To assess the penetrability of light within channels of monolithic structures a ray tracing software has been used. The software has been created under Matlab® environment, using the Solar Concentrating Toolbox SCT developed by CIEMAT [7].

To take into account the sun shape and the beam quality of the concentrating system we have convolved the sun shape according to Abetti [8] with a Gaussian function representing the total optical error of the system. For the purpose of this analysis a total error of 3.2 mrad has been assumed in all the cases. The economy of computation time has been obtained via a hybridization of ray tracing techniques and the well known classical cone optics methodology. The reflected cone for each and every convolved ray has been generated randomly according to the Probability Density Function. The distribution of the reflected rays is a function of the reflected cone but also of the reflecting point at the concentrator surface. To accurately quantify both effects on the distribution we have created as many reflecting points on the reflector as rays representing the reflected cone. Then the total number of rays pointing to the target will be the number of reflecting points X number of rays per reflected cone, and therefore both effects would be represented adequately.

To predict the penetrability as a function of the view angle the study has been focused on parabolic dishes with different rim angles from 20° to 60°. Besides the influence of the view angle, the impact of the reflectivity and the specularity on penetrability has been a matter of this study as well. We have developed a specular reflectance model and a diffuse reflectance model to calculate the maximum and minimum penetrability achievable1.

Pitch length of individual ducts is also one of the key factors to be taken into account while designing. The study of the influence of the pitch length in penetrability is also essential. With SCT software it is possible to calculate the flux distribution by channels, it is possible to discern between close channels and also between walls inside a single channel. Although we have found differences in flux distribution between walls in the same channel and between channels according with the channel position from the center of the focal point, that results are not presented here because the aim of this analysis is finding general information about several design parameters and their influence on penetrability. Because of that, the results presented are mean values from the channels studied, placing their aperture at the focal plane. In total, as many as 250 cases (125 diffuse model and

The diffuse cases have been calculated from the case Reflectivity =0, applying shape factors.

125 specular model) were characterized by combination of five reflectivity values, from 0.1 to 0.9 each 0.2, five rim angles from 20° to 60°, and five pitch lengths from 1 to 5 mm. The results shown in this paper are those with view angles closer to typical designs in solar tower projects with heliostats fields.