Effect of secondary reflector on solar flux intensity and uniformity of a Fresnel concentrator

ABSTRACT

Nowadays, consumption of renewable energies such as solar energies is increasing. Electricity in solar systems is produced using either photovoltaics or concentrated solar power (CSP) systems. Therefore, concentrating technologies are key parts of CSP systems. The lower cost of linear Fresnel collectors makes them attractive for electricity production and thermal energy applications. However, due to partly losing the reflected rays, their optical efficiency is low. By installing a secondary reflector above the absorber tube of Fresnel collectors, lost rays are mostly redirected toward the absorber tube and a higher optical efficiency is achieved. The geometry and configuration of secondary reflectors have an intense influence on the optical efficiency. Therefore, there is a need to determine the most appropriate shape and configuration of secondary reflectors. In this study, four different types and orientations of secondary reflectors including circular, flat, few segmented, and parabolic were investigated and compared optically. All the geometric parameters were optimized for each case individually. To calculate the solar flux intensity and uniformity on the absorber tube, a ray-tracing method was implemented using a mathematical code that was developed based on MATLAB software. All the secondary reflectors were simulated in the code, with variable geometrical parameters. A wide range of geometrical parameters were investigated to obtain the most efficient geometry of every secondary reflector design in flux intensity and flux uniformity in comparison with the other cases. The results showed that few flat segmented collectors increase the flux to 87.9%. In addition, the highest uniformity of solar flux was obtained by using the circular and parabolic secondary reflectors. Considering the manufacturing cost as an important parameter, the flat reflector was the favorable one from the flux quantity point of view. Finally, if all the three criteria, i.e., flux quantity, its distribution, and manufacturing cost, were essential to be considered, the flat reflector might be chosen as the most appropriate secondary reflector.