## 2 Materials and Methods
### In-Field Test Room Analysis
#### 3.1.2 Enthalpy vs. Temperature  
The results of the enthalpy variation as a function of temperature are shown in Figure 5 for PCM X (Infinite RTM 21), in Figure 6 for PCM Y (Infinite RTM 23) and in Figure 7 for PCM Z (Infinite RTM 25). The curves relating to both the melting phase (gray line) and solidification phase (black line) are plotted in these graphs.  
For the PCM X (Figure 5), it can be observed that:  
* During the melting phase (gray line), the melting process starts at \(\sim\)19 \({}^{\circ}\)C, reaches its maximum value between 23 \({}^{\circ}\)C and 25 \({}^{\circ}\)C and ends at 25 \({}^{\circ}\)C.
* The solidification phase (black line) is concentrated over the 21-20 \({}^{\circ}\)C range, thus corresponding to its partial enthalpy peak.
* The hysteresis phenomena are quite evident, with a difference of about 4 \({}^{\circ}\)C between the temperature at which the PCM is fully melted and the temperature at which solidification begins.  
Figure 5: Enthalpy results: cumulative enthalpy (**a**) and areal cumulative enthalpy (**b**) of PCM X (Infinite RTM 21).  
The results of PCM Y (Figure 6) show that:  
* During the melting phase (gray line), the melting process starts at ~23 \({}^{\circ}\)C, reaches its maximum value between 24 \({}^{\circ}\)C and 27 \({}^{\circ}\)C and ends at 28 \({}^{\circ}\)C.
* The solidification phase (black line) is concentrated over the 25-23 \({}^{\circ}\)C range, thus corresponding to its maximum enthalpy variation.
* The hysteresis phenomena are still quite evident with a difference of about 2.5 \({}^{\circ}\)C between the temperature at which the PCM is fully melted (~27.5 \({}^{\circ}\)C) and the temperature at which solidification begins (~25 \({}^{\circ}\)C). However, the hysteresis is smaller than in the PCM X case. From the results of PCM Z (Figure 7), it is possible to highlight that:
* During the melting phase (gray line), the transition starts at 24 \({}^{\circ}\)C and ends at 28 \({}^{\circ}\)C, with the peak at ~28 \({}^{\circ}\)C.
* During the solidification phase (black line), the transition phenomena start at 26 \({}^{\circ}\)C and end at 24 \({}^{\circ}\)C.
* The hysteresis phenomena are quite evident with a difference of about 2 \({}^{\circ}\)C between the temperature at which the PCM is fully melted (~28 \({}^{\circ}\)C) and the temperature at which solidification begins (~26 \({}^{\circ}\)C).  
The obtained values show that the behavior of PCM Y and PCM Z differ less than expected, considering their nominal temperatures of 23 \({}^{\circ}\)C and 25 \({}^{\circ}\)C, respectively, as the phase transitions occur in the same temperature range for both.  
Figure 6: Enthalpy results: cumulative enthalpy (**a**) and areal cumulative enthalpy (**b**) of PCM Y (Infinite RTM 23).  
Figure 7: Enthalpy results: cumulative enthalpy (**a**) and areal cumulative enthalpy (**b**) of Sample Z (Infinite RTM 25).  
Furthermore, the results of these tests show that the melting and solidification enthalpy curves do not end at the same values, and that a variation of between \(\sim\)5-20% is observed. These results can be justified by considering the different velocities between the heating and the cooling tests. When adopting DHFM as the test device, the temperature velocity of the step variation cannot be fully controlled as in DSC methods, and the cooling tests are generally slower than the heating ones because of the different heating and cooling powers of the thermoelectric generator (Peltier cells). As highlighted in [34], an interlab test activity has demonstrated that different tests' velocities may influence the PCM storage capacity to a great extent.