |title=Global warming potential (GWP) of HFC refrigerants |url=https://iifiir.org/en/encyclopedia-of-refrigeration/global-warming-potential-gwp-of-hfc-refrigerants |access-date=2023-09-13 |website=iifiir.org |language=en |archive-date=2023-09-24 |archive-url=https://web.archive.org/web/20230924115907/https://iifiir.org/en/encyclopedia-of-refrigeration/global-warming-potential-gwp-of-hfc-refrigerants |url-status=live }}</ref>||4705||2285
|-
|[[R-134a]]<ref name=":1" />||4060||1470
|-
|[[R-404a]]<ref name=":1" />|| 7258||4808
|}

Devices with R-290 refrigerant (propane) are expected to play a key role in the future.<ref name=":2">{{Cite news |date=2023-09-06 |title=Propane-powered heat pumps are greener |newspaper=[[The Economist]] |url=https://www.economist.com/science-and-technology/2023/09/06/propane-powered-heat-pumps-are-greener |access-date=2023-09-17 |issn=0013-0613 |archive-date=2023-09-17 |archive-url=https://web.archive.org/web/20230917022901/https://www.economist.com/science-and-technology/2023/09/06/propane-powered-heat-pumps-are-greener |url-status=live }}</ref><ref>{{Cite web |last=Everitt |first=Neil |date=2023-09-15 |title=Qvantum plant has 1 million heat pump capacity |url=https://www.coolingpost.com/world-news/qvantum-plant-has-1-million-heat-pump-capacity/ |access-date=2023-09-17 |website=Cooling Post |language=en-GB |archive-date=2023-09-24 |archive-url=https://web.archive.org/web/20230924120053/https://www.coolingpost.com/world-news/qvantum-plant-has-1-million-heat-pump-capacity/ |url-status=live }}</ref> The 100-year GWP of propane, at 0.02, is extremely low and is approximately 7000 times less than R-32. However, the flammability of propane requires additional safety measures: the maximum safe charges have been set significantly lower than for lower flammability refrigerants (only allowing approximately 13.5 times less refrigerant in the system than R-32).<ref>{{cite web |last1=Miara |first1=Marek |title=Heat Pumps with Climate-Friendly Refrigerant Developed for Indoor Installation |url=https://www.ise.fraunhofer.de/en/press-media/press-releases/2019/heat-pumps-with-climate-friendly-refrigerant-developed-for-indoor-installation.html |publisher=Fraunhofer ISE |date=22 October 2019 |access-date=21 February 2022 |archive-date=20 February 2022 |archive-url=https://web.archive.org/web/20220220224636/https://www.ise.fraunhofer.de/en/press-media/press-releases/2019/heat-pumps-with-climate-friendly-refrigerant-developed-for-indoor-installation.html |url-status=live }}</ref><ref>{{Cite web |title=Refrigerant Safety – About Refrigerant Safety, Toxicity and Flammability |url=https://check-mark.co.uk/info/basics/refrigerant-safety/ |access-date=17 April 2024 |website=Checkmark}}</ref><ref>{{Cite web |date=2015-09-01 |title=A2L – Mildly Flammable Refrigerants |url=https://www.acrjournal.uk/features/a2l-mildly-flammable-refrigerants/ |access-date=2024-04-17 |website=ACR Journal}}</ref> This means that R-290 is not suitable for all situations or locations. Nonetheless, by 2022, an increasing number of devices with R-290 were offered for domestic use, especially in Europe.{{Cn|date=February 2024}}

At the same time,{{When|date=February 2024}} HFC refrigerants still dominate the market. Recent government mandates have seen the phase-out of [[Chlorodifluoromethane|R-22]] refrigerant. Replacements such as R-32 and R-410A are being promoted as environmentally friendly but still have a high GWP.<ref>{{Cite web |last=US Environmental Protection Agency |first=OAR |date=14 November 2014 |title=Phaseout of Ozone-Depleting Substances (ODS) |url=https://www.epa.gov/ods-phaseout |url-status=live |archive-url=https://web.archive.org/web/20150924132109/http://www.epa.gov/ozone/title6/phaseout/accfact.html |archive-date=24 September 2015 |access-date=16 February 2020 |website=US EPA |language=en}}</ref> A heat pump typically uses 3&nbsp;kg of refrigerant. With R-32 this amount still has a 20-year impact equivalent to 7 tons of {{CO2}}, which corresponds to two years of natural gas heating in an average household. Refrigerants with a high ODP have already been phased out.{{Cn|date=February 2024}}