Comparison between predicted and actually observed in-service degradation of a nickel pigmented anodized aluminium absorber coating for solar DHW systems

The actual service degradation in optical performance of a nickel pigmented anodized aluminium absorber coating has been investigated in order to bett er validate predicted service life data from accelerated life testing. Samp les from the coating taken from collectors used in solar DHW systems for ti me periods of 10 years or more were analysed for that purpose. The study, w hich was performed by the IEA Working Group: Materials in Solar Thermal Col lectors, utilized results from a comprehensive joint case study on accelera ted life testing previously performed in Task X of the IEA Solar Heating an d Cooling Programme. It could be concluded from the present study that the agreement between degradation data determined for the absorber samples from the DHW systems and that from accelerated life testing from the Task X stu dy was astonishingly good both from a quantitative and a qualitative point of view. For the anodized aluminium coating the results of the present stud y strongly point to the fact that the design of the solar collector with re spect to airtightness is the most crucial factor in determining service lif e. The service life was defined as the period during which the optical perf ormance is not less than 95 % of its original value. The estimated service life is of the order of 30-40 years for the coating in an airtight solar co llector with controlled ventilation of air, whereas in a non-airtight colle ctor with essential uncontrolled ventilation of air, the corresponding life is around 5-10 years. The general conclusion from the study is that the ac celerated life testing method as developed by the Task X group is an effici ent tool in predicting expected service life of absorber coatings and is th erefore to be recommended for qualification of durability of new kinds of a bsorber coatings. (C) 2000 Elsevier Science B.V. All rights reserved.