A heat pump is a device that transfers heat from a lower-temperature source to a higher-temperature destination using an external energy input. Most familiar systems are powered by electricity, although some are driven principally by thermal energy. Heat pumps serve building heating, water heating, and industrial processes; reversible systems also provide cooling. Refrigerators and air conditioners operate on the same physical principle, but their useful output is the removal of heat rather than its delivery. (energy.gov)
Thermodynamic principle
Heat flows spontaneously from a warmer body to a colder one. A heat pump reverses this net direction by consuming an external energy input, consistent with the second law of thermodynamics. Its operation does not create energy: the heat delivered comprises energy extracted from the source together with the energy supplied to operate the device. (openstax.org)
For an idealized work-driven system operating cyclically, the first law of thermodynamics gives
where is heat delivered to the hotter reservoir, is heat extracted from the colder reservoir, and is net work supplied. These quantities are positive magnitudes measured over the same operating interval. Consequently, the delivered heat can exceed the work input without violating energy conservation. (openstax.org)
The source need not feel warm to a person. A heat pump can extract energy from subfreezing outdoor air because its working fluid is made colder than that air. The practical limits arise from the temperature difference, available heat-transfer rate, and equipment characteristics—not simply from whether the source is above or below water’s freezing point. (openstax.org)
Vapor-compression operation
The predominant electrically driven design uses a vapor-compression cycle. A circulating refrigerant passes through four principal components:
- Evaporator. At low pressure, refrigerant absorbs heat from the source and evaporates.
- Compressor. Mechanical work compresses the vapor, raising its pressure and temperature.
- Condenser. The hot refrigerant transfers heat to the destination and ordinarily condenses into liquid.
- Expansion device. A valve or other restriction lowers the refrigerant pressure, producing a colder fluid mixture that returns to the evaporator.
The evaporator and condenser are heat exchangers. Evaporation and condensation exploit the energy associated with a phase change, including latent heat. (nist.gov)
In a reversible building system, a reversing valve redirects refrigerant so that the indoor and outdoor heat exchangers exchange functions. The indoor coil releases heat in heating mode and absorbs it in cooling mode; reversing the system does not mean reversing the compressor’s mechanical rotation. (betterbuildingssolutioncenter.energy.gov)
Performance and theoretical limits
The principal measure of heating performance is the coefficient of performance, or COP:
A heating COP of 4 means that four units of heat are delivered for each unit of work input. COP is not the same as a heat engine’s conversion efficiency: its numerator includes heat collected from an external source. Thus values above one are physically expected. (openstax.org)
For a perfectly reversible heat pump operating between constant-temperature reservoirs, the Carnot cycle establishes the upper limit:
Temperatures must be expressed in kelvins. The relation shows why performance improves as the temperature difference, often called the temperature lift, decreases. Real machines perform below this limit because of irreversible processes and finite temperature differences needed for heat exchange. (openstax.org)
A single COP describes particular operating conditions, not necessarily a whole year. Seasonal assessments account for changing weather and heating loads, while standardized ratings prescribe particular test conditions. In the United States, HSPF2 describes seasonal heating performance and SEER2 describes seasonal cooling performance; both use British thermal units per watt-hour rather than the dimensionless units of COP. Numerical values therefore cannot be compared directly with COP without accounting for units and test definitions. (energy.gov)
Main types
Heat pumps can be classified by their heat source, heat-delivery medium, driving energy, or equipment arrangement. These classifications overlap: for example, an air-source system may deliver either warm air or heated water. (energy.gov)
Air-source heat pumps exchange heat with outdoor air. Air-to-air systems deliver conditioned air through ducts or individual indoor units, including ductless mini-splits. Air-to-water systems deliver heat through a water circuit, which may serve radiators, underfloor heating, or hot-water storage. Their performance is especially sensitive to outdoor temperature because it affects both the source temperature and the building’s heating demand. (energy.gov)
Ground-source heat pumps, often called geothermal heat pumps, exchange heat with the shallow ground through buried piping. Collectors may occupy horizontal trenches or vertical boreholes. The ground generally changes temperature less rapidly than outdoor air, providing a comparatively stable source in winter and sink in summer. These systems use the ground as thermal storage; they do not require a high-temperature geothermal resource. Excavation or drilling usually makes installation more expensive than an equivalent air-source installation. (energy.gov)
Water-source heat pumps use water as the source or sink. Potential sources include surface water, groundwater, wastewater, and shared water circuits. Larger systems can recover otherwise unused heat from factories or data centers. (iea.org)
Absorption heat pumps use a thermal input to drive a different cycle rather than relying principally on a mechanically driven vapor compressor. Their driving heat may come from fuel combustion, steam, solar-heated water, or geothermal-heated water. They still require auxiliary equipment, but their electricity demand can be comparatively small. (energy.gov)
Hybrid systems combine a heat pump with another heater, such as a gas furnace. The division of heating duty depends on equipment capability and operating conditions. Because these systems consume more than one energy carrier, their performance cannot be characterized fully by the heat pump’s COP alone. (energy.gov)
Applications
In buildings, heat pumps provide space heating and, when configured for reversible operation, cooling. Heat may be distributed through air ducts, indoor fan units, radiators, or underfloor circuits. Storage tanks can support domestic hot-water production and provide flexibility in water-based systems. (iea.org)
A heat-pump water heater transfers heat from surrounding air into stored water. Many models include an electric-resistance element for high demand or conditions unsuitable for heat-pump operation. Because the appliance extracts energy from its surroundings, its installation environment forms part of the thermal system: it requires an adequate air supply and an appropriate ambient-temperature range. (bsesc.energy.gov)
Industrial heat pumps recover lower-temperature heat and upgrade it for useful process heating. Their outputs may include hot air, hot water, or steam. Large heat pumps also supply district-heating networks, where a central installation distributes heat to multiple buildings. Industrial waste heat, wastewater, and data-center heat can serve as sources for these applications. (iea.org)
Historical development
The heat-pump concept developed alongside mechanical refrigeration and thermodynamics. In December 1852, William Thomson, later Lord Kelvin, presented a proposal for heating or cooling buildings through a thermodynamic system using air. The distinction between a refrigerator and a heater was increasingly understood as a distinction in which side of the same heat-transfer process supplied the useful service. (calteches.library.caltech.edu)
By the mid-twentieth century, commercial equipment could provide year-round building conditioning, drawing heat from air, water, or the ground. A 1948 account in Caltech’s Engineering and Science described operating installations and discussed challenges that remain relevant: frosting, variable source temperatures, installation cost, and the importance of system layout and air distribution. (calteches.library.caltech.edu)
Subsequent development has included variable-speed compressors and fans, more refined controls, and equipment designed for colder climates. These changes allow systems to match varying loads more closely and broaden the conditions under which air-source heat pumps can supply building heat. (energy.gov)
Constraints and environmental effects
Cold-weather operation presents two distinct challenges: maintaining sufficient heating capacity and maintaining favorable energy performance. Equipment designed for cold climates can operate below freezing, but capability varies by model and operating temperature. Variable-speed operation and supplementary heating can address changing demand, although supplementary energy use affects whole-system performance. (energy.gov)
Frost may accumulate on the outdoor evaporator when its surface is cold enough and the air contains sufficient moisture. Frost obstructs airflow and heat exchange. Many air-source systems periodically reverse operation to warm the outdoor coil and melt the ice; this defrost process temporarily interrupts normal heating and consumes energy. (betterbuildingssolutioncenter.energy.gov)
Economically, high thermodynamic performance does not guarantee the lowest total cost. Installation expense, electricity and fuel prices, necessary distribution-system changes, and available skilled labor all affect deployment. Physical constraints include space, noise, and the suitability of existing building systems. (iea.org)
Electrically driven heat pumps can reduce greenhouse-gas emissions by replacing fossil-fuel heating, especially when supplied with low-emissions electricity. Their environmental assessment must also consider emissions from electricity generation and refrigerant leakage. Widespread electrification of heating can increase winter demand on the electrical grid, making grid planning and demand management part of heat-pump deployment. (iea.org)
Refrigerant selection involves trade-offs among thermodynamic properties, environmental effects, and safety. Some fluorinated refrigerants have high global warming potential if released. Lower-impact alternatives have different constraints: propane is flammable, ammonia is toxic, and carbon dioxide systems operate at high pressures. Refrigerant choice therefore affects the cycle, component design, operating envelope, and safety measures rather than merely substituting one fluid for another. (nist.gov)
References
- 5 Applications of Thermodynamics: Heat Pumps and Refrigerators — College Physics 2eopenstax.org
- Refrigerants and Working Fluidsnist.gov
- The Heat Pump: What It Is, What It Does, and What It May Do Somedaycalteches.library.caltech.edu
- Purchasing Energy-Efficient Residential Air-Source Heat Pumpsenergy.gov
- Heat Pump Equipment Key Product Criteriaenergystar.gov
- Geothermal Heat Pumpsenergy.gov
- How a Heat Pump Works — The Future of Heat Pumpsiea.org
- HVAC Heat Pump Water Heatersbsesc.energy.gov
- Overview of Key Barriers to Accelerating the Deployment of Heat Pumps and Corresponding Policy Solutionsiea.org