Albizzia pollen-inspired phase change capsules accelerate energy storage of packed-bed thermal energy storage system

Packed-bed thermal energy storage (PBTES) system using phase change capsules has been widely applied for thermal energy harvesting and management to alleviate unbalanced energy supply and demand problems. However, the slow thermal energy charging is always a daunting challenge limiting its fast development. Here, a bionic phase change materials (PCMs) capsule by mimicking the natural structure of albizzia pollen is proposed. The heat storage performance and economy of capsules with different internal fin structures are investigated numerically and experimentally. The thermal charging of pollen-type PCMs capsules is the fastest, whose melting time prominently reduces by 19 %, 24 %, 41 % and 61 % compared to the plate-type, ring-type, column-type, and pure PCMs capsules, respectively. That is because the contact area between the skeleton and the capsule wall is increased and the average distance between PCMs and the heat transfer surface is shortened. The pollen-type capsules exhibit the best economy with the exergy efficiency is improved by 15 % compared to pure PCMs capsule. By optimizing the ratio of fin length to radius (L/R) to 5/6, the melting time can be further reduced by 62 % and the exergy efficiency can be improved by 16 % owing to the enhanced heat transfer rate in the center of the pollen-type capsules. A PBTES system is further built based on optimized pollen-based capsules, and faster heat storage rate is successfully demonstrated in experiments. These results provide alternative solutions for optimizing PBTES systems and improving their thermal energy storage performances.