Prediction of the radiative properties of reconstructed alpha-SiC foams used for concentrated solar applications
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Prediction of the radiative properties of reconstructed alpha-SiC foams used for concentrated solar applications Benoit Rousseau1, Simon Guevelou1, Gilberto Domingues1, Jerome Vicente2, Cyril Caliot3 and Gilles Flamant3 1
CNRS UMR 6607, LTN, rue Christian Pauc, 44306 Nantes, France
2
CNRS UMR 7343, IUSTI, 5 rue Enrico Fermi, 13453 Marseille, France
3
CNRS UPR 8521, PROMES, 7 rue de Four Solaire, 66120 Font Romeu Odeillo, France
ABSTRACT A SiC-based ceramic foam applied in solar thermal processes was characterized in detail in terms of its textural parameters and its radiative properties. Scanning electron microscopy and x-ray µ-tomography were first performed to investigate the 3D texture of the sample at several length scales. Infrared reflectance microscopy was also applied to probe the local optical responses on the struts constituting the foam. Based on the whole set of experimental data, a numerical tool (C++) was implemented to reconstruct virtual SiC foams. A Monte Carlo Ray Tracing code (iMorphRad, C++) was then used to compute the normal spectral emittance for the real SiC foam and for another reconstructed SiC foam with similar textural features. The two numerically determined emittances were then compared with previous infrared spectroscopy experimental measurements. This numerical procedure enables us to propose a methodology for the design of SiC foams with prescribed radiative properties. INTRODUCTION Silicon carbide (SiC) foams are receiving growing interest for concentrated solar applications because they can be advantageously applied as volumetric absorbers [1-3]. These foams exhibit large specific surface areas, high permeabilities and high resistances to thermal stress. In central solar power plants, volumetric solar absorbers receive a high and unsteady heat flux (up to 1200 kW/m2[4]) and operate at high temperatures up to 1200℃. SiC is reported to be a refractory material [5] that is well adapted to operate under severe conditions. Upon thermal cycling in air, a thin layer of silica (SiO2) (amorphous or polycrystalline) can grow that passively protects SiC samples [6-9]. This thermal oxidation occurs at temperatures approximately 9001200°C. From an optical viewpoint, the thin SiO2 layer must have a weak impact at thicknesses lower than 50 nm. A key challenge for volumetric solar absorbers is to increase their spectral selectivity. A good solar absorber efficiently captures solar energy in the visible and near infrared spectral range while maintaining low infrared emissivity [10]. This requirement necessitates accurate control of the microscopic mechanisms responsible for the absorption of light for several length scales. In turn, obtaining the desired spectral selectivity also implies control over the fabrication process, leading to the design of the volumetric solar absorber. This task is a difficult challenge for physicists and chemists working in the field of materials science.
This work investigated both the textural and radiative properties of an SiC foam used as a volumetric solar
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