TheInfoList

A heat pump is a system used to heat or cool an enclosed space or domestic water by transferring
thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concepts, such as the internal energy of a system; heat or sensible heat, which are defined as types of energy transfer (as is ...
from a cooler space to a warmer space using the
refrigeration cycle Thermodynamic heat pump cycles or refrigeration cycles are the conceptual and mathematical model A mathematical model is a description of a system A system is a group of Interaction, interacting or interrelated elements that act according to ...
, moving heat in the opposite direction in which
heat transfer Heat transfer is a discipline of thermal engineering Thermal engineering is a specialized sub-discipline of mechanical engineering Mechanical engineering is an engineering Engineering is the use of scientific method, scientific pri ...

would take place without the application of external power. When used to cool a building, a heat pump works like an
air conditioner Air conditioning (also A/C, air conditioner) is the process of removing heat and controlling the humidity Humidity is the concentration of water vapour (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 ...

by transferring heat from inside the building to the outdoors. When used to heat a building, the heat pump operates in reverse: Heat is transferred into the building from the outdoors. Common heat pump types are
air source heat pump An air source heat pump (ASHP) is a type of heat pump A heat pump is a system used to heat or cool an enclosed space or domestic water by transferring thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Therma ...
s,
ground source heat pump A ground source heat pump (also geothermal heat pump) is a heating/cooling system for buildings that uses a type of heat pump A heat pump is a system used to heat or cool an enclosed space or domestic water by transferring thermal energy Ther ...
s, water source heat pumps and exhaust air heat pumps. Heat pumps are also often used in
district heating . District heating (also known as heat networks or teleheating) is a system for distributing heat generated in a centralized location through a system of insulated pipes for residential and commercial heating requirements such as space heater, s ...

systems. The efficiency of a heat pump is expressed as a
coefficient of performance The coefficient of performance or COP (sometimes CP or CoP) of a heat pump, refrigerator or air conditioning system is a ratio of useful heating or cooling provided to work (energy) required. Higher COPs equate to higher efficiency, lower energy ( ...
(COP), or seasonal coefficient of performance (SCOP). The higher the number, the more efficient a heat pump is and the less energy it consumes. When used for space heating these devices are typically much more energy efficient than simple electrical resistance heaters. Heat pumps have a smaller
carbon footprint A carbon footprint is the total greenhouse gas (GHG) emissions caused by an individual, event, organization, service, place or product, expressed as carbon dioxide equivalent. Greenhouse gases, including the carbon-containing gases carbon dio ...

than heating systems burning
fossil fuels A fossil fuel is a hydrocarbon In organic chemistry, a hydrocarbon is an organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen che ...
such as
natural gas Natural gas (also called fossil gas; sometimes just gas) is a naturally occurring hydrocarbon gas mixture consisting of methane and commonly including varying amounts of other higher alkanes, and sometimes a small percentage of carbon dioxid ...

, but those powered by hydrogen are also low-carbon and may become competitors.

# Principle of operation

According to the
second law of thermodynamics The second law of thermodynamics establishes the concept of entropy Entropy is a scientific concept, as well as a measurable physical property that is most commonly associated with a state of disorder, randomness, or uncertainty. The term an ...
, heat will flow spontaneously from a region of higher temperature to a region of lower temperature. Heat will not flow spontaneously from lower temperature to higher, but it can be made to flow in this direction if
work Work may refer to: * Work (human activity), intentional activity people perform to support themselves, others, or the community ** Manual labour, physical work done by humans ** House work, housework, or homemaking * Work (physics), the product of ...

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 heating of water and the interior of buildings.G.F.C. Rogers and Y.R. Mayhew (1957), ''Engineering Thermodynamics, Work and Heat Transfer'', Section 13.1, Longmans, Green & Co Ltd. 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: :$= \frac$ where * $W$ is the
work Work may refer to: * Work (human activity), intentional activity people perform to support themselves, others, or the community ** Manual labour, physical work done by humans ** House work, housework, or homemaking * Work (physics), the product of ...
performed on the
working fluid For fluid power, a working fluid is a gas or liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force ...
by the heat pump’s compressor. * $Q$ is the
heat In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these ...

transferred from the lower-temperature reservoir to the higher-temperature reservoir. * $COP$ is the instantaneous
coefficient of performance The coefficient of performance or COP (sometimes CP or CoP) of a heat pump, refrigerator or air conditioning system is a ratio of useful heating or cooling provided to work (energy) required. Higher COPs equate to higher efficiency, lower energy ( ...
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
unity Unity may refer to: Buildings * Unity Building The Unity Building, in Oregon, Illinois, is a historic building in that city's Oregon Commercial Historic District. As part of the district the Oregon Unity Building has been listed on the National R ...
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. 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 and the interior of the building is at 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 Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concepts, such as the internal energy of a system; heat or sensible heat, which are defined as types of energy transfer (as is ...
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. *As the temperature of the interior of the building rises progressively to 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.

# History

Milestones: * 1748:
William Cullen William Cullen FRS FRS may also refer to: Government and politics * Facility Registry System, a centrally managed Environmental Protection Agency database that identifies places of environmental interest in the United States * Family Res ...

demonstrates artificial refrigeration. * 1834:
Jacob Perkins Jacob Perkins (9 July 1766 – 30 July 1849) was an American inventor, mechanical engineer and physicist. Born in Newburyport, Massachusetts, Perkins was apprenticed to a goldsmith. He soon made himself known with a variety of useful mechanical inv ...

builds a practical
refrigerator A refrigerator (colloquially fridge) is a commercial and home appliance A home appliance, also referred to as a domestic appliance, an electric appliance or a household appliance, is a machine which assists in household A household co ...

with
dimethyl ether Dimethyl ether (DME, also known as methoxymethane) is the organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. D ...
. * 1852:
Lord Kelvin William Thomson, 1st Baron Kelvin, (26 June 182417 December 1907) was a British mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of ...
describes the theory underlying heat pumps. * 1855–1857:
Peter von Rittinger Peter Ritter von Rittinger or Peter von Rittinger (see styling variants at ''Ritter Ritter (German for "knight A knight is a person granted an honorary title A title is one or more words used before or after a person's name, in certain con ...

develops and builds the first heat pump. * 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 . Zogg M.: History of Heat Pumps - Swiss Contributions and International Milestones, Swiss Federal Office of Energy, Berne 2008, online=https://www.aramis.admin.ch/Texte/?ProjectID=45262 * 1928:
Aurel Stodola Aurel Boleslav Stodola (11 May 1859 – 25 December 1942) was a Slovak Slovak may refer to: * Something from, related to, or belonging to Slovakia (''Slovenská republika'') * Slovaks, a Western Slavic ethnic group * Slovak language, an Indo-Europ ...
constructs a closed loop heat pump (water source from
Lake Geneva , pushpin_map = Switzerland , image = Lake Geneva by Sentinel-2.jpg , caption = Satellite image , image_bathymetry = , caption_bathymetry = , location = Switzerland, France , coords = ...

) which provides heating for the
Geneva , neighboring_municipalities= Carouge Carouge () is a Municipalities of Switzerland, municipality in the Canton of Geneva, Switzerland. History Carouge is first mentioned in the Early Middle Ages as ''Quadruvium'' and ''Quatruvio''. In 124 ...

city hall to this day. * 1937-1945: During and after the
First World War World War I, often abbreviated as WWI or WW1, also known as the First World War or the Great War, was a global war A world war is "a war War is an intense armed conflict between states State may refer to: Arts, entertainmen ...

, Switzerland suffered from heavily difficult energy imports and subsequently expanded its hydropower plants. In the period before and especially during the
Second World War World War II or the Second World War, often abbreviated as WWII or WW2, was a global war A world war is "a war War is an intense armed conflict between states State may refer to: Arts, entertainment, and media Literatur ...
, 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 Brown, Boveri & Compagnie (BBC) was a Swiss group of electrical engineering companies. It was founded in Zürich, in 1891 by Charles Eugene Lancelot Brown and Walter Boveri who worked at the Maschinenfabrik Oerlikon. In 1970 BBC took over the ...
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 it was replaced by a new, more efficient heat pump , * 1945: John Sumner, City Electrical Engineer for
Norwich Norwich () is a city and district of Norfolk Norfolk () is a rural and non-metropolitan county A county is a geographical region of a country used for administrative or other purposesChambers Dictionary The ''Chambers Dictionary ...

, installs an experimental water-source heat pump fed central heating system, using a neighboring river to heat new Council administrative buildings. Seasonal efficiency ratio of 3.42. Average thermal delivery of 147 kW and peak output of 234 kW. * 1948: Robert C. Webber is credited as developing and building the first ground heat pump. * 1951: First large scale installation—the
Royal Festival Hall The Royal Festival Hall is a 2,700-seat concert, dance and talks venue within Southbank Centre in London. It is situated on the South Bank of the River Thames, not far from Hungerford Bridge, in the London Borough of Lambeth. It is a Grade I li ...

in London is opened with a
town gas Coal gas is a flammable gaseous fuel made from coal Coal is a combustible black or brownish-black sedimentary rock, formed as stratum, rock strata called coal seams. Coal is mostly carbon with variable amounts of other Chemical element, ...

-powered reversible water-source heat pump, fed by
the Thames The River Thames ( ), known alternatively in parts as the The Isis, River Isis, is a river that flows through southern England including London. At , it is the longest river entirely in England and the Longest rivers of the United Kingdom, se ...
, for both winter heating and summer cooling needs.

# Types

## Air source heat pump

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 heat emitters which release 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. They are normally also used to heat water for washing which is stored in a domestic hot water tank. Air source heat pumps are relatively easy and inexpensive to install and have therefore historically been the most widely used heat pump type. In mild weather, COP may be around 4.0, while at temperatures below around 0 °C (32 °F) an air-source heat pump may still achieve a COP of 2.5. The average COP over seasonal variation is typically 2.5–2.8, with exceptional models able to exceed this in mild climates.

## Geothermal (ground-source) heat pump

A geothermal heat pump (
North American English North American English (NAmE, NAE) is the most generalized variety Variety may refer to: Science and technology Mathematics * Algebraic variety, the set of solutions of a system of polynomial equations * Variety (universal algebra), classes ...
) or ground-source heat pump (
British English British English (BrE) is the standard dialect A standard language (also standard variety, standard dialect, and standard) is a language variety that has undergone substantial codification of grammar and usage and is employed by a populatio ...
) draws heat from the soil or from
groundwater Groundwater is the water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known form ...

which remains at a relatively constant temperature all year round below a depth of about . A well maintained geothermal heat pump will typically have a COP of 4.0 at the beginning of the heating season and a seasonal COP of around 3.0 as heat is drawn from the ground. Geothermal heat pumps are more expensive to install due to the need for the drilling of boreholes for vertical placement of heat exchanger piping or the digging of trenches for horizontal placement of the piping that carries the heat exchange fluid (water with a little antifreeze). A geothermal heat pump can also be used to cool buildings during hot days, thereby transferring heat from the dwelling back into the soil via the ground loop.
Solar thermal collector A solar thermal collector collects heat In thermodynamics, heat is energy in transfer to or from a thermodynamic system, by mechanisms other than Work (thermodynamics), thermodynamic work or Mass transfer, transfer of matter. The various ...
s or piping placed within the tarmac of a parking lot can also be used to replenish the heat underground.

## Exhaust air heat pump

Exhaust air heat pumps extract heat from the exhaust air of a building and require
mechanical ventilation Mechanical ventilation, assisted ventilation or intermittent mandatory ventilationIntermittent Mandatory Ventilation (IMV) refers to any mode of mechanical ventilation where a regular series of breaths are scheduled but the ventilator senses p ...
. There are two classes of exhaust air heat pumps. * 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

A solar-assisted heat pump integrates a heat pump and thermal solar panels in a single system. Typically these two technologies are used separately (or are operated in parallel) to produce
hot water File:Water Heater White.jpg, 300px, A small tank water heater Water heating is a heat transfer process that uses an energy source to heat water above its initial temperature. Typical domestic uses of hot water include cooking, cleaning, bathing, ...
. In this system the solar thermal panel is the low-temperature heat source, and the heat produced feeds the heat pump's evaporator. The goal of this system is to get high COP and then produce energy in a more efficient and less expensive way.

## 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.

## Hybrid heat pump

Hybrid (or twin source) heat pumps draw heat from different sources depending on the outside air temperature. When outdoor air is above 4 to 8 degrees Celsius (40–50 degrees Fahrenheit, depending on ground water temperature) they use air; at colder temperatures they use the ground source. These twin source systems can also store summer heat by running ground source water through the air exchanger or through the building heater-exchanger, even when the heat pump itself is not running. This has two advantages: it functions as a low-cost system for interior air cooling, and (if ground water is relatively stagnant) it increases the temperature of the ground source, which improves the energy efficiency of the heat pump system by roughly 4% for each degree in temperature rise of the ground source.

# Applications

The
International Energy Agency The International Energy Agency (IEA; french: Agence internationale de l'énergie) is a Paris Paris () is the Capital city, capital and List of communes in France with over 20,000 inhabitants, most populous city of France, with an estima ...
estimated that, , there were 800 million heat pumps installed on Earth. They are used in climates with moderate
heating, ventilation, and air conditioning File:Control circuit in household HVAC unit.jpg, The control circuit in a household HVAC installation. The wires connecting to the blue terminal block on the upper-right of the board lead to the thermostat. The fan enclosure is directly behind th ...
(HVAC) needs and may also provide domestic hot water and tumble clothes drying functions. The purchase costs are supported in various countries by consumer rebates.

## Heating and cooling of buildings and vehicles

In HVAC applications, a heat pump is typically a
vapor-compression refrigeration Vapour-compression refrigeration or vapor-compression refrigeration system (VCRS), in which the refrigerant A refrigerant is a working fluid used in the refrigeration cycle of air conditioning Air conditioning (also A/C, air conditioner) ...
device that includes a
reversing valve A reversing valve a type of valve and is a component in a heat pump A heat pump is a system used to heat or cool an enclosed space or domestic water by transferring thermal energy Thermal radiation in visible light can be seen on this hot met ...
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. In cooler climates, the default setting of the reversing valve is heating. The default setting in warmer climates is cooling. Because the two heat exchangers, the condenser and evaporator, must swap functions, they are optimized to perform adequately in both modes. Therefore, the SEER rating, which is the Seasonal Energy Efficiency Rating, of a reversible heat pump is typically slightly less than two separately optimized machines. For equipment to receive the
Energy Star Energy Star (trademarked ''ENERGY STAR'') is a program run by the U.S. Environmental Protection Agency (EPA) and U.S. Department of Energy (DOE) that promotes energy efficiency. The program provides information on the energy consumption of produ ...

rating, it must have a rating of at least 14.5 SEER.

## Water heating

In
water heating Water heating is a heat transfer process that uses an energy source to heat water above its initial temperature. Typical domestic uses of hot water include cooking, cleaning, bathing, and space heating. In industry, hot water and water heated t ...
applications, a heat pump may be used to heat or preheat water for swimming pools or heating potable water for use by homes and industry. Usually heat is extracted from outdoor air and transferred to an indoor water tank, another variety extracts heat from indoor air to assist in cooling the space.

## District heating

Heat pumps can also be used as heat supplier for
district heating . District heating (also known as heat networks or teleheating) is a system for distributing heat generated in a centralized location through a system of insulated pipes for residential and commercial heating requirements such as space heater, s ...

. Possible sources of heat for such applications are
sewage Sewage (or domestic sewage, domestic wastewater, municipal wastewater) is a type of wastewater Wastewater is generated after the use of fresh water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and n ...
water, ambient water (e.g. sea, lake and river water), industrial
waste heat Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing Work (thermodynamics), work. All such processes give off some waste heat as a fundamental result of the laws of thermodynamics. Waste hea ...
,
geothermal energy Geothermal energy is the thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concepts, such as the internal energy of a system; heat or sensible heat, which a ...
,
flue gas Flue gas is the gas exiting to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler Poland Poland ( pl, Polska ), officially the Republic of Poland ( pl, Rzeczpospoli ...
, waste heat from
district cooling District cooling is the cooling equivalent of district heating. Working on broadly similar principles to district heating, district cooling delivers chilled water to buildings like offices and factories needing cooling. In winter, the source for th ...
and heat from solar
seasonal thermal energy storage A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of Earth's orbit around the Sun and Earth's axial tilt relative to the ecliptic plane. I ...
. In Europe, more than 1500 MW of large-scale heat pumps were installed since the 1980s, of which about 1000 MW were in use in Sweden in 2017. Large scale heat pumps for district heating combined with
thermal energy storage Thermal energy storage (TES) is achieved with widely different technologies. Depending on the specific technology, it allows excess thermal energy to be stored and used hours, days, months later, at scales ranging from the individual process, b ...
offer high flexibility for the integration of variable renewable energy. Therefore, they are regarded as a key technology for smart energy systems with high shares of
renewable energy Renewable energy is energy that is collected from renewable resource File:Global Vegetation.jpg, Global vegetation A renewable resource, also known as a flow resource, is a natural resource which will replenish to replace the portion resou ...
up to 100%, and advanced 4th generation district heating systems. They are also a crucial element of cold district heating systems.

## Industrial heating

There is great potential to reduce the energy consumption and related greenhouse gas emissions in industry by application of industrial heat pumps. An international collaboration project completed in 2015 collected totally 39 examples of R&D-projects and 115 case studies worldwide. The study shows that short payback periods of less than 2 years are possible, while achieving a high reduction of CO2 emissions (in some cases more than 50%). High Temperature Heat Pump innovations are emerging to further increase Industrial Heat Pump thermal application range and especially waste heat to energy recovery.

# Performance

When comparing the performance of heat pumps the term 'performance' is preferred to 'efficiency', with
coefficient of performance The coefficient of performance or COP (sometimes CP or CoP) of a heat pump, refrigerator or air conditioning system is a ratio of useful heating or cooling provided to work (energy) required. Higher COPs equate to higher efficiency, lower energy ( ...
(COP) being used to describe the ratio of useful heat movement per work input. An electrical resistance heater has a COP of 1.0, which is considerably lower than a well-designed heat pump which will typically be between COP of 3 to 5 with an external temperature of 10 °C and an internal temperature of 20 °C. A ground-source heat pump will typically have a higher performance than an air-source heat pump. The "Seasonal Coefficient of Performance" (SCOP) is a measure of the aggregate energy efficiency measure over a period of one year which it is very dependent on region climate. One framework for this calculation is given by the Commission Regulation (EU) No 813/2013: A heat pump's operating performance in cooling mode is characterized in the US by either its
energy efficiency ratio The efficiency of air conditioners is often rated by the seasonal energy efficiency ratio (SEER) which is defined by the Air Conditioning, Heating, and Refrigeration Institute in its 2008 standard AHRI 210/240, ''Performance Rating of Unitary Air- ...
(EER) or
seasonal energy efficiency ratio The efficiency of air conditioners is often rated by the seasonal energy efficiency ratio (SEER) which is defined by the Air Conditioning, Heating, and Refrigeration Institute in its 2008 standard AHRI 210/240, ''Performance Rating of Unitary Air ...
(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. Actual performance varies, and it depends on many factors such as installation details, temperature differences, site elevation, and maintenance.

# Operation

Vapor-compression uses a circulating liquid
refrigerant A refrigerant is a working fluid For fluid power, a working fluid is a gas or liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an app ...
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 compressor is a mechanical device that increases the pressure Pressure (symbol: ''p'' or ''P'') is the force In physics, a force is an influence that can change the motion (physics), motion of an Physical object, object. A force ...
, a reservoir, a
reversing valve A reversing valve a type of valve and is a component in a heat pump A heat pump is a system used to heat or cool an enclosed space or domestic water by transferring thermal energy Thermal radiation in visible light can be seen on this hot met ...
which selects between heating and cooling mode, two
thermal expansion valve File:Thermostatic valve in system.svg, 308x308px, The sensing bulb is positioned near the end of the evaporator and ensures enough refrigerant flows to chill the whole evaporator, but not so much that liquid reaches the sensing position. The equali ...
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 saturated vapor 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 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 Saturation, saturated, unsaturation or unsaturated may refer to: Chemistry * Saturation, a property of organic compounds referring to carbon-carbon bonds **Saturated and unsaturated compounds **Degree of unsaturation **Saturated fat or fatty acid ...
, is next routed through an expansion valve where it undergoes an abrupt reduction in pressure. That pressure reduction results in the adiabatic
flash evaporation 246px, A typical flash drum Flash evaporation (or partial evaporation) is the partial vapor that occurs when a saturated liquid stream undergoes a reduction in pressure by passing through a throttling valve or other throttling device. This process ...
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 Thermodynamic heat pump cycles or refrigeration cycles are the conceptual and mathematical model A mathematical model is a description of a system A system is a group of Interaction, interacting or interrelated elements that act according to ...
, 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 Humidity is the concentration of water vapor, water vapour present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation (meteorology), precipitation, d ...

. The ice is melted through defrosting cycle. In 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 of cool the buildings.

## Refrigerant choice

Until the 1990s heat pumps, along with fridges and other related products used
chlorofluorocarbon Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are fully or partly halogenated paraffinParaffin may refer to: Substances * Paraffin wax, a white or colorless soft solid that is used as a lubricant and for other applications * L ...
s (CFCs) as refrigerants that caused major damage to the when released into the
atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in ...

. Use of these chemicals was banned or severely restricted by the
Montreal Protocol The largest Antarctic ozone hole recorded as of September 2006 The Montreal Protocol is an international treaty A treaty is a formal, legally binding written agreement between actors in international law International law, also known ...
of August 1987. Replacements, including
R-134a 1,1,1,2-Tetrafluoroethane (also known as norflurane (INN Inns are generally establishments or buildings where travelers can seek lodging, and usually, food and drink. Inns are typically located in the country or along a highway; before th ...
and
R-410A R-410A, sold under the trademarked names AZ-20, EcoFluor R410, Forane 410A, Genetron R410A, Puron, and Suva 410A, is a zeotropic but near-azeotropic An azeotrope () or a constant boiling point mixture is a mixture of two or more liquids whose pr ...
, are hydrofluorocarbon with similar thermodynamic properties with insignificant
ozone depletion potential The ozone depletion potential (ODP) of a chemical compound is the relative amount of degradation to the ozone layer it can cause, with trichlorofluoromethane (R-11 or CFC-11) being fixed at an ODP of 1.0. Chlorodifluoromethane (R-22), for example, ...
global warming potential Global warming potential (GWP) is the heat In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of ...

. HFC is a powerful greenhouse gas which contributes to climate change. R-410A#Environmental effects More recent refrigerators include difluoromethane (R32) and which do not deplete the ozone and are also far less harmful to the environment.
Dimethyl ether Dimethyl ether (DME, also known as methoxymethane) is the organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their co ...
(DME) has also gained in popularity as a refrigerant.

# Government incentives

## United Kingdom

: heat pumps are taxed at the reduced rate of 5% instead of the usual level of VAT of 20% for most other products.

## United States

### Alternative Energy Credits in Massachusetts

The Alternative Energy Portfolio Standard (APS) was developed in 2008 to require a certain percentage of the Massachusetts electricity supply to be sourced from specific alternative energy sources. In October 2017, the Massachusetts Department of Energy (DOER) drafted regulations, pursuant to Chapter 251 of the Acts of 2014 and Chapter 188 of the Acts of 2016, that added renewable thermal, fuel cells, and waste-to-energy thermal to the APS. Alternative Energy Credits (AECs) are issued as an incentive to the owners of eligible renewable thermal energy facilities, at a rate of one credit per every megawatt-hour equivalent (MWhe) of thermal energy generated. Retail electricity suppliers may purchase these credits to meet APS compliance standards. The APS expands the current renewable mandates to a broader spectrum of participants, as the state continues to expand its portfolio of alternative energy sources.