For the optical analysis, a reliable tool was employed, based on Monte-Carlo Ray-Tracing (MCRT) models, which have been used for both concentrating systems (Fresnel-reflector and parabolic-trough based) which are installed in the premises of SESL at NCSR “Demokritos” and are parts of the “Archimedes” infrastructure.
Below, typical results of the optical analysis are presented, more specifically, the non-uniform incident radiative flux distribution on the absorber, which in this case is a metal tube along which the working fluid flows.
This was imposed as the boundary condition to a second, thermal model, for the simulation of the heat transfer to the working fluid.
Besides, a new optimization method was developed for the geometry of a secondary reflecting surface, which can be used for the enhancement of the optical and, consequently, the overall efficiency of Fresnel-reflector systems.
This is achieved through the capturing and back-reflection of rays which, for a receiver consisting of concentric, evacuated tubes, do not strike the receiver at first, which is the case particularly when the diameter of the tube is comparable to or smaller than the width of the primary reflectors.
Representative results from the modeling of the two systems in the SolTrace environment are presented in the figures below. On the left picture, the geometry and a small number of reflected rays from the mirrors grouped in arrays (subsets) are shown, while on the right picture the rays reflected from individual, uniformly distributed mirrors are shown.
The position of the mirrors corresponds to the focus as this occurs at solar noon.
Respective results from the thermal-hydraulic simulation are shown in the figures below. More specifically, a qualitative picture of the temperature distribution throughout the external surface of the receiver is presented,
for two different incidence angles at summer solstice and for a case with glass mirrors used as the reflective surfaces.
