Measurement Science for Service Life Prediction of Polymeric Components Used in Photovoltaic (PV) Systems

Summary

Due to intensified pricing pressure and rapid growth of photovoltaic (PV) technology, low cost and emerging polymeric materials are being used increasingly in module manufacturing. Because of their relatively recent deployment, little is known about their long-term performance and reliability. Furthermore, standards for quantitatively characterizing the performance and predicting the service lives of polymeric components used in PV systems are lacking, hindering innovation, implementation, and assurance of PV technologies. To address this problem, this project will develop, implement and deliver measurement science for accurate and timely assessment of the long-term performance and lifetime of polymeric components used in PV systems.  This project involves four major thrusts: (1) Advance analytical tools capable of providing crucial data for understanding degradation mechanisms and failure modes of PV polymeric components, laminates, and mini-modules, (2) Construct a state-of-the-art accelerated weathering laboratory device with multiple applied environmental stresses for testing PV components, laminates and mini-modules, (3) Develop validated reliability-based mathematical models for linking field and laboratory exposure results and predicting service lives of  PV components under different environmental conditions, and (4) Develop standards for testing, characterizing, and predicting service life of PV polymeric components.

Description

Objective - To develop and implement measurement science for predicting and validating the lifetime of polymeric components utilized in photovoltaic applications.

What is the new technical idea?  
The US annual energy consumption from solar and other renewable sources exceeded coal for the first time in over 130 years since 2019 [1]. Among the renewable energy sources, the solar market has experienced unprecedented growth. [2-4]. This rapid growth in solar panel installation has come with new challenges regarding module reliability: 1) over 90% of global PV installations are less than 10 years old, and 2) there has been over a 90% reduction in module price in the past decade mainly due to the use of new materials and new technologies. These statistics mean that there is a lack of long-term historical data about module reliability, and even if such data were available, it may not be useful because new materials and components may perform differently than their more expensive predecessors. A literature review from NREL reported that the median rate of degradation for exposure up to 10 years was significantly higher than that of 10 years and longer. [5] A recent worldwide study on nearly 2 GW of PV fields inspection shows that total module defects are above 25%, and the annual increase in the defects related to polymeric components rose to 50% in 2019.[6] Therefore, it is significantly important to study degradation and failure mechanisms, and develop measurement science to accurately predict the field performance of materials in modules, especially for new materials used in the emerging technologies without any historical field data (e.g., bifacial passivated emitter and rear contact (PERC) modules).

The long-term reliability of a PV module is highly affected by the degradation behavior of the polymeric components within the module, such as the encapsulant and backsheet [7]. For example, corrosion, a major field failure mode leading to loss of power, is strongly accelerated by acetic acid, a product from degradation of encapsulant ethylene vinyl acetate (EVA). The cracking and delamination of the backsheet due to degradation can lead to the dielectric breakdown of PV systems and safety concerns as well as lower reliability of PV modules. However, current standardized test methods used for qualifying PV components and modules are only useful for detecting premature failures, and not for predicting service life or ensuring long-term reliability of products. This is problematic because degradation of the PV modules can be non-linear through their lifetime [8]. Additionally, these tests do not apply the relevant environmental stressors simultaneously, therefore, the degradation modes from those tests may not be realistic.

To address this problem, the new technical idea of this project is to develop and transfer measurement science to industry for evaluating and validating the lifetime of polymeric components in PV systems. In the previous phase of this project, we developed methods to expose, characterize, and predict the damage of PV materials and components based on the SPHERE technology and the reliability-based damage model. Due to the interdependent multi-stress effect, complex degradation mechanisms, and a high demand on the precise and accurate control of exposure weathering for service life prediction, there is a need to enhance the exposure capability, deepen the understanding, refine the tests, validate the prediction models, and continue developing the standards for service life prediction. This project consists of four major thrusts: (1) to advance analytical tools capable of providing crucial data for understanding degradation mechanisms and failure modes of PV polymeric components, laminates, and mini-modules, (2) to build up a state-of-the-art accelerated laboratory weathering chamber that applies multiple simultaneous environmental stresses for testing PV components, laminates, and mini-modules, (3) to develop reliability-based models for linking field and laboratory exposure results and predicting service lives of PV components under different environmental conditions, and (4) to develop and improve standards for testing, characterizing, and predicting service life of PV polymeric components.

What is the research plan?  
This project will continue to identify, measure, model, and integrate scientific knowledge of degradation and failure into the development of reliability-based accelerated test methods and service life prediction tools for polymeric components used in PV systems. The research plan in FY21 consists of the following component tasks:

• Engage industry partners and develop research plans to effectively advance and transfer measurement science to stakeholders. NIST will engage industrial members for material selection, sample preparation, and failure identification, and will continually transfer the progress of measurement science in PV components to the PV industry in an effective, timely manner.
• Design and fabricate a state-of-the-art accelerated test facility for PV components, laminates, and mini-modules. Currently no commercial weathering device can provide accurate, well-controlled simultaneous multiple environmental stresses suitable for accelerated testing of PV components and modules. A state-of-the-art integrating sphere-based weathering chamber for PV components testing will be designed and fabricated, functioning with highly uniform and intensive UV irradiance, well-controlled panel temperature, a wide-range of relative humidity, and simultaneous cyclic mechanical loadings. In addition, work will be done to facilitate the use of a commercial 6-port sphere for PV component testing.
• Investigate the interdependent multi-stress effect on degradation mechanisms of PV encapsulation materials, including the interaction between encapsulant degradation and backsheet performance, and the relationship between mechanical stress on backsheet cracking. The experiment is designed to include the effects of encapsulant type (ethylene vinyl acetate (EVA) vs. polyolefins) and sample form (coupon samples vs. free-standing film) on chemical, optical, and mechanical degradation of backsheets. Sample preparation was completed in FY20. Due to COVID-19, the initiation of exposure of these samples was postponed from FY20 to FY21. The exposure may last over 12 months until FY22. 
• Refine and apply the advanced analytical tools for characterizing the degradation of PV components under combined laboratory stresses and in field modules. The protocols for non-destructive optical and chemical characterizations will be further developed, including the existing measurements by UV-visible-near-IR spectroscopy and attenuated total reflection FTIR, and the new confocal Raman spectroscopy for the transparent PV backsheets and coupons. Mechanical and morphological properties of PV components and laminates will also be characterized during exposure. Multiscale cross-sectional chemical, optical, morphological, and mechanical techniques will be further developed based on Raman spectroscopy, scanning electron microscopy, atomic force microscopy, and micro-FTIR for depth profiling and failure analysis of exposed laminated multicomponent coupons, mini-modules, and field modules. The recently NIST-developed “Fragmentation Test” [9-10], a real-time monitoring technique to evaluate the crack propensity of backsheet with laser scanning confocal microscopy combined with in-situ small strain tensile tests, will be further developed and validated with emerging backsheet materials and new field results. X-ray and neutron-based techniques will be explored to study the changes in microstructure and the microcrack formation for aged backsheets under mechanical stress. This work was initiated from an exploratory project in FY19 and planned in the project in FY20. Due to the cancelled beamtime and limited access to laboratory during Covid-19, the work has been postponed to FY21.  
• Initiation of outdoor exposure of transparent backsheets, coupons, and other emerging backsheet materials in Florida, Arizona, and NIST Gaithersburg. The chemical, optical, and mechanical measurements of exposed backsheets and glass/encapsulant/backsheet laminates will be followed to better understand the real-world backsheet degradation. The degradation mechanisms and failure modes will be compared with those exposed on the SPHERE, and the database for the outdoor exposure of various backsheets and PV coupons in different climates will be established.
• Continue the field survey of backsheet performance of NIST PV arrays. PV modules at the NIST Gaithersburg campus will be periodically examined to assess backsheet performance using non-destructive techniques, with the results being compared to degradation progression observed in previous field surveys. Work will be undertaken on irradiance measurement to further quantify the impact of module mounting variables on backsheet degradation and module performance.
• Develop standardized test methods for improving materials testing, laboratory weathering and service life prediction of PV polymers and components. An ASTM draft of “Test Procedure for Using a Laser Confocal Microscope and a Mini-Tensile Tester for Quantifying Surface Cracking Propensity of a Photovoltaic Backsheet” will be initiated based on the NIST developed “Fragmentation Test” [9-10]. The draft standard on “Standard Practice on Solar Array Field Survey on Degradation of Backsheet” will be submitted to E44.09 by working with Case Western Reserve University, UL, and NREL. The NIST SPHERE technology will be first-time incorporated into an ASTM G03 working item, which is “Standard Practice for Operating High Ultraviolet Irradiance Metal Halide Lamp Apparatus for Exposure of Materials”.

References:

1. U.S Energy Information Administration (EIA) Today in Energy, May 2020
   https://www.eia.gov/todayinenergy/detail.php?id=43895
2. PVPS 2019 Snapshot of Global PV Markets”, https://resources.solarbusinesshub.com/images/reports/216.pdf
3. Renew Economy Lean Energy News and Analysis, September 25, 2019 https://reneweconomy.com.au/global-solar-to-drive-double-digit-renewabl…
4. https://irena.org//media/Files/IRENA/Agency/Publication/2018/Mar/IRENA_…
5. Jordan and Kurtz, “Photovoltaic Degradation Rates — An Analytical Review”. NREL/JA-5200-51664 (2012) (http://www.nrel.gov/docs/fy12osti/51664.pdf)
6. Tracy, J. “Durability of Packaging Material in Globally Fielded PV Modules”, NIST/UL PV Materials Durability Workshop (2019) https://www.nist.gov/system/files/documents/2020/01/15/TRACY.pdf
7. Kohl, et al, “Impact of Permeation Properties and Backsheet-Encapsulant Interactions on the Reliability of PV Modules”, ISRN Renewable Energy, Vol. 2012, Article ID 459731 (2012)
8. Jordan, D. C., Silverman, T. J., Sekulic, B., and Kurtz, S. R. (2016) PV degradation curves: non-linearities and failure modes. Prog. Photovolt: Res. Appl., doi: 10.1002/pip.2835.
9. Lyu, Y., Kim, J.H., Fairbrother, A., Gu, X., “Degradation and Cracking Behavior of Polyamide-Based Backsheet Subjected to Sequential Fragmentation Test”, IEEE Journal of Photovoltaics, Vol. 8 No. 6, 1748-1753(2018) https://doi.org/10.1109/JPHOTOV.2018.2863789
10. Lin, C.C., Lyu, Y., Jacobs D., Kim J.H., Wan, K.T., Hunston D., Gu, X., “A Novel Test Method for Quantifying Cracking Propensity of Photovoltaic Backsheets after Ultraviolet Exposure”, Prog Photovolt Res Appl. 1–11 (2018). https://doi.org/10.1002/pip.3038