Heat pump

This article is about devices used to heat and potentially also cool a building using the refrigeration cycle. For more about the theory, see Heat pump and refrigeration cycle. For details of the most common type, see air source heat pump. For a similar device for cooling only, see air conditioner. For heat pumps used to keep food cool, see refrigerator. For other uses, see Heat pump (disambiguation).
External heat exchanger of an air-source heat pump for both heating and cooling
Mitsubishi heat pump interior air handler wall unit
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A heat pump is a device that consumes energy (usually electricity) to transfer heat from a cold heat sink to a hot heat sink. Specifically, the heat pump transfers 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 (e.g. winter); the pump may also be designed to move heat from the house to the warmer outdoors in warm weather (e.g. summer). 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 pressure and temperature rise. When operating as a heater in cold weather, the warmed gas flows to a heat exchanger in the indoor space where some of its thermal energy is 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. For example, an air-to-water heat pump that produces 6kW at a SCOP of 4.62 will give over 4kW of energy into a heating system for every kilowatt of energy that the heat pump uses itself to operate. 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 kWh of thermal energy into a building. The carbon footprint of heat pumps depends on how electricity is generated, but they usually reduce emissions.[8] 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]

  1. ^ "Heat Pump Systems". Energy.gov. Retrieved 2024-03-26.
  2. ^ "Heat Pump Systems". US Department of Energy. Archived from the original on 27 April 2023. Retrieved 27 April 2023.
  3. ^ "Exhaust air heat pumps". Energy Saving Trust. Retrieved 2024-02-22.
  4. ^ a b 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.
  5. ^ IPCC AR6 WG3 Ch11 2022, Sec. 11.3.4.1.
  6. ^ IPCC SR15 Ch2 2018, p. 142.
  7. ^ Everitt, Neil (2023-09-11). "Study proves heat pump efficiency at low temperatures". Cooling Post. Retrieved 2024-01-22.
  8. ^ 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.