Heat pump

A heat pump is a device that uses work to transfer heat from a cool space to a warm space by transferring thermal energy using a refrigeration cycle, cooling the cool space and warming the warm space. In cold weather 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 warm weather. As they transfer heat rather than generating heat, they are more energy-efficient than other ways of heating or cooling a home.<ref>"Heat Pump Systems". US Department of Energy. Archived from the original on 27 April 2023. Retrieved 27 April 2023.</ref>

A gaseous refrigerant is compressed so its temperature rises. 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 transferred to that indoor space, causing the gas to condense to its liquid state. The liquified refrigerant flows to a heat exchanger in the outdoor space where the pressure falls, the liquid 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 pumps are the most common models, while other types include ground source heat pumps, water source heat pumps and exhaust air heat pumps.<ref>"Exhaust air heat pumps". Energy Saving Trust. Retrieved 2024-02-22.</ref> Large-scale heat pumps are also used in district heating systems.<ref name="ieareport">Technology Report: The Future of Heat Pumps. International Energy Agency (Report). November 2022. Archived from the original on 2023-01-06. Retrieved 2023-01-06. License: CC BY 4.0.</ref>

The efficiency of a heat pump is expressed as a coefficient of performance (COP), or seasonal coefficient of performance (SCOP). The higher the number, the more efficient a heat pump is. When used for space heating, heat pumps are typically more energy-efficient than electric resistance and other heaters.

Because of their high efficiency and the increasing share of fossil-free sources in electrical grids, heat pumps are playing a key role in climate change mitigation.<ref>IPCC AR6 WG3 Ch11 2022, Sec. 11.3.4.1.</ref><ref>IPCC SR15 Ch2 2018, p. 142.</ref> Consuming 1 kWh of electricity, they can transfer 1<ref>Everitt, Neil (2023-09-11). "Study proves heat pump efficiency at low temperatures". Cooling Post. Retrieved 2024-01-22.</ref> to 4.5<ref>"How to find the Coefficient of Performance of a Heat Pump". Unacademy. Retrieved 2024-01-22.</ref> kWh of thermal energy into a building. The carbon footprint of heat pumps depends on how electricity is generated, but they usually reduce emissions.<ref>Deetjen, Thomas A.; Walsh, Liam; Vaishnav, Parth (2021-07-28). "US residential heat pumps: the private economic potential and its emissions, health, and grid impacts". Environmental Research Letters. 16 (8): 084024. Bibcode:2021ERL....16h4024D. doi:10.1088/1748-9326/ac10dc. ISSN 1748-9326. S2CID 236486619.</ref> Heat pumps could satisfy over 80% of global space and water heating needs with a lower carbon footprint than gas-fired condensing boilers: however, in 2021 they only met 10%.<ref name="ieareport" />

Principle of operation

A: indoor compartment, B: outdoor compartment, I: insulation, 1: condenser, 2: expansion valve, 3: evaporator, 4: compressor

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 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 work performed on the 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 coefficient of performance, and the work required by a heat pump can be calculated easily by considering an ideal heat pump operating on the reversed Carnot cycle:

If the low-temperature reservoir is at a temperature of 270 K (−3 °C) and the interior of the building is at 280 K (7 °C) the relevant coefficient of performance is 27. This means only 1 joule of work is required to transfer 27 joules of heat from a reservoir at 270 K to another at 280 K. The one joule of work ultimately ends up as thermal energy in the interior of the building so for each 27 joules of heat that are removed from the low-temperature reservoir, 28 joules of heat are added to the building interior, making the heat pump even more attractive from an efficiency perspective.<ref group="note">As explained in Coefficient of performance TheoreticalMaxCOP = (desiredIndoorTempC + 273) ÷ (desiredIndoorTempC - outsideTempC) = (7+273) ÷ (7 - (-3)) = 280÷10 = 28 <ref>"Is there some theoretical maximum coefficient of performance (COP) for heat pumps and chillers?". Physics Stack Exchange. Retrieved 2024-02-22.</ref></ref>

As the temperature of the interior of the building rises progressively to 300 K (27 °C) the coefficient of performance falls progressively to 9. 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.<ref group="note">As explained in Coefficient of performance TheoreticalMaxCOP = (desiredIndoorTempC + 273) ÷ (desiredIndoorTempC - outsideTempC) = (27+273) ÷ (27 - (-3)) = 300÷30 = 10<ref>"Is there some theoretical maximum coefficient of performance (COP) for heat pumps and chillers?". Physics Stack Exchange. Retrieved 2024-02-22.</ref></ref>

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 meters - could improve efficiency in the long run.<ref>Williamson, Chris (2022-10-13). "Heat pumps are great. Let's make them even better". All you can heat. Retrieved 2024-02-22.</ref>

History

Milestones:

1748: William Cullen demonstrates artificial refrigeration.<ref>"The often forgotten Scottish inventor whose innovation changed the world". The National. 2022-04-10. Retrieved 2024-02-21.</ref>
1834: Jacob Perkins builds^{[citation needed]}a practical refrigerator with dimethyl ether.
1852: 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>Banks, David L. (2008-05-06). An Introduction to Thermogeology: Ground Source Heating and Cooling (PDF). Wiley-Blackwell. ISBN 978-1-4051-7061-1. Archived (PDF) from the original on 2016-12-20. Retrieved 2014-03-05.</ref>
1877: In the period before 1875, heat pumps were for the time being pursued for 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 salt works.<ref name="MaZo-HHP2008">"History of Heat Pumping Technologies in Switzerland – Texts". www.aramis.admin.ch. Archived from the original on 2021-11-23. Retrieved 2023-09-14.</ref><ref>Wirth, E. (1955), Aus der Entwicklungsgeschichte der Wärmepumpe, Schweizerische Bauzeitung (in German), vol. 73, pp. 647–650, archived from the original on 2021-11-20, retrieved 2021-11-20{{citation}}: CS1 maint: unrecognized language (link)</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.^{[citation needed]}
1937–1945: During the First World War fuel prices were very high in Switzerland but it had plenty of hydropower.^: 18 In the period before and especially during the 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, Escher Wyss and 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 kW to 6 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 kW, and peak output of 234 kW.<ref name="auto">Electricity supply in the United Kingdom : a chronology – from the beginnings of the industry to 31 December 1985. The Electricity Council. 1987. ISBN 978-0851881058. OCLC 17343802.</ref>
1948: Robert C. Webber is credited as developing and building the first ground-source heat pump.<ref>Banks, David (August 2012). An Introduction to Thermogeology: Ground Source Heating and Cooling. John Wiley & Sons. p. 123.</ref>
1951: First large scale installation—the Royal Festival Hall in London is opened with a town gas-powered reversible water-source heat pump, fed by 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 heat pump

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Ground source heat pump

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Heat recovery ventilation

Exhaust air heat pumps extract heat from the exhaust air of a building and require 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 heat pump

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Water-source heat pump

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>Energy Saving Trust (2019-02-13). "Could a water source heat pump work for you?". Energy Saving Trust. Archived from the original on 2022-10-04. Retrieved 2022-10-04.</ref> The largest water-source heat pump was installed in the Danish town of Esbjerg in 2023.<ref name="Baraniuk 2023 g296">Baraniuk, Chris (2023-05-29). "The 'exploding' demand for giant heat pumps". BBC News. Archived from the original on 2023-09-07. Retrieved 2023-09-19.</ref><ref name="Ristau 2022 i239">Ristau, Oliver (2022-07-24). "Energy transition, the Danish way". DW. Archived from the original on 2023-08-09. Retrieved 2023-09-19.</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>Padavic-Callaghan, Karmela (6 December 2022). "Heat pump uses a loudspeaker and wet strips of paper to cool air". New Scientist. Archived from the original on 2023-01-04. Retrieved 2023-01-04.</ref>

Electrocaloric heat pumps are solid state.<ref>Everitt, Neil (2023-08-14). "Scientists claim solid-state heat pump breakthrough". Cooling Post. Archived from the original on 2023-09-24. Retrieved 2023-09-17.</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>"Heat Pump Systems". U.S. Department of Energy. Archived from the original on 2017-07-04. Retrieved 2016-02-05.</ref> The purchase costs are supported in various countries by consumer rebates.<ref>"Renewable Heat Incentive – Domestic RHI – paid over 7 years". Ground Source Heat Pump Association. Archived from the original on 2018-03-08. Retrieved 2017-03-12.</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 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 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>"Heat Pump Efficiency | Heat Pump SEER Ratings". Carrier. Archived from the original on 2023-01-14. Retrieved 2023-01-14.</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>"COP and SPF for Heat Pumps Explained". Green Business Watch UK. 2019-11-07. Retrieved 2024-02-22.</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>"How it Works — Heat Pump Water Heaters (HPWHs)". www.energystar.gov. Retrieved 2024-01-22.</ref><ref>"Heat-pump hot water systems". Sustainability Victoria. Retrieved 2024-01-22.</ref>

District heating

Large (megawatt-scale) heat pumps are used for district heating.<ref>Baraniuk, Chris (2023-05-29). "The 'exploding' demand for giant heat pumps". BBC News. Archived from the original on 2023-09-07. Retrieved 2023-09-17.</ref> However as of 2022^[update] about 90% of district heat is from fossil fuels. <ref>"District Heating - Energy System". IEA. Retrieved 2024-01-22.</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">David, Andrei; et al. (2017). "Heat Roadmap Europe: Large-Scale Electric Heat Pumps in District Heating Systems". Energies. 10 (4): 578. doi:10.3390/en10040578.</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 limiting climate change by phasing out fossil fuels.<ref name="David" /><ref>Sayegh, M. A.; et al. (2018). "Heat pump placement, connection and operational modes in European district heating". Energy and Buildings. 166: 122–144. doi:10.1016/j.enbuild.2018.02.006. Archived from the original on 2019-12-14. Retrieved 2019-07-10.</ref> They are also a crucial element of systems which can both heat and cool districts.<ref>Buffa, Simone; et al. (2019), "5th generation district heating and cooling systems: A review of existing cases in Europe", Renewable and Sustainable Energy Reviews (in Deutsch), vol. 104, pp. 504–522, doi:10.1016/j.rser.2018.12.059</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>"Home". Annex 35. Retrieved 2024-02-22. </ref><ref>"Industrial Heat Pumps: it's time to go electric". World Business Council for Sustainable Development (WBCSD). Retrieved 2024-02-22.</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>IEA HPT TCP Annex 35 Publications Archived 2018-09-21 at the Wayback Machine</ref><ref>"Application of Industrial Heat Pumps. Annex 35 two-page summary". HPT – Heat Pumping Technologies. Retrieved 2023-12-28.</ref> Industrial heat pumps can heat up to 200°C, and can meet the heating demands of many light industries.<ref>"Norwegian Researchers Develop World's Hottest Heat Pump". Ammonia21. 2021-08-05. Archived from the original on 2022-05-23. Retrieved 2022-06-07.</ref><ref>"Heat pumps are key to helping industry turn electric". World Business Council for Sustainable Development (WBCSD). Archived from the original on 2023-09-24. Retrieved 2022-10-04.</ref> In Europe alone, 15 GW of heat pumps could be installed in 3,000 facilities in the paper, food and chemicals industries.<ref name=ieareport />

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">"Heating and cooling with a heat pump: Efficiency terminology". Natural Resources Canada. 8 September 2022. Archived from the original on 3 April 2023. Retrieved 3 April 2023.</ref> Performance varies, depending on installation details, temperature differences, site elevation, location on site, pipe runs, flow rates, and maintenance.

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 °C and an internal temperature of 20 °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>"Ecodesign requirements for space heaters". European Union Law. Archived from the original on 2021-01-18. Retrieved 2021-01-31.</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) = 0.293 W/W) and larger values indicate better performance.

COP variation with output temperature
Pump type and source	Typical use	35 °C (e.g. heated screed floor)	45 °C (e.g. heated screed floor)	55 °C (e.g. heated timber floor)	65 °C (e.g. radiator or DHW)	75 °C (e.g. radiator and DHW)	85 °C (e.g. radiator and DHW)
High-efficiency air-source heat pump (ASHP), air at −20 °C<ref name="CREN">The Canadian Renewable Energy Network 'Commercial Earth Energy Systems', Figure 29 Archived 2011-05-11 at the Wayback Machine. . Retrieved December 8, 2009.</ref>		2.2	2.0	‐	‐	‐	‐
Two-stage ASHP, air at −20 °C<ref name="TIPC">Technical Institute of Physics and Chemistry, Chinese Academy of Sciences 'State of the Art of Air-source Heat Pump for Cold Region', Figure 5 Archived 2016-04-14 at the Wayback Machine. . Retrieved April 19, 2008.</ref>	Low source temperature	2.4	2.2	1.9	‐	‐	‐
High-efficiency ASHP, air at 0 °C<ref name="CREN"/>	Low output temperature	3.8	2.8	2.2	2.0	‐	‐
Prototype transcritical CO ₂ (R744) heat pump with tripartite gas cooler, source at 0 °C<ref name="STEEN">SINTEF Energy Research 'Integrated CO₂ Heat Pump Systems for Space Heating and DHW in low-energy and passive houses', J. Steen, Table 3.1, Table 3.3 Archived 2009-03-18 at the Wayback Machine. . Retrieved April 19, 2008.</ref>	High output temperature	3.3	‐	‐	4.2	‐	3.0
Ground-source heat pump (GSHP), water at 0 °C<ref name="CREN"/>		5.0	3.7	2.9	2.4	‐	‐
GSHP, ground at 10 °C<ref name="CREN"/>	Low output temperature	7.2	5.0	3.7	2.9	2.4	‐
Theoretical Carnot cycle limit, source −20 °C		5.6	4.9	4.4	4.0	3.7	3.4
Theoretical Carnot cycle limit, source 0 °C		8.8	7.1	6.0	5.2	4.6	4.2
Theoretical Lorentzen cycle limit (CO ₂ pump), return fluid 25 °C, source 0 °C<ref name="STEEN"/>		10.1	8.8	7.9	7.1	6.5	6.1
Theoretical Carnot cycle limit, source 10 °C		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.

heating system	emissions of energy source	efficiency	resulting emissions for thermal energy
heat pump with onshore wind power	11 gCO₂/kWh<ref>"How Wind Can Help Us Breathe Easier". Energy.gov. Archived from the original on 2023-08-28. Retrieved 2023-09-13.</ref>	400% (COP=4)	3 gCO₂/kWh
heat pump with global electricity mix	436 gCO₂/kWh<ref>"Global Electricity Review 2023". Ember. 2023-04-11. Archived from the original on 2023-04-11. Retrieved 2023-09-13.</ref> (2022)	400% (COP=4)	109 gCO₂/kWh
natural-gas thermal (high efficiency)	201 gCO₂/kWh<ref name="Quaschning 2022">Quaschning 2022</ref>	90%^{[citation needed]}	223 gCO₂/kWh
heat pump electricity by lignite (old power plant) and low performance	1221 gCO₂/kWh<ref name="Quaschning 2022"/>	300% (COP=3)	407 gCO₂/kWh

In most settings, heat pumps will reduce CO₂ emissions compared to heating systems powered by fossil fuels.<ref name="UK sabotaging decarbonize">"The UK is sabotaging its own plan to decarbonize heating". Engadget. 27 May 2021. Archived from the original on 2021-06-06. Retrieved 2021-06-06.</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>Deetjen, Thomas A; Walsh, Liam; Vaishnav, Parth (2021-07-28). "US residential heat pumps: the private economic potential and its emissions, health, and grid impacts". Environmental Research Letters. 16 (8): 084024. Bibcode:2021ERL....16h4024D. doi:10.1088/1748-9326/ac10dc. S2CID 236486619.</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>"Can the UK rely on hydrogen to save its gas boilers?". inews.co.uk. 2021-05-21. Archived from the original on 2021-06-06. Retrieved 2021-06-06.</ref><ref>IEA (2022), Global Hydrogen Review 2022, IEA, Paris https://www.iea.org/reports/global-hydrogen-review-2022 Archived 2023-01-10 at the Wayback Machine , License: CC BY 4.0</ref>

Operation

Figure 2: Temperature–entropy diagram of the vapor-compression cycle.

Large heat pump setup for a commercial building

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 8 main components: a compressor, a reservoir, a reversing valve which selects between heating and cooling mode, two thermal expansion valves (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 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 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 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 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 (COP) by 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>Ludwig von Cube, Hans (1981). Heat Pump Technology. Butterworths. pp. 22–23. ISBN 0-408-00497-5. Archived from the original on 2023-04-03. Retrieved 2023-01-02.</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.

Refrigerant choice

Pure refrigerants can be divided into organic substances (hydrocarbons (HCs), chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), hydrofluoroolefins (HFOs), and HCFOs), and inorganic substances (ammonia (NH
₃), carbon dioxide (CO
₂), and water (H
₂O)<ref>Chamoun, Marwan; Rulliere, Romuald; Haberschill, Philippe; Berail, Jean Francois (2012-06-01). "Dynamic model of an industrial heat pump using water as refrigerant". International Journal of Refrigeration. 35 (4): 1080–1091. doi:10.1016/j.ijrefrig.2011.12.007. ISSN 0140-7007.</ref>).<ref>Wu, Di (2021). "Vapor compression heat pumps with pure Low-GWP refrigerants". Renewable and Sustainable Energy Reviews. 138: 110571. doi:10.1016/j.rser.2020.110571. ISSN 1364-0321. S2CID 229455137. Archived from the original on 2023-09-24. Retrieved 2022-11-17.</ref> Their boiling points are usually below −25 °C.<ref>"Everything you need to know about the wild world of heat pumps". MIT Technology Review. Archived from the original on 2023-08-01. Retrieved 2023-09-19.</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,^{[clarification needed]} 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>Miara, Marek (2019-10-22). "Heat Pumps with Climate-Friendly Refrigerant Developed for Indoor Installation". Fraunhofer ISE. Archived from the original on 2022-02-20. Retrieved 2022-02-21.</ref> as most countries have now ratified the Kigali Amendment to ban HFCs.<ref>Rabe, Barry G. (2022-09-23). "Pivoting from global climate laggard to leader: Kigali and American HFC policy". Brookings. Archived from the original on 2022-10-04. Retrieved 2022-10-04.</ref> Isobutane (R600A) and propane (R290) are far less harmful to the environment than conventional hydrofluorocarbons (HFC) and are already being used in air-source heat pumps.<ref>Itteilag, Richard L. (2012-08-09). Green Electricity and Global Warming. AuthorHouse. p. 77. ISBN 9781477217405. Archived from the original on 2021-11-23. Retrieved 2020-11-01.</ref> Propane may be the most suitable for high temperature heat pumps.<ref name=":2" /> Ammonia (R717) and carbon dioxide (R-744) also have a low GWP. As of 2023^[update] smaller CO
₂ heat pumps are not widely available and research and development of them continues.<ref>"Smart CO2 Heat Pump". www.dti.dk. Archived from the original on 2023-01-30. Retrieved 2023-09-17.</ref> A 2024 report said that refrigerants with GWP are vulnerable to further international restrictions.<ref>"Annex 53 Advanced Cooling/Refrigeration Technologies 2 page summary". HPT - Heat Pumping Technologies. Retrieved 2024-02-19.</ref>

Until the 1990s, heat pumps, along with fridges and other related products used chlorofluorocarbons (CFCs) as refrigerants, which caused major damage to the ozone layer when released into the atmosphere. Use of these chemicals was banned or severely restricted by the Montreal Protocol of August 1987.<ref>"Handbook for the Montreal Protocol on Substances that Deplete the Ozone Layer – 7th Edition". United Nations Environment Programme – Ozone Secretariat. 2007. Archived from the original on 2016-05-30. Retrieved 2016-12-18.</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>"Refrigerants – Environmental Properties". The Engineering ToolBox. Archived from the original on 2013-03-14. Retrieved 2016-09-12.</ref> HFCs are powerful greenhouse gases which contribute to climate change.<ref name="R-410A">R-410A#Environmental effects.</ref><ref>Ecometrica.com (27 June 2012). "Calculation of green house gas potential of R-410A". Archived from the original on 2015-07-13. Retrieved 2015-07-13.</ref> Dimethyl ether (DME) also gained in popularity as a refrigerant in combination with R404a.<ref name="mecanica-dme">"R404 and DME Refrigerant blend as a new solution to limit global warming potential" (PDF). March 14, 2012. Archived from the original (PDF) on March 14, 2012.</ref> More recent refrigerants include difluoromethane (R32) with a lower GWP, but still over 600.

refrigerant	20-year GWP	100-year GWP
R-290 propane<ref name="ar6">IPCC_AR6_WG1_Ch7 2021, 7SM-26</ref>	0.072	0.02
R-600a isobutane		3<ref>LearnMetrics (2023-05-12). "List of Low GWP Refrigerants: 69 Refrigerants Below 500 GWP". LearnMetrics. Archived from the original on 2023-06-10. Retrieved 2023-09-13.</ref>
R-32<ref name="ar6"/>	491	136
R-410a<ref name=":1">"Global warming potential (GWP) of HFC refrigerants". iifiir.org. Archived from the original on 2023-09-24. Retrieved 2023-09-13.</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">"Propane-powered heat pumps are greener". The Economist. 2023-09-06. ISSN 0013-0613. Archived from the original on 2023-09-17. Retrieved 2023-09-17.</ref><ref>Everitt, Neil (2023-09-15). "Qvantum plant has 1 million heat pump capacity". Cooling Post. Archived from the original on 2023-09-24. Retrieved 2023-09-17.</ref> The GWP of propane is about 500 times less than conventional HFC refrigerants and thus extremely low. The flammability of propane requires additional security measures. This issue can be targeted with a reduced charge.<ref>Miara, Marek (22 October 2019). "Heat Pumps with Climate-Friendly Refrigerant Developed for Indoor Installation". Fraunhofer ISE. Archived from the original on 20 February 2022. Retrieved 21 February 2022.</ref> By 2022, an increasing number of devices with R-290 were offered for domestic use, especially in Europe.^{[citation needed]}

At the same time,^[when?] HFC refrigerants still dominate the market. Recent government mandates have seen the phase-out of R-22 refrigerant. Replacements such as R-32 and R-410A are being promoted as environmentally friendly but still have a high GWP.<ref>US Environmental Protection Agency, OAR (14 November 2014). "Phaseout of Ozone-Depleting Substances (ODS)". US EPA. Archived from the original on 24 September 2015. Retrieved 16 February 2020.</ref> A heat pump typically uses 3 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.^{[citation needed]}

Government incentives

Financial incentives aim to protect consumers from high fossil gas costs and to reduce greenhouse gas emissions,<ref>"Heat Pumps". IEA. Archived from the original on 2023-09-17. Retrieved 2023-09-17.</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>"Electrifying industrial processes with heat pumps". 22 March 2022. Archived from the original on 2022-08-08. Retrieved 2022-08-09.</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>Department of Energy, Environment and Climate Action, Victoria Government (Australia) (2023-10-11). "Hot water systems for businesses". Victoria Government.</ref><ref>Department of Energy, Environment and Climate Action (Australia), Victoria Government (2023-09-23). "Hot water systems for households". Victoria Government.</ref><ref>New South Wales Climate and Energy Action, New South Wales Government (Australia) (2023-12-08). "Upgrade your hot water system". NSW Government.</ref><ref>Australian Government, Queensland (2023-10-05). "Queensland Business Energy Saving and Transformation Rebates". Queensland Government.</ref><ref>Time To Save (2023-11-21). "Hot Water Rebates in Australia: A Detailed Guide For Businesses". Timetosave.</ref>

Canada

In 2022, the Canada Greener Homes Grant<ref>"Canada Greener Homes Grant". 17 March 2021. Archived from the original on 2022-01-17. Retrieved 2022-01-17.</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>"Coal fired boiler replacement in Beijing rural area". Archived from the original on 2023-03-24. Retrieved 2023-09-14.</ref>

United Kingdom

As of 2022: heat pumps have no Value Added Tax (VAT) although in Northern Ireland they are taxed at the reduced rate of 5% instead of the usual level of VAT of 20% for most other products.<ref>"HMCR rates for goods and services". 11 July 2022. Archived from the original on 2022-07-22. Retrieved 2022-08-24.</ref> As of 2022^[update] the installation cost of a heat pump is more than a gas boiler, but with the "Boiler Upgrade Scheme"<ref>"Apply for the Boiler Upgrade Scheme". Archived from the original on 2023-09-19. Retrieved 2023-09-14.</ref> government grant and assuming electricity/gas costs remain similar their lifetime costs would be similar.<ref>"BBC Radio 4 – Sliced Bread, Air Source Heat Pumps". BBC. Archived from the original on 2022-04-30. Retrieved 2022-04-30.</ref>

United States

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>Shao, Elena. "H. R. 5376 – Inflation Reduction Act of 2022". Congress.gov. U.S. Congress. Archived from the original on 17 November 2022. Retrieved 17 November 2022.</ref>

In 2022, more heat pumps were sold in the United States than natural gas furnaces.<ref>"As Heat Pumps Go Mainstream, a Big Question: Can They Handle Real Cold?". The New York Times. February 22, 2023. Archived from the original on April 11, 2023. Retrieved April 11, 2023.</ref>

In November 2023 Biden's administration allocated 169 million dollars from the Inflation Reduction Act to speed production of heat pumps. It used the Defense Production Act for doing so, because according to the administration, energy that is better for climate, is better also for national security.<ref>Frazin, Rachel (17 November 2023). "Biden administration uses wartime authority to bolster energy efficient manufacturing". The Hill. Retrieved 29 November 2023.</ref>

Notes

References

Sources

IPCC reports

IPCC (2021). Masson-Delmotte, V.; Zhai, P.; Pirani, A.; Connors, S. L.; et al. (eds.). Climate Change 2021: The Physical Science Basis (PDF). Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press (In Press).
- Forster, P.; Storelvmo, T.; Armour, K.; Collins, W. (2021). "Chapter 7: The Earth's energy budget, climate feedbacks, and climate sensitivity Supplementary Material" (PDF). IPCC AR6 WG1 2021.
IPCC (2018). Masson-Delmotte, V.; Zhai, P.; Pörtner, H.-O.; Roberts, D.; et al. (eds.). 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 (PDF). Intergovernmental Panel on Climate Change. https://www.ipcc.ch/sr15/.
- Rogelj, J.; Shindell, D.; Jiang, K.; Fifta, S.; et al. (2018). "Chapter 2: Mitigation Pathways Compatible with 1.5°C in the Context of Sustainable Development" (PDF). IPCC SR15 2018. pp. 93–174.
IPCC (2022). Shula, P. R.; Skea, J.; Slade, R.; Al Khourdajie, A.; et al. (eds.). Climate Change 2022: Mitigation of Climate Change (PDF). Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, UK and New York, New York, USA: Cambridge University Press (In Press). Archived from the original (PDF) on 2022-04-04. Retrieved 2022-05-10.
- IPCC (2022). "Industry" (PDF). IPCC AR6 WG3 2022.

Other

Quaschning, Volker. "Specific Carbon Dioxide Emissions of Various Fuels". Retrieved 22 February 2022.

External links

Media related to Lua error in Module:Commons_link at line 63: attempt to index field 'wikibase' (a nil value). at Wikimedia Commons

Lua error in Module:Authority_control at line 181: attempt to index field 'wikibase' (a nil value).

v t e Heating, ventilation, and air conditioning
Fundamental concepts	Air changes per hour Bake-out Building envelope Convection Dilution Domestic energy consumption Enthalpy Fluid dynamics Gas compressor Heat pump and refrigeration cycle Heat transfer Humidity Infiltration Latent heat Noise control Outgassing Particulates Psychrometrics Sensible heat Stack effect Thermal comfort Thermal destratification Thermal mass Thermodynamics Vapour pressure of water
Technology	Absorption refrigerator Air barrier Air conditioning Antifreeze Automobile air conditioning Autonomous building Building insulation materials Central heating Central solar heating Chilled beam Chilled water Constant air volume (CAV) Coolant Cross ventilation Dedicated outdoor air system (DOAS) Deep water source cooling Demand controlled ventilation (DCV) Displacement ventilation District cooling District heating Electric heating Energy recovery ventilation (ERV) Firestop Forced-air Forced-air gas Free cooling Heat recovery ventilation (HRV) Hybrid heat Hydronics Ice storage air conditioning Kitchen ventilation Mixed-mode ventilation Microgeneration Passive cooling Passive daytime radiative cooling Passive house Passive ventilation Radiant heating and cooling Radiant cooling Radiant heating Radon mitigation Refrigeration Renewable heat Room air distribution Solar air heat Solar combisystem Solar cooling Solar heating Thermal insulation Thermosiphon Underfloor air distribution Underfloor heating Vapor barrier Vapor-compression refrigeration (VCRS) Variable air volume (VAV) Variable refrigerant flow (VRF) Ventilation Water heat recycling
Components	Air conditioner inverter Air door Air filter Air handler Air ionizer Air-mixing plenum Air purifier Air source heat pump Attic fan Automatic balancing valve Back boiler Barrier pipe Blast damper Boiler Centrifugal fan Ceramic heater Chiller Condensate pump Condenser Condensing boiler Convection heater Compressor Cooling tower Damper Dehumidifier Duct Economizer Electrostatic precipitator Evaporative cooler Evaporator Exhaust hood Expansion tank Fan Fan coil unit Fan filter unit Fan heater Fire damper Fireplace Fireplace insert Freeze stat Flue Freon Fume hood Furnace Gas compressor Gas heater Gasoline heater Grease duct Grille Ground-coupled heat exchanger Ground source heat pump Heat exchanger Heat pipe Heat pump Heating film Heating system HEPA High efficiency glandless circulating pump High-pressure cut-off switch Humidifier Infrared heater Inverter compressor Kerosene heater Louver Mechanical room Oil heater Packaged terminal air conditioner Plenum space Pressurisation ductwork Process duct work Radiator Radiator reflector Recuperator Refrigerant Register Reversing valve Run-around coil Sail switch Scroll compressor Solar chimney Solar-assisted heat pump Space heater Smoke canopy Smoke damper Smoke exhaust ductwork Thermal expansion valve Thermal wheel Thermostatic radiator valve Trickle vent Trombe wall TurboSwing Turning vanes Ultra-low particulate air (ULPA) Whole-house fan Windcatcher Wood-burning stove Zone valve
Measurement and control	Air flow meter Aquastat BACnet Blower door Building automation Carbon dioxide sensor Clean air delivery rate (CADR) Control valve Gas detector Home energy monitor Humidistat HVAC control system Infrared thermometer Intelligent buildings LonWorks Minimum efficiency reporting value (MERV) OpenTherm Programmable communicating thermostat Programmable thermostat Psychrometrics Room temperature Smart thermostat Thermographic camera Thermostat Thermostatic radiator valve
Professions, trades, and services	Architectural acoustics Architectural engineering Architectural technologist Building services engineering Building information modeling (BIM) Deep energy retrofit Duct cleaning Duct leakage testing Environmental engineering Hydronic balancing Kitchen exhaust cleaning Mechanical engineering Mechanical, electrical, and plumbing Mold growth, assessment, and remediation Refrigerant reclamation Testing, adjusting, balancing
Industry organizations	AHRI AMCA ASHRAE ASTM International BRE BSRIA CIBSE Institute of Refrigeration IIR LEED SMACNA UMC
Health and safety	Indoor air quality (IAQ) Passive smoking Sick building syndrome (SBS) Volatile organic compound (VOC)
See also	ASHRAE Handbook Building science Fireproofing Glossary of HVAC terms Warm Spaces World Refrigeration Day Template:Home automation Template:Solar energy

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Heat pump

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Air-source heat pump

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Heat recovery ventilation

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Improvement of coefficient of performance (COP) by subcooling

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