Mechanism-driven design of gradient porous metal foam/PCM composites in a triplex-tube helical-coiled thermal energy storage unit

Phase change material latent heat thermal energy storage systems are often limited by low thermal conductivity and temperature non-uniformity under rapid charging and discharging. To address this issue, this study proposes a mechanism-driven gradient porous metal foam/phase change material composite in a triplex-tube helical-coiled latent heat thermal energy storage unit to match local heat transfer mechanisms with non-uniform thermal driving forces during melting. A three-dimensional transient model considering flow resistance and local thermal non-equilibrium was developed to compare uniform, single-gradient, and dual-gradient radial porosity structures. Results show that the positive gradient reduces the complete melting time to 344 s, about 16% and 23% shorter than the uniform and negative-gradient cases, while the high-low-high dual-gradient further reduces it to 333 s; by contrast, the low-high-low gradient suppresses convection and prolongs melting. The temperature field reveals a trade-off between charging rate and thermal uniformity. Overall, gradient porosity better balances conductivity enhancement, convective transport, and temperature uniformity, providing guidance for compact, high-power-density active thermal energy storage systems and showing good adaptability under solar charging.