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

Journal Article · Mon Oct 10 04:00:00 EDT 2005 · Journal of Atmospheric and Oceanic Technology, 22(10):1531-1540

DOI:https://doi.org/10.1175/JTECH1782.1· OSTI ID:876870

Reda, I; Hickey, J R; Long, Charles N; Myers, D; Stoffel, T; Wilcox, S; Michalsky, Joseph J; Dutton, Ellsworth G; Nelson, D W

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 common summation calibration method, or component sum, 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 responsivity. Most operational pyranometers that are in use for solar radiation measuring networks are calibrated using the summation method since it is possible to calibrate many pyranometers simultaneously. From this arises the importance of correcting the summation method results to account for the thermal offset error. In this article, we describe a method of using a blackbody system to calculate the net-longwave responsivity of pyranometers, which is largely responsible for the offset error. This longwave responsivity is then used to correct the pyranometer’s shortwave responsivity during the summation method calibrations and thereby substantially reduces the effect of the offset error on the final pyranometer responsivity. Practical procedures for performing this calibration procedure along with its limitations and remaining uncertainties are given.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (US)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 876870
• Report Number(s): PNNL-SA-42233; KP1704010
• Journal Information: Journal of Atmospheric and Oceanic Technology, 22(10):1531-1540, Journal Name: Journal of Atmospheric and Oceanic Technology, 22(10):1531-1540 Journal Issue: 10 Vol. 22
• Country of Publication: United States
• Language: English

Related Subjects

14 SOLAR ENERGY
CALIBRATION
PYRANOMETERS
SKY
SOLAR ENERGY
SOLAR RADIATION
THERMOCOUPLES