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? Over the past decade, the PV market has experienced unprecedented growth.  For the first time, solar exceeded both natural gas and wind for new electrical generating capacity in 2016. Total installed U.S. solar PV capacity is expected to nearly triple over the next 5 years.  With such a large investment in newly installed PV systems, there is a growing demand from manufacturers, investors, and customers to assure the reliability of module safety and fulfill the warranty conditions, especially for emerging, low cost PV products. Despite the fact that the majority of PV system failures are related to inverters, the temporary energy production loss due to inverter failures during the lifetime of PV systems is much less than the dramatic, permanent energy production loss due to higher degradation rates of PV modules.  A literature review from NREL reported that the individual module degradation rate could be as high as 4%/year, but the median and average degradation rates were only 0.5 %/year and 0.8 %/year, respectively.  However, the median rate of degradation for exposure up to 10 years was significantly higher than that of 10 years and longer. A recent study on degradation rates of PV modules in hot-dry desert climates over a period of 12 years showed that about 50% of PV modules had degradation rates over 1 %/year.  Additionally, with the emphasis on cost reductions and using new technology, the lifetime of new PV products is uncertain.
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 back-sheet. 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 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, which is problematic because degradation of the PV modules can be non-linear through their lifetime.  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 have 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 consists of the following component tasks:
- Engage industry partners and develop research plans to directly develop 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 a direct, 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 possibly with 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 to facilitate modelling efforts. Previous work in the project did not extensively examine the influence of temperature on reciprocity behavior of polymer degradation, and the effect of mechanical stress on failure behaviors is also lacking. New experiments will include the effect of phase transitions on the activation energy of the backsheet chemical and optical degradation. The dependence of crack formation on mechanical loading during UV exposure below and above glass transition temperature of backsheet materials will be examined.
- Refine and apply the advanced analytical tools for characterization of degradation under multiple simultaneous stresses. The protocols for non-destructive optical and chemical characterizations will be developed, including the measurements by UV-visible-near-IR spectroscopy, confocal Raman spectroscopy, and confocal fluorescence spectroscopy. Mechanical and morphological properties of PV components and laminates will also be characterized during exposure. Cross-sectional chemical, optical, morphological and mechanical techniques will be further developed based on Raman spectroscopy, scanning electron microscopy, confocal fluorescence microscopy and micro-FTIR for depth profiling and failure analysis of exposed laminated multicomponent coupons, mini-modules, and modules from the field. Wedge-based and blister-based adhesion tests will be developed to identify the weakest interface and interfacial fracture energy for PV multilayers and laminates, providing crucial data for understanding the relationship between delamination and degradation of the PV components and laminates.
- Outdoor exposure will continue at outdoor sites in Florida, Arizona, and NIST Gaithersburg. These sites will allow for chemical, optical, and mechanical measurements of backsheet and glass/encapsulant/backsheet laminates to better understand degradation in the field. The degradation mechanisms and failure modes will be compared with those exposed on the SPHERE, and the damage will be used to validate the service life prediction models developed based on accelerated laboratory exposure.
- 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 an FY17 Q1 field survey. Work will be undertaken with Division 732 on irradiance measurement and view factor calculation to further quantify the impact of module mounting variables on backsheet degradation and module performance
- Validate mathematical models for predicting service life of PV components with outdoor exposure results. The reliability-based methodology and cumulative damage models will be used to quantitatively link the critical properties of PV components from the accelerated laboratory exposure to those collected in the field. Mathematical models for describing the kinetics of physical and chemical degradation, linking laboratory and field exposure results, and predicting service life of PV components will be established and validated.
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3 Wohlgemuth, “Reliability of PV Systems”, Proc. of SPIE Vol. 7048 704802
4 Jordan and Kurtz, “Photovoltaic Degradation Rates — An Analytical Review”. NREL/JA-5200-51664 (2012) (http://www.nrel.gov/docs/fy12osti/51664.pdf)
5 Tamizh and Mani, “Degradation Rates, Safety Failures and Reliability Failures of Fielded PV
Modules: Lessons Learned in Hot-Dry Desert Climates”, 2nd Atlas/NIST PV Materials
Durability Workshop (2013) (http://www.nist.gov/el/building_materials/upload/tamiznew.pdf)
6 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)
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