UV, Temperature, and Humidity: Task Group 5
Task Group 5 is focused on the effects of ultraviolet (UV) radiation, which for many photovoltaic (PV) materials is coupled to temperature and humidity.
Light (especially UV radiation) can cause changes in modules including discoloration, reduced adhesion at interfaces, embrittlement, light-induced degradation, and solarization. The components affected by UV in PV modules includes the: encapsulant, backsheet, edge seal, glass, cable jacketing, connectors, and the junction-box. Few institutions have the capability to irradiate full-sized modules because the application of high-fidelity AM1.5 light in large areas is expensive. An often-used strategy has been to apply the stresses to either material coupons or a smaller version of a module (“mini-modules”).
Task Group 5 aims to standardize the test conditions (including the UV sources, UV irradiance, temperature, and humidity) used to age specimens of all sizes. Future focus for the task group may include weathering of hot modules (to account for mounting and/or climate), the use of water spray during UV weathering, UV weathering as part of an accelerated-aging test sequence, UV weathering as part of a combined stress testing, UV weathering used in a diurnal-based accelerated test sequence. Including the issues of delamination and subsequent corrosion, Task Group 5’s scope overlaps Task Group 3’s. It remains to be determined if there is interaction between UV and abrasion of coatings, front-glass, or backsheet.
Having surveyed equipment that is being used for UV exposure testing in the industry, experiments were conducted to compare the degradation of samples aged using different light sources. Studies include the determination of the activation energy (and corresponding acceleration factor) for ethylene-vinyl acetate discoloration, surveying the weathering of encapsulant attachment strength, surveying the weathering of the adhesion of edge seals (used in thin-film modules).
A subsequent second study is being used to validate the International Electrotechnical Commission’s (IEC’s) weathering standard, IEC TS 62788-7-2:2017, for contemporary encapsulant and backsheet materials. In addition to a back-sheet round-robin test (examining discoloration and cracking) using different UV-test sources and source spectra, a four-cell mini-module test program was completed in Japan in collaboration with Task Group 3, comparing test sequences including separate UV exposure, thermal cycling, and dynamic mechanical load steps. Efforts have merged between task groups 2, 3, and 5 in Japan to focus on encapsulant adhesion in modules. A separate Task Group 5 effort is ongoing in China, with a round-robin experiment examining encapsulation and backsheets.
Related standards efforts in progress include: a UV aging standard examining the optical durability of encapsulation, IEC 62788-1-7; the PV materials and components weathering standard, IEC 62788-7-2; the PV encapsulation materials standard series, IEC 62788-1; the backsheet and frontsheet materials standard series, IEC 62788-2; and the guidelines for qualifying PV modules, components, and materials for operation at high temperature, IEC TS 63126 ED1.
The interlaboratory study examining the attachment strength of encapsulation being organized through the Americas group is summarized in:
Degradation in PV Encapsulant Strength of Attachment: An Interlaboratory Study Towards a Climate-Specific Test (paper) (presentation)
Authors: Miller, D. C., Annigoni, E., Ballion, A., Bokria, J. G., Bruckman, L. S., Burns, D. M., Chen, X., Feng., J., French, R. H., Fowler, S., Honeker, C. C., Kemp, M. D., Khonkar, H., Köhl, M., Perret-Aebi, L.-E., Phillips, N. H., Scott., K. P., Sculati-Meillaud, F., and Wohlgemuth, J. H.
Conference: 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC)
The interlaboratory study examining the optical durability of encapsulation being organized through the Americas group is summarized in:
Degradation in Photovoltaic Encapsulant Transmittance: Results of the First PVQAT TG5 Artificial Weathering Study
Authors: Miller, D. C., Bokria, J. G., Burns, D. M., Fowler, S., Gu, X., Hacke, P. L., Honeker, C. C., Kempe, M. D., Köhl, M., Phillips, N. H., Scott., K. P., Singh, A., Suga, S., Watanabe, S., and Zielnik, A. F.
Journal: Progress in Photovoltaics: Research and Applications
IEC Quality Assurance TG5: UV, Temperature and Humidity
Authors: Miller, D. C., Bath, J., Köhl, M., and Shioda, T.
Workshop: Sophia Workshop on PV Module Reliability
The interlaboratory experiment examining backsheet being organized through the European group is summarized in:
Round Robin Testing of Various Back-Sheets for PV-Modules with Different Ultra-Violet Radiation Sources and Sample Temperatures
Authors: Köhl, M., Ballion, A., Lee, Y.-H., Wu, H.-S., Scott, K. P., Glick, S., Hacke, P., and Koo, H. J.
Conference: 31st European Photovoltaic Solar Energy Conference and Exhibition
While some materials are posted on the pbworks website, please contact your regional leader to more closely follow the activities.
David Miller — National Renewable Energy Laboratory, “ Task Group 5 X,” including a variety of international members
Tsuyoshi Shioda — Mitsui Chemicals, including Japan
Carol Chen — China National Electric Apparatus Research Institute, including China
Other leaders and regional groups are welcome.