In the past decade, a testing procedure on accelerated lifetime testing (ALT) of absorber surfaces for solar thermal flat plate collectors has been established [7-9]. This qualification procedure is based on a general methodology for service lifetime assessment of solar energy materials worked out within the framework of Task 10 , Task 27 , and the Working Group MSTC  of the IEA Solar Heating and Cooling Programme. In the development of the procedure it was assumed that the intended use of the absorber surface to be qualified was in single-glazed flat plate collectors for domestic hot water (DHW) production. The absorber surface should be considered to be qualified if it meets the requirement of a design service life of 25 years with a maximum loss in the optical performance leading to a reduction in the overall DHW system performance of 5%.
Within this project a thickness insensitive solar selective (TISS) paint based on polyurethane (PU) binder applied on a copper substrate [13,14] was tested with regards to its thermal stability. Thus, onlypartone oftheALTmethodwas exercised. Tests that cover the long-term stability with regards to high humidity and condensation (part two) and atmospheric corrosion caused by sulfur dioxide (part three) have not been investigated.