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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 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, the junction-box, and the PV cells. 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.

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 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 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 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. 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 62788-7-2; 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, IEC 62788-1; the backsheet and frontsheet materials standard series, IEC 62788-2; and the extended stress testing of PV materials, and components, IEC 63209.

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., 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., 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 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

Other leaders and regional groups are welcome.