Effect of directional dependency of wall reflectivity and incident concentrated solar flux on the efficiency of a cavity solar receiver

摘要

Managing the optical properties of a cavity solar receiver to create spectral and directional selectivities is a solution to improve receiver efficiencies. A reduction in the incident solar power lost by reflection and by emission in a solar receiver allows the absorption of the solar flux to be maximized. This report investigates the influence of the cavity walls directional reflectivity on the thermal radiative efficiency of a cubic cavity solar receiver. A Monte Carlo ray-tracing method is used to calculate the power lost by reflections and by emission with respect to the incident radiation angular distribution and the bidirectional reflectance distribution function of the cavity walls. To study the influence of the directional dependency of the incident flux on the radiative efficiency, four patterns are considered: collimated, diffuse, focused, and Themis incidences. The directional-hemispherical reflectivity for the bottom wall (face to aperture) and lateral walls are distinguished. For diffuse walls, the absorption efficiency is primarily affected by the lateral walls reflectivity because of the back reflection losses. For specular walls, the driving parameter is the bottom wall reflectivity. In addition, the radiative efficiency with thermal emission was studied for the Themis configuration and a slightly weakest dependency of the efficiency on the lateral walls reflectivity was found.