Three-dimensional modelling and analysis of solar radiation absorption in porous volumetric receivers

This work addresses the three-dimensional modelling and analysis of solar radiation absorption in a porous volumetric receiver using the Monte Carlo Ray Tracing (MCAT) method. The receiver is composed of a solid matrix of homogeneous porous material and isotropic properties, bounded on its side by a cylindrical wall that is characterized through a diffuse albedo. The Henyey-Greenstein phase function is used to model the radiation scattering inside the porous media. The effect of the angle of incidence, optical thickness (porosity, pores size and height of the receiver), asymmetry factor of the phase function and wall properties on the solar radiation absorption in the porous media is studied in order to obtain the receiver efficiency as a function of these parameters. The model was validated by comparing the results for a simple geometry composed of a long slab of finite thickness with the values available in the literature, and then tested with a cylindrical receiver using a parabolic dish as concentration system with a concentration factor of 500. A peak of absorbed solar radiation of 156 MW M-3 and an absorption efficiency of 90.55% were obtained for a phase function asymmetry factor of 0.4 (forward scattering) and scattering albedo and extinction coefficient of 0.54 and 100 m(-1), respectively. The results for the diffuse reflectance, diffuse transmittance and absorption are also presented. The model developed in this work is useful to obtain and understand the energy absorption distribution in porous volumetric receivers coupled to solar concentration systems, when different porous structures and geometric parameters are used.