Expanded graphite as thermal conductivity enhancer for paraffin wax being used in thermal energy storage systems

Abstract

Phase change material such as paraffin wax has very low thermal conductivity which leads to many defects upon its practical utilization in thermal energy storage system. In this paper, study on paraffin/expanded graphite (EG) composite has been carried out to enhance the thermal conductivity of pure paraffin (base material). EG (supporting material) with its worm-like structure has been introduced as thermal conductivity enhancer in paraffin/EG composite. A type of paraffin wax with melting temperature (Tm = 60-62 °C) has been investigated. Three samples of paraffin/EG composite have been prepared with weight percentage of EG as 5%, 10% and 15%. Self-absorption technique has been introduced by preparing the samples in a cylindrical vessel at temperature slightly more than melting temperature of paraffin for a certain period of time until maximum incorporation of paraffin wax was achieved into the porous structure of expanded graphite. Thermal characteristics have been investigated by differential scanning calorimeter which showed no change in the melting temperature of base material, however a slight change in phase transition temperature was observed, and measured latent heat of the composite was a little lower than theoretical latent heat. Thermal conductivity of each sample of paraffin/EG composite determined by laser flash method has been achieved 4 times, 6 times and 6.5 times higher than that of pure paraffin for 5%, 10% and 15% paraffin/EG composite, respectively, and as quantity of EG was increased, thermal conductivity of the composite got increased as well. Results obtained by scanning electron microscopy indicated uniform mingling of paraffin wax and expanded graphite in the composite. Form-stability has been confirmed through liquid leakage test which showed no leakage of paraffin from composite. This work is equally significant to be engaged in thermal energy storage systems, cooling of electronic devices and thermal management of batteries.