Renewable energy generation is required to achieve net-zero energy buildings. Solar photovoltaic (PV) arrays typically offer the best means for providing this energy source. The decision of which photovoltaic product to select and how each system is designed, operated, and maintained depends, in large part, on the electrical performance information provided to the decision makers (e.g., the PV array owner, facilities manager, financer). Furthermore, additional utilization of 2nd and 3rd generation photovoltaic devices in conditions that differ from conventional bright outdoor light, such as applications in indoor/low light environments for powering internet-of-things sensors, installation where lighting is diffuse, or situations where operation is improved with concentrated illumination, requires new measurement scales and characterization methods. The creation of a NIST-designed and calibrated standard reference solar cell has established an SI-traceable reference instrument that will decrease the measurement uncertainty in electrical performance ratings of solar devices, thus giving more confidence to those specifying systems. This reference instrument is also a first step towards creating standard reporting conditions (SRC) other than the often-utilized “1-sun” or air mass 1.5 illumination condition. With the in-house development of the differential spectral responsivity method, performance of these NIST reference cells can be measured and calibrated under almost any lighting condition, enabling NIST to calibrate solar cells under unique conditions that no other laboratory in the world offers as of today. This effort also sets up NIST to lead a committee to write new standards on characterization of solar cells under non-standard reporting conditions. NIST will also complete work to capture high-quality performance data from field sites. Two sites that have previously been instrumented will be maintained as will a meteorological station. Data from the experiments will be published for use by outside researchers.
Objective - To develop and improve the measurement science to: (1) accurately characterize the electrical and optical performance of solar photovoltaic cells, (2) design a standard reference cell with appropriate calibrations under a standard reporting condition or an ad-hoc reporting condition as deemed necessary by the end user, and (3) explore the efficiency of new generations of PV technologies under a variety of lighting conditions or for energy harvesting to power internet of things (IoT) devices.
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 develop standard reference instruments, measurement methods and new standards for the latest challenges in this field. 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 (leading to a patent granted in 2018), (3) a variety of solar simulators and temperature dependent I-V measurement stations for obtaining the electrical performance of single junction, multijunction, and other non-traditional PV cells and modules, (4) a custom hyperspectral imaging system capable of performing electroluminescence imaging of solar cells from micron scale to dimensions of up to 150 mm, and (5) 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. With regard to a measurement service, a reference solar cell has been fabricated and tested, and a comprehensive uncertainty budget has been developed for it. Initially, the majority of the progress noted above was achieved while focusing on applications to single-junction, monocrystalline silicon (mono-Si) PV cells. However, in the last few years significant progress has been made towards measuring and characterizing other emerging PV technologies such as multijunction solar cells, and this work will continue. Additionally, novel PV materials that have improved performance under low light and non-standard reporting conditions 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, a very high quality set of PV and meteorological data has been collected over the last several years, and factors such as 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 have been considered to produce a detailed and reliable data set. Such an approach allows the greatest utility for using the data for effectively evaluating and improving PV system simulation models, for providing datasets that can be confidently used when learning how to use 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 for quantifying the impact of using data from more typical PV field monitoring systems when investigating issues such as fault detection and service lifetime predictions. A large amount of this dataset has been published to a public website, (https://pvdata.nist.gov/), and more of that data will be evaluated and made ready for online dissemination or directly to collaborators over the next year.
What is the research plan?
Efforts will continue to establish a NIST measurement service for calibration of 20 mm x 20 mm, silicon-based reference solar cells under the standard reporting conditions (SRC) through the use of the spectral response measurement system. The focus in FY 20 will be on continuous fabrication and evaluation of the reference cells with emphasis on temperature dependent spectral responsivity and I-V measurements, electroluminescence measurements, improving the repeatability of the monochromator calibration procedure, automated beam scanning experiments, performing additional rounds of inter-lab comparisons and lastly, making revisions to the quality manual according to the new NIST requirements. Also, additional improvements to the current measurement system will be made such as replacing some of the older or unserviceable equipment (e.g., lock-in and transimpedance amplifiers), reprogramming new equipment, and attempting to expand the wavelength scale up to 2500 nm.
In addition to the reference cell work, steady progress has also been made on multijunction solar cell measurements. Next year’s efforts will focus on performing detailed I-V curve measurements with procedures in place to account for luminescent coupling effects and other complications in such measurements. With the new hyperspectral imaging system and the planned addition of photoluminescence measurement capability, additional work will be done on measurements related to electroluminescence and photoluminescence properties of these devices and their effect on the overall cell performance. To test the accuracy and reliability of the new in-house multijunction solar simulator, an inter-lab measurement comparison with a national lab such as the Naval Research Lab will also be organized.
Energy harvesting from ambient lighting conditions for the purpose of providing power for IoT sensors and devices is a new area that will continue to be explored in more depth in FY20. To properly determine the performance of solar cells and modules under indoor low-light conditions, a new I-V curve measurement system will be put in place, comprised of a remote-controlled light source, an electronic switch, and a stage accommodating both a reference solar cell and a test PV module. A program will be written to control the light levels and set them to a desired level and perform proper I-V curve measurements and parameter extraction under any given lighting conditions by properly taking into account the spectral mismatch correction factor. After this activity, an I-V curve model/data fitting program will be written and implemented to use the collected data for making predictions over an extended set of irradiance levels and ambient temperatures.
Finally, as the PV field data collection efforts begin to ramp down, the team will maintain some core capabilities such as the weather station and roof-top I-V and irradiance measurements while it will work with existing internal and external collaborators to complete collection, compiling, reducing, and making available quality data for various research and modeling needs.