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Solar Accuracy to the 3/10000 Degree

Ibrahim Reda works at NREL's Solar Radiation Research Laboratory, where he has developed Solar Position Algorithm software.
Photo by Dennis Schroeder, NREL

Solar Accuracy to the 3/10000 Degree

Revolutionary calibration technique carries world standard.

Say the word "pedigree" and most people think thoroughbred horses or purebred puppies. However, a metrologist such as the National Renewable Energy Laboratory's (NREL) Senior Scientist and Metrology Laboratory Manager Ibrahim Reda thinks calibration traceability to national and international standards. Quality research requires, among many things: accuracy, repeatability, defensibility, and calibration traceability. Calibration traceability is research data's pedigree. The data have more value because you can validate the origin and you can predict its future accuracy with greater confidence.

Defining the Broadband Outdoor Radiometer Calibration Process

Measurement accuracy is critical to our smart grid future. "The smart grid has to know precisely what your budget is for each resource you are using — oil, coal, solar, wind. Accuracy is translated into dollars; more accurate is more attractive and less risky to investors," explained Ibrahim Reda.

The NREL Metrology Laboratory, which is located at the Solar Radiation Research Laboratory, developed a process called Broadband Outdoor Radiometer Calibration. Chet Wells, Daryl Myers, and Tom Stoffel started this process in the late 1970s to calibrate radiometers, such as pyranometers and pyrheliometers, which measure the sun's energy. Radiometers are utilized across the globe for a variety of purposes including:

  • Photovoltaic (PV) research on solar cell efficiency: evaluating how a particular cell performs under certain atmospheric conditions
  • Atmospheric science: predicting weather and advancing international and regional climate simulations and projections
  • Agriculture: advancing knowledge of photosynthesis and its effect on crop production
  • Solar resource data: determining if a parcel of land is suitable for a PV field or soybeans
  • Renewable energy applications: determining current and future development and investment.

Considering the numbers of radiometers in use around the world, and the copious amounts of solar data they generate for the specific purpose of making fiscal decisions, it becomes ever more critical to ensure a world standard in instrument accuracy, precision, and calibration traceability.

Position of Sun: Accuracy to the 3/10000 Degree

Ibrahim Reda pioneered an algorithm that tracks the position of the sun to within 0.0003 degrees of accuracy through the year 6000. Now that's accuracy.

"We characterize the instruments based on the solar angle," Reda said. "It's vital that instruments get a precise read on the amount of energy they are getting from the sun at a precise solar angle."

Photo of a man in a green coat, jeans, tennis shoes and sunglasses installing a silver and white instrument that stands on a tripod and has many cords leading away from it on a rooftop under a clear blue sky.

Ibrahim Reda installs reference radiometers to perform the annual NREL-Pyrheliometer Comparison (NPC), on the deck of NREL's Solar Radiation Research Laboratory.
Photo by Dennis Schroeder, NREL

The process, which has been accredited under the International Organization for Standardization (ISO) 17025, adds unique value for NREL, other national laboratories, industry, and academic partners because it validates the lab's solar measurements, and stands as proof that they conform to international standards. By reducing uncertainty, validating calibration traceability, and using an accredited world-standard process, NREL increases stakeholder's confidence in the bankability of its data and research results.

NREL Delivers World-Class Solar Measurements

NREL is the only institution in the world that currently provides ISO 17025-accredited, radiometer calibrations that are traceable to the World Radiometric Reference (WRR)—calibrations that feature both unmatched characterization versus sun angle range as well as low uncertainty. The WRR, which is the international standard from which all radiometer calibrations are traceable, is maintained by the World Meteorological Organization in Switzerland, and is transferred to the world's scientific community once every five years during the International Pyrheliometer Comparisons gathering in Davos. Ibrahim Reda and Tom Stoffel regularly participate in this outdoor calibration exercise to ensure that NREL's calibration standards are directly traceable to the WRR. Their efforts in guaranteeing NREL's radiometer calibrations and solar energy measurements help the laboratory maintain its world-class pedigree.

Learn more about NREL's Metrology Laboratory at the Solar Radiation Research Laboratory.

Learn more about NREL's Spectrum of Clean Energy Innovation and how the laboratory's capabilities emulate the nature of the innovation process.

The NREL Spectrum of Clean Energy Innovation

Issue 3

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