Methods to Characterize Effective Thermal Conductivity, Diffusivity and Thermal Response in Different Classes of Composite Phase Change Materials

Abstract

The phase change materials (PCMs) used in devices for thermal energy storage (TES) and management are often characterized by low thermal conductivity, a limit for their applicability. Composite PCMs (C-PCM), which combine active phase (proper PCM) with a passive phase with high conductivity and melting temperature have thus been proposed. The paper deals with the effect of length-scale on thermal characterization methods of C-PCM. The first part of the work includes a review of techniques proposed in the scientific literature. Up to now, special focus has been given to effective thermal conductivity and diffusivity at room or low temperature, at which both phases are solid. Conventional equipment has been used, neglecting length-scale effect in cases of coarse porous structures. An experimental set-up developed to characterize the thermal response of course porous C-PCMs also during active phase transition at high temperature is then presented. The setup, including high temperature-heat flux sensors and thermocouples to be located within samples, has been applied to evaluate the thermal response of some of the above C-PCMs. Experimental test results match Finite Elements (FE) simulations well, once a proper lattice model has been selected for the porous passive phase. FE simulations can then be used to estimate temperature difference between active and passive phase that prevents considering the C-PCM as a homogeneous material, to describe it by effective thermo-physical properties. In the engineering field, under these conditions, the design steps for TES systems design cannot be simplified by considering C-PCMs as homogeneous materials in FE codes.

Numerical and Experimental Evaluation of Thermal Conductivity: An Application to Al-Sn Alloys

“…From an experimental point of view, many techniques are available for characterizing thermal conductivity. The choice of the appropriate method depends on the material class (metallic, polymeric, ceramic), their melting temperature, their expected conductivity/diffusivity, and, in the case of composite materials, their scale length [14]. The latter factor implies that the specimen size for the specific test must allow us to consider the composite material as homogeneous.…”

“…The latter factor implies that the specimen size for the specific test must allow us to consider the composite material as homogeneous. Therefore, for example, laser flash analysis (LFA, [15,16]) can be applied to a composite material with a sub-millimetric phase size, while a nonconventional experimental setup, like the one proposed by Gariboldi et al in [14], must be designed according to the specific material structure.…”

Methods for Determining the Thermal Transfer in Phase-Changing Materials (PCMs)

“…The use of these materials offers a maximum benefit when applied to buildings insulated with lightweight material as they have small intrinsic storage capacity. Thus, in order to achieve isolation with light energy efficiency, the following conditions must be met: reduced external energy exchange ensured by thermal insulation; the use of renewable energy sources; the reduction of energy requirements by using phase-changing materials [1][2][3][4][5].…”

“…There are numerous programs for analysis using the finite elements method: SAP 05, COSMOS, COMSOL, ANSYS, NASTRAN, PATRAN, IMAGE-3D, NOSAP, etc., some of them being specialized in solving a certain type of problem, others having a more general character. Under these circumstances, it is necessary to identify the best modeling program using FEM that allows the thermal transfer to be modeled for a PCM of the butyl acetate type that is encapsulated in polymethyl methacrylate (PMMA) [1].…”