The optical efficiency based on a surface reflectance measurements is 0.94, the gap between reflective surface and the absorber fin is 4 mm and the absorptance is 0.95. The first gap loss (green rays) is detected at an incident angle of 44 degrees which is depicted as a decrease in the optical efficiency seen in Fig. 6.

In Fig. 6 gap losses separate into roughly two ranges. A flat response occurs between 80 and 100 degrees and an abrupt efficiency drop occurs at 44 degrees. To show that the gap loss is the cause

of the optical efficiency drops, another simulation is run in which there is no gap loss. See Fig. 7. The graph depicts a rounded distribution with no abrupt jump in efficiency at any nominal angle.

To understand the nature of gap loss, the gap loss is plotted over the incident angles in Fig. 8. The gap losses are separated into roughly two levels.

The reflectivity is now changed to 0.7 to achieve an optical efficiency curve that has a shape that is more of a dished appearance around the 90 degree incident angle. See Fig. 9.