## 2 Characterization Methods for Phase Change Materials
### Flucmetric Method  
A fluxmeter bench, shown as a schematic diagram in Figure 3, was used to experimentally characterize the two studied PCM, paraffin RT58 and polymer PEG6000. Similar fluxmeter bunches have been used in previous studies to characterize samples of pure PCM [11; 12; 13; 14], or mortars containing encapsulated PCM [15; 16]. The fluxmeter bench consisted of a poly (methyl methacrylate) (PMMA) brick filled with PCM, which was heated or cooled by two heat exchangers. The PMMA brick measured 210 mm \(\times\) 140 mm \(\times\) 18 mm and the thickness of the PMMA walls was 4 mm. Two heat exchangers applied thermal loads on the large sides of the sample. The four lateral sides represented 17% of the total surface of the sample and were enclosed with an insulation layer of extruded polystyrene foam, in order to limit thermal losses through these sides.  
Figure 1: Principle of the DSC method.  
Measurements of PCM temperature and heat transfers between the large sides enabled the accurate characterization of the PCM inside the brick. The heat flux was monitored by two fluxmeters located between the outer brick wall and the heat exchanger. The fluxmeters had a sensitivity of 177 \(\mu\)V/(W\(\cdot\)m\({}^{-2}\)) and, once calibrated, the accuracy of the measurement was close to 3%. A T-type thermocouple, with an accuracy of 0.1 \({}^{\circ}\)C, was inserted inside the brick to monitor the PCM temperature. Fluxmeters were also implemented on the lateral sides, except on the top side to pass the thermocouple wire, to quantify the thermal losses through the insulated sides.  
Four experiments were carried out to experimentally characterize RT58 and PEG6000. For each experiment, the same temperature was applied to the brick sample by the two heat  
Figure 3: Experimental apparatus of the fluxmeter bench (sectional view).  
Figure 2: Principle of the STEP method.  
exchangers. For the two first experiments, a heating/cooling ramp was applied between 20 \({}^{\circ}\)C and 70 \({}^{\circ}\)C. The first experiment had a fast heating/cooling ramp of 4 h, and the second experiment had a slow heating/cooling ramp of 18 h. The two last experiments consisted of two step loads with a sharp variation of the heat exchangers' temperature. The first step load enabled the simulation of a fast charging process, with a quasi-instantaneous increase in the heat exchangers' temperature from 20 \({}^{\circ}\)C to 70 \({}^{\circ}\)C. Conversely, the second step load imitated a fast discharging, with a sharp decrease in the temperature from 70 \({}^{\circ}\)C to 20 \({}^{\circ}\)C.  
The solid and liquid densities of the PCM were determined by weighing. The PMMA brick was weighed, the first time empty, and the second time once the PCM had been poured inside the brick in a liquid state at 70 \({}^{\circ}\)C. To fill the brick sample, it was necessary to incorporate 215 g of RT58 and 285 g of PEG6000. With the inner volume of the brick and the mass difference between the empty brick and the brick containing the PCM, it was possible to compute the liquid density of the PCM. During the solidification process, the PCM height inside the brick decreased with the shrinkage of the PCM. The solid density was deduced from the height difference inside the brick between the liquid and solid states. Table 1 presents the results obtained for the solid and liquid densities of RT58 and PEG6000. Concerning RT58, the results were in good agreement with the values given by the manufacturer: \(\rho_{S}\) = 770 kg.m\({}^{-3}\) and \(\rho_{L}\) = 880 kg.m\({}^{-3}\).