Todd Krajewski

Assessing field performance of flexible PV modules for moisture induced degradation from accelerated testing

The work presented here is aimed at assessing the field performance of flexible photovoltaic (PV) modules with regard to moisture-induced degradation through the use of accelerated testing. A semi-empirical framework for such assessment is developed using a combination of empirical and analytical models. Testing at different temperature and humidity conditions is carried out for representative flexible copper indium gallium selenide (CIGS) based PV module configurations. Test results highlight the limitations of such accelerated testing and it is shown that for the high-moisture-barrier systems tested, the test times can become prohibitive for establishing humidity dependence through accelerated testing. Through the use of empirical models based on the theory of rate processes, a method is proposed to translate test results to field performance using typical meteorological year (TMY) data.

An in-situ method of monitoring flexible CIGS modules moisture induced degradation and lifetime prediction

Moisture induced degradation is a challenge facing flexible CIGS modules designs as the polymeric barriers usually have higher water vapor transmittance rate (WVTR) than rigid glass barriers. In the process of module design, the expected lifetime is a key point in selecting the most appropriate barriers. However, the correlation between common acceleration tests and module lifetime is still not well known. This work introduces a method of monitoring flexible CIGS module degradation in-situ by recording sheet resistance when a module is exposed to humidity and temperature. An empirical Hallberg-Peck model was employed to estimate the module lifetime.

Establishing the long term moisture barrier performance of the edge seal from accelerated testing

In this work a novel and relatively inexpensive test technique is developed for characterizing moisture barrier performance of the edge seal in PV modules in a manner that is decoupled from other components of the PV module. A theoretical framework is developed to conjecture appropriate equations and functional forms for moisture breakthrough time in accelerated tests as well as in the field for an edge seal with desiccant embedded in it. Results from accelerated testing are shown to fit the proposed functional forms under various conditions thereby validating the theoretical framework. These equations and functional forms enable the prediction of long term moisture barrier performance of the edge seal using historical meteorological data (TMY data) in conjunction with results from accelerated tests. In the specific case of edge seal tested on certain MiaSole glass-glass modules these results establish that the edge seal considered is sufficient to prevent moisture ingress in the module, beyond the intended service life of the module, even for the most aggressive environmental conditions evaluated in this study. Further this study identifies paths to extend this work to the more complex structure involved in building CIGS based flexible products.

Predicting edge seal performance from accelerated testing

Degradation in performance of a PV module attributable to moisture ingress has received significant attention in PV reliability research. Assessment of field performance of PV modules against moisture ingress through product-level testing in temperature-humidity control chambers poses challenges. Development of a meaningful acceleration factor model is challenging due to different rates of degradation of components embedded in a PV module, when exposed to moisture. Test results are typically a convolution of moisture barrier performance of the edge seal and degradation of laminated components when exposed to moisture. It is desirable to have an alternate method by which moisture barrier performance of the edge seal in its end product form can be assessed in any given field conditions, independent of particular cell design. In this work, a relatively inexpensive test technique was developed to test the edge seal in its end product form in a manner that is decoupled from other components of the PV module. A theoretical framework was developed to assess moisture barrier performance of edge seal with desiccants subjected to different conditions. This framework enables the analysis of test results from accelerated tests and prediction of the field performance of the edge seal. Results from this study lead to the conclusion that the edge seal on certain Miasole glass-glass modules studied is effective for the most aggressive weather conditions examined, beyond the intended service.