Design of thermal conductivity, permeability, and heat storage behavior of Composite Phase Change Materials based on metallic TPMS lattices

Abstract

The paper focuses on composites based on inner Al-alloy sheet-based Triply Periodic Minimal Surfaces (TPMSs) structures, which can be manufactured with high porosity (ε), leading to two non-interconnected domains. These conditions favor designing Composite Phase Change Materials (C-PCMs) in which the void domains are filled by the same or different PCMs, which enable thermal energy storage in the form of latent heat. In this paper, we demonstrate that C-PCMs can be designed based on models of their effective thermophysical properties. To this aim, the effective thermal conductivity (λ_eff) of various Al-based TPMS structures, among which Primitive-Schwarz (PS), Gyroid (G), Diamond (D), and I-graph and wrapped package graph (I-WP), was calculated and analytically modeled. Different filling phases, such as tin, paraffins, or water, were considered to evaluate the influence of the thermal conductivity ratio of the two C-PCM phases at different ε. Furthermore, the transient thermal behavior of Al-based PS C-PCMs was numerically simulated in the extreme cases of temperature ramp or constant heat flux inputs. Low-conductive paraffin and high-conductive tin were selected as filling materials. The results of the analyses revealed that in the first case (temperature ramp), the hybrid Al/paraffin C-PCM exhibited fast phase change, corresponding to peak-type thermal power storage and higher differences between Al and paraffin phase temperatures. Constant heat flux led to a more gradual paraffin melting and heat storage. Instead, the choice of the boundary conditions is less influential on the fully metallic C-PCM response. Combining two different PCMs with the PS lattice further modulates the thermal response of C-PCMs, making them appealing for Temperature Management (TEM) purposes. Finally, an analytical model of the permeability of PS structures was developed based on numerical simulations and compared to highly scattered literature data. Permeability estimation allows the calculation of the Rayleigh-Darcy parameter, setting thresholds for the onset of the convection of the molten PCM phase within the TPMS skeleton, which modifies the thermal response of the C-PCM.