Comparison of different types of secondary mirrors for solar application

Two-stage solar concentrators make solar beams downwards providing flexible choices for energy utilization. Five types of secondary mirrors (a flat mirror, an ellipsoidal mirror, a hyperboloidal mirror with upper/lower sheet and a paraboloidal mirror) are compared. Effects of geometry parameters and concentrator precisions on the optical performance are analyzed using Advanced System Analysis Program. The results indicate that concentrators with a flat mirror or hyperboloidal mirror with lower sheet are more sensitive to rim angle or relative location. The secondary mirror is better a convex surface especially when rim angle is more than 90°. A flat mirror or hyperboloidal mirror with lower sheet performs better with higher redirect focal points. A hyperboloidal mirror with upper sheet is the best however numerical aperture changing. The intercept factors decreased with the increase of random errors or optical errors. Both the fabrication and assemblage requirements for a concentrator with a hyperboloidal mirror with lower sheet are the strictest. Experiments are carried out based on a hyperboloidal mirror with upper sheet. The experiments results are in accordance with the ray-tracing results. Therefore, further studies on optimization of the two-stage concentrators using the ray-tracing model can be conducted.