Heat pump
A heat pump is a device that uses mechanical or thermal energy to transfer heat from one space to another. The mechanical heat pump, also known as a Cullen engine, uses electric power to transfer heat by compression. Specifically, it transfers thermal energy by means of a heat pump and refrigeration cycle, cooling one space and warming the other. Heat pumps driven by thermal energy are known as absorption heat pumps.
In winter, a heat pump can move heat from the cool outdoors to warm a house; in summer, it may also be designed to move heat from the house to the warmer outdoors. As it transfers rather than generates heat, it is more energy-efficient than heating by gas boiler.
In a typical vapor-compression heat pump, a gaseous refrigerant is compressed so its pressure and temperature rise. When the pump operates 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 space, causing the gas to condense into a liquid. 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. 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. Large-scale heat pumps are also used in district heating systems.
Because of their high efficiency and the increasing share of fossil-free sources in electrical grids, heat pumps are playing a role in climate change mitigation. At a cost of 1 kWh of electricity, they can transfer 1 to 4.5 kWh of thermal energy into a building. The carbon footprint of heat pumps depends on how electricity is generated, but they usually reduce emissions. 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%, the boycott of Russian natural gas has accelerated the need to shift toward alternative, 3 million European heat pumps were sold in 2023. Although sales have grown significantly, adoption remains limited. In 2025, REPowerEU provides a clear roadmap to transition to this high efficient and flexible air conditioning system.
Operation
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.The heat pump works by the use of reverse cycle conditioning. Liquid refrigerant flows through coils in the cooling unit and absorbs heat, becoming a gas which is then compressed to further raise the temperature. This gaseous refrigerant is pumped into more coils in the heating unit, where a fan blows air over the coil to absorb the heat and liquefy the refrigerant. Most heat pumps are capable of transferring heat in both directions.
The amount of work required to provide an amount of heat Q to a higher-temperature reservoir such as the interior of a building, while extracting heat from a lower-temperature reservoir such as ambient air is:
where
- is the work performed on the working fluid by the heat pump's compressor.
- is the heat released in the higher-temperature reservoir.
- is the instantaneous coefficient of performance for the heat pump at the temperatures prevailing in the reservoirs at one instant.
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 maximum theoretical coefficient of performance is 28. This means 1 joule of work delivers 28 joules of heat to the interior. The one joule of work ultimately ends up as thermal energy in the interior of the building and 27 joules of heat are moved from the low-temperature reservoir.
- As the temperature of the interior of the building rises progressively to the coefficient of performance falls progressively to 10. This means each joule of work is responsible for transferring 9 joules of heat out of the low-temperature reservoir and into the building. Again, the 1 joule of work ultimately ends up as thermal energy in the interior of the building so 10 joules of heat are added to the building interior.
History
Milestones:;1748
;1834
;1852
;1855–1857
;1877
;1928
;1937–1945
;1945
;1948
;1951
;2019
Types
Air-source
Ground source
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
Water-source
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. The largest water-source heat pump was installed in the Danish city of Esbjerg in 2023.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.Electrocaloric heat pumps are solid state.
Applications
The International Energy Agency estimated that, as of 2021, heat pumps installed in buildings have a combined capacity of more than 1000 GW. They are used for heating, ventilation, and air conditioning and may also provide domestic hot water and tumble clothes drying. The purchase costs are supported in various countries by consumer rebates.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 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 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.
Heating seasonal performance factor or Seasonal Performance Factor 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.