Comparison-Based Optical Assessment of Hyperboloid and Ellipsoid Reflectors in a Beam-Down Solar Tower System With Linear Fresnel Heliostats

Beam-down concentrating solar tower (BCST) is known for its merits in easy installation and maintenance as well as lower convection heat loss of the central receiver (CR) when comparing to a traditional concentrated solar tower system. A point-line-coupling-focus (PLCF) BCST system using linear Fresnel heliostat (LFH) as the first stage concentrator (heliostat) and hyperboloid/ellipsoid reflector as the tower reflector (TR) is proposed and theoretically analyzed and compared in this paper. Theoretical investigation on the ray concentrating mechanism with two commonly used reflector structures, namely, hyperboloid and ellipsoid, is conducted utilizing Monte Carlo ray-tracing (MCRT) method. The objective of this study is to reveal the achievable optical performance of these types of TRs in the PLCF system considering the effect of LFH tracking errors on TR astigmatism as well as the differences of optical efficiency factors and power transmission in a large-scale biomimetic layout. Results indicate that the ellipsoid system is superior in terms of interception efficiency over the hyperboloid system due to smaller astigmatism at the CR aperture, especially at larger facet tracking error. However, the ellipsoid reflector shows significantly lower TR shading efficiency resulting from the larger TR surface area compared to that of the hyperboloid reflector. The total optical efficiency of the hyperboloid system is always better than that of the ellipsoid system, and this efficiency gap decreases as the ratio epsilon increases. The hyperboloid TR is proved to be more promising and practical for the PLCF system.