Editing Heat pump

{{use dmy dates|date=November 2024}}
{{Short description|System that transfers heat from one space to another}}
{{About|devices used to heat and potentially also cool buildings or water using the refrigeration cycle|other uses|Heat pump (disambiguation)}}
[[File:Heat pump unit.webp|thumb|External heat exchanger of an [[air source heat pump|air-source heat pump]] for both heating and cooling]]
[[File:Mitsubishi Heat Pump interior air handler.agr.jpg|thumb|[[Mitsubishi Electric]] heat pump interior air handler wall unit]]
{{Sustainable energy}}
A '''heat pump''' is a device that uses [[electricity]] to transfer heat from a colder place to a warmer place. Specifically, the heat pump transfers [[thermal energy]] using a [[heat pump and refrigeration cycle]], cooling the cool space and warming the warm space.<ref>{{Cite web |title=Heat Pump Systems |url=https://www.energy.gov/energysaver/heat-pump-systems |access-date=2024-03-26 |website=Energy.gov |language=en}}</ref> In winter a heat pump can move heat from the cool outdoors to warm a house; the pump may also be designed to move heat from the house to the warmer outdoors in summer. As they transfer heat rather than generating heat, they are more energy-efficient than heating by [[gas boiler]].<ref>{{Cite web |title=Gas boiler vs heat pump: which is right for you? - British Gas |url=https://www.britishgas.co.uk/heating/guides/air-source-heat-pumps-vs-boilers.html |access-date=2025-01-18 |website=www.britishgas.co.uk |language=en-GB}}</ref> 

A gaseous [[refrigerant]] is [[Compression (physics)|compressed]] so its pressure and temperature rise. When operating as a heater in cold weather, the warmed gas flows to a [[heat exchanger]] in the indoor space where some of its thermal energy is [[heat transfer|transferred]] to that indoor space, causing the gas to [[Condensation|condense]] into a liquid. The liquified refrigerant flows to a heat exchanger in the outdoor space where the pressure falls, the liquid [[Evaporation|evaporates]] and the temperature of the gas falls. It is now colder than the temperature of the outdoor space being used as a heat source. It can again take up energy from the heat source, be compressed and repeat the cycle.

[[Air source heat pump]]s are the most common models, while other types include [[ground source heat pump]]s, [[Water-source heat pumps|water source heat pumps]] and [[exhaust air heat pump]]s.<ref>{{Cite web |title=Exhaust air heat pumps |url=https://energysavingtrust.org.uk/advice/exhaust-air-heat-pumps/ |access-date=2024-02-22 |website=Energy Saving Trust |language=en-GB}}</ref> Large-scale heat pumps are also used in [[district heating]] systems.<ref name="ieareport">{{cite report |url=https://www.iea.org/reports/the-future-of-heat-pumps |title=Technology Report: The Future of Heat Pumps |date=November 2022 |access-date=2023-01-06 |archive-url=https://web.archive.org/web/20230106161159/https://www.iea.org/reports/the-future-of-heat-pumps |archive-date=2023-01-06 |url-status=live |website=International Energy Agency}} License: CC BY 4.0.</ref>

Because of their high efficiency and the increasing share of fossil-free sources in electrical grids, heat pumps are playing a role in [[climate change mitigation]].<ref>{{Harvnb|IPCC AR6 WG3 Ch11|2022|loc=Sec. 11.3.4.1}}.</ref><ref>{{harvnb|IPCC SR15 Ch2|2018|p=142}}.</ref> Consuming 1&nbsp;kWh of electricity, they can transfer 1<ref>{{Cite web |last=Everitt |first=Neil |date=2023-09-11 |title=Study proves heat pump efficiency at low temperatures |url=https://www.coolingpost.com/world-news/study-proves-heat-pump-efficiency-at-low-temperatures/ |access-date=2024-01-22 |website=Cooling Post |language=en-GB}}</ref> to 4.5&nbsp;kWh of thermal energy into a building. The [[carbon footprint]] of heat pumps [[Life-cycle greenhouse gas emissions of energy sources|depends on how electricity is generated]], but they usually reduce emissions.<ref>{{Cite journal |last1=Deetjen |first1=Thomas A. |last2=Walsh |first2=Liam |last3=Vaishnav |first3=Parth |date=2021-07-28 |title=US residential heat pumps: the private economic potential and its emissions, health, and grid impacts |journal=Environmental Research Letters |language=en |volume=16 |issue=8 |page=084024 |bibcode=2021ERL....16h4024D |doi=10.1088/1748-9326/ac10dc |issn=1748-9326 |s2cid=236486619 |doi-access=free}}</ref> Heat pumps could satisfy over 80% of global space and water heating needs with a lower carbon footprint than gas-fired [[condensing boiler]]s: however, in 2021 they only met 10%.<ref name="ieareport" />

== Principle of operation ==

[[File:Refrigerator-cycle.svg|thumb|A: indoor compartment, B: outdoor compartment, I: insulation, 1: condenser, 2: expansion valve, 3: evaporator, 4: compressor]]{{Main|Heat pump and refrigeration cycle|Vapor-compression refrigeration}}

Heat flows spontaneously from a region of higher temperature to a region of lower temperature. Heat does not flow spontaneously from lower temperature to higher, but it can be made to flow in this direction if [[Work (physics)|work]] is performed. The work required to transfer a given amount of heat is usually much less than the amount of heat; this is the motivation for using heat pumps in applications such as the heating of water and the interior of buildings.<ref name="R&M">G. F. C. Rogers and Y. R. Mayhew (1957), ''Engineering Thermodynamics, Work and Heat Transfer'', Section 13.1, Longmans, Green & Company Limited.</ref>

The amount of work required to drive an amount of heat Q from a lower-temperature reservoir such as ambient air to a higher-temperature reservoir such as the interior of a building is:
<math display="block">W = \frac{ Q}{\mathrm{COP}}</math>
where
* <math>W </math> is the [[Mechanical work|work]] performed on the [[Working fluid selection|working fluid]] by the heat pump's compressor.
* <math> Q </math> is the [[heat]] transferred from the lower-temperature reservoir to the higher-temperature reservoir.
* <math>\mathrm{COP}</math> is the instantaneous [[coefficient of performance]] for the heat pump at the temperatures prevailing in the reservoirs at one instant.

The coefficient of performance of a heat pump is greater than one so the work required is less than the heat transferred, making a heat pump a more efficient form of heating than electrical resistance heating. As the temperature of the higher-temperature reservoir increases in response to the heat flowing into it, the coefficient of performance decreases, causing an increasing amount of work to be required for each unit of heat being transferred.<ref name=R&M/>

The [[Heat pump and refrigeration cycle#Coefficient of performance|coefficient of performance, and the work required]] by a heat pump can be calculated easily by considering an ideal heat pump operating on the [[Carnot cycle#Reversed Carnot cycle|reversed Carnot cycle]]:
*If the low-temperature reservoir is at a temperature of {{cvt|270|K|C}} and the interior of the building is at {{cvt|280|K|C}} the maximum theoretical coefficient of performance is 28. This means 1 joule of work delivers 28 joules of heat to the interior. The one joule of work ultimately ends up as [[thermal energy]] in the interior of the building and 27 joules of heat are moved from the low-temperature reservoir.{{refn|group=note|1=As explained in [[Coefficient of performance]] TheoreticalMaxCOP = (desiredIndoorTempC + 273) ÷ (desiredIndoorTempC - outsideTempC)  = (7+273) ÷  (7 - (-3)) = 280÷10 = 28 <ref name="physics.stackexchange.com">{{Cite web |title=Is there some theoretical maximum coefficient of performance (COP) for heat pumps and chillers? |url=https://physics.stackexchange.com/questions/350074/is-there-some-theoretical-maximum-coefficient-of-performance-cop-for-heat-pump |access-date=2024-02-22 |website=Physics Stack Exchange |language=en}}</ref>}}
*As the temperature of the interior of the building rises progressively to {{cvt|300|K|C}} the coefficient of performance falls progressively to 10. This means each joule of work is responsible for transferring 9 joules of heat out of the low-temperature reservoir and into the building. Again, the 1 joule of work ultimately ends up as thermal energy in the interior of the building so 10 joules of heat are added to the building interior.{{refn|group=note|1= As explained in [[Coefficient of performance]]  TheoreticalMaxCOP = (desiredIndoorTempC + 273) ÷ (desiredIndoorTempC - outsideTempC)  = (27+273) ÷  (27 - (-3)) = 300÷30 = 10<ref name="physics.stackexchange.com"/>}}

This is the theoretical amount of heat pumped but in practice it will be less for various reasons, for example if the outside unit has been installed where there is not enough airflow. More data sharing with owners and academics—perhaps from [[heat meter]]s—could improve efficiency in the long run.<ref>{{Cite web |last=Williamson |first=Chris |date=2022-10-13 |title=Heat pumps are great. Let's make them even better |url=https://medium.com/all-you-can-heat/heat-pumps-are-great-lets-make-them-even-better-d508b8e3a751 |access-date=2024-02-22 |website=All you can heat |language=en}}</ref>

== History ==
Milestones:

;1748
: [[William Cullen]] demonstrates artificial refrigeration.<ref>{{Cite web |date=2022-04-10 |title=The often forgotten Scottish inventor whose innovation changed the world |url=https://www.thenational.scot/culture/20058191.william-cullen-time-scottish-inventor-brought-cold/ |access-date=2024-02-21 |website=The National |language=en}}</ref>
;1834
: [[Jacob Perkins]] patents a design for a practical refrigerator using [[dimethyl ether]].<ref>{{cite book |last1=Bathe |first1=Greville |last2=Bathe |first2=Dorothy |title=Jacob Perkins, his inventions, his times, & his contemporaries |date=1943 |publisher=The Historical Society of Pennsylvania |page=149}}</ref>
;1852
: [[William Thomson, 1st Baron Kelvin|Lord Kelvin]] describes the theory underlying heat pumps.<ref name="MaZo-HHP2008" />
;1855–1857
: [[Peter von Rittinger]] develops and builds the first heat pump.<ref>{{Cite book| url=http://www.researchandmarkets.com/reports/2174113/an_introduction_to_thermogeology_ground_source.pdf|title=An Introduction to Thermogeology: Ground Source Heating and Cooling|last=Banks|first=David L.|publisher=Wiley-Blackwell| isbn=978-1-4051-7061-1| author-link=David L. Banks|date=2008-05-06|access-date=2014-03-05|archive-date=2016-12-20|archive-url=https://web.archive.org/web/20161220214051/http://www.researchandmarkets.com/reports/2174113/an_introduction_to_thermogeology_ground_source.pdf|url-status=live}}</ref>
;1877
: In the period before 1875, heat pumps were for the time being pursued for [[Vapor-compression evaporation|vapour compression evaporation]] (open heat pump process) in salt works with their obvious advantages for saving wood and coal. In 1857, Peter von Rittinger was the first to try to implement the idea of vapor compression in a small pilot plant. Presumably inspired by Rittinger's experiments in Ebensee, Antoine-Paul Piccard from the University of Lausanne and the engineer J. H. Weibel from the Weibel–Briquet company in Geneva built the world's first really functioning vapor compression system with a two-stage piston compressor. In 1877 this first heat pump in Switzerland was installed in the [[Bex|Bex salt works]].<ref name="MaZo-HHP2008">{{Cite web |title=History of Heat Pumping Technologies in Switzerland – Texts |url=https://www.aramis.admin.ch/Texte/?ProjectID=45262 |url-status=live |archive-url=https://web.archive.org/web/20211123223806/https://www.aramis.admin.ch/Texte/?ProjectID=45262 |archive-date=2021-11-23 |access-date=2023-09-14 |website=www.aramis.admin.ch}}</ref><ref>{{citation |last=Wirth |first=E. |title=Aus der Entwicklungsgeschichte der Wärmepumpe, Schweizerische Bauzeitung |date=1955 |url=https://www.e-periodica.ch/digbib/view?pid=sbz-002:1955:73#3306 |volume=73 |issue=52 |pages=647–650 |access-date=2021-11-20 |archive-url=https://web.archive.org/web/20211120111740/https://www.e-periodica.ch/digbib/view?pid=sbz-002:1955:73#3306 |url-status=live |language=German |archive-date=2021-11-20}}</ref>
;1928
: [[Aurel Stodola]] constructs a closed-loop heat pump (water source from [[Lake Geneva]]) which provides heating for the [[Geneva]] city hall to this day.<ref name="d-ex22731">{{cite news |last=Randall |first=Ian |date=2022-07-31 |title=Heat pumps: The centuries-old system now at the heart of the Government's energy strategy |newspaper=[[Daily Express]] |url=https://www.express.co.uk/news/science/1648227/heat-pumps-centuries-old-concept-rooted-in-British-energy-strategy |access-date=2024-03-16 }}</ref>{{Unreliable source?|date=January 2025}}
;1937–1945
: During the [[World War I|First World War]], fuel prices were very high in Switzerland but it had plenty of [[hydropower]].<ref name="MaZo-HHP2008"/>{{Rp|page=18}} In the period before and especially during the [[World War II|Second World War]], when neutral Switzerland was completely surrounded by fascist-ruled countries, the coal shortage became alarming again. Thanks to their leading position in energy technology, the Swiss companies [[Sulzer (manufacturer)|Sulzer]], [[Escher Wyss & Cie|Escher Wyss]] and [[Brown, Boveri & Cie|Brown Boveri]] built and put in operation around 35 heat pumps between 1937 and 1945. The main heat sources were lake water, river water, groundwater, and waste heat. Particularly noteworthy are the six historic heat pumps from the city of Zurich with heat outputs from 100&nbsp;kW to 6&nbsp;MW. An international milestone is the heat pump built by Escher Wyss in 1937/38 to replace the wood stoves in the City Hall of Zurich. To avoid noise and vibrations, a recently developed rotary piston compressor was used. This historic heat pump heated the town hall for 63 years until 2001. Only then was it replaced by a new, more efficient heat pump.<ref name="MaZo-HHP2008"/>
;1945
: John Sumner, City Electrical Engineer for [[Norwich]], installs an experimental water-source heat pump fed central heating system, using a nearby river to heat new Council administrative buildings. It had a seasonal efficiency ratio of 3.42, average thermal delivery of 147&nbsp;kW, and peak output of 234&nbsp;kW.<ref name="auto">{{Cite book |title=Electricity supply in the United Kingdom : a chronology – from the beginnings of the industry to 31 December 1985 |date=1987 |publisher=The Electricity Council. |isbn=978-0851881058 |oclc=17343802}}</ref>
;1948
: Robert C. Webber is credited as developing and building the first ground-source heat pump.<ref>{{cite book |last1=Banks |first1=David |title=An Introduction to Thermogeology: Ground Source Heating and Cooling |date=August 2012 |publisher=John Wiley & Sons |pages=123 }}</ref>
;1951
: First large scale installation—the [[Royal Festival Hall]] in London is opened with a [[Coal gas|town gas]]-powered reversible water-source heat pump, fed by [[River Thames|the Thames]], for both winter heating and summer cooling needs.<ref name="auto"/>
;2019
: The [[Kigali Amendment]] to phase out harmful refrigerants takes effect.

==Types==

===Air-source===
{{Excerpt|Air source heat pump}}

===Ground source===
{{Excerpt|Ground source heat pump}}

=== Heat recovery ventilation ===
{{Main|Heat recovery ventilation}}
Exhaust air heat pumps extract heat from the exhaust air of a building and require [[Ventilation (architecture)|mechanical ventilation]]. Two classes exist:
* Exhaust air-air heat pumps transfer heat to intake air.
* Exhaust air-water heat pumps transfer heat to a heating circuit that includes a tank of domestic hot water.

=== Solar-assisted ===
{{Excerpt|Solar-assisted heat pump}}

===Water-source===
[[File:Pond Loop Being Sunk.jpg|thumb|upright=1.4|Water-source heat exchanger being installed]]

A [[water-source heat pump]] works in a similar manner to a ground-source heat pump, except that it takes heat from a body of water rather than the ground. The body of water does, however, need to be large enough to be able to withstand the cooling effect of the unit without freezing or creating an adverse effect for wildlife.<ref>{{Cite web |last=Energy Saving Trust |date=2019-02-13 |title=Could a water source heat pump work for you? |url=https://energysavingtrust.org.uk/could-water-source-heat-pump-work-you/ |url-status=live |archive-url=https://web.archive.org/web/20221004170847/https://energysavingtrust.org.uk/could-water-source-heat-pump-work-you/ |archive-date=2022-10-04 |access-date=2022-10-04 |website=Energy Saving Trust |language=en-GB}}</ref> The largest water-source heat pump was installed in the Danish town of Esbjerg in 2023.<ref name="Baraniuk 2023 g296">{{cite web | last=Baraniuk | first=Chris | title=The 'exploding' demand for giant heat pumps | website=BBC News | date=2023-05-29 | url=https://www.bbc.com/news/business-65321487 | access-date=2023-09-19 | archive-date=2023-09-07 | archive-url=https://web.archive.org/web/20230907053141/https://www.bbc.com/news/business-65321487 | url-status=live }}</ref><ref name="Ristau 2022 i239">{{cite web | last=Ristau | first=Oliver | title=Energy transition, the Danish way | website=DW | date=2022-07-24 | url=https://www.dw.com/en/danish-port-city-to-play-key-role-in-europes-wind-energy-plans/a-62464055 | access-date=2023-09-19 | archive-date=2023-08-09 | archive-url=https://web.archive.org/web/20230809045025/https://www.dw.com/en/danish-port-city-to-play-key-role-in-europes-wind-energy-plans/a-62464055 | url-status=live }}</ref>

=== Others===
A thermoacoustic heat pump operates as a [[thermoacoustic heat engine]] without refrigerant but instead uses a standing wave in a sealed chamber driven by a loudspeaker to achieve a temperature difference across the chamber.<ref>{{Cite web |first=Karmela |last=Padavic-Callaghan |date=6 December 2022 |title=Heat pump uses a loudspeaker and wet strips of paper to cool air |url=https://www.newscientist.com/article/2349973-heat-pump-uses-a-loudspeaker-and-wet-strips-of-paper-to-cool-air/ |access-date=2023-01-04 |website=New Scientist |language=en-US |archive-date=2023-01-04 |archive-url=https://web.archive.org/web/20230104132907/https://www.newscientist.com/article/2349973-heat-pump-uses-a-loudspeaker-and-wet-strips-of-paper-to-cool-air/ |url-status=live }}</ref>

Electrocaloric heat pumps are solid state.<ref>{{Cite web |last=Everitt |first=Neil |date=2023-08-14 |title=Scientists claim solid-state heat pump breakthrough |url=https://www.coolingpost.com/world-news/scientists-claim-solid-state-heat-pump-breakthrough/ |access-date=2023-09-17 |website=Cooling Post |language=en-GB |archive-date=2023-09-24 |archive-url=https://web.archive.org/web/20230924115904/https://www.coolingpost.com/world-news/scientists-claim-solid-state-heat-pump-breakthrough/ |url-status=live }}</ref>

==Applications==
The [[International Energy Agency]] estimated that, as of 2021, heat pumps installed in buildings have a combined capacity of more than 1000 GW.<ref name="ieareport" /> They are used for [[heating, ventilation, and air conditioning]] (HVAC) and may also provide domestic hot water and tumble clothes drying.<ref>{{cite web|url = http://energy.gov/energysaver/heat-pump-systems|title = Heat Pump Systems|publisher = U.S. Department of Energy|access-date = 2016-02-05|archive-date = 2017-07-04|archive-url = https://web.archive.org/web/20170704201055/https://energy.gov/energysaver/heat-pump-systems|url-status = live}}</ref> The purchase costs are supported in various countries by consumer rebates.<ref>{{cite web|url =http://www.gshp.org.uk/RHI_Domestic.html|title =Renewable Heat Incentive – Domestic RHI – paid over 7 years|website =Ground Source Heat Pump Association|access-date =2017-03-12|archive-date =2018-03-08|archive-url =https://web.archive.org/web/20180308103938/http://www.gshp.org.uk/RHI_Domestic.html|url-status =live}}</ref>

===Space heating and sometimes also cooling===
In HVAC applications, a heat pump is typically a [[vapor-compression refrigeration]] device that includes a [[reversing valve]] and optimized heat exchangers so that the direction of ''heat flow'' (thermal energy movement) may be reversed. The reversing valve switches the direction of refrigerant through the cycle and therefore the heat pump may deliver either heating or cooling to a building.

Because the two heat exchangers, the condenser and evaporator, must swap functions, they are optimized to perform adequately in both modes. Therefore, the [[SEER|Seasonal Energy Efficiency Rating]] (SEER in the US) or [[European seasonal energy efficiency ratio]] of a reversible heat pump is typically slightly less than those of two separately optimized machines. For equipment to receive the [[Energy Star|US Energy Star]] rating, it must have a rating of at least 14 SEER. Pumps with ratings of 18 SEER or above are considered highly efficient. The highest efficiency heat pumps manufactured are up to 24 SEER.<ref>{{Cite web |title=Heat Pump Efficiency {{!}} Heat Pump SEER Ratings |url=https://www.carrier.com/residential/en/us/products/heat-pumps/heat-pump-efficiency/ |access-date=2023-01-14 |website=Carrier |language=en |archive-date=2023-01-14 |archive-url=https://web.archive.org/web/20230114133418/https://www.carrier.com/residential/en/us/products/heat-pumps/heat-pump-efficiency/ |url-status=live }}</ref>

[[Heating seasonal performance factor]] (in the US) or [[Seasonal Performance Factor]] (in Europe) are ratings of heating performance. The SPF is Total heat output per annum / Total electricity consumed per annum in other words the average heating COP over the year.<ref>{{Cite web |date=2019-11-07 |title=COP and SPF for Heat Pumps Explained |url=https://greenbusinesswatch.co.uk/cop-vs-spf |access-date=2024-02-22 |website=Green Business Watch UK |language=en-GB}}</ref>

====Window mounted heat pump====
[[File:Window heat pump.webp|thumb|Saddle-style window mounted heat pump 3D sketch]]
Window mounted heat pumps run on standard 120v AC outlets and provide heating, cooling, and humidity control. They are more efficient with lower noise levels, condensation management, and a smaller footprint than [[Packaged terminal air conditioner|window mounted air conditioners]] that just do cooling.<ref>{{cite web |date=11 June 2024 |title=Why This Window Heat Pump is Genius – Undecided with Matt Ferrell |url=https://undecidedmf.com/why-this-window-heat-pump-is-genius/}}</ref>

===Water heating===
In [[water heating]] applications, heat pumps may be used to heat or preheat water for swimming pools, homes or industry. Usually heat is extracted from outdoor air and transferred to an indoor water tank.<ref>{{Cite web |title=How it Works — Heat Pump Water Heaters (HPWHs) |url=https://www.energystar.gov/products/water_heaters/high_efficiency_electric_storage_water_heaters/how_it_works |access-date=2024-01-22 |website=www.energystar.gov |language=en}}</ref><ref>{{Cite web |title=Heat-pump hot water systems |url=https://www.sustainability.vic.gov.au/energy-efficiency-and-reducing-emissions/save-energy-in-the-home/water-heating/choose-the-right-hot-water-system/heat-pump-water-heaters |access-date=2024-01-22 |website=Sustainability Victoria |language=en}}</ref>

===District heating===
Large (megawatt-scale) heat pumps are used for [[district heating]].<ref>{{Cite news |last=Baraniuk |first=Chris |date=2023-05-29 |title=The 'exploding' demand for giant heat pumps |language=en-GB |work=[[BBC News]] |url=https://www.bbc.com/news/business-65321487 |access-date=2023-09-17 |archive-date=2023-09-07 |archive-url=https://web.archive.org/web/20230907053141/https://www.bbc.com/news/business-65321487 |url-status=live }}</ref> However {{As of|2022|lc=y}} about 90% of district heat is from [[fossil fuel]]s.<ref>{{Cite web |title=District Heating – Energy System |url=https://www.iea.org/energy-system/buildings/district-heating |access-date=2024-01-22 |website=IEA |language=en-GB}}</ref> In Europe, heat pumps account for a mere 1% of heat supply in district heating networks but several countries have targets to decarbonise their networks between 2030 and 2040.<ref name=ieareport /> Possible sources of heat for such applications are [[sewage]] water, ambient water (e.g. sea, lake and river water), industrial [[waste heat]], [[geothermal energy]], [[flue gas]], waste heat from [[district cooling]] and heat from solar [[seasonal thermal energy storage]].<ref name="David">{{cite journal |last1=David |first1=Andrei|display-authors=etal |date=2017 |title=Heat Roadmap Europe: Large-Scale Electric Heat Pumps in District Heating Systems |journal=[[Energies]] |volume=10 |issue= 4|pages=578 |doi=10.3390/en10040578 |doi-access=free }}</ref> Large-scale heat pumps for district heating combined with [[thermal energy storage]] offer high flexibility for the integration of [[variable renewable energy]]. Therefore, they are regarded as a key technology for [[Climate change mitigation|limiting climate change]] by [[Fossil fuel phase-out|phasing out fossil fuels]].<ref name="David" /><ref>{{cite journal |last1=Sayegh |first1=M. A. |display-authors=etal |date=2018 |title=Heat pump placement, connection and operational modes in European district heating |url=http://bura.brunel.ac.uk/handle/2438/15834 |url-status=live |journal=Energy and Buildings |volume=166 |pages=122–144 |doi=10.1016/j.enbuild.2018.02.006 |bibcode=2018EneBu.166..122S |archive-url=https://web.archive.org/web/20191214210133/http://bura.brunel.ac.uk/handle/2438/15834 |archive-date=2019-12-14 |access-date=2019-07-10}}</ref> They are also a crucial element of [[cold district heating|systems which can both heat and cool districts]].<ref>{{citation |author=Buffa |first=Simone |title=5th generation district heating and cooling systems: A review of existing cases in Europe |date=2019 |periodical=[[Renewable and Sustainable Energy Reviews]] |volume=104 |pages=504–522 |language=de |doi=10.1016/j.rser.2018.12.059 |display-authors=etal |doi-access=free|bibcode=2019RSERv.104..504B }}</ref>

===Industrial heating===
There is great potential to reduce the energy consumption and related greenhouse gas emissions in industry by application of industrial heat pumps, for example for [[process heat]].<ref>{{Cite web |title=Home |url=https://heatpumpingtechnologies.org/annex35/ |access-date=2024-02-22 |website=Annex 35 |language=en}}</ref><ref>{{Cite web |title=Industrial Heat Pumps: it's time to go electric |url=https://www.wbcsd.org/wgthg |access-date=2024-02-22 |website=World Business Council for Sustainable Development (WBCSD) |language=en-GB }}{{Dead link|date=March 2025 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> Short payback periods of less than 2 years are possible, while achieving a high reduction of {{CO2}} emissions (in some cases more than 50%).<ref>[https://heatpumpingtechnologies.org/publications/?search=Annex+35 IEA HPT TCP Annex 35 Publications] {{Webarchive|url=https://web.archive.org/web/20180921153253/https://heatpumpingtechnologies.org/publications/?search=Annex+35 |date=2018-09-21 }}</ref><ref>{{Cite web |title=Application of Industrial Heat Pumps. Annex 35 two-page summary. |url=https://heatpumpingtechnologies.org/publications/application-of-industrial-heat-pumps-annex-35-two-page-summary/ |access-date=2023-12-28 |website=HPT – Heat Pumping Technologies |language=en}}</ref> Industrial heat pumps can heat up to 200&nbsp;°C, and can meet the heating demands of many [[Light industry|light industries]].<ref>{{Cite web |date=2021-08-05 |title=Norwegian Researchers Develop World's Hottest Heat Pump |url=https://ammonia21.com/norwegian-researchers-develop-worlds-hottest-heat-pump/ |access-date=2022-06-07 |website=Ammonia21 |language=en-US |archive-date=2022-05-23 |archive-url=https://web.archive.org/web/20220523024019/https://ammonia21.com/norwegian-researchers-develop-worlds-hottest-heat-pump/ |url-status=live }}</ref><ref>{{Cite web |title=Heat pumps are key to helping industry turn electric |url=https://www.wbcsd.org/3w9qh |access-date=2022-10-04 |website=World Business Council for Sustainable Development (WBCSD) |language=en-GB |archive-date=2023-09-24 |archive-url=https://web.archive.org/web/20230924115906/https://www.wbcsd.org/Pathways/Energy/News/Heat-pumps-are-key-to-helping-industry-turn-electric |url-status=live }}</ref> In Europe alone, 15&nbsp;GW of heat pumps could be installed in 3,000 facilities in the paper, food and chemicals industries.<ref name=ieareport />

==Performance==
{{Main|Coefficient of performance}}
The performance of a heat pump is determined by the ability of the pump to extract heat from a low temperature environment (the ''source'') and deliver it to a higher temperature environment (the ''sink'').<ref name="nrc">{{Cite web|date=8 September 2022|access-date=3 April 2023|title=Heating and cooling with a heat pump: Efficiency terminology|url=https://natural-resources.canada.ca/energy-efficiency/energy-star-canada/about/energy-star-announcements/publications/heating-and-cooling-heat-pump/6817#b6|publisher=Natural Resources Canada|archive-date=3 April 2023|archive-url=https://web.archive.org/web/20230403181606/https://natural-resources.canada.ca/energy-efficiency/energy-star-canada/about/energy-star-announcements/publications/heating-and-cooling-heat-pump/6817#b6|url-status=live}}</ref> Performance varies, depending on installation details, temperature differences, site elevation, location on site, pipe runs, flow rates, and maintenance.  

In general, heat pumps work most efficiently (that is, the heat output produced for a given energy input) when the difference between the heat source and the heat sink is small. When using a heat pump for space or water heating, therefore, the heat pump will be most efficient in mild conditions, and decline in efficiency on very cold days. Performance metrics supplied to consumers attempt to take this variation into account.

Common performance metrics are the SEER (in cooling mode) and seasonal coefficient of performance (SCOP) (commonly used just for heating), although SCOP can be used for both modes of operation.<ref name=nrc/> Larger values of either metric indicate better performance.<ref name=nrc/> When comparing the performance of heat pumps, the term ''performance'' is preferred to ''efficiency'', with [[coefficient of performance]] (COP) being used to describe the ratio of useful heat movement per work input.<ref name=nrc/> An [[electrical resistance heater]] has a COP of 1.0, which is considerably lower than a well-designed heat pump which will typically have a COP of 3 to 5 with an external temperature of 10&nbsp;°C and an internal temperature of 20&nbsp;°C. Because the ground is a constant temperature source, a ground-source heat pump is not subjected to large temperature fluctuations, and therefore is the most energy-efficient type of heat pump.<ref name=nrc/>

The "seasonal coefficient of performance" (SCOP) is a measure of the aggregate energy efficiency measure over a period of one year which is dependent on regional climate.<ref name=nrc/> One framework for this calculation is given by the Commission Regulation (EU) No. 813/2013.<ref>{{CELEX|id=32013R0813|text=Commission Regulation (EU) No 813/2013 of 2 August 2013 implementing Directive 2009/125/EC of the European Parliament and of the Council with regard to ecodesign requirements for space heaters and combination heaters}}</ref>

A heat pump's operating performance in cooling mode is characterized in the US by either its [[energy efficiency ratio]] (EER) or [[seasonal energy efficiency ratio]] (SEER), both of which have units of BTU/(h·W) (note that 1 BTU/(h·W)&nbsp;= 0.293 W/W) and larger values indicate better performance.

{| class="wikitable" style="text-align:center; margin-right:auto"
|+ COP variation with output temperature
|-
! scope="col" | Pump type and source
! scope="col" | Typical use
! scope="col" | 35&nbsp;°C <br /> (e.g. heated [[screed]] floor)
! scope="col" | 45&nbsp;°C <br /> (e.g. heated screed floor)
! scope="col" | 55&nbsp;°C <br /> (e.g. heated timber floor)
! scope="col" | 65&nbsp;°C <br /> (e.g. radiator or [[Water heating|DHW]])
! scope="col" | 75&nbsp;°C <br /> (e.g. radiator and DHW)
! scope="col" | 85&nbsp;°C <br /> (e.g. radiator and DHW)
|-
|style="text-align:left"| High-efficiency air-source heat pump (ASHP), air at −20&nbsp;°C<ref name="CREN">The Canadian Renewable Energy Network [http://dsp-psd.pwgsc.gc.ca/Collection/M92-251-2002E.pdf 'Commercial Earth Energy Systems', Figure 29] {{Webarchive|url=https://web.archive.org/web/20110511210543/http://dsp-psd.pwgsc.gc.ca/Collection/M92-251-2002E.pdf |date=2011-05-11 }}. . Retrieved December 8, 2009.</ref>
|style="text-align:left"|
| 2.2
| 2.0
| ‐
| ‐
| ‐
| ‐
|-
|style="text-align:left"| Two-stage ASHP, air at −20&nbsp;°C<ref name="TIPC">Technical Institute of Physics and Chemistry, Chinese Academy of Sciences [http://repository.tamu.edu/bitstream/handle/1969.1/5474/ESL-IC-06-11-312.pdf?sequence=4 'State of the Art of Air-source Heat Pump for Cold Region', Figure 5] {{Webarchive|url=https://web.archive.org/web/20160414101842/http://repository.tamu.edu/bitstream/handle/1969.1/5474/ESL-IC-06-11-312.pdf?sequence=4 |date=2016-04-14 }}. . Retrieved April 19, 2008.</ref>
|style="text-align:left"| Low source temperature
| ''2.4''
| 2.2
| 1.9
| ‐
| ‐
| ‐
|-
|style="text-align:left"| High-efficiency ASHP, air at 0&nbsp;°C<ref name="CREN"/>
|style="text-align:left"| Low output temperature
| ''3.8''
| 2.8
| 2.2
| 2.0
| ‐
| ‐
|-
|style="text-align:left"| Prototype transcritical {{chem|CO|2}} (R744) heat pump with tripartite gas cooler, source at 0&nbsp;°C<ref name="STEEN">SINTEF Energy Research [http://www.r744.com/knowledge/papers/files/pdf/pdf_379.pdf 'Integrated CO<sub>2</sub> Heat Pump Systems for Space Heating and DHW in low-energy and passive houses', J. Steen, Table 3.1, Table 3.3] {{webarchive|url=https://web.archive.org/web/20090318233012/http://www.r744.com/knowledge/papers/files/pdf/pdf_379.pdf |date=2009-03-18 }}. . Retrieved April 19, 2008.</ref>
|style="text-align:left"| High output temperature
| 3.3
| ‐
| ‐
| ''4.2''
| ‐
| 3.0
|-
|style="text-align:left"| Ground-source heat pump (GSHP), water at 0&nbsp;°C<ref name="CREN"/>
|style="text-align:left"|
| 5.0
| 3.7
| 2.9
| 2.4
| ‐
| ‐
|-
|style="text-align:left"| GSHP, ground at 10&nbsp;°C<ref name="CREN"/>
|style="text-align:left"| Low output temperature
| ''7.2''
| 5.0
| 3.7
| 2.9
| 2.4
| ‐
|-
|style="text-align:left"| Theoretical [[Carnot cycle]] limit, source −20&nbsp;°C
|style="text-align:left"|
| 5.6
| 4.9
| 4.4
| 4.0
| 3.7
| 3.4
|-
|style="text-align:left"| Theoretical [[Carnot cycle]] limit, source 0&nbsp;°C
|style="text-align:left"|
| 8.8
| 7.1
| 6.0
| 5.2
| 4.6
| 4.2
|-
|style="text-align:left"| Theoretical [[Transcritical cycle|Lorentzen cycle]] limit ({{chem|CO|2}} pump), return fluid 25&nbsp;°C, source 0&nbsp;°C<ref name="STEEN"/>
|style="text-align:left"|
| 10.1
| 8.8
| 7.9
| 7.1
| 6.5
| 6.1
|-
|style="text-align:left"| Theoretical [[Carnot cycle]] limit, source 10&nbsp;°C
|style="text-align:left"|
| 12.3
| 9.1
| 7.3
| 6.1
| 5.4
| 4.8
|}

===Carbon footprint===
The [[carbon footprint]] of heat pumps depends on their individual efficiency and how electricity is produced. An increasing share of low-carbon energy sources such as wind and solar will lower the impact on the climate.

{| class="wikitable"
|-
! heating system !! emissions of energy source !! efficiency || resulting emissions for thermal energy
|-
| heat pump with onshore wind power || 11&nbsp;g{{CO2}}/kWh<ref>{{Cite web |title=How Wind Can Help Us Breathe Easier |url=https://www.energy.gov/eere/wind/articles/how-wind-can-help-us-breathe-easier |access-date=2023-09-13 |website=Energy.gov |language=en |archive-date=2023-08-28 |archive-url=https://web.archive.org/web/20230828230748/https://www.energy.gov/eere/wind/articles/how-wind-can-help-us-breathe-easier |url-status=live }}</ref>|| 400% (COP=4) || 3&nbsp;g{{CO2}}/kWh
|-
| heat pump with global electricity mix || 436&nbsp;g{{CO2}}/kWh<ref>{{Cite web |date=2023-04-11 |title=Global Electricity Review 2023 |url=https://ember-energy.org/latest-insights/global-electricity-review-2023/ |url-status=live |archive-url=https://web.archive.org/web/20230411233554/https://ember-climate.org/insights/research/global-electricity-review-2023/ |archive-date=2023-04-11 |access-date=2023-09-13 |website=Ember |language=en-US}}</ref> (2022) || 400% (COP=4) || 109&nbsp;g{{CO2}}/kWh
|-
| natural-gas thermal (high efficiency)|| 201&nbsp;g{{CO2}}/kWh<ref name="Quaschning 2022">{{harvnb|Quaschning|2022}}</ref> || 90%{{Cn|date=September 2023}}|| 223&nbsp;g{{CO2}}/kWh
|-
| heat pump<br />electricity by lignite (old power plant)<br />and low performance || 1221&nbsp;g{{CO2}}/kWh<ref name="Quaschning 2022"/> || 300% (COP=3) || 407&nbsp;g{{CO2}}/kWh
|}

In most settings, heat pumps will reduce {{CO2}} emissions compared to heating systems powered by [[fossil fuels]].<ref name="UK sabotaging decarbonize">{{Cite web|title=The UK is sabotaging its own plan to decarbonize heating|url=https://www.engadget.com/air-source-heat-pumps-uk-120044198.html|access-date=2021-06-06|website=Engadget |date=27 May 2021 |archive-date=2021-06-06|archive-url=https://web.archive.org/web/20210606155712/https://www.engadget.com/air-source-heat-pumps-uk-120044198.html|url-status=live}}</ref> In regions accounting for 70% of [[world energy consumption]], the emissions savings of heat pumps compared with a high-efficiency gas boiler are on average above 45% and reach 80% in countries with cleaner electricity mixes.<ref name=ieareport /> These values can be improved by 10 percentage points, respectively, with alternative refrigerants. In the United States, 70% of houses could reduce emissions by installing a heat pump.<ref>{{cite journal |last1=Deetjen |first1=Thomas A |last2=Walsh |first2=Liam |last3=Vaishnav |first3=Parth |journal=Environmental Research Letters |volume=16 |issue=8|title=US residential heat pumps: the private economic potential and its emissions, health, and grid impacts |date=2021-07-28 |page=084024 |doi=10.1088/1748-9326/ac10dc |bibcode=2021ERL....16h4024D |s2cid=236486619 |doi-access=free }}</ref><ref name=ieareport /> The rising share of renewable electricity generation in many countries is set to increase the emissions savings from heat pumps over time.<ref name=ieareport />

Heating systems powered by green hydrogen are also low-carbon and may become competitors, but are much less efficient due to the energy loss associated with hydrogen conversion, transport and use. In addition, not enough green hydrogen is expected to be available before the 2030s or 2040s.<ref>{{Cite web|date=2021-05-21|title=Can the UK rely on hydrogen to save its gas boilers?|url=https://inews.co.uk/news/can-the-uk-rely-on-hydrogen-to-save-its-gas-boilers-1014119|access-date=2021-06-06|website=inews.co.uk |archive-date=2021-06-06|archive-url=https://web.archive.org/web/20210606155711/https://inews.co.uk/news/can-the-uk-rely-on-hydrogen-to-save-its-gas-boilers-1014119|url-status=live}}</ref><ref>IEA (2022), Global Hydrogen Review 2022, IEA, Paris https://www.iea.org/reports/global-hydrogen-review-2022 {{Webarchive|url=https://web.archive.org/web/20230110225147/https://www.iea.org/reports/global-hydrogen-review-2022 |date=2023-01-10 }} , License: CC BY 4.0</ref>

==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]]
{{multiple image
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 | image1 = Large heat pump setup.webp
 | height1 = 70px
 | caption1 = Large heat pump setup for a commercial building

 | image2 = Large heat pump setup 2.webp
 | height2 = 70px
 | caption2 = Wiring and connections to a central air unit inside
}}

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}}

==Government incentives==
Financial incentives aim to protect consumers from high fossil gas costs and to reduce [[greenhouse gas emissions]],<ref>{{Cite web |title=Heat Pumps |url=https://www.iea.org/energy-system/buildings/heat-pumps |access-date=2023-09-17 |website=IEA |archive-date=2023-09-17 |archive-url=https://web.archive.org/web/20230917022539/https://www.iea.org/energy-system/buildings/heat-pumps |url-status=live }}</ref> and are currently available in more than 30 countries around the world, covering more than 70% of global heating demand in 2021.<ref name=ieareport />

===Australia===
Food processors, brewers, petfood producers and other industrial energy users are exploring whether it is feasible to use renewable energy to produce industrial-grade heat. Process heating accounts for the largest share of onsite energy use in Australian manufacturing, with lower-temperature operations like food production particularly well-suited to transition to renewables.

To help producers understand how they could benefit from making the switch, the Australian Renewable Energy Agency (ARENA) provided funding to the Australian Alliance for Energy Productivity (A2EP) to undertake pre-feasibility studies at a range of sites around Australia, with the most promising locations advancing to full feasibility studies.<ref>{{Cite web|url=https://arena.gov.au/blog/electrifying-industrial-processes-with-heat-pumps/|title=Electrifying industrial processes with heat pumps.|date=22 March 2022|access-date=2022-08-09|archive-date=2022-08-08|archive-url=https://web.archive.org/web/20220808190958/https://arena.gov.au/blog/electrifying-industrial-processes-with-heat-pumps/|url-status=live}}</ref>

In an effort to incentivize energy efficiency and reduce environmental impact, the Australian states of Victoria, New South Wales, and Queensland have implemented rebate programs targeting the upgrade of existing hot water systems. These programs specifically encourage the transition from traditional gas or electric systems to heat pump based systems.<ref>{{Cite web |last=Department of Energy, Environment and Climate Action |first=Victoria Government (Australia) |date=2023-10-11 |title=Hot water systems for businesses |url=https://www.energy.vic.gov.au/businesses/victorian-energy-upgrades-businesses/hot-water-systems |work=Victoria Government |language=en-AU}}</ref><ref>{{Cite web |last=Department of Energy, Environment and Climate Action (Australia) |first=Victoria Government |date=2023-09-23 |title=Hot water systems for households |url=https://www.energy.vic.gov.au/households/victorian-energy-upgrades-for-households/hot-water-systems |work=Victoria Government |language=en-AU}}</ref><ref>{{Cite web |last=New South Wales Climate and Energy Action |first=New South Wales Government (Australia) |date=2023-12-08 |title=Upgrade your hot water system |url=https://www.energy.nsw.gov.au/households/rebates-grants-and-schemes/household-energy-saving-upgrades/upgrade-your-hot-water |work=NSW Government |language=en-AU}}</ref><ref>{{Cite web |last=Australian Government |first=Queensland |date=2023-10-05 |title=Queensland Business Energy Saving and Transformation Rebates |url=https://www.business.qld.gov.au/running-business/energy-business/energy-efficiency-rebate |work=Queensland Government |language=en-AU}}</ref><ref>{{Cite web |last=Time To Save |date=2023-11-21 |title=Hot Water Rebates in Australia: A Detailed Guide For Businesses |url=https://www.timetosave.com.au/hot-water-rebates-in-australia/ |work=Timetosave |language=en-AU}}</ref>

===Canada===
In 2022, the Canada Greener Homes Grant<ref>{{Cite web|url=https://www.nrcan.gc.ca/energy-efficiency/homes/canada-greener-homes-grant/23441|title=Canada Greener Homes Grant|date=17 March 2021|access-date=2022-01-17|archive-date=2022-01-17|archive-url=https://web.archive.org/web/20220117215119/https://www.nrcan.gc.ca/energy-efficiency/homes/canada-greener-homes-grant/23441|url-status=live}}</ref> provides up to $5000 for upgrades (including certain heat pumps), and $600 for energy efficiency evaluations.

=== China ===
Purchase subsidies in rural areas in the 2010s reduced burning coal for heating, which had been causing ill health.<ref>{{Cite web |title=Coal fired boiler replacement in Beijing rural area |url=https://hpa.ehpa.org/coal-fired-boiler-replacement-in-beijing-rural-area-2/ |access-date=2023-09-14 |archive-date=2023-03-24 |archive-url=https://web.archive.org/web/20230324214156/https://hpa.ehpa.org/coal-fired-boiler-replacement-in-beijing-rural-area-2/ |url-status=live }}</ref>

In the 2024 report by the [[International Energy Agency]] (IEA) titled "The Future of Heat Pumps in China," it is highlighted that China, as the world's largest market for heat pumps in buildings, plays a critical role in the global industry. The country accounts for over one-quarter of global sales, with a 12% increase in 2023 alone, despite a global sales dip of 3% the same year.<ref name=":0">{{Cite web |title=Executive summary – The Future of Heat Pumps in China – Analysis |url=https://www.iea.org/reports/the-future-of-heat-pumps-in-china/executive-summary |access-date=2024-04-12 |website=IEA |language=en-GB}}</ref>

Heat pumps are now used in approximately 8% of all heating equipment sales for buildings in China as of 2022, and they are increasingly becoming the norm in central and southern regions for both heating and cooling. Despite their higher upfront costs and relatively low awareness, heat pumps are favored for their energy efficiency, consuming three to five times less energy than electric heaters or fossil fuel-based solutions. Currently, decentralized heat pumps installed in Chinese buildings represent a quarter of the global installed capacity, with a total capacity exceeding 250 GW, which covers around 4% of the heating needs in buildings.<ref name=":0" />

Under the Announced Pledges Scenario (APS), which aligns with China's carbon neutrality goals, the capacity is expected to reach 1,400 GW by 2050, meeting 25% of heating needs. This scenario would require an installation of about 100 GW of heat pumps annually until 2050. Furthermore, the heat pump sector in China employs over 300,000 people, with employment numbers expected to double by 2050, underscoring the importance of vocational training for industry growth. This robust development in the heat pump market is set to play a significant role in reducing direct emissions in buildings by 30% and cutting PM2.5 emissions from residential heating by nearly 80% by 2030.<ref name=":0" /><ref>IEA (2024), The Future of Heat Pumps in China, IEA, Paris <nowiki>https://www.iea.org/reports/the-future-of-heat-pumps-in-china</nowiki>, Licence: CC BY 4.0</ref>

===European Union===
To speed up the deployment rate of heat pumps, the European Commission launched the Heat Pump Accelerator Platform in November 2024.<ref>{{cite web| url=https://energy.ec.europa.eu/topics/energy-efficiency/heat-pumps/heat-pump-accelerator-platform_en | title= The Heat Pump Accelerator Platform| date=2024|access-date=2024-11-27|website= European Commission}}</ref> It will encourage industry experts, policymakers, and stakeholders to collaborate, share best practices and ideas, and jointly discuss measures that promote sustainable heating solutions.<ref>{{cite web| url=https://energy.ec.europa.eu/topics/energy-efficiency/heat-pumps_en | title= Heat pumps| date=2024|access-date=2024-11-27|website= European Commission}}</ref>

===United Kingdom===

Until 2027 fixed heat pumps have no Value Added Tax (VAT).<ref>{{Cite web |date=2024-01-31 |title=Energy-saving materials and heating equipment (VAT Notice 708/6) |url=https://www.gov.uk/guidance/vat-on-energy-saving-materials-and-heating-equipment-notice-7086 |access-date=2025-01-19 |website=GOV.UK |language=en}}</ref> {{As of|2022}} the installation cost of a heat pump is more than a gas boiler, but with the "Boiler Upgrade Scheme"<ref>{{Cite web |title=Apply for the Boiler Upgrade Scheme |url=https://www.gov.uk/apply-boiler-upgrade-scheme |access-date=2023-09-14 |archive-date=2023-09-19 |archive-url=https://web.archive.org/web/20230919162720/https://www.gov.uk/apply-boiler-upgrade-scheme |url-status=live }}</ref> government grant and assuming electricity/gas costs remain similar their lifetime costs would be similar on average.<ref>{{Cite web |title=BBC Radio 4 – Sliced Bread, Air Source Heat Pumps |url=https://www.bbc.co.uk/programmes/m0016pvn |url-status=live |archive-url=https://web.archive.org/web/20220430081415/https://www.bbc.co.uk/programmes/m0016pvn |archive-date=2022-04-30 |access-date=2022-04-30 |website=BBC |language=en-GB}}</ref> However lifetime cost relative to a gas boiler varies considerably depending on several factors, such as the quality of the heat pump installation and the tariff used.<ref>{{Cite web |last=Lawrence |first=Karen |date=2024-05-03 |title=Air source heat pump costs and savings |url=https://www.which.co.uk/reviews/ground-and-air-source-heat-pumps/article/ground-and-air-source-heat-pumps/air-source-heat-pump-costs-and-savings-akySY6N5Y6Dd |access-date=2024-06-07 |website=Which? |language=en}}</ref> In 2024 England was criticised for still allowing new homes to be built with gas boilers, unlike some other counties where this is banned.<ref>{{Cite web |title=Clean Heat without the Hot Air: British and Dutch lessons and challenges |url=https://ukerc.ac.uk/news/clean-heat-without-the-hot-air-british-and-dutch-lessons-and-challenges/ |access-date=2024-06-07 |website=UKERC |language=en}}</ref>

===United States===
{{Further|Environmental policy of the Joe Biden administration|Climate change in the United States}}{{Update|part=section|date=January 2025|reason=talks about 2023 in future tense}}
The High-efficiency Electric Home Rebate Program was created in 2022 to award grants to State energy offices and Indian Tribes in order to establish state-wide high-efficiency electric-home rebates. Effective immediately, American households are eligible for a tax credit to cover the costs of buying and installing a heat pump, up to $2,000. Starting in 2023, low- and moderate-level income households will be eligible for a heat-pump rebate of up to $8,000.<ref>{{cite web |author=Shao |first=Elena |title=H. R. 5376 – Inflation Reduction Act of 2022 |url=https://www.congress.gov/bill/117th-congress/house-bill/5376/text |url-status=live |archive-url=https://web.archive.org/web/20221117004205/https://www.congress.gov/bill/117th-congress/house-bill/5376/text |archive-date=17 November 2022 |access-date=17 November 2022 |website=Congress.gov |date=16 August 2022 |publisher=U.S. Congress}}</ref>

In 2022, more heat pumps were sold in the United States than natural gas furnaces.<ref>{{cite news |url=https://www.nytimes.com/interactive/2023/02/22/climate/heat-pumps-extreme-cold.html |title=As Heat Pumps Go Mainstream, a Big Question: Can They Handle Real Cold? |date=February 22, 2023 |newspaper=[[The New York Times]] |access-date=April 11, 2023 |archive-date=April 11, 2023 |archive-url=https://web.archive.org/web/20230411162035/https://www.nytimes.com/interactive/2023/02/22/climate/heat-pumps-extreme-cold.html |url-status=live }}</ref>

In November 2023 Biden's administration allocated 169 million dollars from the [[Inflation Reduction Act]] to speed up production of heat pumps. It used the Defense Production Act to do so, in a stated bid to advance national security.<ref>{{cite news |last1=Frazin |first1=Rachel |date=17 November 2023 |title=Biden administration uses wartime authority to bolster energy efficient manufacturing |agency=The Hill |url=https://thehill.com/policy/energy-environment/4315744-biden-admin-wartime-authority-bolster-energy-efficient-manufacturing/ |access-date=29 November 2023}}</ref>

== Notes ==
{{reflist|group=note}}

==References==
{{Reflist}}

===Sources===
====IPCC reports====
*{{Cite book |ref={{harvid|IPCC AR6 WG1|2021}} |author=IPCC |author-link=IPCC |year=2021 |title=Climate Change 2021: The Physical Science Basis |series=Contribution of Working Group I to the [[IPCC Sixth Assessment Report|Sixth Assessment Report]] of the Intergovernmental Panel on Climate Change |display-editors=4 |editor1-first=V. |editor1-last=Masson-Delmotte |editor2-first=P. |editor2-last=Zhai |editor3-first=A. |editor3-last=Pirani |editor4-first=S. L. |editor4-last=Connors |editor5-first=C. |editor5-last=Péan |editor6-first=S. |editor6-last=Berger |editor7-first=N. |editor7-last=Caud |editor8-first=Y. |editor8-last=Chen |editor9-first=L. |editor9-last=Goldfarb |editor10-first=M. I. |editor10-last=Gomis |publisher=Cambridge University Press (In Press) |place= |language=en |isbn= |url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Full_Report.pdf}}
**{{Cite book
|ref= {{harvid|IPCC AR6 WG1 Ch7|2021}}
|chapter=Chapter 7: The Earth's energy budget, climate feedbacks, and climate sensitivity Supplementary Material
|chapter-url= https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter07_SM.pdf
|last1=Forster |first1=P. 
|last2=Storelvmo |first2=T. 
|last3=Armour|first3=K. 
|last4=Collins|first4=W.
|title= {{Harvnb|IPCC AR6 WG1|2021}}
|year=2021
}}
<!-- ## -->
*{{Cite book   |ref= {{harvid|IPCC SR15|2018}} <!-- ipcc:20200312 -->
 |author= IPCC |author-link= IPCC
 |year= 2018
 |title= Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty
 |display-editors= 4
 |editor-first1= V.     |editor-last1= Masson-Delmotte
 |editor-first2= P.     |editor-last2= Zhai
 |editor-first3= H.-O.  |editor-last3= Pörtner
 |editor-first4= D.     |editor-last4= Roberts
 |editor-first5= J.     |editor-last5= Skea
 |editor-first6= P. R.  |editor-last6= Shukla
 |editor-first7= A.     |editor-last7= Pirani
 |editor-first8= W.     |editor-last8= Moufouma-Okia
 |editor-first9= C.     |editor-last9= Péan
 |editor-first10= R.    |editor-last10= Pidcock
 |editor-first11= S.    |editor-last11= Connors
 |editor-first12= J. B. R. |editor-last12= Matthews
 |editor-first13= Y.    |editor-last13= Chen
 |editor-first14= X.    |editor-last14= Zhou
 |editor-first15= M. I. |editor-last15= Gomis
 |editor-first16= E.    |editor-last16= Lonnoy
 |editor-first17= T.    |editor-last17= Maycock
 |editor-first18= M.    |editor-last18= Tignor
 |editor-first19= T.    |editor-last19= Waterfeld
 |publisher= Intergovernmental Panel on Climate Change
 |isbn= <!-- not issued? -->
 |url= https://www.ipcc.ch/site/assets/uploads/sites/2/2019/06/SR15_Full_Report_High_Res.pdf
}} https://www.ipcc.ch/sr15/.
<!-- ## -->
**{{Cite book  |ref= {{harvid|IPCC SR15 Ch2|2018}} <!-- ipcc:20200312 -->
 |year= 2018
 |chapter= Chapter 2: Mitigation Pathways Compatible with 1.5°C in the Context of Sustainable Development
 |chapter-url= https://www.ipcc.ch/site/assets/uploads/sites/2/2019/05/SR15_Chapter2_High_Res.pdf
 |display-authors= 4
 |first1= J.     |last1= Rogelj |author1-link=Joeri Rogelj
 |first2= D.     |last2= Shindell
 |first3= K.     |last3= Jiang
 |first4= S.     |last4= Fifta
 |first5= P.     |last5= Forster
 |first6= V.     |last6= Ginzburg
 |first7= C.     |last7= Handa
 |first8= H.     |last8= Kheshgi
 |first9= S.     |last9= Kobayashi
 |first10= E.    |last10= Kriegler
 |first11= L.    |last11= Mundaca
 |first12= R.    |last12= Séférian
 |first13= M. V. |last13= Vilariño
 |title= {{Harvnb|IPCC SR15|2018}}
 |pages= 93–174
}}
*{{Cite book |ref={{harvid|IPCC AR6 WG3|2022}} |author=IPCC |author-link=IPCC |year=2022 |title=Climate Change 2022: Mitigation of Climate Change |series=Contribution of Working Group III to the [[IPCC Sixth Assessment Report|Sixth Assessment Report]] of the Intergovernmental Panel on Climate Change |display-editors=4 |editor1-first=P. R. |editor1-last=Shula |editor2-first=J. |editor2-last=Skea |editor3-first=R. |editor3-last=Slade |editor4-first=A. |editor4-last=Al Khourdajie |editor5-first=R. |editor5-last=van Diemen |editor6-first=D. |editor6-last=McCollum |editor7-first=M. |editor7-last=Pathak |editor8-first=S. |editor8-last=Some |editor9-first=P. |editor9-last=Vyas |editor10-first=R. |editor10-last=Fradera |editor11-first=M. |editor11-last=Belkacemi |editor12-first=A. |editor12-last=Hasija |editor13-first=G. |editor13-last=Lisboa |editor14-first=S. |editor14-last=Luz |editor15-first=J. |editor15-last=Malley |publisher=Cambridge University Press (In Press) |place=Cambridge, UK and New York, New York, USA |isbn= |url=https://report.ipcc.ch/ar6wg3/pdf/IPCC_AR6_WGIII_FinalDraft_FullReport.pdf |access-date=2022-05-10 |archive-date=2022-04-04 |archive-url=https://web.archive.org/web/20220404161910/https://report.ipcc.ch/ar6wg3/pdf/IPCC_AR6_WGIII_FinalDraft_FullReport.pdf |url-status=dead}}
** {{Cite book |ref={{harvid|IPCC AR6 WG3 Ch11|2022}} |chapter=Industry |chapter-url=https://www.ipcc.ch/report/ar6/wg3/downloads/report/IPCC_AR6_WGIII_Chapter11.pdf |author=IPCC |author-link=IPCC |year=2022 |title={{Harvnb|IPCC AR6 WG3|2022}} }}
<!-- ## -->

==== Other ====
* {{cite web |ref= {{harvid|Quaschning|2022}} |last1=Quaschning |first1=Volker |title=Specific Carbon Dioxide Emissions of Various Fuels |url=https://www.volker-quaschning.de/datserv/CO2-spez/index_e.php |access-date=22 February 2022}}

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