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

Summary:

Standards for quantitatively characterizing the performance and predicting the service lives of polymeric components used in photovoltaic (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 involvesfour major thrusts:

  1. Advance analytical tools capable of providing crucial data for understanding degradation mechanism and failure mode of PV polymeric components, laminates and mini-modules
  2. Construct a state-of-the-art accelerated laboratory weathering device with multiple applied environmental stresses for testing PV components, laminates and mini-modules
  3. Develop reliability-based models for linking field and laboratory exposure results and predicting service lives of PV components under different environmental conditions
  4. Develop standards for testing, characterization, and service life prediction for PV polymeric materials and components.

Description:

Objective - To develop and implement measurement science for predicting 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. Within the last year, PV developers installed roughly 40 GW of panels worldwide – a 22 % increase compared to 2012. According to the NPD Solarbuzz Marketbuzz report, the annual installed capacity is projected to reach 100 gigawatts (GW) with annual PV module revenues rising to $50 billion by 2018. With the large influx of new PV systems, plant owners, investors and customers increasingly demand clear answers about reliability, especially PV system lifetime. 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.[4,7] 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 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. Additionally, these tests do not apply the relevant environmental stressors simultaneously.

To address this problem, the new technical idea of this project is to develop and transfer measurement science to industry for evaluating the lifetime of polymeric components in PV systems. Based on the success of our previous study on PV materials, this new project will focus on the multilayer polymeric components, laminates, and mini-modules. 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 device with simultaneous multiple applied 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, characterization, and service life prediction for PV polymeric materials and components.

What is the research plan?

This project will 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 for developing and transferring measurement science to stakeholders The NIST-industry consortium on Service Life Prediction of Polymers in Photovoltaic Systems was initiated in 2013. The current members include suppliers of polymeric components for PV systems, cell and module manufacturers, and end-users. NIST will engage industrial members for material selection, sample preparation, failure identification, and continually receive input and feedback to develop project and experimental plans in PV component research. Meanwhile, through this consortium, the progress of measurement science in PV components will be transferred to 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 device for PV mini-module accelerated laboratory 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 and strain sphere for PV components testing.
  • Develop advanced analytical tools for characterization of degradation under multiple simultaneous stresses A factorial experiment will be designed with simultaneous UV irradiance, elevated temperature, and moisture for a mechanistic and kinetic study on degradation of PV components, laminates, and mini-modules during exposure to accelerated laboratory conditions as well as real-time field exposure. Non-destructive optical and chemical characterizations will be developed by UV-visible-near-IR spectroscopy, confocal Raman spectroscopy, and fluorescence spectroscopy. Mechanical, electrical, and morphological properties of PV components and laminates will also be characterized during exposure. Cross-sectional chemical and mechanical mapping techniques will be developed. The electrical performance and spectral response of PV mini-modules will be measured with help of EL Energy and Environment Division. 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. 
  • Develop mathematical models for linking laboratory and field exposure results and predicting service life of PV components The degradation mechanism of PV components exposed in the accelerated laboratory will be compared to those exposed outdoors. The correlation will be only assessed for systems that exhibit the same degradation mechanism. 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. 

Reference Documents:

  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).

Major Accomplishments:

Outcomes:

  • Draft standards for PV materials testing and accelerated laboratory weathering of PV polymeric materials.
  • 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.