A heat pump consists of a closed circuit through which a special fluid (refrigerant) flows. This fluid takes on a liquid or gaseous state according to temperature and pressure conditions.
This closed circuit consists of:
| Electric Heat Pump | Gas Absorption Heat Pump |
| a compressor | a generator an absorber |
| a condenser | a condenser |
| an expansion valve | a series of restrictors |
| an evaporator | an evaporator |
The condenser and the evaporator consist of heat exchangers, i.e. special tubes placed in contact with service fluids (which may be water or air) in which the refrigerant flows. The latter transfers heat to the condenser (the high temperature side) and takes it away from the evaporator (the low temperature side).
During operation, the refrigerant, inside the circuit, undergoes the following transformations:
| Electric Heat Pump | Gas Absorption Heat Pump |
| Condensation: refrigerant flowing from the compressor passes from a gaseous to liquid state, giving off heat to the outside. | Condensation: refrigerant flowing from the generator passes from a gaseous to liquid state, giving off heat to the external fluid (water or air). |
| Expansion: passing through the expansion valve, the liquid refrigerant cools and is partially transformed into vapour. | Expansion: passing through the restrictors, i.e. suitably calibrated narrowings, the refrigerant is cooled. |
| Evaporation: the refrigerant absorbs heat and evaporates completely. | Evaporation: the refrigerant absorbs heat from the external fluid (water or air) and evaporates completely, returning to a gaseous state. |
| Compression: the refrigerant, in a gaseous state and at low pressure, coming from the evaporator, is taken to a high pressure; during compression it is heated, absorbing a certain amount of heat. | Absorber: the refrigerant is absorbed by an absorbing fluid, making it liquid once more Generator: the liquid solution of the refrigerant and absorbing is heated in the generator by means of a gas burner, separating the refrigerant, which evaporates, increasing in temperature and pressure. |
| All of these transformations together make up the electric heat pump’s cycle: the compressor provides the refrigerant with energy, the refrigerant absorbs heat in the evaporator from the surrounding medium, and through the condenser, transfers it to the medium to be heated. | All of these transformations together make up the gas heat pump’s cycle: by supplying energy with the gas burner (methane/LPG) the refrigerant in the evaporator absorbs heat from the external fluid and, through the condenser, transfers it to the medium to be heated. |