Afshin Andreas

Recent Progress in Reducing the Uncertainty in and Improving Pyranometer Calibrations

The Measurements and Instrumentation Team within the Distributed Energy Resources Center at the National Renewable Energy Laboratory, NREL, calibrates pyranometers for outdoor testing solar energy conversion systems. The team also supports climate change research programs. These activities led NREL to improve pyranometer calibrations. Low thermal-offset radiometers measuring the sky diffuse component of the reference solar irradiance removes bias errors on the order of 20 Watts per square meter (W/m 2 ) in the calibration reference irradiance. Zenith angle dependent corrections to responsivities of pyranometers removes 15 to 30 W/m 2 bias errors from field measurements. Detailed uncertainty analysis of our outdoor calibration process shows a 20% reduction in the uncertainty in the responsivity of pyranometers. These improvements affect photovoltaic module and array performance characterization, assessment of solar resources for design, sizing, and deployment of solar renewable energy systems, and ground-based validation of satellite-derived solar radiation fluxes.

Solar Position Algorithm for Solar Radiation Applications (Revised)

This report is a step-by-step procedure for implementing an algorithm to calculate the solar zenith and azimuth angles in the period from the year -2000 to 6000, with uncertainties of ?0.0003/. It is written in a step-by-step format to simplify otherwise complicated steps, with a focus on the sun instead of the planets and stars in general. The algorithm is written in such a way to accommodate solar radiation applications.

An Extensive Comparison of Commercial Pyrheliometers under a Wide Range of Routine Observing Conditions

In the most comprehensive pyrheliometer comparison known to date, 33 instruments were deployed to measure direct normal solar radiation over a 10-month period in Golden, Colorado. The goal was to determine their performance relative to four electrical-substitution cavity radiometers that were calibrated against the World Radiometric Reference (WRR) that is maintained at the World Radiation Center in Davos, Switzerland. Because of intermittentcabling problems with one of the cavity radiometers, the average of three windowed, electrical-substitution cavity radiometers served as the reference irradiance for 29 test instruments during the 10-month study. To keep the size of this work manageable, comparisons are limited to stable sunny conditions, passing clouds, calm and windy conditions, and hot and cold temperatures. Other variables could have been analyzed, or the conditions analyzed could have employed higher resolution. A more complete study should be possible now that the instruments are identified; note that this analysis was performed without any knowledge on the part of the analyst of the instruments’ manufacturers or models. Apart from the windowed cavities that provided the best measurements, two categories of performance emerged during the comparison. All instruments exceeded expectations in that they measured with lower uncertainties than the manufacturers’ own specifications. Operational 95% uncertainties for the three classes of instruments, which include the uncertainties of the open cavities used for calibration, were about 0.5%, 0.8%,and 1.4%.The open cavitiesthat wereused for calibrationof allpyrheliometers havean estimated 95% uncertainty of 0.4%‐0.45%, which includes the conservative estimate of 0.3% uncertainty for the WRR.

Optical Radiation Measurements for Photovoltaic Applications: Instrumentation Uncertainty and Performance

Evaluating the performance of photovoltaic (PV) devices in the laboratory and in the field requires accurate knowledge of the optical radiation stimulating the devices. We briefly describe the radiometric instrumentation used for characterizing broadband and spectral irradiance for PV applications. Spectral radiometric measurement systems are used to characterize solar simulators (continuous and pulsed, or flash sources) and natural sunlight. Broadband radiometers (pyranometers and pyrheliometers) are used to assess solar resources for renewable applications and develop and validate broadband solar radiation models for estimating system performance. We describe the sources and magnitudes of uncertainty associated with calibrations and measuremens using these instruments. The basic calibration and measurement uncertainty associated with this instrumentaion are based on the guidlines described in the International Standards Organization (ISO) and Bureau INternationale des Poids et Mesures (BIPM) Guide to Uncertainty in Measurement. The additional contributions to uncertainty arising from the uncertainty in characterization functions and correction schemes are discussed and ilustrated. Finally, empirical comparisons of several solar radiometer instrumentation sets illustrate that the best measurement accuracy for broadband radiation is on the order of 3%, and spectrally dependent uncertainty for spectroradiometer systems range from 4% in the visible to 8% to 10% in the ultraviolet and infrared.

Pulse analysis spectroradiometer system for measuring the spectral distribution of flash solar simulators

Flashing artificial light sources are used extensively in photovoltaic module performance testing and plant production lines. There are several means of attempting to measure the spectral distribution of a flash of light; however, many of these approaches generally capture the entire pulse energy. We report here on the design and performance of a system to capture the waveform of flash at individual wavelengths of light. Any period within the flash duration can be selected, over which to integrate the flux intensity at each wavelength. The resulting spectral distribution is compared with the reference spectrum, resulting in a solar simulator classification.

Quantifying the impact of incidence-angle dependence on solar radiometric calibration

Evaluating photovoltaic cells, modules, arrays, and system performance relies on accurate measurements of the solar radiation resources available for power conversion. Measuring solar resources accurately can lead to a reduction in the investment risks associated with installing and operating solar energy systems. The National Renewable Energy Laboratorys Solar Radiation Research Laboratory collects and disseminates solar irradiance data and provides calibrations of broadband radiometers that are traceable to the international standards. It is essential that radiometric data are traceable to the international system of units, e.g., through the World Radiometer Reference and World Infrared Standard Group. This paper demonstrates the importance and application of an existing approach that ultimately reduces the uncertainty of radiometric measurements. Almost all commercially available broadband radiometers use a single responsivity value that is generated at a 45° solar zenith angle (incident angle) based on outdoor calibrations or transfers between radiometers inside integrating spheres or that responsivity is generated using normal incident radiation based on indoor calibrations using lamps and comparisons to reference radiometers to compute measured irradiance data. However, based on our experience and that of other experts in the radiometric science community, this method introduces increased uncertainty to the data. If a single responsivity value is used, the radiometer will overestimate or underestimate the irradiance data compared to the reference irradiance. This was demonstrated in Myers [1], Reda [2], and Reda et al. [3]. Further, by using responsivity as a function of solar zenith angle, the uncertainty for some instruments in the responsivity value can be reduced by as much as 50% compared to using a single responsivity calculated at 45° [2, 3].

Shortwave radiometer calibration methods comparison and resulting solar irradiance measurement differences: A user perspective

Banks financing solar energy projects require assurance that these systems will produce the energy predicted. Furthermore, utility planners and grid system operators need to understand the impact of the variable solar resource on solar energy conversion system performance. Accurate solar radiation data sets reduce the expense associated with mitigating performance risk and assist in understanding the impacts of solar resource variability. The accuracy of solar radiation measured by radiometers depends on the instrument performance specification, installation method, calibration procedure, measurement conditions, maintenance practices, location, and environmental conditions. This study addresses the effect of different calibration methods provided by radiometric calibration service providers, such as NREL and manufacturers of radiometers, on the resulting calibration responsivity. Some of these radiometers are calibrated indoors and some outdoors. To establish or understand the differences in calibration methodology, we processed and analyzed field-measured data from these radiometers. This study investigates calibration responsivities provided by NRELs broadband outdoor radiometer calibration (BORCAL) and a few prominent manufacturers. The BORCAL method provides the outdoor calibration responsivity of pyranometers and pyrheliometers at 45° solar zenith angle, and as a function of solar zenith angle determined by clear-sky comparisons with reference irradiance. The BORCAL method also employs a thermal offset correction to the calibration responsivity of single-black thermopile detectors used in pyranometers. Indoor calibrations of radiometers by their manufacturers are performed using a stable artificial light source in a side-by-side comparison between the test radiometer under calibration and a reference radiometer of the same type. In both methods, the reference radiometer calibrations are traceable to the World Radiometric Reference (WRR). These different methods of calibration demonstrated +1% to +2% differences in solar irradiance measurement. Analyzing these differences will ultimately help determine the uncertainty of the field radiometer data and guide the development of a consensus standard for calibration. Further advancing procedures for precisely calibrating radiometers to world reference standards that reduce measurement uncertainty will allow more accurate prediction of solar output and improve the bankability of solar projects.

Solar Resources Measurements in Elizabeth City, North Carolina - Equipment Only: Cooperative Research and Development Final Report, CRADA Number CRD-07-217

Site-specific, long-term, continuous, and high-resolution measurements of solar irradiance are important for developing renewable resource data. These data are used for several research and development activities consistent with the NREL mission: establish a national 30-year climatological database of measured solar irradiances; provide high quality ground-truth data for satellite remote sensing validation; support development of radiative transfer models for estimating solar irradiance from available meteorological observations; provide solar resource information needed for technology deployment and operations. Data acquired under this agreement will be available to the public through NRELs Measurement & Instrumentation Data Center - MIDC (www.nrel.gov/midc). The MIDC offers a variety of standard data display, access, and analysis tools designed to address the needs of a wide user audience (e.g., industry, academia, and government interests).

Southern Nevada Renewable Resource Assessment

University of Nevada, Las Vegas Renewable Energy Center (UNLV-REC) currently monitors three meteorological stations in southern Nevada under the direction of the National Renewable Energy Laboratory (NREL) and is funded by the Nevada Southwest Energy Partnership (NSWEP). The three station locations are Eldorado Valley, UNLV-REC Solar Site, and Nevada Power Company Clark Station. The installation dates for each of the locations were October of 2004 for Eldorado Valley station, August of 2003 for the UNLV-REC Solar Site, and March of 2006 for the Nevada Power Clark Station. Publicly available data from each site have been archived since installation completion. This paper discusses the installation of the equipment for each site and images of the setup. The data that is being collected between the sites is also compared. Data comparisons between the sites include net monthly solar energy; monthly peak direct normal irradiance (DNI), average daily wind speed, monthly wind roses, and average monthly dry bulb temperatures. The recently measured data is also compared to resource maps developed by NREL and to TMY data. With these meteorological resources, microclimatic variations can be studied for the area and used as a renewable energy resource for renewable installations in southern Nevada.Copyright