Numerical study on the thermal energy storage performance of graphite matrix composite with phase change in shell-in-tube: Effects of bulk density and wall temperature

Phase change materials (PCMs) offer notable solutions in thermal energy storage applications, that can contribute to a sustainable environment and a low carbon footprint. However, the low thermal conductivity of PCM restricts its usage in thermal energy storage systems. In this study, a 2-D numerical investigation is per-formed on a graphite matrix composite with phase change in a shell-in-tube geometry to overcome the low thermal conductivity issue of PCM for solar energy and waste heat recovery applications beyond the previous efforts in the literature. The validation of the numerical models is provided by data obtained from the experi-mental setup. The effects of various bulk densities (0, 23, 50, 100, and 143 kg/m3) of graphite matrix on storage performance were tested under three isothermal wall temperatures (65 degrees C, 75 degrees C, and 85 degrees C). Thermal perfor-mance is evaluated through melting time, liquid fraction, enhancement ratio, total stored thermal energy, Stefan number, and energy storage rates. Results show that graphite matrix composites illustrate the remarkable po-tential for thermal energy storage. It is observed that there is an ideal bulk density value (100 kg/m3), and the effect of bulk density is diminished beyond an ideal value of 100 kg/m3 depending on similar thermophysical properties. The difference between the ideal and maximum bulk densities is just 8 % for the energy storage rate. Melting time is reduced by about 44 times for 100 kg/m3 as compared to pure paraffin in terms of abundant thermal paths to diffuse the heat in the storage medium. On the other side, for 100 kg/m3(Twall = 75 degrees C), the total stored thermal energy is decreased slightly by about 6.29 % in terms of decreased PCM mass, but the energy storage rate is increased by 76 times compared to the pure paraffin case, considering the enhanced liquid fraction. Moreover, the effect of increasing wall temperature is greater at lower bulk density values due to lower thermal properties. For 100 kg/m3 and 23 kg/m3, the energy storage rate is increased by 225.3 % and 298.9 %, respectively, with the 20 degrees C increase in wall temperature (from Twall = 85 degrees C to Twall = 65 degrees C).