To perform the accelerated ageing tests a Votsch Industrietechnik climate chamber of type VC 4033 MH was used. In the chamber it is possible to run tests within a temperature range of 25 to 90oC while regulating the relative humidity between 10 to 95 %. The sample holder was water cooled to make sure that condense was formed on the samples. A thermostat bath of type Techne Tempunit TU-16D controlled that the temperature of the cooling water was about five degrees lower than the temperature of the chamber.

Condensation, high temperature and aggressive airborne pollutant tests are recommended for solar absorber coatings. The condensation test is however the most important test to perform on the type of absorbers investigated in this work because previous experience shows that alumina can be sensitive to moisture [7]. To be able to compare accelerated ageing test results from different laboratories some kind of test procedure has to be defined. There exists no European standardized procedure to test the durability of a solar absorber surface, but some assessment methods have been proposed by the IEA Task X Working Group, designated ISO/DIN 12952.

One assessment method is a condensation test where the temperature of the environment is set to 40°C and the relative humidity to 95 % [4]. The sample temperature should be a few degrees colder than the surroundings to ensure that condensation occurres on the sample surface. Tested samples should be assessed after 80, 150, 300 and 600 hours according to the following performance criterion:

PC = — Aasol + Q.25Astherm < 0.05

where Aasoi is the difference in normal solar absorptance before and after the test and Astherm is the difference in normal thermal emittance. The 0.25 factor reduces the importance of a change in thermal emittance compared to a change in solar absorptance. The testing cycle should continue up to 600 hours as long as the PC value is less than 0.05, if not, it is terminated. A PC value of more than 0.05 indicates that the surface is not condense proof and has poor resistance towards moisture and hence there is no use in continuing the test. Note that it is possible to get a negative PC value, which indicates an actual improvement of the optical selective properties of the surface.