Preparation and thermal characterization of expanded graphite/paraffin composite phase change material

Expanded graphite (EG)/paraffin composite phase change materials (PCMs), with mass fraction of EG varying from 0 to 10 wt.%, were prepared and characterized. Polarizing optical microscope investigation showed that compact EG networks formed gradually with increase in the mass fraction of EG. These networks provided thermal conduction paths which enhanced the thermal conductivity of the composite PCMs, e.g., an addition of 10 wt.% EG resulting in a more than 10-fold increase in the thermal conductivity compared to that of pure paraffin. Thermal characterization of the composite PCMs with a differential scanning calorimeter (DSC) revealed the effect of the porous EG on the phase change behavior of paraffin. The shifts in the phase change temperatures were observed. The maximum deviation of the melting/freezing points of the composite PCMs from that of pure paraffin was 1.2 °C whereas that of the peak melting/freezing temperature was 5.6 °C. The DSC investigation also showed an anomaly in the latent heat of the paraffin in the composite PCMs in that it first increased and then decreased with increase in the EG fraction. Heat storage/retrieval tests of the composite PCMs in a latent thermal energy storage system showed that the heat storage/retrieval durations for EG(10)/paraffin(90) composite were reduced by 48.9% and 66.5%, respectively, compared to pure paraffin, which indicated a great improvement in the heat storage/retrieval rates of the system.