Standard testing of absorber surface durability according to ISO 22975-3 versus measured thermal and high-humidity stress of absorber surface at extreme test sites

One key component in solar thermal collectors, the solar absorber, is addressed in this paper. A well-established durability testing method for the solar absorber is the procedure described in ISO 22975-3. This standard testing procedure is used to determine the long-term behavior and service life of selective solar absorbers for use in vented flat-plate solar collectors. The thermal and humidity stress on different samples of solar absorbers during long-term outdoor exposure at different extreme sites with harsh conditions, in tropical, alpine, arid and maritime climates, were monitored. The samples were optically characterized before and after the outdoor exposure. For collectors placed in marine environments, chloride ions from sodium chloride are considered to be a major corrosion agent in these regions. To compare the effect of microclimate and ambient conditions on the corrosivity for selected outdoor exposure sites, two sets of standard corrosion coupons (aluminum, carbon steel, copper and zinc) were mounted inside and outside the solar thermal collector, respectively. After the evaluation of the measured stress factors, temperature and humidity, it was found that the corresponding testing time of the procedure for high temperature and resistance to condensed water of the absorber surface test was less than that specified by the standard, ISO 22975-3. Therefore, the standard testing procedure is testing at higher thermal and humidity loads than the investigated extreme test sites.