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

Summary

Due to intensified pricing pressure, increased demand for economic circularity, and rapid growth of photovoltaic (PV) technology, emerging 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 long-term performance and predicting the service lives of polymeric components used in PV systems are still 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, with focus on emerging PV encapsulants, backsheets, and balance of system materials such as connectors and junction boxes.  This project involves four major thrusts: (1) advance analytical tools capable of providing crucial data for understanding degradation mechanisms and failure modes of emerging polymeric components and retrieved field modules, (2) refine and validate a state-of-the-art accelerated laboratory testing chamber with simultaneous UV, temperature, humidity, and robust cyclic strains for long-term durability tests of emerging PV backsheets, (3) develop lifetime prediction models and finite element backsheet cracking analysis under different environmental conditions, and (4) develop standards for testing, characterizing, and predicting the service life of PV polymeric components.

Description

Objective 
To develop and implement measurement science for accurately testing, characterizing, predicting, and validating the lifetime of emerging polyolefin-based encapsulants, coextruded or coated backsheets, and other polymeric components utilized in photovoltaic systems.

Technical Idea
Driven by decarbonization initiatives of the power sector and rising demand for resilient renewable energy, 2022 became another record year for global solar PV generation additions, accounting for approximately 65% of global renewable energy capacity growth, according to a study conducted by the International Renewable Energy Agency (IRENA) [1, 2]. Although the PV module scorecard by PV Evolutions Labs (PVEL) indicates overall module quality has improved, the percentage of manufacturers that experienced a failure also increased. Almost one-third of the bills of materials (BOMs) tested suffered at least one failure during testing [3]. The rapid growth in solar installations can lead to new challenges regarding module reliability, because 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. 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.  Additionally, these tests do not apply relevant environmental stressors simultaneously, therefore, the degradation modes from those tests may not be realistic.

In the previous phase of this project (by FY20), we developed an extensive characterization technique for the study of long-term performance of commonly used backsheets, and successfully established a linkage between accelerated laboratory testing and outdoor optical performance for a polyethylene terephthalate (PET)-based backsheet. One outcome of this phase was to uncover a potential driver of premature solar panel failures for commonly used polyamide backsheets [4], enabling module manufacturers to stop using this type of backsheet for any new modules. Since FY21, we have focused on developing test methods for assessing long-term durability of transparent backsheets, which are critical to the application of emerging bifacial PV technology. In FY23, we initiated the development of a non-destructive chemical depth-profiling technique based on confocal Raman miscroscopy. Knowledge generated from these efforts has been directly transferred to industries through the NIST/industry PV Consortium, helping the industry better understand new degradation modes of emerging transparent backsheets and making better choices of new materials for bifacial PV technologies.

The technical idea of this project is to develop measurement science for accurately evaluating and validating the service life of polymeric components in PV emerging systems, with focus on emerging PV encapsulants, backsheets and connectors.  This project consists of four major thrusts in FY25: (1) advance analytical tools capable of providing crucial data for understanding degradation mechanisms and failure modes of emerging polymeric components and retrieved field and modules, (2) refine and validate a state-of-the-art accelerated laboratory testing chamber with simultaneous UV, temperature, humidity and robust cyclic strains for long-term durability tests of emerging PV backsheets, (3) develop lifetime prediction models and FEM multi-physics models for predicting PV module performance and backsheet cracking under different climates, and (4) develop standards for testing, characterizing, and predicting the service life of emerging PV polymeric components and transfer to the PV industry. 

Research Plan 
In FY25, 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 emerging polymeric components used in PV systems.  The research plan consists of the following tasks:

• Engage industry partners to advance and transfer measurement science to stakeholders in an effective, timely manner. FY25 engagements will include analysis of retrieved modules with inner-layer cracking of backsheets by working with Southern Company, NREL and UL, and continued investigation of long-term performance of coated PV backsheets that are the mainstream in module manufacturing.
• Validate and deploy a robust state-of-the-art, accelerated laboratory testing chamber with simultaneous UV, temperature, humidity, and cyclic mechanical strain for long-term durability testing of emerging PV backsheets. The arrival of key components for driving the dynamic strains were severely delayed (~ 6 months) in FY22 due to restrained supply chains. During FY23, this new load device was tested and a mismatch between the design and the fabrication was identified, which has delayed the validation of this device. Unfortunately, the mismatch problem was not solved due to the complexity of the problem and the restrained time of the personnel in FY24. In FY25, we plan to re-design the device, and substantially improve this capability for long-term performance testing of emerging backsheets. Currently, no commercial accelerated laboratory device can offer such a capability. If successful, a similar chamber will be designed and fabricated in FY26 in a new 6-port SPHERE device for multi-stressed backsheet testing.
• Continuously develop advanced in-situ techniques for characterizations of structure, property, and performance of emerging PV polymers under deformation using Raman microscope, scanning electron microscope (SEM), atomic force microscope (AFM), and X-ray scattering. From FY22 to FY24, we successfully achieved and validated the direct observation of phase transformation in crack tips of an aged PVDF-based backsheet using in-situ tensile testing with synchrotron X-ray scattering, and tested the feasibility of applying in-situ tensile testing under Raman microscope. In FY25, we plan to carry out a more systematic study based on this method for a better understanding of backsheet cracking under mechanical stress. The NIST-developed “Fragmentation Test” [5, 6], will be further refined and validated with emerging backsheet materials and new field results. 
• Complete a refined simulation for global moisture content in PV modules and submit a paper for

the WERB review. 

• Initiate a reciprocity study on degradation of a PV backsheet using high irradiance 6-port SPHERE by comparing the results from 2m NIST SPHERE. The dataset will be critical to the development of accelerated laboratory testing with metal halide light source.

• Continue to develop a synchrotron X-ray scattering-based microscale mapping technique for depth-dependent degradation study of aged glass/encapsulant/backsheet system. Preliminary data obtained in FY24 indicate that this advanced method is promising for providing unique spatially resolved depth-dependent degradation information for PV components.
• Complete 3.5-year outdoor exposure for 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 monitored to better understand real-world backsheet degradation. Degradation mechanisms and failure modes will be compared with those exposed in the SPHERE, and the dataset for outdoor exposure of various backsheets and PV coupons exposed in different climates will be collected.
• Continue field surveys for backsheet performance of two NIST PV arrays. In FY25, we will resume the field survey of PV modules on the NIST Gaithersburg campus using non-destructive techniques, including color, gloss, portable ATR-FTIR and Raman, with the results being compared to degradation progression observed in previous field surveys.

Further develop standardized test methods for improving materials testing, accelerated laboratory testing and service life prediction of PV polymers and components. In FY24, the ASTM G03 work item, “Standard Practice for Operating High Ultraviolet Irradiance Metal Halide Lamp Apparatus for Exposure of Materials”, which addresses the light sources such as those used by the NIST SPHERE, was balloted in the ASTM subcommittee with comments. In FY25, we will continue to work on the revision of this document for the re-ballot. In addition, we will continuously provide SPHERE-based data for the development of IEC 62788-1-6 IEC 62788-7-2.

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REFERENCES:

¹Renewable Energy Market Update - Outlook for 2022 and 2023 (May 2022). https://www.iea.org/reports/renewable-energy-market-update-may-2022

² Solar dominated renewable energy capacity growth in 2022 – IRENA (March 22, 2023) https://www.pv-tech.org/solar-dominated-renewable-energy-capacity-growth-in-2022-irena/#:~:text=Solar%20energy%20accounted%20for%20about,%25%20year%2Don%2Dyear.

³ PVEL releases ninth edition of PV Module Reliability Scorecard (May 23, 2023) https://pv-magazine-usa.com/2023/05/23/pvel-releases-ninth-edition-of-pv-module-reliability-scorecard/

⁴ https://www.eurekalert.org; https://www.nist.gov/news-events/news/2020/03/nist-study-uncovers-potential-driver-premature-solar-panel-failures

⁵ 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

⁶ 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