# Preparation and Characterization of CPCM for Thermal Energy Storage Unit
## 3 Results and Discussion
### Thermal Stability of CPCMs  
To indicate their thermal stability, thermal cycling tests for CPCM3 and CPCM9 are carried out. Figure 8 shows their DSC results after 50 cycle tests and 100 cycle tests. Before/after 100 cycle tests, the melting temperature of CPCM3 and its corresponding latent heat are 41.94 \({}^{\circ}\)C/42.03 \({}^{\circ}\)C and 168.28 kJ/kg/167.59 kJ/kg, respectively, and the solidification temperature and its corresponding latent heat are 40.93 \({}^{\circ}\)C/41.44 \({}^{\circ}\)C and 164.34 kJ/kg/163.47 kJ/kg, respectively. For CPCM9, they are 41.80 \({}^{\circ}\)C/42.03 \({}^{\circ}\)C and 170.65 kJ/kg/169.78 kJ/kg, respectively, in the melting process, and 41.44 \({}^{\circ}\)C/41.44 \({}^{\circ}\)C and 167.70 kJ/kg/166.67 kJ/kg, respectively, in the solidification process. It can be concluded that the phase change temperature and latent heat of CPCM3 and CPCM9 are almost constant during the thermal cycling test of 100 times, and their thermal stability is good. Figure 9 shows the TG images of LA, CPCM3, and CPCM9 to indicate their thermal stability more. Below 130 \({}^{\circ}\)C, the mass loss rate of the CPCMs is less than 1% due to free-water evaporation [30,31]. When the heating temperature is increased to more than 130 \({}^{\circ}\)C, the volatilization and thermal decomposition of LA gradually occur. When the heating temperature is about 200-250 \({}^{\circ}\)C, the mass loss rate of the CPCMs reaches the maximum and the thermal decomposition of EG occurs. The weight loss rate of pure LA almost reaches 100%, that of CPCM3 is 74%, and that of CPCM9 is 84%. This indicates that EG can form a physical protective barrier on the LA absorbed in EG [32, 33, 34] to improve the thermal stability of a CPCM, which is expected by the TESU of the ASHP heating system. \begin{table}
\begin{tabular}{c c} \hline \hline
**Chemical Groups** & **Absorption Peaks (cm\({}^{-1}\))** \\ \hline Hydroxy-oh stretching vibration peak & 3449.13, 3432.30, 3425.60 \\ \hline Methylene C-H antisymmetric stretching vibration & 2953.86, 2917.41, 2871.09, 2955.31, 2917.61, \\ and symmetric stretching vibration peaks & 2955.01, 2916.91, 2850.45, 2848.64 \\ \hline Carbonyl C=O stretching vibration peak & 1701.83, 1698.54, 1700.24 \\ \hline C-H bending vibration peak of saturated & 1464.87, 1410.92, 1303.91, 1249.02, 1465.57, \\ alkyl group & 1302.46, 1466.21, 1326.79 \\ \hline C-O stretching vibration peak & 1114.23, 1082.21, 1109.59, 1194.54 \\ \hline Unsaturated hydrocarbon group =C-H & 939.23, 935.90, 938.02 \\ out-of-plane rocking vibration peak & 777.94, 720.68, 718.27, 723.94 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Different chemical groups and their corresponding absorption peaks. Figure 7: Absorption peaks of different materials. (**a**) LA absorption peak; (**b**) CPCM absorption peak. When a CPCM is used in the TESU, its volumetric expansion rate should also be considered, and a smaller volumetric expansion rate is expected during the solid-liquid transition process [35; 36]. The volume expansion rate is defined by Equation (5). \[\beta=(V_{2}-V_{1})/V_{1} \tag{5}\]  
where \(\beta\) represents the volume expansion rate (%), \(V_{1}\) represents the initial volume (m\({}^{3}\)), and \(V_{2}\) represents the volume after the temperature change (m\({}^{3}\)). Figure 10 shows the densities of the CPCMs and LA in solid and liquid states. Because some air fills in loose particles of a solid CPCM, the CPCM density in the solid state is less than that in the liquid state. When the phase change material is changed from a liquid state to a solid state, the volumetric expansion rate of LA is 2.480%, and the volumetric expansion rates of the CPCMs are all less than that of pure LA. Among them, the volumetric expansion rate of CPCM3 is the smallest at 0.714%. The relative error of the density measurement in the solid state and liquid state is 1% and 0.1%, respectively. Figure 8: Cyclic DSC curve: (**a**) CPCM3; (**b**) CPCM9. Figure 9: TG test curves of cycling test.