# Preparation and Characterization of CPCM for Thermal Energy Storage Unit
## 3 Results and Discussion
### DSC Test Results  
Because more energy is required by LA from an ordered state (solid state) to a disordered state (liquid state) [23], the melting temperature \(T_{\text{onset}}\) of a CPCM in the melting process is always higher than the solidification temperature \(T_{\text{onset}}\) of a CPCM in the solidifying process, and their difference is defined as supercooling. According to the onset definitions of the melting process (\(T_{\text{onset}}\)) and supercooling [24; 25], Figure 6a shows the DSC results of LA and how to determine the supercooling degree, Figure 6b shows the DSC results of 10 CPCMs, and Table 1 gives their phase transition temperature, latent heat, and supercooling degree. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{ \begin{tabular}{c} **Sample** \\ **Name** \\ \end{tabular}  } & \multirow{2}{*}{**M(LA)/M(EG)**} & \multirow{2}{*}{ \begin{tabular}{c} **EG** \\ **(Mesh)** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Temperature** \\ **(\({}^{\circ}\)C)** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Latent Heat** \\ **(kJ/kg)** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Temperature** \\ **(\({}^{\circ}\)C)** \\ \end{tabular}  } & \multirow{2}{*}{\ \begin{tabular}{c} **Latent Heat** \\ **(kJ/kg)** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **(\({}^{\circ}\)C)** \\ \end{tabular}  } \\ \hline CPCM1 & 6:4 & 100 & 41.38 & 107.04 & 40.32 & 102.25 & 1.06 \\ CPCM2 & 7:3 & 100 & 41.71 & 134.67 & 40.53 & 130.62 & 1.18 \\ CPCM3 & 8:2 & 100 & 41.94 & 168.28 & 40.98  
The phase transition temperature of CPCMs should increase with LA content in the melt process, and that of LA should be the highest one. The situation should be contrary in the solidification process, but the change rule is not as obvious as expected. This reason could be indicated by the Clausius-Clapeyron equation shown in Equation (3). The absorbed processes of LA in EG cannot be completely identical among CPCMs, which results in the different fullness of LA in each cavity. When the temperature of a CPCM is changed, the air pressure in the cavity space of a CPCM will also change, which leads to different pressure effects on the phase transition temperature. In addition, the phase transition temperature may be affected by the thermal conductivity and specific heat of a CPCM. \[\frac{dp}{dT}=\frac{\Delta H_{c}}{T\Delta V} \tag{3}\]  
where \(dp/dT\) is the rate of change in pressure with temperature; \(\Delta H_{c}\) is the phase transition enthalpy; \(T\) is the phase transition equilibrium temperature; and \(\Delta V\) is the specific volume change during the phase transition. The supercooling degree of LA is the largest one, 1.79 \({}^{\circ}\)C, but the CPCM supercooling degree increases when the LA mass content decreases and the EG content increases, which could be caused by more air-filled cavities of the CPCM with more content of EG. When the CPCM with more air is heated, the air pressure in the cavities increases, which leads to a larger Tonset in the melt process, and it is contrary in the solidification process. Because the porosity of CPCMs with 200-mesh EG is greater, as shown in Figure 3, their supercooling degree is larger than that of CPCMs based on 100-mesh EG in the same condition. The relation between the latent heat of the CPCMs, \(\Delta H_{c}\), the latent heat of LA, \(\Delta H_{m}\), and the mass fraction of EG, \(\varphi\), can be expressed by Equation (4). The latent heat of the CPCMs gradually increases with the LA mass content [23; 26; 27]. The latent heat of the CPCMs shown in the melt process is larger than that shown in the solidification process. The mesh effect of EG on the latent heat of the CPCMs is not obvious.