Heat pumps, air conditioning and refrigeration: Engineering methods

Air, ground, fridges, air conditioners and heat-driven chillers

What this tab covers The engineering methods that can do this conversion, compared on efficiency, scale, economics, limitations and reliability.

All compression machines share the same cycle; they differ in heat source, output temperature and size. Absorption machines swap electricity for heat.

Comparison

Method	Efficiency	Scale	Economics	Limitations	Reliability
Air-source heat pumps (air-to-air and air-to-water)¹⁴	Household COP around 4 in mild conditions, lower in deep cold and during defrost	2 to 20 kW homes; larger commercial units	Cheapest heat pump to install	Output and COP fall as outside temperature falls; cold-climate models are designed to perform at temperatures as low as -15 C (5 F)	Defrost cycles and outdoor fans are the main wear points
Ground- and water-source heat pumps¹⁴	Higher COP than air-source, especially in cold climates	Homes to 100 MW-class district systems	Installation can cost several times air-source; 5 to 10 year payback	Needs land or drilling	Indoor parts up to 24 years, ground loop 50+ years
Air conditioners³	Cooling COP typically several times the electricity input; limited by the indoor-outdoor gap	Room units to large chillers	Mass-produced; air-to-air units are about 40% cheaper to make in China than in Japan	Adds to summer peak power demand	Refrigerant leaks and compressor wear
Refrigerators and freezers¹	Ideal COP about 4.8 at freezer conditions (calculated); real units much lower	About 0.1 kW compressors	Low cost per unit; running cost dominated by electricity	Large temperature lift; door openings and insulation set the load	Sealed systems often last a decade or more
Large and industrial heat pumps¹³	COP above 3 for a 30 to 50 C lift	MW to hundreds of MW; Japan had almost 650 MW of industrial heat pumps in 2024	About 10 times the capital cost of a gas boiler	Units above 160 C expected commercially by 2027; most industrial heat below 200 C still at demonstration stage for heat pumps	Industrial compressors with long service intervals
Absorption chillers and heat pumps³⁵	COP 0.70 to 0.79 single stage, 1.35 to 1.42 two stage	5 to 3,000 refrigeration tons; a 200 MW absorption heat pump at Qianxi, China recovers steel mill heat	1,800 to 6,000 USD per ton installed; attractive only where heat is cheap or free	Large and heavy per kW; needs cooling towers	Few moving parts; corrosion and crystallisation of the salt solution are the main risks

On efficiency, ground- and water-source units beat air-source, and electric compression beats absorption by a wide margin, but absorption wins when driven by heat that would otherwise be wasted. Air-source dominates sales because it is cheapest and easiest to fit. Industrial heat pumps are where the economics are hardest: capital is high, so they need long running hours, low lifts and a favourable electricity-to-gas price ratio.

How each method works

Air-source heat pumps (air-to-air and air-to-water)

An outdoor fan coil extracts heat from air; air-to-air units blow warm air indoors, air-to-water units heat radiator or underfloor water and hot-water tanks. Almost 85% of heat pumps sold for buildings worldwide are air-source.

Ground- and water-source heat pumps

Fluid circulates in buried loops, boreholes or a lake or river, where temperatures stay steadier than air.

Air conditioners

A heat pump run in cooling mode, usually a split system with an outdoor compressor and indoor fan coil. Japan has 2.8 units per household.

Refrigerators and freezers

A small hermetic compressor drives the same cycle, holding food at about 4 C and -18 C and rejecting heat from coils at the back.

Large and industrial heat pumps

Screw or turbo compressors lift waste heat or river water to process steam or district heating temperatures.

Absorption chillers and heat pumps

Waste heat, steam or exhaust drives a lithium bromide-water cycle (chilling to about 4 C, 40 F, and above) or an ammonia-water cycle (below that).

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
Heat Pump Systems US Department of Energy, Energy Saver, 2025
Combined Heat and Power Technology Fact Sheet Series: Absorption Chillers for CHP Systems US Department of Energy, 2017