Combined experimental and numerical determination of the asymmetry factor of scattering phase functions in porous volumetric solar receivers

Modelling of solar radiation propagation and absorption in porous media is a crucial part in the modelling of porous volumetric receivers in concentrated solar power (CSP) plants. The radiative properties of the porous media should be known in detail for accurate receiver modelling. In this work, an experimental study and a numerical model are combined aiming to estimate the asymmetry factor of Henyey-Greenstein (HG) scattering phase function in the visible spectral range for porous volumetric receivers made of open cell silicon carbide (SiC) ceramic foam. For the experimental, the hemispherical diffuse reflectance of five different samples is measured using a collimated light source and an integrating sphere. For the numerical modelling, an algorithm based on a three dimensional Monte Carlo Ray Tracing (MCRT) method was developed to simulate radiation propagation and absorption in porous media for the same conditions of the experimental apparatus. The asymmetry factor is determined by adjusting its value in the numerical model in order to minimize the difference between measured and simulated values. Results show that the solar radiation scattering in open-cell SiC ceramic foams is slightly backwards, being the optimum asymmetry factor of the Henyey-Greenstein phase function approximately -0.25, with a mean bias error of 0.0045% and a root mean square difference of 0.2926% between modelled and measured values of diffuse reflectance. Experimental results were also compared with the phase function for packing of spheres and with the isotropic scattering phase function. The first overpredicts the diffuse reflectance, while the second function underpredicts it.The authors acknowledge the support of the Portuguese National Science Foundation – FCT (Fundação para a Ciência e a Tecnologia) – through the Grant No. SFRH/BD/115923/2016. The authors also acknowledge the funding provided by the European Union through the European Regional Development Fund, included in the COMPETE 2020 (Operational Program Competitiveness and Internationalization) through the ICT project (UID/GEO/04683/2013) with the reference POCI-01-0145-FEDER-007690. Acknowledgements are also addressed to LANIK ceramic foam company for providing the samples, and to Sérgio Aranha, to Sandra Velez and to Teresa Foito for the help provided in setting up the experimental apparatus and procedure