Refrigerators and Heat Pumps
A refrigerator or heat pump is a reversed heat engine: it uses work W to extract heat Qc from a cold region and deliver heat Qh = Qc + W to a warmer one. A refrigerator is rated by COPR = Qc/W and a heat pump by COPHP = Qh/W; the second law limits these to the Carnot values Tc/(Th − Tc) and Th/(Th − Tc).
Curious
Heat naturally flows from hot to cold (see Second Law). To move it the other way you must do work, just as a water pump must work to lift water uphill. The coils at the back of a refrigerator feel warm because that is where the heat taken from inside, plus the energy of the motor, is released.
A heat pump for a house is the same machine pointed the other way: it extracts heat from outdoor air or the ground and delivers it indoors. Because it moves heat rather than creating it, a heat pump can deliver three or four times more heat than the electrical energy it uses, without breaking energy conservation (see First Law).
Exploring
| Stage of vapour-compression cycle | What happens |
|---|---|
| Compressor | Work raises refrigerant vapour pressure and temperature |
| Condenser | Hot vapour releases heat to the warm side and condenses |
| Expansion valve | Throttling drops pressure and temperature (enthalpy constant) |
| Evaporator | Cold liquid absorbs heat from the cold side and boils |
Deep Dive
Thermodynamic limits. Applying the second law to a cyclic device exchanging heat with two reservoirs gives Qh/Th ≥ Qc/Tc, which bounds the COP (see Carnot Cycle). Since COPHP = COPR + 1, a heat pump always delivers at least as much heat as the work it consumes. Performance falls as the temperature lift grows, which is why air-source heat pumps lose capacity in very cold weather and ground-source units, with steadier source temperatures, often perform better.
Throttling. The expansion valve exploits the Joule–Thomson effect discovered by Joule and Kelvin: an isenthalpic expansion of a real fluid cools it below its inversion temperature (see Enthalpy). Cascades of such stages, as in the Linde process, liquefy air, and helium liquefaction by Kamerlingh Onnes in 1908 opened low-temperature physics (see Heike Kamerlingh Onnes).
Refrigerants. Chlorofluorocarbons (CFCs), introduced in the 1930s, were found to deplete stratospheric ozone (Molina and Rowland, 1974; Nobel Prize in Chemistry 1995) and were phased out under the 1987 Montreal Protocol. Their replacements, hydrofluorocarbons, are potent greenhouse gases being phased down under the 2016 Kigali Amendment, prompting a shift to lower-impact refrigerants such as CO2, ammonia and hydrocarbons, each with engineering trade-offs.
Concept Relationship Map
Requires: heat engines, second law, latent heat
Enables: cryogenics, air conditioning, building heating
Commonly confused with: “over 100% efficiency”
Sources
- Thomson, W. (1852). Proceedings of the Philosophical Society of Glasgow, 3, 269–272.
- Çengel, Y.A., Boles, M.A. (2015). Thermodynamics: An Engineering Approach (8th ed.). McGraw-Hill.
- Molina, M.J., Rowland, F.S. (1974). “Stratospheric sink for chlorofluoromethanes.” Nature, 249, 810–812.