Performance improvement of heat storage tank systems: Capsule innovations inspired by golden barrel cactus structure

In packed-bed thermal energy storage systems, limited heat transfer rates constrain both charging and discharging efficiency, especially for smooth phase change material (PCM) capsules. This limitation adversely affects overall thermal performance and poses challenges for practical large-scale implementation, particularly in maintaining the non-directionality and ease of aggregation. Inspired by cactus structures, this study presents ribbed cactus PCM capsules as a novel design to improve the thermal performance of PCM capsules. The flow and heat transfer of smooth and ribbed cactus PCM capsules in constrained channels are first quantified to assess the impact of ribbed geometry on thermal performance. A systematic analysis of rib number, depth, and width demonstrates the ribbed design's markedly enhanced heat transfer. The results show that the novel ribbed cactus PCM capsule significantly enhances PCM melting and heat transfer by increasing surface area and delaying flow separation. Surface ribs raise the separation angle from 97.8° to 91.8° for smooth capsules to 120.5°–105.9°, improving convective performance and wake stability. The innovative ribbed cactus PCM capsule design significantly improves heat transfer, reaching 320 K in 1254 s—about 32.8% faster than the smooth capsule. Total heat transfer rises from 18.9 W to 40.8 W, representing an enhancement of ∼116%, while the charging time is reduced by approximately 33% compared to the smooth capsule.