Durability of aluminium based solar selective absorbers under condensed water

ABSTRACT Sustainable building technology requires the use of solar collectors for heating and cooling purposes. The collectors’ efficiency is mainly influenced by the solar absorber performance. Besides the optical properties of the absorber coating, i.e. high absorption of the incident solar radiation and low heat emission, the degradation due to environmental factors should be considered when designing solar absorbers. However, there is a lack of knowledge on the degradation mechanisms. The current standard methodology from ISO 22975-3, created within TASK 10 IEA Solar and Heating Programme only considers the optical degradation. This study focuses on the performance and durability of eight types of aluminium multi-layered absorbers. The combined effect of high humidity (95% RH), condensation and temperature (40°C and 60°C) was studied. The test samples were measured after different testing time intervals to assess the surface/interface (TEM), chemical (EDX) and optical modifications (UV-Vis-NIR and FTIR spectroscopy). The results revealed that the tested conditions are not strongly influencing the optical properties (solar absorptance and thermal emittance) in the case of the samples with anti-reflective layer and/or humidity protection barrier. All these types are qualified according to ISO 22975-3. However, strong colour modifications, layers alteration and chemical composition changes are recorded. The colour stability of the absorber should be a requirement, especially for architectural integration of solar collectors. Moreover, this study has shown that for modern aluminium based coatings, the predominant degradation process is not the formation of aluminium hydroxide, but the reduction of the antireflective layer thickness. Keywords: accelerated test procedure, lifetime assessment, spectral selectivity, aluminium based solar selective absorbers, degradation mechanism