# Heat pumps, air conditioning and refrigeration: Engineering methods

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

Source page: https://www.materialinsights.org/energy/e_heat_pumps/methods/

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)**^[1](https://build-up.ec.europa.eu/sites/default/files/content/thefutureofheatpumps.pdf)^[4](https://www.energy.gov/energysaver/heat-pump-systems) | 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 pumps**^[1](https://build-up.ec.europa.eu/sites/default/files/content/thefutureofheatpumps.pdf)^[4](https://www.energy.gov/energysaver/heat-pump-systems) | 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 conditioners**^[3](https://heatpumpingtechnologies.org/articles/heat-pumps-step-up-what-the-ieas-heat-pump-monitor-2026-means-for-heating-industry-and-energy-security/) | 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 freezers**^[1](https://build-up.ec.europa.eu/sites/default/files/content/thefutureofheatpumps.pdf) | 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 pumps**^[1](https://build-up.ec.europa.eu/sites/default/files/content/thefutureofheatpumps.pdf)^[3](https://heatpumpingtechnologies.org/articles/heat-pumps-step-up-what-the-ieas-heat-pump-monitor-2026-means-for-heating-industry-and-energy-security/) | 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 pumps**^[3](https://heatpumpingtechnologies.org/articles/heat-pumps-step-up-what-the-ieas-heat-pump-monitor-2026-means-for-heating-industry-and-energy-security/)^[5](https://www.energy.gov/sites/prod/files/2017/06/f35/CHP-Absorption%20Chiller-compliant.pdf) | 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

1. [The Future of Heat Pumps](https://build-up.ec.europa.eu/sites/default/files/content/thefutureofheatpumps.pdf) International Energy Agency, 2022
2. [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)](https://heatpumpingtechnologies.org/articles/heat-pumps-step-up-what-the-ieas-heat-pump-monitor-2026-means-for-heating-industry-and-energy-security/) IEA Heat Pumping Technologies TCP, 2026
3. [Heat Pump Systems](https://www.energy.gov/energysaver/heat-pump-systems) US Department of Energy, Energy Saver, 2025
4. [Combined Heat and Power Technology Fact Sheet Series: Absorption Chillers for CHP Systems](https://www.energy.gov/sites/prod/files/2017/06/f35/CHP-Absorption%20Chiller-compliant.pdf) US Department of Energy, 2017

## Other sections

- [Summary](https://www.materialinsights.org/energy/e_heat_pumps/index.md)
- [Overview](https://www.materialinsights.org/energy/e_heat_pumps/overview/index.md)
- [Correlation](https://www.materialinsights.org/energy/e_heat_pumps/correlation/index.md)
- [Science and limits](https://www.materialinsights.org/energy/e_heat_pumps/science/index.md)
- [Value chain](https://www.materialinsights.org/energy/e_heat_pumps/valuechain/index.md)
- [Unit economics](https://www.materialinsights.org/energy/e_heat_pumps/economics/index.md)
- [Equipment and suppliers](https://www.materialinsights.org/energy/e_heat_pumps/equipment/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_heat_pumps/links/index.md)