Standards for quantitatively characterizing the performance and predicting the service lives of polymeric materials and components used in photovoltaic (PV) systems are lacking, hindering innovation, development, and implementation of PV technologies. To address this problem, this project will develop and transfer measurement science for evaluating the performance and lifetime of polymers in PV systems, including accelerated aging tools and standards involving the application of simultaneous multiple stresses, characterization of degradation mechanisms, and mathematical models describing critical parameters for prediction of long-term performance and service lives.
Objective: By FY 2014, to develop and implement measurement science for predicting the lifetime of polymeric materials utilized in photovoltaic applications.
What is the new technical idea? The success of photovoltaic (PV) technologies will ultimately depend on a clear demonstration of the long-term reliability of PV products. However, many components of current PV systems, such as encapsulants, front sheets, backsheets, edge seals and junction box adhesives are based on polymers, which are susceptible to environmental and mechanical attack. Current standardized test methods used for qualifying PV polymers are only useful for detecting premature failures or comparing the performance of one product against another, and not for predicting service life or ensuring long-term reliability of products. Additionally, these tests do not apply the relevant environmental stressors simultaneously, hence, knowledge of synergistic/antagonistic relationships between the environmental factors is lacking.
The new technical idea is to develop and transfer measurement science to industry for evaluating the lifetime of polymeric materials in PV systems. This project will specifically develop: (1) a state-of-the-art accelerated laboratory weathering device with multiple applied environmental stresses (UV radiation, temperature, and moisture) for testing PV polymers, components, and mini-modules, (2) advanced analytical tools capable of providing crucial data for understanding degradation mechanisms and failure modes of PV polymeric materials, components and modules (3) reliability-based models for linking field and laboratory exposure results and predicting service lives of PV polymeric materials under different environmental conditions, and (4) standards for testing, characterization, and service life prediction for polymers used in PV systems.
What is the research plan? This project will identify, measure, model and integrate scientific knowledge of degradation and failure into the development of standardized characterizations and accelerated test methods for polymers used in PV applications. The research plan consists of the following component tasks:
- Engage industry partners and develop R&D road map. A consortium on Service Life Prediction of Polymers in Photovoltaic Systems was initiated in FY 13. The current members include suppliers of polymeric components for PV systems, cell and module manufacturers, and end-users. Through this consortium, NIST will receive continual input and feedback from PV industry stakeholders in developing project and experimental plans in PV material research.
- Design and fabricate a state-of-the-art PV accelerated test facility. Currently no commercial weathering device can provide accurate, well-controlled simultaneous multiple environmental stresses suitable for accelerated testing of PV materials and modules. The NIST SPHERE is an integrating sphere-based weathering device in which panel temperature, relative humidity, spectral UV irradiance, and spectral UV wavelength can be independently and accurately controlled. Major modifications will be made to the existing SPHERE to provide additional capabilities, including elevated temperature exposure and mini-module exposures.
- Characterize degradation under multiple simultaneous stresses. To develop scientific-based standards for service life prediction of polymers used in PV systems, a factorial experiment will be designed with multiple simultaneous stresses for a mechanistic study on degradation of PV materials and mini-modules during exposure to accelerated laboratory conditions as well as real-time field exposure. Chemical, optical, mechanical, electrical and morphological properties of PV materials and components will be characterized using advanced spectroscopic, microscopic and thermo-mechanical techniques. In-situ adhesion tests under elevated temperature and relative humidity will be developed.
- Design, fabricate, and expose model mini-PV modules. Mini-modules that are similar in construction to commercial modules with known field history will be designed and fabricated (with the assistance of industry partners) for use in accelerated SPHERE and field testing. Innovative SPHERE exposure, measurement of relevant properties, and characterization of degradation and failure modes will be performed. Non-destructive techniques for characterization of polymer degradation in mini-modules will be developed.
- Develop mathematical models describing degradation kinetics and mechanisms towards service life prediction of PV polymer Quantitative service life prediction of PV materials and components will be based on mathematical modeling of the data from laboratory accelerated testing and the field. The reliability-based predictive methodology that was developed for pure polymers1,2 will be applied. Models describing the degradation kinetics, mechanisms and correlations between laboratory accelerated testing and the field testing will be established. These kinetic models provide critical parameters for the ultimate reliability-based predictive models and software for service life prediction of PV polymers.
 Gu, et al., "Linking Accelerated Laboratory Test with Outdoor Performance for a Model Epoxy Coating System" in Service Life Prediction for Polymeric Materials: Global Perspectives, Eds: J. Martin, R. Ryntz, J. Chin, R. Dickie, Springer Press, 2009.
 Meeker, et al., “A Statistical Model for Linking Field and Laboratory Exposure Results for a Model Coating,” Proceedings of 4th International Symposium on Service Life Prediction: Global Perspectives, Key Largo, Florida (2008).
- Fabrication and assembly of new SPHERE environmental chambers with enhanced elevated temperature and simultaneous UV irradiation and humidity control.
- Development of new test methods to characterize adhesion between different module components, including blister-based adhesion tests and in-situ adhesion tests under elevated temperature and relative humidity conditions.
- New knowledge delivered to PV industry and standard committees (ASTM E44, IEC TC 82 and UL) through NIST PV consortium, presentations in conferences, and regular standard subcommittee teleconferences on the importance of applying simultaneous UV radiation, temperature and moisture to acceptance criteria testing of encapsulant materials. Recommendations to revisions of current qualification test standard.
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