Use of Integrated Day-Long Stagnation Temperature for Measuring Changes in Solar Collector Materials Properties

Abstract

Measurements of the maximum temperatures reached by solar-energy-absorbing surfaces provide a useful method for detecting possible degradation in the optical and heat transfer properties of materials used in collectors. This investigation shows that a method based on integrating the absorber temperature rise over ambient and solar irradiation on a daily basis has advantages over alternative test methods based on steady-state measurements of either absorber stagnation temperature or collector thermal efficiency. A nonsteady-state analytical model is developed and used to evaluate the sensitivity of the proposed test method to material degradation and environmental effects. Outdoor data are presented to validate the analytical model and depict typical results that can be expected using the test procedure.