Tom Stoffel

Atmospheric longwave irradiance uncertainty: Pyrgeometers compared to an absolute sky‐scanning radiometer, atmospheric emitted radiance interferometer, and radiative transfer model calculations

Because atmospheric longwave radiation is one of the most fundamental elements of an expected climate change, there has been a strong interest in improving measurements and model calculations in recent years. Important questions are how reliable and consistent are atmospheric longwave radiation measurements and calculations and what are the uncertainties? The First International Pyrgeometer and Absolute Sky-scanning Radiometer Comparison, which was held at the Atmospheric Radiation Measurement programs Southern Great Plains site in Oklahoma, answers these questions at least for midlatitude summer conditions and reflects the state of the art for atmospheric longwave radiation measurements and calculations. The 15 participating pyrgeometers were all calibration-traced standard instruments chosen from a broad international community. Two new chopped pyrgeometers also took part in the comparison. An absolute sky-scanning radiometer (ASR), which includes a pyroelectric detector and a reference blackbody source, was used for the first time as a reference standard instrument to field calibrate pyrgeometers during clear-sky nighttime measurements. Owner-provided and uniformly determined blackbody calibration factors were compared. Remarkable improvements and higher pyrgeometer precision were achieved with field calibration factors. Results of nighttime and daytime pyrgeometer precision and absolute uncertainty are presented for eight consecutive days of measurements, during which period downward longwave irradiance varied between 260 and 420 W m−2. Comparisons between pyrgeometers and the absolute ASR, the atmospheric emitted radiance interferometer, and radiative transfer models LBLRTM and MODTRAN show a surprisingly good agreement of <2 W m−2 for nighttime atmospheric longwave irradiance measurements and calculations.

Uncertainty estimates for global solar irradiance measurements used to evaluate PV device performance

Abstract Broadband (0.3 – 3.0 μm) global solar irradiance measurements are used in the evaluation of solar energy conversion devices. The uncertainty attached to such measurements is important in evaluating whether conclusions associated with the measurements are statistically valid. A standardized uncertainty analysis method, developed over the past 15 years in the arena of consensus standards and professional society organizations, is described and applied. The results of the uncertainty analysis for the instrument calibration and field data measurement process indicate that the total measurement uncertainty in pyranometry (i.e. the measurement of global solar irradiance) can approach 5%. Thus comparisons of results between laboratories using different pyranometers can have a total uncertainty of up to 10%. Statistically valid conclusions on a conversion devices performance may be drawn only if such results account for known bias errors or exceed the uncertainty limits derived using this methodology.

Using a Blackbody to Calculate Net Longwave Responsivity of Shortwave Solar Pyranometers to Correct for Their Thermal Offset Error during Outdoor Calibration Using the Component Sum Method

Abstract Thermopile pyranometers’ thermal offset has been recognized since the pyranometer’s inception. This offset is often overlooked or ignored because its magnitude is small compared to the overall solar signal at higher irradiance. With the demand of smaller uncertainty in measuring solar radiation, recent publications have described a renewed interest in this offset, its magnitude, and its effect on solar measurement networks for atmospheric science and solar energy applications. Recently, it was suggested that the magnitude of the pyranometer thermal offset is the same if the pyranometer is shaded or unshaded. Therefore, calibrating a pyranometer using a method known as the shade/unshade method would result in accurate responsivity calculations because the thermal offset error is canceled. When using the component sum method for the pyranometer calibration, the thermal offset error, which is typically negative when the sky is cloudless, does not cancel, resulting in an underestimated shortwave resp...

Pyrgeometer calibration at the National Renewable Energy Laboratory (NREL)

Abstract Pyrgeometers are used to measure longwave terrestrial radiation. Regular pyrgeometer calibration against an internationally recognized standard is required in order to measure the longwave radiation consistently at different sites around the globe. At present, there is no internationally recognized standard to calibrate pyrgeometers. A well-characterized blackbody is, however, an accepted approach. This paper describes a method of establishing a precise blackbody reference and using it to calibrate a group of four transfer reference pyrgeometers. The group is then deployed outdoors to evaluate the precision of the blackbody calibration. The results from the outdoor data shows that the percentage mean-square-error of each transfer reference pyrgeometer is 0.12%, 0.07%, 0.46%, and 0.10% with a resultant percentage root-mean-square of 0.43%. The errors are calculated with respect to the average of the irradiance readings of the transfer reference pyrgeometers. To minimize the number of transfer reference pyrgeometers and to allow more space for calibrating test pyrgeometers, a sub-set of the transfer reference pyrgeometers is then used to calibrate a test pyrgeometer outdoors. The calibration of the test pyrgeometer resulted in reducing its error from +4.00% to ±0.32% with respect to the irradiance measured by the sub-set of the transfer reference pyrgeometers. The outdoor calibration method can minimize the calibration cost resulting from using the lengthy and costly blackbody calibration because many pyrgeometers can be calibrated at the same time. Appendix A shows a diagram that describes the papers concept.

Uncertainty Estimate for the Outdoor Calibration of Solar Pyranometers: A Metrologist Perspective

Abstract: Pyranometers are used outdoors to measure solar irradiance. By design, this of radiometer can measure the total hemispheric (global) or diffuse (sky) irradiance when the detector is unshaded or shaded from the sun disk, respectively. These measurements are used in a variety of applications including solar energy conversion, atmospheric studies, agriculture, and materials science. Proper calibration of pyranometers is essential to ensure measurement quality. This paper describes a step-by-step method for calculating and reporting the uncertainty of the calibration, using the guidelines of the ISO “Guide to the Expression of Uncertainty in Measurement” or GUM, that is applied to the pyranometer calibration procedures used at the National Renewable Energy Laboratory (NREL). The NREL technique characterizes a responsivity function of a pyranometer as a function of the zenith angle, as well as reporting a single calibration responsivity value for a zenith angle of 45°. The uncertainty analysis shows that a lower uncertainty can be achieved by using the response function of a pyranometer determined as a function of zenith angle, in lieu of just using the average value at 45°. By presenting the contribution of each uncertainty source to the total uncertainty; users will be able to troubleshoot and improve their calibration process. The uncertainty analysis method can also be used to determine the uncertainty of different calibration techniques and applications, such as deriving the uncertainty of field measurements.

Current issues in terrestrial solar radiation instrumentation for energy, climate, and space applications

Reductions of uncertainty in terrestrial solar radiation measurements are needed to validate the Earths radiation balance derived from satellite data. Characterization of solar energy resources for renewable technologies requires greater time and spatial resolution for economical technology deployment. Solar radiation measurement research at the National Renewable Energy Laboratory addresses calibrations, operational characteristics, and corrections for terrestrial solar radiation measurements. We describe progress in measurements of broadband diffuse-sky radiation, and characterization of field instrument thermal offsets and spectral irradiance. The need and prospects for absolute references for diffuse and long-wave terrestrial solar radiation measurements are discussed. Reductions in uncertainty of broadband irradiance measurements from tens of watts per square meter to a few (one to two) watts per square meter are reported, which reduce time and labor to quantify and identify trends in artificial optical radiation sources, terrestrial solar radiation, and the Earths radiation budget.

Chapter 1 – Terms and Definitions

What is solar-resource assessment? What are the key scientific and engineering elements associated with solar-forecasting? These are important questions to be addressed by solar-project developers, financiers, scientists, engineers, and electricity-grid operators as they prepare for higher penetrations of centralized and distributed solar-power generation into the nation’s electrical-power grid. The purpose of this chapter is to address these questions by providing the reader with foundational information and an appreciation for the terminology used in this uniquely comprehensive volume on solar-energy resources. Complex interactions of solar radiationwith the Earth-atmosphere-ocean form the basis for understanding the weather-driven nature of this abundant source of renewable energy. Specifically, the effects of clouds, aerosols, and other atmospheric constituents on photons leaving the Sun and reaching the Earth’s surface continue to be the subject of active research, as highlighted in this book.

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).

An overview of NREL's PV solar radiation research task activities and results

This paper presents an overview of the recent activities and results of the Photovoltaics (PV) Solar Radiation Research task of NREL’s PV Advanced Research and Development (PVAR&D) Project. Topics covered include the Atmospheric Optical Calibration System (AOCS) and instrumentation systems for monitoring and characterizing the solar irradiance available to PV systems. Both types of instrumentation systems and activities are required for a thorough understanding of PV device performance and design.