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.[1] 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.[2]
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.[3] Large-scale heat pumps are also used in district heating systems.[4]
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.[5][6] Consuming 1 kWh of electricity, they can transfer 1[7] to 4.5[8] kWh of thermal energy into a building. The carbon footprint of heat pumps depends on how electricity is generated, but they usually reduce emissions.[9] 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%.[4]
Principle of operation
[
edit
]
A: indoor compartment, B: outdoor compartment, I: insulation, 1: condenser, 2: expansion valve, 3: evaporator, 4: compressorHeat 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.[10]
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:
W = Q C O P {\displaystyle W={\frac {Q}{\mathrm {COP} }}}
W {\displaystyle W}
work performed on the working fluid by the heat pump's compressor.Q {\displaystyle Q}
heat transferred from the lower-temperature reservoir to the higher-temperature reservoir.C O P {\displaystyle \mathrm {COP} }
coefficient of performance for the heat pump at the temperatures prevailing in the reservoirs at one instant.where
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.[10]
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:
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.[13]
History
[
edit
]
Milestones:
Types
[
edit
]
Air-source heat pump
[
edit
]
This section is an excerpt from Air source heat pump
Heat pump on balcony of apartmentAn air source heat pump (ASHP) is a heat pump that can absorb heat from air outside a building and release it inside; it uses the same vapor-compression refrigeration process and much the same equipment as an air conditioner, but in the opposite direction. ASHPs are the most common type of heat pump and, usually being smaller, tend to be used to heat individual houses or flats rather than blocks, districts or industrial processes.[21]
Air-to-air heat pumps provide hot or cold air directly to rooms, but do not usually provide hot water. Air-to-water heat pumps use radiators or underfloor heating to heat a whole house and are often also used to provide domestic hot water.
An ASHP can typically gain 4 kWh thermal energy from 1 kWh electric energy. They are optimized for flow temperatures between 30 and 40 °C (86–104 °F), suitable for buildings with heat emitters sized for low flow temperatures. With losses in efficiency, an ASHP can even provide full central heating with a flow temperature up to 80 °C (176 °F).[22]
As of 2023 about 10% of building heating worldwide is from ASHPs. They are the main way to phase out gas boilers (also known as "furnaces") from houses, to avoid their greenhouse gas emissions.[23]
Air-source heat pumps are used to move heat between two heat exchangers, one outside the building which is fitted with fins through which air is forced using a fan and the other which either directly heats the air inside the building or heats water which is then circulated around the building through radiators or underfloor heating which releases the heat to the building. These devices can also operate in a cooling mode where they extract heat via the internal heat exchanger and eject it into the ambient air using the external heat exchanger. Some can be used to heat water for washing which is stored in a domestic hot water tank.[24]
Air-source heat pumps are relatively easy and inexpensive to install, so are the most widely used type. In mild weather, coefficient of performance (COP) may be between 2 and 5, while at temperatures below around −8 °C (18 °F) an air-source heat pump may still achieve a COP of 1 to 4.[25]
While older air-source heat pumps performed relatively poorly at low temperatures and were better suited for warm climates, newer models with variable-speed compressors remain highly efficient in freezing conditions allowing for wide adoption and cost savings in places like Minnesota and Maine in the United States.[26]While older air-source heat pumps performed relatively poorly at low temperatures and were better suited for warm climates, newer models with variable-speed compressors remain highly efficient in freezing conditions allowing for wide adoption and cost savings in places like Minnesota and Maine in the United States.
Ground source heat pump
[
edit
]
This section is an excerpt from Ground source heat pump
A heat pump in combination with heat and cold storageA ground source heat pump (also geothermal heat pump) is a heating/cooling system for buildings that use a type of heat pump to transfer heat to or from the ground, taking advantage of the relative constancy of temperatures of the earth through the seasons. Ground-source heat pumps (GSHPs) – or geothermal heat pumps (GHP), as they are commonly termed in North America – are among the most energy-efficient technologies for providing HVAC and water heating, using far less energy than can be achieved by burning a fuel in a boiler/furnace or by use of resistive electric heaters.
Efficiency is given as a [27] OtherwiseEfficiency is given as a coefficient of performance (CoP) which is typically in the range 3 – 6, meaning that the devices provide 3 – 6 units of heat for each unit of electricity used. Setup costs are higher than for other heating systems, due to the requirement to install ground loops over large areas or to drill bore holes, and for this reason, ground source is often suitable when new blocks of flats are built.Otherwise air-source heat pumps are often used instead.
Heat recovery ventilation
[
edit
]
Exhaust air heat pumps extract heat from the exhaust air of a building and require mechanical ventilation. Two classes exist:
Solar-assisted heat pump
[
edit
]
Water-source heat pump
[
edit
]
Water-source heat exchanger being installedA 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.[31] The largest water-source heat pump was installed in the Danish town of Esbjerg in 2023.[32][33]
Others
[
edit
]
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.[34]
Electrocaloric heat pumps are solid state.[35]
Applications
[
edit
]
The International Energy Agency estimated that, as of 2021, heat pumps installed in buildings have a combined capacity of more than 1000 GW.[4] They are used for heating, ventilation, and air conditioning (HVAC) and may also provide domestic hot water and tumble clothes drying.[36] The purchase costs are supported in various countries by consumer rebates.[37]
Space heating and sometimes also cooling
[
edit
]
Heat pump on balcony of apartmentIn 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.[38]
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.[39]
Water heating
[
edit
]
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.[40][41]
District heating
[
edit
]
Large (megawatt-scale) heat pumps are used for district heating.[42] However as of 2022 about 90% of district heat is from fossil fuels. [43] 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.[4] 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.[44] 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.[44][45] They are also a crucial element of systems which can both heat and cool districts.[46]
Industrial heating
[
edit
]
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.[47][48] Short payback periods of less than 2 years are possible, while achieving a high reduction of CO2 emissions (in some cases more than 50%).[49][50] Industrial heat pumps can heat up to 200°C, and can meet the heating demands of many light industries.[51][52] In Europe alone, 15 GW of heat pumps could be installed in 3,000 facilities in the paper, food and chemicals industries.[4]
Performance
[
edit
]
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).[53] 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.[53] Larger values of either metric indicate better performance.[53] 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.[53] 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.[53]
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.[53] One framework for this calculation is given by the Commission Regulation (EU) No. 813/2013.[54]
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 °CCO
2
CO
2
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[58] 400% (COP=4) 3 gCO2/kWh heat pump with global electricity mix 436 gCO2/kWh[59] (2022) 400% (COP=4) 109 gCO2/kWh natural-gas thermal (high efficiency) 201 gCO2/kWh[60] 90%[citation needed
] 223 gCO2/kWh heat pumpIn most settings, heat pumps will reduce CO2 emissions compared to heating systems powered by fossil fuels.[61] 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.[4] 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.[62][4] The rising share of renewable electricity generation in many countries is set to increase the emissions savings from heat pumps over time.[4]
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.[63][64]
Operation
[
edit
]
An internal view of the outdoor unit of an Ecodan air source heat pumpLarge 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[65] 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
[
edit
]
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.[66]
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
[
edit
]
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)[67]).[68] Their boiling points are usually below −25 °C.[69]
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,[70] as most countries have now ratified the Kigali Amendment to ban HFCs.[71] 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.[72] Propane may be the most suitable for high temperature heat pumps.[73] Ammonia (R717) and carbon dioxide (R-744) also have a low GWP. As of 2023 smaller CO
2 heat pumps are not widely available and research and development of them continues.[74] A 2024 report said that refrigerants with GWP are vulnerable to further international restrictions.[75]
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.[76]
Replacements, including R-134a and R-410A, are hydrofluorocarbons (HFC) with similar thermodynamic properties with insignificant ozone depletion potential (ODP) but had problematic GWP.[77] HFCs are powerful greenhouse gases which contribute to climate change.[78][79] Dimethyl ether (DME) also gained in popularity as a refrigerant in combination with R404a.[80] More recent refrigerants include difluoromethane (R32) with a lower GWP, but still over 600.
refrigerant 20-year GWP 100-year GWP R-290 propane[81] 0.072 0.02 R-600a isobutane 3[82] R-32[81] 491 136 R-410a[83] 4705 2285 R-134a[83] 4060 1470 R-404a[83] 7258 4808Devices with R-290 refrigerant (propane) are expected to play a key role in the future.[73][84] 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.[85] 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.[86] 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
[
edit
]
Financial incentives aim to protect consumers from high fossil gas costs and to reduce greenhouse gas emissions,[87] and are currently available in more than 30 countries around the world, covering more than 70% of global heating demand in 2021.[4]
Australia
[
edit
]
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.[88]
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.[89][90][91][92][93]
Canada
[
edit
]
In 2022, the Canada Greener Homes Grant[94] provides up to $5000 for upgrades (including certain heat pumps), and $600 for energy efficiency evaluations.
China
[
edit
]
Purchase subsidies in rural areas in the 2010s reduced burning coal for heating, which had been causing ill health.[95]
United Kingdom
[
edit
]
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.[96] As of 2022 the installation cost of a heat pump is more than a gas boiler, but with the "Boiler Upgrade Scheme"[97] government grant and assuming electricity/gas costs remain similar their lifetime costs would be similar.[98]
United States
[
edit
]
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.[99]
In 2022, more heat pumps were sold in the United States than natural gas furnaces.[100]
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.[101]
Notes
[
edit
]
References
[
edit
]
Sources
[
edit
]
IPCC reports
[
edit
]
Other
[
edit
]
Studio Harmony/Shutterstock
By now, you've learned that air-source heat pumps use an outdoor fan to bring air over refrigerant-filled coils. Two sets of these coils transfer this heat indoors where it's then blown away from the coils by a second fan, and distributed through your home as cool goodness. Some air-source heat pump systems consist of a single packaged unit containing both sets of coils in one box. This box is then installed on the roof of a building with the ductwork extending through the wall. A lot of larger systems for commercial buildings are installed in this way.
Home heat pumps are usually split systems with an outdoor and an indoor component installed through the wall. Depending on the type of system, it may have one or more indoor components to distribute heat.
Advertisement
Ground-source heat pumps (also known as geothermal heat pumps) are a little different. They absorb heat from the ground or an underground body of water and transfer it indoors — or vice versa. The most common type of ground-source heat pump transfers heat directly from the ground by absorbing it through buried pipes filled with water or a refrigerant.
These liquid-pumping pipes can be either closed-loop or open-loop systems, and they operate pretty much exactly how they sound. In a closed-loop system, the same refrigerant or water circulates through the pipes repeatedly. In an open-loop system, water is pumped out of the underground water source, like a well or a man-made lake. From there, the heat is extracted from the water and that water returns to the well or surface lake. More water is then pumped from the well to extract more heat in a continuous open loop.
If that's not enough to blow your mind, consider the absorption heat pump — air-source pumps that are powered by natural gas, solar power, propane or geothermal-heated water rather than by electricity. Absorption pumps can be used for large-scale applications but are now available for homes.
The main difference between a standard air-source heat pump and an absorption pump is that instead of compressing a refrigerant, an absorption pump absorbs ammonia into water, and then a low-power pump pressurizes it. The heat source then boils the ammonia out of the water — and the process starts all over again.
When you go to check out an absorption heat pump, it helps to know how they're rated. Manufacturers rate them using a measurement called a coefficient of performance (COP), which sounds pretty complicated. But all you need to know is to look for a COP above 1.2 for heating and above 0.7 for cooling. And don't worry, we'll discuss ratings for standard heat pumps a little later.
Air-source, ground-source and absorption heat pumps are the most common kinds of heat pumps, but they won't work in every situation. Read on to learn about other types of heat pumps.
Advertisement