Accelerated life testing of solar absorber coatings:: Testing procedure and results

Abstract

A procedure for accelerated life testing of solar absorber surfaces was developed within the framework of the working group MSTC ( Materials in Solar Thermal Collectors) of the IEA- SHCP ( International Energy Agency- Solar Heating and Cooling Programme). The intensive material and micro-climatic investigations on solar thermal collectors and solar systems within the preceding IEA Task X (1985–1991) as well as further studies in this field by the group itself formed a basis for this work. The procedure was formulated as a standard, and submitted to ISO at the beginning of 1997 as a ‘Draft Proposal’. It carries the designation ISO/CD 12592,2 ‘Solar Energy — Materials for flat-plate collectors — Qualification test procedures for solar surface durability’. The proposed standard describes in detail a procedure for the examination of the long-term stability of solar absorber coatings used in flat-plate collectors for domestic hot water systems. The minimum lifetime of the absorber surface is estimated to be 25 years. Possible degradation caused by the thermal load, by condensation and high humidity are considered, as well as by air pollutants (sulphur dioxide). In order to examine the feasibility and reliability of the standard procedure, a ‘Round Robin’ test was performed in a further project of the working group MSTC. Within this ‘Round Robin’ the durability of five different absorber coatings was tested in accordance with the proposed standard by three independent laboratories. The outcome of this test is that all of the laboratories obtained the same results for each of the coatings. As the sulphur dioxide test described in the standard procedure was performed by only one laboratory, a comparison of the results cannot be made. Therefore, this article will be limited to the accelerated ageing tests regarding the resistance to high temperature and to high humidity and condensation. The limitation is made for both the description of the testing method and the presentation of the results of the ‘Round Robin’ test.