Numerical Analysis of Radiation Attenuation in Volumetric Solar Receivers Composed of a Stack of Thin Monolith Layers

In a volumetric receiver installed in solar tower power plants, the absorber operates as a convective heat exchanger, absorbing concentrated solar radiation and transferring thermal energy to a fluid flowing through it. Radiation absorption is related to the absorber geometry, the optical properties of the absorber surface, and the incident radiation intensity distribution, which depends on the heliostat field configuration. In order to minimize light reflection and thermal emission from the receiver frontal surface, a well-designed volumetric absorber requires a geometry that promotes radiation penetration. In addition, it should encourage a high heat transfer to the fluid to increase the thermal efficiency of the absorber. This paper analyses the radiation attenuation in an original volumetric absorber, applying a Monte Carlo ray-tracing method. The proposed absorber consists of a stack of thin multi-channel monoliths with square cross section channels in which the relative position between consecutive layers is shifted in the transversal direction. The study includes the influence of surface optical properties (absorptivity and reflectivity), the geometrical characteristics of the structure (length, wall thickness and spacing between elements) and the incident radiation profile (related to different heliostat field configurations). As result, a general expression for describing the transmittance inside the absorber as a function of the depth length is proposed.