An Experimental Investigation of the Caloric Properties for the Composite Phase-change Material Paraffin Wax/Expanded Graphite

Efficient thermal energy storage (TES) is one of the pillars on the way to the reduction of greenhouse gas emissions. Energy density is a key parameter for the phase change materials (PCMs) for the TES systems. Therefore, the caloric properties of the new generation of composite PCMs require a deeper examination. Here we show that vacuum impregnation of the expanded graphite (EG) matrix with industrial paraffin wax (PW) contributes to an increase in the value of enthalpy of the diffuse phase transition for the composite PCM PW/EG in comparison with pure PW. For this purpose, the PW/EG samples were explored in terms of the effective heat capacity over a wide temperature range. The mass fraction of the EG in the PW varied in the range from 3.2 to 6.5 %. The effect of the EG on the PW caloric properties, measured during heating and cooling, was different. It was found that during PW crystallization the exothermic process of adsorption of molecules of liquid PW on the surface of EG pores occurred. Therefore, it is a reason for the significant increase in the crystallization enthalpy of the PW/ EG samples vs. theoretical value. On the contrary, the EG matrix suppresses thermal fluctuations that contribute to the destruction of metastable structures in the liquid PW. It may explain the decrease in the melting enthalpy of the PW/ EG samples vs. the theoretical value. The increase in the EG mass fraction above the lower concentration of the PCM stability - 3.2% - does not lead to a significant variation in the value of phase transition enthalpy. The obtained effects indicate the expediency for further search of the optimal EG concentration in the various organic PCMs. The industrial implementation of the new generation of PCMs containing EG will contribute to improving the efficiency of the TES units.