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

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 uses of PV technologies. To address this problem, this project will produce 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 prediction of long-term performance and service lives.

Objective:  By FY2014, 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 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 attacks. Current standardized test methods used for qualifying PV polymers are only useful for detecting premature failures or comparing the performance of one 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 standards and tools 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 and components, and PV mini-modules, (2)  advanced  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 characterization 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 is being built up. The interested industrial stakeholders 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 program and experimental plans in PV material research.
• 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, in-situ electrical bias, and 4-cell 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 fundamental mechanistic study on degradation of PV materials and components during exposure under multiple simultaneous stresses will be carried out. A factorial experiment will be designed to assess the effects of key environmental factors on the main degradation mechanisms of the component materials or 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 explored.
• 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 modules and novel adhesion tests for identifying the weakest interfacial adhesion will be developed.
• Develop and validate service life prediction models. 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 NIST service life prediction reliability-based predictive methodology that was developed for pure polymers^[1],^[2]  will be applied to link the relevant PV material property data.  The correlation between time-to-failure in accelerated testing and time-to-failure in field will be established.
  

[1] 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.

[2] 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).

Recent Results:

Outcomes:

• R&D road map for development of needed measurement science for predicting the service lives of PV polymeric materials, components, and modules, with input from industry partners and end-users, submitted to WERB (Gu, 2012).
• New design of improved SPHERE technology for elevated temperature testing with simultaneous UV irradiation and accurate humidity control (Byrd, Stanley, 2012).
• CRADA packages signed and returned by interested industrial stakeholders. New consortium of Service Life Prediction of Polymers in Photovoltaic Systems scheduled to start in September, 2012. (Chin, Gu, 2012)

• Best technical poster award in 2012 NREL PV Module Reliability Workshop. (Gu, 2012)
• Establishment of SPHERE-based accelerated laboratory exposure and outdoor exposure for selected PV encapsulant and backsheet materials. Characterizations of initial critical properties (chemical, physical, optical, and mechanical) for PV materials and components have been carried out. (Gu, Watson, Stanley, Nguyen, Sung, Forster, White, 2012).
• Contract awarded to Smithsonian Institute for Spectral Irradiance Measurements, which will provide real-time outdoor radiometric data in 5 locations in the US (Florida, Arizona, Colorado, Wisconsin and Maryland) with 15 years history. The data will be incorporated into NIST Standard Reference Data (SRD) system. (White, Byrd, Watson, 2012)

Standards and Codes:  NIST staff are actively serving on ASTM E44 (Solar, Geothermal and Other Alternative Energy Sources), UL STPs (Standard Technical Panel) and IEC TC 82 (Solar Photovoltaic Energy Systems) committees to provide inputs to qualification and durability test standards for PV system components as well as systems. Xiaohong Gu is the team leader in IEC82/WG2 weathering group. Draft standards developed in this research will be introduced in these standards development organizations. New or revised performance-based standards for accelerated aging and assessment of long-term durability and service lives of polymeric materials used in PV systems will be produced and transferred, providing guidance for material selection and acceptance criteria for PV industries.