## 4 Discussion  
This paper presents the results of a set of experiments performed with an innovative refrigerant-PCM-water heat exchanger as the evaporator of a standard vapour compression refrigeration system with variable cooling capacity. Three distinct operating modes were tested: charging, discharging, and simultaneous charging and discharging. In PCM charging and discharging modes, the RPW-HEX acts as a TES tank while in simultaneous charging and discharging mode it acts as a heat exchanger between the refrigerant and the HTF. The parameters analysed in all experiments were the PCM temperature profiles, the cold energy stored in the PCM, the energy charging or discharging rate of the PCM, and the duration of the processes in the three modes. In the charging mode (Mode 1), different compressor powers ranging from 15% to 75% with regards to the maximum power capacity were tested for a fixed evaporation set-point of \(-10\)\({}^{\circ}\)C. The results showed that a complete PCM charging was not possible, regardless the compressor cooling power. This is because of the stratification of PCM temperatures which makes it practically impossible to cool down the PCM located at the top part of the RPW-HEX well below the solidification temperature  
Figure 23: Influence of the HTF flow rate on the instantaneous cold energy stored in the PCM in Mode 3b with the HTF inlet temperature at 12\({}^{\circ}\)C.  
Figure 24: Influence of the HTF flow rate on the total cold energy stored in the PCM and time to reach equilibrium in Mode 3b with the HTF inlet temperature at 12\({}^{\circ}\)C.  
of around 5 \({}^{\circ}\)C. A complete charging could still be possible if a lower evaporation set-point and a low compressor power were used, but this option is not attractive from a practical point of view. The duration of the charging process was considerably reduced from around 32 min to 3 min by increasing the compressor power from 15% to 75%. Nevertheless, the cold energy stored did not depend considerably on the compressor power, and the maximum amount of 294 kJ of cold energy stored in the PCM was achieved at a relatively low compressor power of only 30%. In spite of this, the charging at 60% compression power could be preferred because the charging duration is reduced by 60% with respect to the duration at 30% compressor power, while the reduction of the cold energy stored in the PCM is of only 7%. In the discharging mode (Mode 2), different HTF inlet temperatures (9 \({}^{\circ}\)C, 12 \({}^{\circ}\)C, and 15 \({}^{\circ}\)C) and also different flow rates (50 L\(\cdot\)h\({}^{-1}\), 100 L\(\cdot\)h\({}^{-1}\), and 150 L\(\cdot\)h\({}^{-1}\)) were tested for the PCM initially completely charged. The results showed that the discharging duration decreased from 25 min to 9 min by increasing the HTF flow rate from 50 L\(\cdot\)h\({}^{-1}\) to 150 L\(\cdot\)h\({}^{-1}\) at constant HTF inlet temperature of 12 \({}^{\circ}\)C because of an increase in the heat transfer rate from 0.2 kW to 0.55 kW, respectively. Both the discharging duration and the heat transfer rate vary from 18 min to 8 min and from 0.3 kW to 0.65 kW by varying the inlet HTF temperature from 9 \({}^{\circ}\)C to 15 \({}^{\circ}\)C, respectively. Moreover, the PCM temperature profile showed a stratification of the HTF inside the RPW-HEX, with a temperature difference between the top and bottom parts of the PCM around 3 \({}^{\circ}\)C.  
In the simultaneous charging and discharging mode (Mode 3), two different cases were considered: Mode 3a, for which the PCM was initially completely discharged (liquid state), and Mode 3b, for which the PCM was initially completely charged (solid state).