Reflector Material Effects on the Outdoor Thermal Response of Helical-Absorber Parabolic Trough Collectors

This study presents a short-term outdoor comparison of mirror-glass and AISI 304 stainless-steel reflectors in parabolic trough collectors equipped with identical helical copper absorbers. Both configurations were operated simultaneously using the same collector geometry, fixed inclination angle, water-supply arrangement, measurement schedule, and instrumentation. Solar irradiance, inlet and outlet water temperatures, absorber temperature, and reflector temperature were recorded over three consecutive experimental days, namely 24–26 October 2025. The results were evaluated using temperature rise and time-dependent temperature output because the gravity-assisted system was not equipped with a flow meter or active flow-control device, preventing reliable calculation of useful heat gain and thermal efficiency. The descriptive results showed that the mirror-glass configuration produced a modestly higher overall temperature response and lower variation among the three daily mean values, although it did not outperform stainless steel at every measurement time or in every daily average. The observed difference is interpreted primarily in terms of the expected higher specular reflectivity and lower optical scattering of mirror glass, which can increase the solar radiation intercepted by the absorber. However, the conclusions are limited by the three-day testing period, absence of verified mass-flow data, lack of direct reflectivity measurements, and unquantified cosine losses associated with fixed operation without automatic tracking. The findings therefore provide configuration-specific guidance for reflector selection rather than a generalized ranking of collector performance.