Improving the thermal response flexibility of 2- and 3-phase composite phase change materials by metallic triply periodic minimal surface structures

The percolation of organic Phase Change Materials (PCMs) into metallic skeletons produces Composite PCMs (C-PCMs). This paper explores Al-Si-Mg alloy Sheet-based Primitive-Schwarz (PS) Triply Periodic Minimal Surface (TPMS) C-PCMs filled by paraffines, comparing them with C-PCMs built with inverse Body-Centred Cubic (BCC) structures. The aim is to derive guidelines for improving the thermal response flexibility of these systems. The lattice geometrical features and C-PCM properties are calculated and modelled as a function of porosity (e), proportional to storable energy. For e > 0.8, the Effective Thermal Conductivity (lambda(eff)) of PS-based C-PCMs is higher than that of BCC-based, reaching 68 % of the maximum theoretical value. Design considerations are used to define a set of feasible C-PCMs whose thermal response is numerically simulated. The PS favours shorter transients than BCC (6.3 % less for e =0.8). The e increase, and, consequently, lambda(eff) reduction, in PS-based C-PCMs raises both storage potential and storage times (542 s vs 694 s for e = 0.8 vs 0.9). Minor changes in the storage times can be obtained by lattice size variation at constant e. The peculiarity of sheet-based TPMSs of splitting the volume into non-interconnected subdomains is exploited to design 3-phase C-PCMs, employing two PCMs having different melting temperatures.