Following the procedure for testing the thermal stability of a solar absorber coating with regards to its thermal stability , the optical properties solar absorptance a and thermal emittance e of the absorber coating have to be measured in a first step. By use of the optical properties measured, the maximum stagnation temperature Tmax of a collector equipped with the absorber to be tested can be found in Table 1 of Reference . Tmax is a vital measure as it is used to determine the estimated thermal load the absorber is exposed to during 25 years of operation; and the estimated thermal load the absorber will be exposed to predetermines the testing conditions (i. e., testing times and temperature levels) for the accelerated lifetime test.
As degradation indicator the so-called performance criterion is used. The performance criterion (pc) is determined by the change of the optical properties of an absorber surface caused by the load the absorber was exposed to: pc = — Da + 0.05 De. A pc of 0.05 will cause a reduction in the overall DHW system performance of 5%.
In the case of the TISS painted absorber investigated, it was found that the test had to start with the exposure of absorber samples at a temperature level of227 ° C (Table 2 of Reference ). According to the procedure the test was interrupted after 36, 75, 150, and 300 h to determine the degree of degradation (i. e., pc). As the pc was found to be less than 0.05 after each interruption the test was continued up to a total exposure time of600 h. After 600 h at 227 °C the pc was found to be smaller than 0.01. Therefore, no additional exposure test had to be performed and the coating could be qualified ifthe adhesion of the samples from the 227 °C test still is good enough. However, the crosscut test performed on the aged samples lead to a complete spalling of the paint from the copper substrate and thus the TISS painted absorber could not be qualified.
However, this result is not really surprising as according to the manufacturer thermal loads above 200 °C exceed the thermal stability of the polyurethane resin binder, which was used for making the TISS paint coating. The result of this test highlights a weak point of the ALT procedure. Higher testing temperatures will shorten the time needed for testing on the one hand but will increase the risk of triggering degradation processes that would never occur at the lower working temperature levels.