Numerical investigation of PCM/graphite matrix thermal energy storage characteristics in tube-in-shell geometry

For sustainable clean energy, a green environment, and low carbon emission goals on a global scale, latent heat thermal energy storage (LHTES) is a priority research topic among energy storage methods. One of the most critical factors determining the storage performance (W/kg) in latent heat thermal energy storage systems is the melting/solidification behavior. In this thesis, a numerical parametric study has been performed on the melting performance of a paraffin/graphite matrix composite thermal energy storage unit in a tube in shell geometry. Numerical studies are carried out for five different graphite matrix bulk densities (0, 23, 50, 100, 143 kg/m3) under two constant surface temperatures (75℃ and 85℃) boundary conditions. Paraffin is used as a phase change material (PCM). The performance of the PCM/graphite matrix composite storage unit with different arrangements and operating conditions are investigated in terms of time-dependent local temperature variations, temperature field, liquid fraction/melting behavior, melting time, and amount of stored energy in comparison with the pure PCM. The accuracy of the numerical models is verified with an experimental setup. The results show that the PCM/graphite matrix composite storage unit has significant potential in latent heat storage. It was observed that the melting time decreased by about 44 times, and the storage performance increased by 40 times with the increase in bulk density compared to the pure paraffin state due to the increasing thermal diffusion effect. However, for values above 100 kg/m3 bulk density, the PCM/graphite matrix performance is limited.