Gas Absorption Heat Pumps

Gas Absorption/Adsorption Heat Pumps (GAHP) use a combination of a carrier and refrigerant which have a natural affinity. GAHP’s use heat to change the composition of the carrier & refrigerant solution and by changing the solution state (liquid-gas-liquid), heat equating to the latent heat of vaporisation is transferred.

The most common solution mixture is ammonia as the refrigerant and water as the carrier. The diagram below demonstrates the process.

  • Generator:

    Heat is applied in the generator to heat the ammonia and water solution and increase its pressure which causes the solution to separate into strong ammonia vapour and a week ammonia/water solution. The strong ammonia vapour is forced into the Condenser 2. Whilst the weak ammonia solution is circulated into the Absorber (5) via an expansion valve (which maintains pressure differential).

  • Condenser:

    The high temperature high pressure ammonia vapour enters the condenser where it condenses due to the space heating/process heating media passing over the exchanger. Because the vapour has changed state and become a liquid it effectively gives up its latent heat of vapourisation into the heating media (e.g. space heating system water). The liquid now proceeds to the Expansion Valve 3.

  • Expansion Valve:

    The ammonia liquid, still at high pressure, passes through the expansion valve where the pressure is reduced down stream.

  • Evaporator:

    The reduced pressure ammonia changes state from liquid to vapour i.e. it effectively boils and evaporates. This results in the solution getting very cold and effectively ‘sucking?heat from evaporator heat source (air or water) which gets colder as the solution gets warmer. If the source is ambient air or ground warmed water, the heat is from a renewable source. The ammonia vapour is now warmer but still at low pressure. It now passes to the Absorber (5).

  • Absorber:

    In the absorber the weak ammonia solution from the Generator (1.) Recombines with the warmed vapour from the Evaporator (4.) having just passed through a second Expansion Valve (7). As the ammonia vapour and weak ammonia solution recombine the vapour changes into a liquid, releasing additional heat into the system.

  • Solution Pump:

    The recombined ammonia solution is pumped back from the Absorber (5) back to the Generator (1) where the process continues.

  • Second Expansion Valve:

    This controls the flow of weak ammonia solution between the Generator and Absorber (5) and hence maintains the pressure differential between the low and high pressure sides of the process.

Hence the GAHP process uses pressure differentials and temperature changes to move heat via change of state in a similar but different way to the vapour compression cycle.

Absorption devices mostly use naturally occurring chemicals for the solution mixture where the components mix volumetrically.

Adsorption devices often use a chemically manufactured absorbant refrigerant and the interaction is on the surface of the absorbant rather than within its volume.