Figure 1 represents the DX-SAHP system studied in this work, which consists of the following components: thermodynamic solar collectors, a hermetic refrigeration compressor, a plate heat exchanger as condenser, an electronic expansion valve (EEV), a hot water storage tank with an auxiliary supplier of heat, heat exchangers inside the building and two water pumps.

A thermodynamic solar collector essentially consists of a flat aluminium plate without any glazing or back insulation and with a selective surface. It presents a centrally located tube running longitudinally where refrigerant gets evaporated thanks to heat absorbed not only from solar radiation, but also from the environment. The evaporated refrigerant is sucked in by the compressor, which drives it to the condenser in elevating its pressure and temperature. The energy rejected by the refrigerant in the condenser heats the water pumped to the storage tank. This tank

supplies hot water to heat exchangers in the building, which achieve and keep thermal comfort conditions in rooms.

The process undergone by the refrigerant can be represented by the vapour compression cycle illustrated in Figure 2. Here, 1-2 represents evaporation and superheating of the refrigerant at evaporating pressure. The isentropic compression process is represented by 2-3, however we are interested in the polytropic compression process 2-3’. After reaching condensation pressure, the refrigerant condenses until becoming subcooled liquid (point 4). Then, the refrigerant is pumped through the EEV and the throttling process takes place. At point 1, we have a two-phase fluid at evaporating temperature, which is lower than ambient air temperature.