## 3 Results and Discussion
### Thermal Properties of the Composite PCM and Energy Density  
Figure 8 shows a DSC curve of the Sample S8 composite PCM tested over a temperature range of 50 \({}^{\circ}\)C to 750 \({}^{\circ}\)C. The fluctuation at -50 \({}^{\circ}\)C shown in the DSC curve is due to the unsteady status of the instrument at the beginning. With increasing temperature, a peak occurs at 483 \({}^{\circ}\)C with an enthalpy of 22.27 J/g. This is probably due to the partial crystal transition of SiC from 6H-SiC to 15R-SiC reported by [PERSON] et al. [40], who studied the sputtering effects in hexagonal silicon carbide and found such a transition at -400 \({}^{\circ}\)C. With a further increase in the temperature, a second peak occurs at 710.1 \({}^{\circ}\)C. This is the solid-liquid phase transition of the eutectic carbonate PCM, which has an enthalpy of 110.2 J/g. The DSC cure of the pure eutectic K\({}_{2}\)CO\({}_{3}\)-Na\({}_{2}\)CO\({}_{3}\) salt is also displayed in Figure 8. One can see that the pure eutectic K\({}_{2}\)CO\({}_{3}\)-Na\({}_{2}\)CO\({}_{3}\) salt has a phase change enthalpy of  
Figure 6: XRD Patterns of the composite PCM and individual components. Figure 7: XRD patterns of Sample S8 with and without 100 thermal cycles. 250.4 J/g at 709.7 \({}^{\circ}\)C. It can be found that the measured enthalpy of the composite (Sample S8) is lower than the theoretical enthalpy (136.47 kJ) which could be calculated by the weight percentage of the salt. [PERSON] et al. [41] and [PERSON] [42] have both studied the experimental latent heat of the composite PCM compared to the theoretical values. They found that the structure of the additives could affect the crystal properties or the phase change process of the PCM. The heat capacity of the composite PCM at different temperatures was measured and is shown in the inset of Figure 8. [PERSON] et al. [43] have reported that the heat capacity of MgO at 1000 K is 1.27 J\(\cdot\)g\({}^{-1}\)K\({}^{-1}\) (51.23 J\(\cdot\)mol\({}^{-1}\)K\({}^{-1}\)). [PERSON] et al. [44] have measured a variety of sintered SiC materials and noted a heat capacity of 1.2 J\(\cdot\)g\({}^{-1}\) K\({}^{-1}\) at 1000 \({}^{\circ}\)C. However, studies about the heat capacity of the eutectic salt K\({}_{2}\)CO\({}_{3}\)-Na\({}_{2}\)CO\({}_{3}\) are limited. [PERSON] et al. [45] have estimated that the heat capacity of the eutectic salt K\({}_{2}\)CO\({}_{3}\)-Na\({}_{2}\)CO\({}_{3}\) is \(\sim\)1.65 J\(\cdot\)g\({}^{-1}\)K\({}^{-1}\) given the heat capacities of the pure salts (those of Na\({}_{2}\)CO\({}_{3}\) and K\({}_{2}\)CO\({}_{3}\) are 1.79 and 1.51 J\(\cdot\)g\({}^{-1}\)K\({}^{-1}\)[46]). The heat capacity of the composite PCM could be estimated by the values of the components. However, the realistic heat capacity of the material is temperature dependent. Therefore, the measured results at various temperatures might be different from the estimated values. With the measured heat capacity and the phase change enthalpy, the energy density of the composite PCM can be calculated by using the equation  
\[Q=\int_{T_{o}}^{T_{m}}C_{s}\mathrm{d}T+\Delta H_{m}+\int_{T_{m}}^{T_{1}}C_{1} \mathrm{d}T \tag{1}\]  
where \(Q\) is the total thermal energy stored in a unit mass of the composite PCM; \(C_{s}\) and \(C_{l}\) are heat capacities of the composite PCM before and after the phase change, respectively; \(T_{1}\), \(T_{0}\) and \(T_{m}\) denote, respectively, the upper bound temperature, lower bound temperature, and the phase change temperature of the composite PCM; and \(\Delta H_{m}\) represents the latent heat of the phase change process. As discussed above, the composite PCM under study has a phase change temperature of 710.1 \({}^{\circ}\)C. We take a temperature range between 550 \({}^{\circ}\)C and 750 \({}^{\circ}\)C for the energy density calculation. The reasons for this are: first, the composite PCM is intended for high temperature applications, e.g., for the next generation of concentrated solar power generation; second, we intend to give a conservative estimation of the energy density.