A heat pump is a device that uses mechanical or thermal energy to transfer heat from one space to another. The mechanical heat pump 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.
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, functioning in the same manner as an air conditioner. 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. License: CC BY 4.0.
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 alternatives; 3 million European heat pumps were sold in 2023. Although sales have grown significantly, adoption remains limited.
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 gained; this is the motivation for using heat pumps in applications such as the 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 & Company Limited.
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, meaning, for example, that they can heat and cool a space.
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 of a heat pump is greater than one so the work required is less than the heat released, 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 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.
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.
History Milestones:
1748 William Cullen demonstrates artificial refrigeration. 1834 Jacob Perkins patents a design for a practical refrigerator using dimethyl ether. 1852 Lord Kelvin describes the theory underlying heat pumps. 1855–1857 Peter von Rittinger develops and builds the first heat pump. 1877 Prior to 1875, heat pumps were being pursued for vapor-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, Switzerland, 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. 1928 In 1928, Slovak engineer Aurel Stodola designed an early water-source heat pump system for the Geneva City Hall using Lake Geneva as a thermal source. This pioneering work laid the foundation for large-scale heat pump applications, followed ten years later by the 100 kW Limmat river system for Zurich City Hall. 1937–1945 During the First World War, fuel prices were high in Switzerland but it had plenty of hydropower. In the period before and especially during the Second World War, when neutral Switzerland was 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 then 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. 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. 1948 Robert C. Webber is credited as developing and building the first ground-source heat pump. 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 river, for both winter heating and summer cooling needs. 2019 The Kigali Amendment to phase out harmful refrigerants takes effect.
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 (HVAC) 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 (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.
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 coefficient of performance (COP) over the year.
Window mounted heat pump
Window mounted heat pumps in the United States run on standard 120V AC outlets and provide heating, cooling, and humidity control. They are more efficient with lower noise levels, condensation management, and a smaller footprint than window mounted air conditioners that only cool.
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.
District heating