Modeling and porosity configuration for enhanced metal foam and cryogenic phase change material composite performance

Cold energy storage provides an effective approach for recovering intermittent cryogenic energy, particularly in liquefied natural gas regasification, thereby reducing exergy losses and improving energy efficiency. However, the low thermal conductivity of phase change materials and inefficient use of storage capacity hinder performance. This study employs both local thermal non-equilibrium and local thermal equilibrium models to simulate n-pentane solidification within copper foam. Considering finite interfacial conductance, the non-equilibrium model predicts a solidification time of 240 s, over twice that of the equilibrium model, and shows better agreement with experiments while revealing larger temperature gradients near the wall. A new cold storage performance factor is proposed to jointly evaluate storage density and time efficiency, providing a comprehensive metric for assessing storage performance. Among uniform porosity cases, 0.7 provides the best overall performance, achieving a peak of 350 kJ/(m3·s) and surpassing 0.9 and 0.5 by 30 % and 45 %, respectively. Furthermore, this study demonstrates that a graded porosity design with porosity increasing from 0.7 at the outer layer to 0.9 at the inner layer optimizes both thermal conductivity and thermal inertia, resulting in a 5 % and 11 % improvement in the performance factor compared to uniform and decreasing porosity gradients. This design also highlights the importance of porosity optimization in enhancing efficiency but identifies challenges with intensified solid–liquid interface undulations due to spatial variations in thermal conductivity. The results establish design principles for optimizing porosity in cryogenic cold energy storage systems, offering guidance for improving solidification rate, storage density, and system efficiency.