Performance investigation on the cascaded packed bed thermal energy storage system with encapsulated nano-enhanced phase change materials for high-temperature applications

Latent heat thermal energy storage (LHTES) systems based on phase change materials (PCMs) offer a reliable way to address the intermittency of renewable energy sources. The low thermal conductivity of PCMs, however, limits the practical use of LHTES systems. This work aims to investigate the effect of mixing graphene (Gr) and copper (Cu) nanoparticles at volume fractions varying from 0 % to 5 % with a PCM on the single charging/discharging performance of a cascaded packed bed LHTES system. The effect of the charge inlet temperature on the cyclic performance is also investigated by considering three cascaded configurations with encapsulated copper-nanoparticle-enhanced PCMs (Cu-NEPCMs) and six inlet temperatures ranging from 390 °C to 440 °C. A concentric-dispersion numerical model is developed to evaluate thermal performance. Synthetic PCMs with melting temperatures in the range of 320°C–380 °C are adopted as the heat storage medium, and molten salt is used as the heat transfer fluid. As the volume fraction of the Cu and Gr nanoparticles increases from 0 % to 5 %, the cascaded Cu-NEPCM and Gr-NEPCM storage systems exhibit decreases in the melting time by 14.4 % and 5.6 % and in the solidification time by 13.9 % and 6.5 %, respectively. As the inlet temperature increases from 390 °C to 440 °C, the optimal cascaded packed bed configuration among the three shows enhancements in the total energy storage in the bed, energy recovered by the salt from the bed, capacity ratio, and total utilization ratio by 82.2 %, 85.6 %, 20.3 %, and 50.5 %, respectively. This work provides insights into the design and operation of LHTES systems with enhanced performance.