Using triply periodic minimal surfaces (TPMS)-based metal foams structures as skeleton for metal-foam-PCM composites for thermal energy storage and energy management applications

Organic phase change materials (PCMs), such as paraffin wax, have shown great potential for their utilization in latent heat thermal energy storage systems (LHTES). However, due to their low thermal conductivity, they are often hybridized with high thermal conductivity metal foam, resulting in metal foam-PCM composites (MFPCMs) with enhanced heat transfer features. Conventional metal foam is usually idealized using the Kelvin cell. Owing to the recent advances in 3D printing, any complicated topology can however be easily manufactured, which paves a pathway for other complex cell types to be utilized in such energy systems other than Kelvin cell. In this exploratory work, three Triply Periodic Minimal Surfaces (TPMS), i.e., Gyroid, I-graph and wrapped package-graph (IWP), and Primitive cells, are (to the best of the knowledge of the present authors), used for the first time, as skeleton for MFPCMs composites to enhance the effective thermal conductivity of conventional PCMs. Transient numerical simulations were performed to compare the thermal energy storage performance of the used TPMS-based PCM composites with their counterpart based on the conventional Kelvin cell. All structures were tested at the same porosity level and unit cell size of 90% and 7 mm, respectively. Simulations were done under two boundary conditions namely isothermal and isoheat flux conditions. Steady state simulations were also performed to assess the effective thermal conductivity of the used MFPCM composites at temperatures below the melting temperature of used PCM (pure conduction). The obtained results stress that the effective thermal conductivity of MFPCMs strongly depends on the cell type and its unique architecture and not only on the cell porosity where significant increase in the effective thermal conductivity of the MFPCMs composites was achieved when the three TPMS structures are used. Under isothermal condition while considering the Kelvin-based MFPCM as the baseline case, the PCM melting time was reduced by approximately 31% for the Gyroid, 40.3% for the IWP, and 35.3% for the Primitive-based MFPCMs. In isoflux case, the PCM melting time did not show dependence on the type of metal foam structure. However, by considering the temperature homogeneity as a performance indicator (quantified by the maximum and minimum temperature difference in PCM domain), Kelvin-based MFPCM showed the highest value for the difference (least homogenous) whereas, IWP-based MFPCM on average was almost 5 K lesser than the Kelvin-based MFPCM during the entire melting process. Therefore, TPMS structures showed superior performance than the Kelvin cell, let alone than the case of PCM alone, which makes them promising candidates for potential utilization in LHTES applications.