Thermal performance of structured packed bed with encapsulated phase change materials

•Three differently structured packed bed thermal energy storage units are compared.•Packed bed thermal energy storage units with face center cubic packing is preferred.•Effects of the Reynolds number, temperature and Prandtl number are investigated.•Variation of Prandtl number and thermal characteristics are not directly correlated.•The thermal conductivity of heat transfer fluid positively affects heat transfer. Packed bed latent heat storage system with simple construction and efficient heat transfer has been extensively applied in the field of renewable energy. This study aimed to select a proper structured packing format by comparing the thermal performance of different structured packed bed thermal energy storage (TES) configurations, with encapsulated paraffin as the phase change material (PCM). The 3D numerical models of the structured packed beds were constructed, and the numerical results are in agreement with the experimental ones. The performance indexes, including the average heat storage rate, total energy, discharging time and construction cost, were evaluated under consistent working condition. The analysis results indicate that the packed bed TES configuration with face centre cubic packing (FCC) possesses superior characteristics, such as high charging rate, low investment cost, large total heat storage energy and stable output temperature, compared with that with body center cubic packing or simple cubic packing. A detailed parametric investigation of the packed bed TES configuration with FCC is implemented to analyse the effect of Reynolds number (Re), inlet temperature and Prandtl number (Pr) of the heat transfer fluid (HTF). Results showed that the uniform extraction of heat is obtained by the packed bed with small inlet Re. The larger the temperature difference between PCM and HTF is, the faster the discharging rate is and the bigger the total heat recovery energy is. By selecting water, fuel oil JP-4, ethylene glycol and hydraulic oil as HTFs, the change in the HTF's Pr is determined to be irrelevant to the thermal characteristic variations of the structured packed bed TES configuration, whereas the thermal conductivity of the HTF positively affects the heat transfer inside the structured packed bed.