In order to be able to predict expected service life of the component and its materials from the results of accelerated ageing tests, the degradation factors under service conditions need to be assessed by measurements. If only the dose of a particular environmental stress is important then the distribution or frequency function of a degradation factor is of interest.

For measurement of microclimatic variables relevant in the assessment of durability of the static solar materials studied in Task 27, various kinds of climatic data during outdoor ex­posure at different test sites are monitored such as global solar irradiation, UV-radiation, surface temperatures, air humidity, precipitation, time of wetness, wind conditions, and atmospheric corrosivity. Such data will be used to predict expected deterioration in per­formance over time by making use of degradation models developed from results of accel­erated tests. Some results from the measurement of microclimatic data are shown in Table 6 and Figure 5.

Figure 5 Microclimatic data measured during outdoor exposure of solar fagade absorbers at ISE in the IEA Task 27 study. Left diagram: Surface temperature frequency histograms for a black painted and a black chrome absorber; Right diagram: UVA and UVB light doses versus exposure time

Accelerated life testing means to quantitatively assess the sensitivity to the various degra­dation factors on the overall deterioration of the performance of the component and its ma­terials.

Figure 6 Change in thermal emittance observed for some reference solar fagade absorber materials during outdoor testing and during accelerated corrosion testing. The corrosivity dose in terms of metallic mass loss of copper at an exposure time is also given for the different tests to illustrate that outdoor performance of those absorbers can be predicted by making use of the equivalent corrosivity dose approach.

Mathematical models are then set up to characterize the different degradation mecha­nisms identified and from the accelerated life test results the parameters of the assumed model for degradation are determined and the service life then estimated.

In Figure 6 is illustrated how the principle of equivalent corrosivity dose in accelerated cor­rosion testing can nicely be adopted in the prediction of the long-term outdoor performance of some solar fagade absorbers. A prerequisite for this is that the accelerated corrosion test correctly simulates the predominating corrosion mechanism occurring under normal outdoor conditions.