Fast charging of thermal energy storage systems enabled by phase change materials mixed with expanded graphite

Abstract Solar thermal energy harvesting, storage and conversion are crucial parameters to have an effective renewable energy technology that can lead to sustainable and grid-independent technologies as well as lessening of global carbon footprints. Phase change materials (PCMs) are capable of storing and releasing great amounts of thermal energy through utilization of latent heat of fusion as result of material phase change. PCM suffers mainly from low rate of response to demand that is mainly due to low thermal conductivity of PCMs. This drawback could be improved by mixing the PCMs with matrix of high thermal conductive materials. The purpose of this paper is to test the efficacy of expanded graphite (EG) matrix combined with PCM at various percentages of 0–90% by volume in terms of response rate to demand and energy storage capacity PCM-EG composite samples were designed and prepared, and tests were performed under controlled solar radiation (using solar simulator) to study the heating cycle. Temperature data were recorded by means of thermocouples and infrared camera. Time-dependent results were analyzed under various testing conditions. The results show that the tested 80% EG-20% PCM composite showed 7 times faster response rate to charging when compared to 10% EG and 90% PCM mixture, while about 30% energy storage reduction was observed as the penalty. There must be a sweet spot where response rate improvement and energy storage reduction are at their optimum condition for a particular application.