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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 with temperature and humidity.

Light (especially UV radiation) can cause changes in PV modules including discoloration, reduced adhesion at interfaces, embrittlement, light-induced degradation (“UV-LID”, “LETID”, and “LID”), and solarization. The components affected by UV in modules include the: encapsulant, backsheet, edge seal, glass, cable jacketing, connectors, the junction-box, and the PV cells. Few institutions have the capability to irradiate full-sized modules because the application of light with high-fidelity to the AM1.5 spectrum over 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, filters, irradiance level, temperature, and humidity) used to age specimens of all sizes. Topics of concern to the task group 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, the scope of Task Group 5 overlaps that of Task Group 3. It remains to be determined if there is interaction between UV and abrasion of coatings, front-glass, or backsheet.

Progress Update

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 the discoloration of poly(ethylene-co-vinyl acetate) (“EVA”) encapsulant, surveying the weathering of encapsulant strength of attachment, surveying the weathering of the adhesion of edge seals (used in thin-film modules).

A 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 validating the weathering of encapsulants in IEC 62788-1-7. 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 study 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 merged between task groups 2, 3, and 5 in Japan to focus on encapsulant adhesion in modules. The "TG5 X" group includes participants from the Americas, the former TG5 Europe, and other international groups.

Standards Activities

Recently published standards (with project team members and input from TG5) include: the measurement of optical transmittance of encapsulants, IEC 62788-1-4; a UV aging standard examining the optical durability of encapsulants, IEC 62788-1-7; the PV materials and components weathering standard, IEC TS 62788-7-2; and the methods for qualifying PV modules, components, and materials for operation at high temperature, IEC TS 63126 ED1. Related standards efforts in progress include: the PV encapsulation materials standard series under IEC 62788-1; the backsheet and frontsheet materials standard series under IEC 62788-2; and the extended stress testing of PV materials and components, IEC 63209-2.

Related Resources

The interlaboratory study that examined the attachment strength of encapsulation, organized through TG5 X is summarized in:

Degradation in Photovoltaic Encapsulation Strength of Attachment: Results of the First PVQAT TG5 Artificial Weathering Study
Authors: Miller D.C., Alharbi F., Andreas A., Bokria J.G., Burns D.M., Bushong J., Chen X.-X., Dietz D., Fowler S, Gu X., Habte A., Honeker C.C., Kempe M.D., Khonkar H., Köhl M., Phillips N.H., Rivera J., Scott K.P., Singh A., and Zielnik, A.F.
Journal: Progress in Photovoltaics: Research and Applications

The interlaboratory study that examined the optical durability of encapsulation, organized through TG5 X 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

Task Leaders

While some materials are posted on the pbworks website, please contact the TG5 leaders 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, combined TG2, TG3 and TG5 in Japan

Other leaders and regional groups are welcome.