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November 7th, 2011

Ladies and Gentlemen—Start your pyrheliometers!

Published on November 7th, 2011 | By: Eileen De Guire
I

Pyreheliometers are pointed toward the sun for their yearly calibrationat NREL. Accurate measurement of solar energy is needed for due diligence of photovoltaic installations. Credit: NREL.

If you have one, you probably were at the 16th annual convening of the NREL Pyrheliometer Comparisons in October in Golden, Colo.

If you don’t have one, it helps to know that a pyrheliometer is a type of radiometer for measuring the sun’s solar energy, and like all instruments, it needs to be calibrated regularly in order for its measurements to be valuable.

And, valuable they are. Solar energy measurements are used by almost anyone in the sunshine business: researchers, universities, the solar industry, and the bankers, venture capitalists and financiers who invest in them.

Calibrations are done against standards, and the international standard for solar energy is kept in Davos, Switzerland, at the World Radiation Center. Every five years instruments are calibrated against it in Davos. NREL is the only facility in the world that holds an annual event for intermediate recalibrations. According to the website, the goal of the gathering is to “transfer the World Radiometric Reference to international, national, and regional researchers.”

In the news release, NREL group manager Tom Stoffel says, “It’s all about traceability.” He continues, “This is all part of due diligence. … [if] you’re proposing a $30 million concentrating-solar-power plant, exactly how much direct sun are you going to get?”

The due diligence requirements have become very demanding since the 1980s. Twenty-five years ago, daily updates on the sun’s intensity in a few zones across the country were typical. Fifteen years ago, hourly reports became available. Now, according to the news release, “The gold standard is once-a-second updates at the exact spot where the solar installation is envisioned.”

NREL keeps a solar calendar that dates back to 1981 with data on the sun’s radiation starting at sunrise. DOE, through its Climate Research Facility in Lamont, Okla., maintains radiometers at 25 sites, three of which are internationally based and two that are mobile (and currently deployed overseas). The CRF, through its Atmosphereic Radiation Measurement program, has 20 years of solar and atmospheric radiation data. These days, data is taken every 60 seconds.

CRF electronic technician, Craig Webb was one of the participants at this year’s calibration event. In the news story, he sums up, “They want everybody to be tied to the same base so we can measure accurately the watts from the sun. That’s why we’re here every year.”


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