Analysis of natural convection coupled with thermal radiation in novel high-concentration nonimaging-optics-based system for multi-junction solar cells

The design of solar concentrators based on nonimaging optics provides an inexpensive, but highly powerful concentrating system with large angular tolerance and uniform cell illumination. However, multi-junction tandem-cell-based concentrators require high levels of concentration to become cost effective. The two-mirror design is capable of working at an average concentration over 800 suns with local concentrations below 2000 suns without a homogenizing kaleidoscope. With this level of concentration it is essential to analyze the thermal effects of the unit at different operating conditions. In this paper we analyze the coupled effects of natural convection and surface thermal radiation inside the air-filled enclosure formed by the primary mirror and the cover of the concentrator that holds the secondary mirror. A coordinate transformation is applied to the governing equations to generate a mapping from a parabolic domain to a rectangular domain. The results show an increase in the total Nusselt number with a decrease in the maximum temperature of the secondary mirror.