4.3 Imaging Concentrators

Imaging concentrators are used to achieve the highest temperatures that are currently achievable with a solar thermal system. Imaging concentrators enable a very large aperture area with a small absorber area, effectively reducing thermal losses at high temperatures. Ray tracing is used to evaluate such concentrating collectors during the design process. By (often digitally) drawing careful geometric reflections within a concentrating collector system, the distribution and angles of incidence of radiation on the absorber can be determined. This is a useful tool for both imaging and nonimaging concentrators, and can be used to show how active tracking systems on imaging concentrators (where the incident radiation is always perpendicular to the aperture) enable a much wider aperture with reduced reflectors compared with nonimaging systems. The typical concentration ratios of the currently existing large-scale systems that are using imaging concentrator technology (Ivanpah, SEGS, etc.) range from in the tens to as high as the low to mid hundreds.

Parabolic imaging concentrators are probably the most studied, both analytically and experimentally. To understand how these collectors are designed, it is necessary to understand the geometric properties of a parabola. 

When light is reflected from the parabolic mirror onto a receiver to produce the optical image, the main parameters considered in the energy analysis are image size (width) and intensity of radiation within that image. Various models and examples of estimating those parameters are provided in the following reading.