Thermal investigation and parametric analysis of cascaded latent heat storage system enhanced by porous media

Cascaded latent heat storage (CLHS) has become a promising solution to the recovery and utilization of thermal energy. Many studies optimized the inlet temperature and velocity of heat transfer fluid to improve the heat transfer rate for the charging process of CLHS systems with pure phase change materials (PCMs), but the effect is not significant, mainly because the thermal resistance of the system comes from the PCMs side due to the low thermal conductivity of pure PCMs. Porous media composite phase change materials (CPCMs) have been shown to have promising prospects in a single PCM system, but no research precedent has been found for the application of porous media CPCM in the CLHS system. Besides, when CPCMs are encapsulated in the cavities of the CLHS system instead of pure PCMs, the difference in the degree of impact between the porosity of a porous media and boundary conditions on the thermal performance of a CLHS system is still unclear. The purpose of this article is to significantly improve the heat transfer rate by applying the CPCM to the CLHS system and to comprehensively compare the impact of porosity and HTF inlet boundary conditions on the charging/discharging time and energy/exergy charged/discharged rates of a CLHS system with CPCMs through two sensitivity analysis methods. When the porosity reduces from 0.97 to 0.92, the charging time and the discharging time decrease by 74.50 % and 73.39 % respectively; the average energy charge rate and average exergy charge rate increase by about 4.11 times; the average energy and exergy discharge rates increase by 5.76 times and 5.8 times respectively. Sensitivity analysis shows that the porosity of a porous media is the most important factor affecting the charging and discharging rates of a CLHS system, but its impact on energy and exergy charge/discharge rates is not monotonic. The effect of the inlet temperature and velocity of the HTF on a CLHS system is complex, and simply changing a single variable may not be effective in optimizing thermal performance parameters. This study provides a comprehensive performance enhancement strategy for the thermal energy storage and utilization processes of CLHS systems, which have great potential in fields such as solar energy collection, waste heat recovery, and building heating.