On the use of effective thermophysical properties to predict the melting process of composite phase change materials with coarse structures

Composite PCMs combining metallic foam and paraffin are widely used as phase change materials (PCMs) to tailor the properties of pure PCMs and enhance the thermal energy storage/release. For the complex composites structures, the transient thermal response prediction by direct simulation (DS) is not easy in term of geometry generation and computation. The volume-averaged model (1T model) considering the composite PCMs as homogeneous media is sometimes used to deal with thermal transport in Composite PCMs, not always with a sufficiently good local description of non-steady conditions. The paper carries out a set of cases where a composite PCM modelled as an open-pore body-centred cell made of Aluminium (Al) filled with paraffin (i) to investigate the combined effects of the geometry of the unit cell (side length, porosity), the composite sample (sample height) and boundary conditions (heat input) on the heat response; (ii) to identify the local/overall errors in temperature and volume fraction of liquid PCM (and thus of stored heat) induced by the use of 1T model for various geometry/heat flux combinations. Analytical equations are proposed to predict the maximum temperature difference between Al and PCM as well as the maximum temperature difference calculated by applying the 1T or DS model as a function of the open cell structure geometry and heat flux. The main novelty introduced in the paper is the analytical model used to quantify the maximum local error on molten PCM volume fraction for the 1T model, and thus on heat stored/released. The model supplies good local thermal response predictions for fine structures and lower heat flux input. Nevertheless, errors in the volume fraction of molten PCMs predicted for the whole sample are far lower and the 1T model can be easily applied in a wider range of geometry/conditions.