Phase Change Materials (PCMs) have been widely investigated as a cooling solution due to their significant latent heat capacity. However, the current PCMs generally suffer a low thermal conductivity, thus hindering the application of PCMs. Composite Phase Change Materials (CPCMs) filling with high thermal conductivity materials have been proposed to solve this issue. Nevertheless, the latent heat of the CPCMs decreases with the mass fraction of fillings, thus leading to a lower allowable working time under safe operating temperature. Therefore, an optimal filling mass fraction of CPCMs is in urgent needed to improve the application of CPCMs. In this study, we developed a one-dimensional conduction heat transfer model of CPCMs to predict the optimal filling mass fraction of CPCMs to realize the maximum allowable working time. The filling mass fraction was introduced into the model and the relationship between the thermal conductivity and latent heat was built. We adopted paraffin as the matrix material and Expanded Graphite (EG) as the thermal conductivity enhancer. The allowable working time of the CPCMs as the function of filling mass fraction was obtained. Based on the principle of the maximum allowable working time, the optimal filling mass fraction was calculated. Comparative experiments were also conducted to validate the accuracy of the prediction model. The parameters which affect the maximum allowable working temperature were also investigated, including input heat flux, safe temperature, and height of CPCMs. The results show that a higher heat flux and height requires a larger filling mass fraction, and it’s opposite for the safe temperature.

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