Absorption heat pump
Heat pumps transfer heat from a lower temperature source to one of a higher temperature. This is the opposite of the natural flow of heat and is the same process that is used to extract heat from a fridge.
Generally heat pumps work using compression, and are powered by electricity. A refrigerant fluid is run through the lower temperature source. The fluid ‘absorbs’ heat and boils, even at temperatures below 0° C. The resulting gas is then compressed, which increases its temperature further. The gas is passed into heat exchanger coils, where it condenses, releasing its latent heat. The process then repeats.
Absorption heat pumps work on a similar basis, with a refrigerant that boils at low temperature and pressure, however, in this case, the refrigerant gas (generally ammonia) is then absorbed in a solution (the ‘absorber’, generally water) which is then heated in the ‘generator’ so that the refrigerant evaporates again, but this time at a higher pressure and temperature. It is then condensed through a heat exchanger, heating ‘cool’ return water from the building, and the process then begins again.
The heat source is generally gas fuelled and so they can be referred to as Gas Absorption Heat Pumps (GAHP). This process is more efficient than a traditional gas-powered boiler.
Other heat sources can be used, such as combined heat and power plant (CHP), solar heated water, (although this requires specialist flat plate collectors that raise the temperature of the water above that normally required), geothermal heat, district heat networks and so on.
Absorption heat pumps are most efficient when supplying low-temperature hot water, such as for underfloor heating. They may be used in combination with conventional boilers to produce higher-temperature water.
Unlike some refrigerants used in compression heat pumps, ammonia is not an ozone depleting gas or a global warming gas. However it is flammable and toxic, and so units are generally hermetically sealed rather than engineered systems and are located outside.
Absorption chillers and absorption refrigerators work on the same principal as absorption heat pumps, but with the flow of heat reversed, so that heat is absorbed from the interior and rejected to the exterior.
 Related articles on Designing Buildings Wiki:
- Absorption refrigeration.
- Air handling unit.
- Air source heat pumps.
- Chilled beam.
- Chiller unit.
- Coefficient of Performance CoP.
- Dynamic thermal modelling of closed loop geothermal heat pump systems.
- Earth-to-air heat exchangers.
- Exhaust air heat pump.
- Fan coil unit.
- Geothermal pile foundations.
- Ground energy options.
- Ground source heat pumps.
- Heat pump.
- Solar thermal heating.
- Thermal labyrinths.
- Water source heat pumps.
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