## 3 Result and Discussion
### Thermal Storage Performance Analysis  
The phase change temperature and thermal energy storage properties of pristine PEG and PEG/GNPs composites were measured by the DSC technique to explore the effect of GNPs on the thermal storage performance of composites. The melting and solidifying DSC curves of PEG and PEG/GNPs composites are shown in Figures 4 and 5, respectively. The detailed calorimetric results of the DSC experiments are tabulated in Table 1, including starting melting/solidifying temperature (\(T_{\text{ms}}/T_{\text{ss}}\)), end melting/solidifying temperature (\(T_{\text{me}}/T_{\text{se}}\)), peak melting/solidifying temperature (\(T_{\text{mp}}/T_{\text{sp}}\)), endothermic/exothermic enthalpy (\(\Delta H_{\text{m}}/\Delta H_{\text{c}}\)). Obviously, for all samples, the conspicuous endothermic and exothermic peak are both exhibited in the melting and solidifying process, which represent the solid-liquid phase change of the pure PEG. As can be observed from Table 1, compared with pristine PEG, the \(T_{\text{s}}\) of the composites is not significantly influenced by the addition of GNPs. Notably, the composites present lower melting temperatures than that of pure PEG, which is possible attributed to the incorporation of GNPs. When the GNPs are uniformly dispersed into the PEG matrix, the intimate interaction between GNPs and PEG, such as surface tension forces, \(\pi\)-\(\pi\) interactions, and capillary forces, will confine the mobility of PEG molecules, resulting in the decline of phase change temperature. Additionally, the thermal conductivity of composites can be enhanced significantly with the addition of GNPs, which leads to a rapid thermal response. Similar results were founded in a previous study [34]. Therefore, it is demonstrated that the decrease in \(T_{\text{m}}\) between pure PEG and composites can be ascribed to the comprehensive effects of these factors. Figure 4: Melting differential scanning calorimeter (DSC) curves of the pure PEG and PEG/GNPs composites with the different contents of GNPs. Figure 3: X-ray diffraction (XRD) patterns of GNPs, pure PEG and PEG/GNPs composites with the different contents of GNPs. High latent heat is an imperative factor in PCMs, since it is directly related to the capacity of PCMs for energy storage. As can been seen in Table 1, the \(\Delta H_{\text{m}}\) of the composites decrease with the increasing mass fraction of the GNPs. One reason for this latent heat loss is that some of the PEG volume is replaced by the GNPs, which do not undergo a phase change. Moreover, it is noticeable that the measured latent heat values of the composites in the process of melting are slightly lower than the arithmetically calculated values by the following equation:  
\[\Delta H_{\text{nano-composite}}=\mu\times\Delta H_{\text{PEG}} \tag{1}\]  
where \(\Delta H_{\text{nano- composites}}\) is the calculated latent heat value of the PEG/GNPs composite, \(\mu\) and \(\Delta\)HPEG are the PEG weight percentage of composite and the latent heat of pure PEG, respectively. The measured and calculated latent heat of the composites in the process of melting are compared in Table 2. As can be seen, the relative errors between the measured and calculated values have a tendency to increase with the increasing mass fraction of GNPs. The measured latent heat capacity of the composites doped with 2 wt. % GNPs are 4.4% lower when compared to the calculated values. This possibility is ascribed to the physical interactions between GNPs and PEG, such as van der Waals force and hydrophobic-hydrophobic interaction, which can restrict the mobility of PEG molecular chains during the process of crystallization, as a result, the phase change enthalpy of PEG decreases [34]. We did not increase the GNPs mass fraction beyond 2 wt. % to avoid further decrease in the latent melting heat.