The best approach in validating an estimated service life from accelerated testing is to make use the results from the accelerated life tests to predict expected change in material properties or component performance versus service time and then by long-term service tests check whether the predicted change in performance with time is actually observed or not.

The results of validation tests therefore can be used to revise a predicted service life and form the starting point also for improving the component tested with respect to environ­mental resistance, if so required. It should be remembered that the main objective of ac­celerated life testing is to try to identify those failures which may lead to an unacceptable short service life of a component. In terms of service life, the main question is most often, whether it is likely or not, that the service life is above a certain critical value.

In the case studies of Task 27 outdoor tests at different test sites are performed for meas­urement of microclimatic variables and for validating predicted loss in outdoor performance from accelerated test results. Tests are performed by CSTB in Grenoble (France), ENEA in Rome (Italy), INETI in Lisbon (Portugal), ISE in Freiburg (Germany), NRELin Colo — rado/Florida/Arizona (USA), SP in Boras (Sweden), SPF-hSr in Rapperswil (Switzerland) and Vattenfall in Alvkarleby (Sweden). In Figure 7 a view of the test site at INETI in Lisbon is shown.

Conclusions

The work in IEA Task 27 on durability assessment of static solar energy materials has shown that it is possible to employ a systematic approach in the evaluation of the expected

service life of the materials studied. Based on the work performed recommended test pro­cedures will be worked out for qualification of new materials with respect to durability.

Figure 7 Results from outdoor exposure of antireflective glazing materials performed at SPF-HSR Rapperswil, Switzerland. The decrease in the solar transmittance with time is due to soiling effects, which vary very much with exposure site.

For recommended durability test procedures to be accepted as international standards, it is of utmost importance to demonstrate their relevance for predicting real in-service long­term performance. We think that the work of Task 27 will meet this requirement.

Acknowledgement

The authors sincerely want to thank the colleagues and participants in the work of Task 27 on the static solar materials for contributions to this paper: Michael Kohl and Volker Kubler (Fraunhofer ISE Freiburg), Ole Holk (DTU Copenhagen), Gary Jorgensen (NREL, Golden Colorado), Bjorn Karlsson (Vattenfall Utvecklings AB Alvkarleby), Manuel Lopes Prates (INETI Lisbon), Kenneth Moller (SP Boras), Marie Brogren, Arne Roos, Anna Werner (Uni­versity Uppsala), Michele Zinzi (ENEA Rome), and Michele Ghaleb (CSTB Genoble)

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