Representative elementary volumes required to characterize the normal spectral emittance of silicon carbide foams used as volumetric solar absorbers

The objective of this work is to determine the representative elementary volumes (REVs) required to accurately characterize the normal spectral emittances of SiC open-cell foams and their dependency on the textural features. Because the REVs determined for porosity are often used as radiative REVs, these REVs are also investigated through a parallel study. For this purpose, several numerical tools have been combined. First, a numerical foam generator that provides realistic textures and that enables control over textural parameters such as the porosity and the cell size distribution is presented. Second, a Monte Carlo ray-tracing (MCRT) code that considers the 3D geometry of a foam and its intrinsic optical properties facilitates the computation of their normal spectral emittances. Both numerical tools are validated through comparison with the textural and radiative properties experimentally measured on a real reference α-SiC foam sample using X-ray μ-tomography and infrared spectroscopic measurements, respectively. Then, a set of 3D virtual α-SiC open-cell foams with prescribed textural features (porosity in the range of 0.4-0.9 and cell sizes in the range of 0.5-1.7 mm) were investigated, and a large number of MCRT simulations were performed to determine their respective REVs for both porosity and emittance. The findings indicate that the minimal REV for porosity cannot be directly used for determining the normal spectral emittance. Some practical considerations are then proposed to characterize the normal spectral emittance of α-SiC open-cell foams. Finally, a linear relationship that links the normal spectral emittance to the porosity and the intrinsic optical properties of an α-SiC open-cell foam is presented. The results indicate that this relationship is independent of the cell size.