Objective - To develop and improve the measurement science to: (1) accurately characterize the electrical performance of solar photovoltaic cells, and (2) design and maintain PV systems by providing high quality meteorological, PV module, and PV system performance data. These data can be used to improve PV system computer models, educate users of those models, contribute to the establishment of an IEC module energy rating, and assist efforts for identifying best practices for collecting and using field verification data.
What is the new technical idea? The technical ideas are to improve and implement state-of-the-art methods for characterizing PV cells and to collect high quality, high utility, operational data of deployed PV modules and arrays. NIST has been successful in developing (1) a hybrid monochromator + light-emitting diode (LED) based spectral response measurement technique, (2) a new combinatorial-based method for evaluating a cell’s photocurrent versus irradiance relationship, and (3) an approach for quantifying the spectral dependence of charge carrier lifetimes. Progress has also been made on rounding out NIST’s eventual suite of PV cell characterization capabilities. A solar simulator based facility has been constructed and commissioned for quantifying a cell’s current versus voltage relationship and its operating temperature dependency. A custom hyperspectral imaging system with macro and micro imaging capability has also been recently added and will be used as a tool for understanding a cell’s performance via electroluminescence measurements. With regard to a measurement service, a reference solar cell has been fabricated and will go through extensive characterization in the next fiscal year. The majority of the progress noted above was achieved while focusing on applications to single-junction, monocrystalline silicon (mono-Si) PV cells. The application of these measurement capabilities to other cell technologies, to multiple junction cells, and to cells with different construction features remains to be completed. Additionally, novel PV materials that capture low light levels are finding expanded use in powering sensors and controls for building operations, and work is needed to best capture their performance under the expected environmental conditions. In all cases, steps will be pursued that minimize the measurement uncertainties.
With regard to collecting field data, the technical idea is to be on the extreme end with respect to measurement uncertainties, diversity of measurements (especially meteorological), measurement redundancy, measurement resolution (i.e., at the module, string, and/or circuit levels), sampling frequency, data capture rates, and curation of the deployed instruments. Such an approach allows the greatest utility for using the data for effectively evaluating and improving PV system computer models, for providing datasets that can be confidently used when learning how to use the commercially-available PV modeling tools, for analyzing the impact of local PV on the electrical grid and how to better estimate the local PV generation several minutes to a full day in advance, and to quantify the impact of using data from the more typical PV field monitoring when investigating such issues as fault detection and service lifetime predictions.
What is the research plan?
Efforts will continue to establish a NIST measurement service for fully characterizing the electrical performance of photovoltaic cells. The focus will be largely in four areas: (1) quantifying and reducing the measurement uncertainty of the existing characterization test facilities, (2) incrementally expanding the population of PV cells that can be accurately characterized using the current suite of test facilities, (3) finalizing the fabrication of a standard silicon-based reference PV device and testing its environmental stability, and (4) studying measurement challenges associated with multijunction solar cell characterization and finding quantitative solutions to improve the accuracy of such measurements. The new hyperspectral imaging system will be used extensively for this study.
To assist the PV community’s efforts to improve modeling of a PV system’s output, NIST will collect and use data from four field-sites and a rooftop weather and PV module test station to evaluate discrepancies between model predictions and measured data. NIST has largely automated the process of collecting, conducting quality checks, and archiving the large amount of data that are continuously generated at the field sites. A private data portal is being ported to the public so the data can be used by outside researchers and modelers. Separate instrumentation to track and measure the sun’s spectral irradiance and more granular string, and module performance measuring instrumentation has been installed and is being used to validate new models and standards. NIST will utilize data from the field arrays and rooftop test station in validating and tuning predictive irradiance and array performance models. These high-quality data sets will cover a minimum of three years of operation and the lessons learned in comparing the measured output with the model predictions will continue to be used to improve best practices for PV system modeling and for creating and maintaining outdoor test stations. These best practices are planned to be collaboratively developed by the PV Performance Modeling Collaborative /(http://pvpmc.org) and/or the IEA Photovoltaic Power Systems (PVPS) Programme Task 13.