Heat pumps, air conditioning and refrigeration: Science and limits

How using work to move heat beats burning fuel

What this tab covers What physically happens in the conversion, step by step, the science behind it, and the hard limits that physics sets on how well it can ever work.

A heat pump uses work, usually from an electric motor, to move heat from a cold place to a warmer one. It delivers several units of heat for each unit of electricity because most of the heat comes from outdoor air, ground or water, not from the electricity.

The conversion, step by step

Motor and inverter
Electrical → Mechanical (shaft work) · Losses are small next to the cycle; the inverter lets the compressor run at part speed¹Evaporator
Low-temperature heat (outdoor air, ground or water) → Latent heat in evaporated refrigerant · Not a loss step: this is where the free ambient heat enters¹Compressor
Mechanical work plus low-pressure vapour → Hot, high-pressure vapour · Raising pressure raises the refrigerant's condensing temperature above the room or water temperature¹Condenser
Hot refrigerant vapour → Useful heat into a room, radiator water or a process · Typical household unit: about 4 units of heat per unit of electricity (COP about 4)¹Expansion valve
High-pressure liquid → Cold low-pressure liquid and vapour · Throttling wastes some potential work; this is one reason real COP is well below the ideal¹

The science

The refrigerant boils at a low temperature, soaking up heat from outside even on a cold day. The compressor squeezes the vapour, which heats it, and in the condenser it gives that heat up as it turns back to liquid. The valve drops the pressure and the loop repeats. An air conditioner or refrigerator is the same machine pointed the other way: the cold side is the useful one. Energy is conserved (heat out equals heat taken in plus electrical work), so the coefficient of performance (COP, useful heat or cooling divided by electricity) can exceed 1. Absorption chillers replace the compressor with a 'thermal compressor': a salt solution (lithium bromide with water as refrigerant, or water with ammonia as refrigerant) absorbs refrigerant vapour, and heat from steam, hot water or exhaust boils it out again at high pressure. Only a small pump needs electricity.

Limits set by science

Limit	Value	Why it exists	Best today
Carnot COP for heating	COP max = Th / (Th - Tc), temperatures in kelvin. For 0 C outside (273 K) and 35 C underfloor water (308 K): 308 / 35 = about 8.8. For -10 C outside (263 K) and 55 C radiators (328 K): 328 / 65 = about 5.0 (calculated)¹	The smaller the temperature lift, the more heat each unit of work can move. This is why low-temperature emitters and ground sources raise COP, and why cold snaps lower it.	A typical household COP of about 4 is roughly 45% of the 8.8 ideal at 0 C to 35 C (calculated). An EU-funded project aims to demonstrate large heat pumps with COP up to 8 where the lift is small.
Carnot COP for cooling	COP max = Tc / (Th - Tc). A freezer at -18 C (255 K) rejecting heat at 35 C (308 K): 255 / 53 = about 4.8 (calculated)¹	Refrigerators lift heat across a wide gap, so their ideal COP is lower than a heat pump's at mild conditions.	Real appliances reach a fraction of this because of compressor losses, throttling and the need for finite temperature differences in heat exchangers.
Ideal absorption chiller COP	COP max = (1 - T0/Tg) x Te/(T0 - Te). Driving heat at 90 C (363 K), heat rejected at 35 C (308 K), chilled water at 7 C (280 K): 0.15 x 10 = about 1.5 (calculated)⁵	An absorption chiller behaves like a heat engine driving a heat pump, so both Carnot factors multiply. Hotter driving heat allows two-stage designs.	Single-stage lithium bromide units: COP 0.70 to 0.79. Two-stage units driven by higher-temperature heat: 1.35 to 1.42.
Industrial temperature ceiling	Up to about 150 C output already achievable where waste heat near 100 C is the source; 160 to 200 C units are at prototype stage¹³	Refrigerants and compressor lubricants limit output temperature; COP falls as the lift grows (above 3 with a 30 to 50 C lift).	Machines delivering 120 C were first demonstrated around 2022; commercial roll-out of units above 160 C is expected by 2027.

Sources

The Future of Heat Pumps International Energy Agency, 2022
Heat Pumps Step Up: What the IEA's Heat Pump Monitor 2026 Means for Heating, Industry and Energy Security (summary of IEA Heat Pump Monitor 2026, https://www.iea.org/reports/heat-pump-monitor-2026) IEA Heat Pumping Technologies TCP, 2026
Combined Heat and Power Technology Fact Sheet Series: Absorption Chillers for CHP Systems US Department of Energy, 2017