NREL Pyrheliometer Comparisons: November 4 & 29, 2021 (NPC-2021)

Research output: NRELTechnical Report


Accurate measurements of direct normal (beam) solar irradiance from pyrheliometers are important for developing and deploying solar energy conversion systems, for improving our understanding of Earth's energy budget for climate change studies, and for other science and technology applications involving solar flux. Providing these measurements places many demands on the quality system used by the operator of commercially available radiometers. Maintaining accurate radiometer calibrations that are traceable to an international standard is the first step in producing research-quality solar irradiance measurements. In 1977, the World Meteorological Organization (WMO) established the World Radiometric Reference (WRR) as the international standard for the measurement of direct normal solar irradiance (Frohlich 1991). The WRR is an internationally recognized, detector-based measurement standard determined by the collective performance of six electrically self-calibrated absolute cavity radiometers comprising the World Standard Group (WSG). Various countries, including the United States, have contributed these specialized radiometers to the Physikalisch-Meteorologisches Observatorium Davos - World Radiation Center (PMOD/WRC) to establish the WSG. As with all measurement systems, Absolute Cavity Radiometers (ACR) are subject to performance changes over time. Therefore, PMOD/WRC in Davos, Switzerland, hosts an quinquennial International Pyrheliometer Comparison (IPC) event for transferring the WRR to participating radiometers by invitation. The National Renewable Energy Laboratory (NREL) has represented the U.S. Department of Energy (DOE) in each IPC since 1980. And NREL has developed and maintained a select group of absolute cavity radiometers with direct calibration traceability to the WRR, and it uses these reference instruments to calibrate pyrheliometers and pyranometers using the International Organization for Standardization (ISO) 17025-accredited Broadband Outdoor Radiometer Calibration (BORCAL) process (Reda et al. 2008). To fill the gap between each IPC, NREL pyrheliometer comparisons (NPCs) are held annually at the Solar Radiation Research Laboratory (SRRL) in Golden, Colorado. Open to all ACR owners and operators, each NPC provides an opportunity to determine the unique WRR transfer factor (WRR-TF) for each participating pyrheliometer. By adjusting all subsequent pyrheliometer measurements by the appropriate WRR-TF, the solar irradiance data are traceable to the WRR. NPC-2021 was held November 4 and 29, 2021. Participants operated 6 ACRs to simultaneously measure clear-sky direct normal solar irradiance during this period. The Transfer Standard Group (TSG) of reference radiometers for NPC-2021 consisted of four NREL radiometers with direct traceability to the WRR, each having participated in the Thirteenth International Pyrheliometer Comparisons (IPC-XIII) in the fall of 2021 (delayed one year due to COVID-19). As a result of NPC-2021, each participating absolute cavity radiometer was assigned a new WRR-TF, which is computed as the reference irradiance determined by the TSG divided by the observed irradiance from the participating radiometer. The performance of the TSG during NPC-2021 was consistent with previous comparisons, including IPC-XIII. The measurement performance of the TSG allowed the transfer of the WRR to each participating radiometer with an estimated uncertainty of +/- 0.33% with respect to the International System of Units. The comparison protocol is based on data collection periods called runs. Each measurement run consists of an electrical self-calibration requiring five minutes for the Automatic Hickey-Frieden (AHF) cavities, a series of 100 solar irradiance measurements at 15-second intervals, and a post-calibration. more than 800 reference irradiance measurements during NPC-2021. The clear-sky daily maximum direct normal irradiance level was 1,007 Wm-2. Ancillary environmental conditions (e.g., broadband turbidity, ambient temperature, relative humidity, wind speed, precipitable water vapor, and spectral data) collected at SRRL during the comparison are available at NPCs are planned annually at the SRRL to ensure worldwide homogeneity of solar radiation measurements traceable to the WRR.
Original languageAmerican English
Number of pages23
StatePublished - 2022

NREL Publication Number

  • NREL/TP-1900-84231


  • absolute cavity radiometer
  • solar irradiance
  • WSG


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