Long-term Performance of Polymeric Materials in Energy Applications

By FY2015, this project will develop and implement measurement science to characterize chemical, mechanical, optical and electrical property changes in polymers used in energy generation, storage, and transmission applications, following exposure to radiation, electric fields, temperature and moisture. The project focus will be on developing standardized methods to characterize property changes for use in service life-time prediction of polymeric materials for photovoltaic and electrical cable applications.

Objective:  By FY2015, to develop and implement measurement science to characterize chemical, mechanical, optical and electrical property changes in polymers used in energy generation, storage, and transmission applications, following exposure to radiation, electric fields, temperature and moisture. The project focus will be on developing standardized methods to characterize property changes for use in service life-time prediction of polymeric materials for photovoltaic and electrical cable applications

What is the new technical idea? 

Polymers are used in numerous renewable energy applications for generation, transmission, and storage of energy.  Examples include photovoltaic module components, battery and fuel cell membranes, insulation for electrical wires and cables, and wind turbine blades.  The commonality between these applications is that there is continuous exposure to temperature, moisture, radiation (including ultra-violet and other electromagnetic frequencies), electrical fields, or ionizing radiation in nuclear power plants.   Standard test methods currently used for qualifying polymers for use in these applications are typically not scientifically-based, and are generally useful only for detecting early failures, not for predicting service life or ensuring long-term reliability of products.  Additionally, many qualification tests do not apply the relevant environmental stressors simultaneously, hence, knowledge of synergistic/antagonistic relationships between the environmental factors is lacking. 

The new technical ideas in this project are (1) to develop and introduce protocols for characterizing materials used in energy applications as a function of exposure to relevant environmental stressors (individually and in combination), (2) to develop new knowledge on the relationship between fundamental material properties and product performance, and (3) to develop and apply models describing long-term material performance during environmental exposure.  This research will provide the technical foundation needed for the development of industry performance and test standards that will be used to qualify the long-term reliability and determine the operating condition of polymers used in energy applications.

What is the research plan?  

Polymers used in two alternative energy technologies--- photovoltaics (PV) and nuclear power will be the focus since PV is one of the world's fastest growing energy technologies, while the safety concern of existing nuclear power facilities is urgent.  One critical challenge for both technologies is that standards or guidelines for quantitatively characterizing the properties and service life of new polymeric componentsused in these applications is lacking, hindering the innovation, development, assurance, and acceptance of these technologies. Examples in PV modules include the encapsulant materials surrounding and binding the solar cells, and the front and back sheets enclosing the encapsulant/solar cell package. Examples in electrical cables include insulation, shielding, and jacket materials.  This project will identify, measure, model and integrate scientific knowledge of degradation and failure into the development of standard characterization and accelerated testing for polymers used in PV and electrical cables applications. This will also lead to standard performance methods to assess the remaining service lives of these polymeric materials in real-time operation for their respective end-application.  For example, in existing nuclear plants, there is a need to evaluate electrical cable condition and make decisions on cable replacement. The research plan consists of three major technical tasks, as follows.

Task 1:  Characterize - Critical material properties in these applications include chemical, optical, mechanical and electrical properties.  Methodologies and metrologies for probing these properties in select materials, which will be chosen to represent the variety of materials encountered in PV and electric cable applications, will be developed and used to assess material behavior as a function of environmental exposure. Non-destructive evaluation techniques for assessing material performance are preferred, since they provide in-situ data on material properties without compromising the integrity or function of the system.  These methodologies and metrologies will be presented to various standards committees (see below) for incorporation into the relevant testing standards for PV and electrical cable applications. 

Task 2:  Expose – Standard protocols will be developed and implemented for exposure of select materials to relevant environmental stressors, including temperature, moisture, radiation, and electrical loads.  Both accelerated laboratory tests and field tests will be carried out.  Laboratory experiments will be designed to assess the effects of key environmental factors on critical material properties for polymers used in PV and electric cable applications. Simultaneous multiple environmental stressors, such as temperature and moisture, will be applied with UV radiation or ionizing radiation to PV polymers and electrical cables, respectively. University of Maryland at College Park will contribute technical expertise and facilities for the ionizing radiation exposure of electrical cables; Techniques developed in Task 1 will be used to assess material properties as a function of exposure to application-specific environmental conditions. 

Task 3: Model – The results of Tasks 1 and 2 will be incorporated into models connecting fundamental material properties, accelerated laboratory testing, and field performance results.  Both statistical modeling and numerical calculations will be developed. For both PV and electrical cables, modeling will be essential to validate acceptance criteria that are developed for condition monitoring tests and also will be used to predict long-term performance of new products. The developed models will be incorporated into test protocols currently under development in various standards committees (see below).        

Standards and Codes:

PIs will work with International Electrochemical Commission (IEC) Technical Committee 82: Solar Photovoltaic Energy Systems, in harmonization with UL committees, as well as American Society of Testing and Materials (ASTM) G03.08 Service Life Prediction, Weathering and Durability.  Xiaohong Gu currently serves as a team leader in  the  weathering task group of IEC 82/WG2, working on modification of accelerated testing protocols for PV materials based on IEC 61215, and other standards for testing and measurement of the performance, durability and safety of PV materials.

Outputs:

• Four invited presentations at conferences on reliability of polymers for energy applications, highlighting the need for a scientific foundation for standards in characterizing long-term reliability of PV and cable materials.
• Research proposals to US Nuclear Regulatory Commission (NRC) on for the development of condition monitoring methods for aging electrical cables.
• Consortium-building workshops held at NIST (September, 2010; March, 2011), providing critical information on technical issues involved in accelerated aging, reliability, and service life prediction of polymers used in PV applications that was used to generate Technical Statement of Work for a proposed PV consortium.  
• Technical Statement of Work developed, with industry input, for a proposed NIST/industry consortium on the reliability of polymers for PV applications.
• Session organized on cable degradation and lifetime for ANSI Nuclear Energy Standards Coordination Collaborative (NESCC) Meeting at NIST (March 1, 2011) and proposed Task Group on Electric Cable Aging for later meeting (July 2011). These outputs are vehicles to assess gaps in cable condition monitoring and qualification standards in the nuclear industry and to gain support and research direction from end-users and potential partners for cable aging/condition monitoring metrology research.

Outcomes:

• Advanced measurement tools for polymers in energy applications developed by combining spectroscopic and microscopic techniques for characterization of polymer degradation under exposure to relevant environmental stressors, providing foundation for non-destructive testing of PV materials in FY2012.

• New knowledge on degradation mechanisms and methods of characterizing property changes in select model PV polymeric materials, which will provide guidance for refining reliability-based accelerated aging protocols using additional materials in FY2012.