## 4 Results of the Fluxmetric Method  
For the FM method, the results of the four experiments carried out on RT58 and PEG6000 are presented in Figure 8. The 4 h fast heating/cooling rate and the 18 h slow heating/cooling ramp are plotted in Figure 8A,B, respectively. Figure 8C shows the experiment with a charging step load, and Figure 8D shows the experiment with a discharging step load. For all the experiments performed, the thermal losses through the insulation layer in mineral wool, measured by the lateral fluxmeters, represented only between 2% and 3% of the total monitored heat flux. As the heat flux occurred almost entirely between the brick sample and the heat exchangers, only the temperature of the heat exchangers influenced the thermal behavior of the sample. A first overall comparison, between RT58 and PEG6000 on the four experiments, showed a greater heat flux for PEG6000. This observation is validated with Figure 9 which compares the total energy exchanged for each PCM between 20 \({}^{\circ}\)C and 70 \({}^{\circ}\)C, under the various heat loads tested. In Figure 9, red bars represent the heating cycles and blue bars represent the cooling cycles. The 4 h ramp, 18 h ramp and step loads are plotted with a clear shade, a medium shade and a dark shade, respectively. The mean total energy stored, PMMA brick included, equaled 83.9 kJ for RT58 and 103.4 kJ for PEG6000. The energy density was deduced by removing the sensible energy stored inside the PMMA brick and dividing the remaining energy by the mass of PCM incorporated inside the brick. Figure 8: Heat flux and temperature measurements of RT58 and PEG6000 for a 4 h ramp (**A**), 18 h ramp (**B**), charging step load (**C**), and discharging step load (**D**). Results showed an energy density, between 20 \({}^{\circ}\)C and 70 \({}^{\circ}\)C, of 289.6 kJ\(\cdot\)kg\({}^{-1}\) for RT58 and 282.8 kJ\(\cdot\)kg\({}^{-1}\) for PEG6000. This meant that if only considering the energy density, RT58 and PEG6000 had similar thermal performances. However, as PEG6000 has a greater mass density than RT58, the energy stored inside the same volume is greater for PEG6000. This leads to a potential duration of DHW production (Figure 8D), where the PCM temperature is above 40 \({}^{\circ}\)C, being 22.7% longer for PEG6000. Therefore, to store a given amount of energy, PEG6000 can enable a more compact design, not because of a higher mass energy density, but due to a greater volumetric energy density. Figure 10 compares RT58 and PEG6000 to show the higher volumetric energy density for PEG6000, and presents the evolution of the heat stored inside the sample, as a function of the PCM temperature. The 18 h heating/cooling ramp was selected because slow rates minimized the thermal gradient inside the sample. With a fast heating/cooling rate, as for the charging and discharging step loads, the PCM temperature will not be representative of the measured heat flux. Moreover, slow heating/cooling rates highlight the different thermal behaviors, which are dependent on the temperature level and the phase change process, melting or solidification, of the PCM. Figure 10 clearly illustrates the supercooling phenomenon during the cooling process of PEG6000 with a short reheating of the PCM. For RT58, Figure 8A,B show a main heat flux peak between 50 and 60 \({}^{\circ}\)C, but also a minor heat flux peak between 40 \({}^{\circ}\)C and 50 \({}^{\circ}\)C. This minor heat flux peak is not mentioned by the manufacturer but has previously been observed experimentally in the literature on DSC experiments [47; 48; 49]. This minor heat flux peak may come from a solid-solid phase change process of the paraffin, or from a solid-liquid phase change process of another material, as the commercial paraffin might not be composed exclusively of one material.