Experimental study on the thermal performance of a large 100 MJ cascaded packed-bed thermal energy storage system with macro-encapsulation of phase change materials

While Phase Change Materials (PCMs) offer high theoretical heat storage capacity, their practical performance in Thermal Energy Storage (TES) systems is often limited by operational constraints such as thermocline, cut-off temperatures, and operating time. To address these challenges, this study introduces a novel macro-encapsulation method to fabricate 8600 stainless steel PCM capsules with validated high-temperature resistance and leak-proof integrity. Using this method, we established a 100 MJ cascaded packed-bed TES system and systematically evaluated its thermal performance under real-world operating conditions. Subsequent experimental analysis compared the effects of varying inlet temperatures and outlet cut-off thresholds on charging/discharging efficiency. Experimental results indicate that during charging process, increasing the inlet temperature within the TES device's range not only enhances thermal storage capacity but also improves energy storage efficiency. For instance, raising the charging inlet temperature from 120 °C to 180 °C increased thermal storage efficiency from 70.4 % to 97.3 %, with effective thermal energy storage density rising from 21.43 kWh/m³ to 53.77 kWh/m³. Conversely, during discharging process, reducing the inlet temperature not only enhances the discharge power but also increases the released heat. To fully release the stored heat, the discharge inlet temperature should be below the lowest phase-change temperature of PCM within TES devices. The discharge cut-off temperature is determined by practical application scenarios, with higher cut-off temperatures imposing stricter requirements for outlet temperature during discharging process, resulting in less released heat and lower thermal energy storage density. This work has applicability in thermal storage and utilization contexts and serves as a valuable reference for the practical design of packed-bed TES systems.