Linear aplanatic Fresnel reflector for practical high-performance solar concentration

Abstract

We propose the linear aplanatic Fresnel reflector (LAFR) as a novel optical design that is suitable for solar thermal power plants. It comprises a segmented primary reflector and a totally stationary secondary mirror and receiver. We show that it is capable of attaining the high intercept factors of conventional parabolic troughs at high concentration, while offering the practical and lower-cost benefits of linear Fresnel reflectors. The fact that dual-mirror Fresnel concentrators can be aplanatic had been overlooked, mainly due to the perception that Fresnel concentrators offer only one degree of design freedom (specifically, the shape of the secondary mirror). The problem is that the two leading orders of geometric aberration must be eliminated to achieve aplanatism, and hence two degrees of design freedom are necessary. After identifying the solar tracking strategy of the segmented primary reflector as the additional degree of design freedom, we derive analytic relations for its placement and curvature. The secondary mirror shape follows directly from the basic aplanatic formalism. We demonstrate that practical high-performance LAFR designs can achieve intercept factors as high as 0.84 at concentration values of ~26.