High-Temperature Linear Radiation-Cavity Solar Collector with a Fresnel Concentrator.

A model solar-thermal concentrator-collector that is 16th scale in cross section relative to a system considered to be of practical dimensions for power generation has been constructed and tested. It consists of an Archimedes i.e., Fresnel mirror-concentrator, 5 cm ID x 1.5 m long glass receiver pipe, 0.3 cm thick graphite absorber, and gaseous Ar heat transfer medium. On clear days direct solar flux ca. 800 Wsq.m., stagnation i.e., no-flow condition temperatures of ca. 370 C have been reached at the center of the experimental device, where end effects are minimal. Experiments have been performed with gas flow rates up to ca. 100 litersmin 200 RE 2000, resulting in a bulk fluid temperature rise in the axial flow direction of up to 285 C. These experiments have been utilized in a study of heat transfer processes within the collector. A computer model of this system which takes into account most of the influential variables e.g., combined forced-free convection, establishment of velocity and temperature profiles, and axial heat conduction has been developed. Its behavior agrees well with observations. Also, after ca. 200 h of high-temperature operation, materials degradation was measured and found to be very low.