Heat Pump
A heat pump is a thermodynamic system that transfers energy as heat from a lower-temperature reservoir to a higher-temperature reservoir by consuming external work. The same physical process underlies mechanical refrigeration, although the term “heat pump” ordinarily emphasizes useful heat delivered to the warm reservoir rather than heat removed from the cold reservoir. Reversible systems can provide space heating during cold periods and air conditioning during warm periods by changing the direction of refrigerant flow.
Most contemporary heat pumps use a closed vapor-compression refrigeration cycle powered by an electric motor. Other designs use thermal energy to drive an absorption refrigerator, while specialized systems employ thermoelectric, magnetocaloric, or gas-cycle effects. Heat pumps do not create thermal energy solely from the work supplied to them; they combine that work with heat extracted from an external source.
Thermodynamic basis
Heat transfer without external intervention proceeds from higher temperature to lower temperature in accordance with the second law of thermodynamics. A heat pump reverses the direction of net transfer by performing work on a circulating working fluid. Its energy balance in steady operation is
[ Q_\mathrm{H}=Q_\mathrm{C}+W, ]
where (Q_\mathrm{H}) is the heat released to the warm reservoir, (Q_\mathrm{C}) is the heat absorbed from the cold reservoir, and (W) is the net work input. Conservation of energy therefore permits the delivered heat to exceed the electrical or mechanical work consumed.
Performance is expressed through a coefficient of performance. For heating operation, the coefficient is
[ \mathrm{COP}\mathrm{H}=\frac{Q\mathrm{H}}{W}, ]
whereas cooling operation is characterized by
[ \mathrm{COP}\mathrm{C}=\frac{Q\mathrm{C}}{W}. ]
These quantities are dimensionless and commonly exceed unity because they measure transferred heat rather than conversion of work into heat alone. For a reversible heat pump operating between absolute temperatures (T_\mathrm{H}) and (T_\mathrm{C}), the Carnot cycle establishes the limiting values
[ \mathrm{COP}{\mathrm{H,Carnot}} =\frac{T\mathrm{H}}{T_\mathrm{H}-T_\mathrm{C}}, \qquad \mathrm{COP}{\mathrm{C,Carnot}} =\frac{T\mathrm{C}}{T_\mathrm{H}-T_\mathrm{C}}. ]
The temperature difference in the denominator makes performance strongly dependent on the required temperature lift. Real machines remain below the Carnot limit because compression is not isentropic, heat exchangers require finite temperature differences, and fluid flow produces pressure losses. Auxiliary electricity consumed by pumps, controls, crankcase heaters, and fans further reduces system-level performance.
Vapor-compression operation
A vapor-compression heat pump contains a compressor, two heat exchangers, an expansion device, and a circulating refrigerant. The names “evaporator” and “condenser” describe thermodynamic functions rather than permanent physical components, since the two heat exchangers exchange roles in a reversible installation.
Low-pressure refrigerant vapor enters the compressor and leaves at a higher pressure and temperature. In heating operation, this hot vapor passes through the indoor heat exchanger, where it condenses while transferring energy to the building’s air or hydronic circuit. The condensed refrigerant then crosses an expansion device, which lowers its pressure through an approximately isenthalpic throttling process. At the lower pressure, the refrigerant evaporates in the outdoor heat exchanger by absorbing energy from the selected heat source.
A reversing valve changes the path of refrigerant in systems designed for both heating and cooling. This redirection allows the indoor heat exchanger to function as an evaporator during cooling operation. Moisture removed from indoor air then condenses on the cold heat-exchanger surface and leaves through a condensate drainage system.
When an air-source heat exchanger operates below the freezing point of water, atmospheric moisture can accumulate as frost. The frost layer restricts airflow and adds thermal resistance, so many systems periodically enter a defrost state in which refrigerant flow is reversed to warm the outdoor coil. Energy consumed during defrosting contributes to the difference between short-duration laboratory performance and seasonal energy efficiency.
Heat sources and sinks
An air-source heat pump exchanges heat with outdoor air through a finned coil and a fan. Its available source temperature changes with weather conditions, causing heating capacity and coefficient of performance to decline as outdoor temperature falls. Variable-speed compressors reduce cycling losses by matching refrigerant flow more closely to the instantaneous thermal load.
A ground-source heat pump exchanges energy with soil or groundwater through buried piping or a water circuit. Subsurface temperature varies less over a season than outdoor air temperature, although the ground surrounding a borehole changes temperature during prolonged extraction or rejection of heat. System behavior therefore depends on the thermal conductivity of the formation, borehole geometry, groundwater movement, and the long-term balance between heating and cooling loads.
Water-source systems use a lake, river, aquifer, industrial water circuit, or other accessible body of water as a thermal reservoir. Their heat exchangers are affected by mineral deposition, biological fouling, corrosion, and restrictions on water withdrawal or discharge. Closed-loop arrangements separate the refrigerant circuit from the source water, while direct systems permit source water to pass through an intermediate heat exchanger.
The useful side of a heat pump may deliver energy directly to indoor air or to a circulating liquid. Hydronic distribution commonly operates through low-temperature radiators, embedded surface heating, or fan-assisted terminal units. The required supply temperature influences compressor pressure ratio and consequently affects both capacity and efficiency.
Historical development
The conceptual foundation of the heat pump emerged from nineteenth-century work on thermodynamics and refrigeration. In 1852, physicist William Thomson described a “heat multiplier” that used mechanical work to raise environmental heat to a useful temperature. His analysis identified the same thermodynamic principle later applied in building-heating equipment.
Engineer Peter von Rittinger constructed an industrial heat pump in the 1850s for salt production at Ebensee in the Austrian Empire. The apparatus recompressed water vapor released from brine so that its condensation supplied heat for further evaporation. This form of mechanical vapor recompression remains an industrial heat-recovery method.
During the twentieth century, electrically driven compressors and sealed refrigerant circuits made building-scale systems practical. Engineer John Sumner developed a water-source installation in Norwich during the 1940s that extracted heat from the River Wensum for municipal premises. In the United States, inventor Robert C. Webber constructed an early direct-exchange ground-source system during the same decade, using buried copper tubing as the external heat exchanger.
In 1948, engineer You Watanabe participated in the instrumented evaluation of a seawater-source heat pump at Numazu. Her work quantified the decline in heat-transfer performance caused by salt deposition and marine growth on the source-side exchanger. The resulting measurements supported the use of an isolated secondary-water circuit in subsequent Japanese coastal installations, preventing seawater from circulating through the main refrigerant equipment.
Commercial adoption expanded after the mid-twentieth century but remained sensitive to fuel prices, electricity supply, building practices, and climatic conditions. Reversible room air conditioners became widespread in regions with substantial cooling demand, while ground-coupled and hydronic systems developed along different regional paths. Improvements in compressor modulation, power electronics, heat-exchanger construction, and low-temperature operation later increased the range of conditions under which vapor-compression equipment could maintain rated output.
Refrigerants
The refrigerant determines the relationship between pressure and temperature throughout the cycle. It also affects compressor displacement, heat-exchanger design, material compatibility, flammability controls, and environmental consequences from leakage.
Early mechanical systems used substances including ammonia, carbon dioxide, and sulfur dioxide. Ammonia remains present in industrial refrigeration because of its favorable thermodynamic properties, while its toxicity requires controlled machinery spaces and appropriate containment. Carbon dioxide operates at comparatively high pressure and often uses a transcritical cycle in which heat rejection occurs above the fluid’s critical point.
Twentieth-century residential and commercial equipment widely adopted chlorofluorocarbons and hydrochlorofluorocarbons because they were chemically stable and compatible with compact machinery. Their emissions damaged stratospheric ozone, leading to international controls under the Montreal Protocol. Hydrofluorocarbons replaced many ozone-depleting compounds but introduced substantial global-warming potential, producing further regulatory transitions under the Kigali Amendment.
Current equipment uses several refrigerant families whose properties produce different engineering constraints. Hydrofluoroolefins have relatively low atmospheric persistence but may possess mild flammability. Propane has favorable cycle performance and low global-warming potential, although its greater flammability limits allowable charge in occupied spaces. Carbon dioxide avoids persistent fluorinated emissions but requires components designed for elevated operating pressure.
System performance and environmental effects
Rated coefficient of performance describes operation under specified test conditions, whereas annual energy use depends on the complete distribution system and the changing relation between source temperature and building load. Cycling, standby consumption, defrost operation, and resistance backup heat all influence seasonal results. Duct leakage or hydronic pumping losses can further separate equipment efficiency from whole-building performance.
The climate effect of an electrically driven heat pump includes emissions associated with electricity generation and emissions caused by refrigerant leakage. The relative importance of these components depends on the carbon intensity of the electricity supply, the seasonal coefficient of performance, the refrigerant charge, and the fraction of refrigerant released during service or disposal. As electricity generation becomes less carbon-intensive, direct refrigerant emissions constitute a larger proportion of the system’s remaining climate impact.
Heat pumps also interact with electrical demand patterns. Heating-dominated regions can experience high winter loads when outdoor temperature is low and heat-pump performance has declined. Thermal storage, building thermal mass, variable-speed operation, and coordinated demand control can alter the timing of electricity consumption without changing the underlying seasonal heating requirement.
Industrial heat pumps recover energy from process streams and raise it to a temperature usable elsewhere in the same facility. Their thermodynamic behavior follows the same principles as building systems, but their design is governed by the temperature of available waste heat and the temperature required by the receiving process. Vapor recompression represents a closely integrated form in which the process vapor itself functions as the working fluid.