Objective - To develop and improve the measurement science to: (1) accurately characterize the electrical performance of solar photovoltaic cells, and (2) accurately and consistently predict the electrical performance of planned and existing installed arrays by improving the modeling algorithms and the user's correct application of those algorithms as well as identifying and communicating best practices for efficiently collecting and using field weather and PV system verification data.
What is the new technical idea? NIST will improve both laboratory characterization of PV cells and model predictions of PV system output. NIST has been successful in developing a hybrid monochrometer + light-emitting diode (LED) based spectral response measurement technique and applying it to single-junction, monocrystalline silicon (mono-Si) PV cells. Although a prevalent technology, single-junction mono-Si cells are the easiest cell type to characterize. The application of the technique to other cell technologies and to multiple junction cells will be evaluated and steps taken to minimize the measurement uncertainty. NIST will also explore using LEDs and a flux addition technique to quantify the degree of linearity of a cell's current output with respect to irradiance, which is another important component to cell characterization. This approach should be less onerous than the approach specified in the authoritative ASTM standard.
With regard to PV system output, the new technical idea is to quantify the benefits of high quality field data, obtained from NIST testbeds, for: (1) comparing the predictive capabilities of different PV system modeling algorithms, (2) making improvements to existing prediction algorithms and developing new ones, and (3) quantifying the impact from using progressively lower quality field measurements (e.g., less time resolution, fewer field instruments/measurements, instruments with higher uncertainties) for model comparisons and for model inputs.
What is the research plan? The research plan for the solar cell electrical performance characterization effort starts with obtaining test samples by partnering with national laboratories, in and outside the US, and various commercial PV companies. The hybrid spectral response technique will be evaluated to identify the best implementation of the technique for each of the obtained PV technologies. Using those same samples, a flux addition approach that uses a minimum of two LEDs will be explored for the purpose of quantifying a cell's (non) linearity – that is, the relationship between a solar cell's photocurrent output with light intensity. To investigate the electrical properties of solar cells (iso-types or multijunction devices) under concentrated lighting conditions, custom optical parts will be used to irradiate the cell with intensities of up to 100 sun-equivalent. The concentrated light output will be measured with a new integrating sphere technique with calibration traceability through the NIST FEL lamps. To complement the cell characterization effort by helping to explain observed macro performance differences, a test method for measuring charge carrier lifetimes within the cell will also be investigated. Finally, in better rounding out an eventual measurement service, the capability for measuring a cell's temperature coefficients will be added, the potential for enhancing the spectral response measurements of small cells (i.e., 2 cm x 2 cm or smaller) will be explored, and a reported technique for quantifying the impact of reflections on a cell's IV curve measurement while using an absolute cavity radiometer to measure the solar simulator's irradiance will be investigated.1 Prior to offering the measurement service, the researchers will participate in cell characterization intercomparisons with other leading laboratories, including an intercomparison organized by the Bureau International des Poids et Mesures (BIPM). These intercomparisons will aid NIST in evaluating its progress towards optimizing its hybrid technique. Finally, cell characterizations and equipment calibrations will be conducted periodically for the sister project in the program entitled "Measurement Science for Service Life Prediction of Polymers Used in Photovoltaic Systems."
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 test station to evaluate discrepancies between model predictions and measured data. NIST will automate the process of collecting, conducting quality checks, and archiving the large amount of data that are continuously generated at the field sites and then used by NIST and outside researchers and modelers. Separate instrumentation to track and measure the sun's spectral and broadband irradiance will be monitored to develop best approaches for collecting data that are required as input to different models. NIST will then utilize data from the field arrays and rooftop test station in developing detailed computer models for each of the four field sites. The resulting high-quality data covering a minimum of two years of operation and the lessons learned in comparing the measured output with the model predictions will 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.
NIST is conducting long-term monitoring of three larger photovoltaic arrays, a rooftop meteorological station, and a rooftop PV module testbed, all located on the NIST Gaithersburg campus. Monitoring of a fourth array is slated to start in FY16. For each site, data from multiple instruments are being collected either every minute or every second. The electrical performance of small samples of solar cells and modules are also being measured as part of developing improved characterization methods, to provide needed input data for in-house PV modeling efforts, and as part of interlaboratory comparisons.
A tool is being developed to translate the collected weather data into the appropriate formats for immediate use by PV system and building performance computer models. Information on the instruments used to gather the long-term monitoring data is being summarized and then posted to allow public access by those individuals interested in such details.
Data from the solar photovoltaic arrays, rooftop weather station, and PV module testbed are being saved to a central NIST server. Long-term storage is planned while the best means for allowing user-friendly and moderately fast public downloads of the data, once quality checked and formatted, is being developed and implemented. All other data sets will be stored for multiple years on office computers that are backed up on a regular basis.
Data from the solar photovoltaic arrays, rooftop weather station, and PV module testbed will be publically available after the data have been quality checked, organized, and any data losses/caveats noted. For the immediate future, data sets will be distributed on a request by request basis. A web page is being established that will be updated with static data summaries. All other generated data will be available to the public upon request. The publicizing of the generated data will be achieved mainly through its inclusion in technical papers and conference/event presentations.
- Ahn, S. K. et al., "Establishment of a primary reference solar cell calibration technique in Korea: methods, results and comparison with WPVS qualified laboratories," Metrologia, Vol. 51, 2014, pp. 139-147.