Accelerating solar thermal energy storage via graded triply periodic minimal surface structures

Solar thermal energy storage (STES), which couples solar thermal conversion with latent heat storage of phase change materials (PCMs), offers a promising route to the continuous utilization of intermittent solar radiation and plays an important role in the decarbonization of heat supply. However, conventional STES systems are often limited by slow charging rates and low overall efficiency, primarily because of poor light penetration and insufficient heat transfer within PCMs. To address these limitations, this study proposes architecturally graded triply periodic minimal surface (TPMS) skeletons for rapid and efficient STES. Numerical simulations combined with experimental measurements are conducted to compare a pure PCM structure, a conventional cubic-strut skeleton structure, and four TPMS-based structures. Among them, the IWP-strut configuration achieves the fastest charging performance, with a melting time of 420.6 s, representing reductions of 47.6 % and 10.0 % compared with a surface-based pure PCM system and a conventional cubic-strut skeleton structure, respectively. This enhancement is attributed to improved light penetration, and intensified natural convection. Further optimization using a graded pore-size distribution reduces the melting time to 404.3 s, corresponding to an additional 3.9 ∼ 6.9 % reduction relative to uniform-pore designs, while increasing the solar thermal energy storage efficiency to 83.9 %. The improvement arises from the redistribution of solar energy absorption toward deeper regions, which alleviates surface-localized heating and enhances energy delivery to diffusion-limited lower PCMs regions. These findings demonstrate that graded TPMS skeletons provide an effective passive structural strategy for accelerating solar thermal energy storage and improving storage efficiency.