Shortwave radiometers such as pyranometers, pyrheliometers, and photovoltaic cells are calibrated with traceability to consensus reference, maintained by Absolute Cavity Radiometers (ACRs). The ACR is an open cavity with no window that measures the extended broadband spectrum of the terrestrial direct solar beam irradiance, unlike shortwave radiometers that cover a limited range of the spectrum. The difference between the two spectral ranges may lead to calibration bias that can exceed 1%. This article describes a method to reduce the calibration bias resulting from using broadband ACRs to calibrate shortwave radiometers by using an ACR with Schott glass window to measure the reference broadband shortwave irradiance in the terrestrial direct solar beam from 0.3 μm to 3 μm. Reducing the calibration bias will result in lowering the historical solar irradiance by at least 0.9%. The published results in this article might raise the awareness of the calibration discrepancy to the users of such radiometers, and open a discussion within the solar and atmospheric science community to define their expectation from such radiometers to the radiometers’ manufacturers and calibration providers.
Shortwave radiometers such as pyranometers, pyrheliometers, and photovoltaic cells are calibrated with traceability to the World Radiometric Reference (WRR) (ISO, 1990) [
As will be described in section 2 below, the reference broadband shortwave irradiance (WSW) is calculated by subtracting the measured reference WLW from the broadband irradiance (WBB), measured by broadband ACR (ACRBB) (i.e. WSW = WBB − WLW). The reference WSW is then used to calibrate an ACR with Schott glass window (ACRSW), which is then used to calibrate a set of shortwave radiometers. The difference between using ACRBB and ACRSW to calibrate the radiometers is shown in Section 3 below.
The shaded and unshaded pyrgeometers, the two ACRs (ACRBB and ACRSW), and fiveshortwave radiometers, two thermopile pyrheliometers (Eppley Laboratory, Inc., model NIP and Kipp and Zonen model CHP1), two thermopile pyranometers (Eppley Laboratory, Inc., model PSP and Kipp and Zonen model CMP22), and one photodiode pyranometer (Kipp and Zonen model SP Lite2),were set up outdoors at the National Renewable Energy Laboratory’s (NREL) Solar Radiation Research Laboratory (SRRL), which is at an elevation of 1828.8 meters above sea level. Data was collected simultaneously from all radiometers under clear sky conditions every 30 seconds, from June 15 to June 18, 2016. The following subsections describe the calibration of the ACRSW. The ACRBB was calibrated with traceability to WRR during NREL Pyrheliometer Comparisons, NPC-2014 [
measure the reference longwave irradiance in the solar beam (WLW) as:
where,
- Wunsh is the longwave irradiance measured by the unshaded pyrgeometer (W・m−2)
- Wsh is the longwave irradiance measured by the shaded pyrgeometer with the Schott glass shading mechanism, see
- Θ is the solar zenith angle (˚).
Detailed derivation of the equation is presented in Reda et al., 2015.
The broadband direct beam irradiance (WBB) is measured by the ACRBB that was calibrated during NREL Pyrheliometer Comparison (NPC-2014) with traceability to WRR [
where,
- M is the sensitivity reciprocal (1/Sensitivity) of the ACRSW (W/m2/mV). Similar to
the ACRBB the ACRSW self-calibrates every hour to calculate M [
- Vtp is the thermopile output voltage when the shutter of the ACRSW is open (mV) (i.e. the direct solar beam is incident on the thermopile’s receiving junctions).
- V0 is the thermopile output voltage when the cavity is self-calibrating and the shutter is closed [i.e. no solar irradiance (mV)].
During the calibration, each measurement data point consists of simultaneous recording of the following parameters:
- Output voltage from the test radiometer, V (µV).
- Reference irradiance measured by ACRBB and ACRSW, WBB and WSW (W・m−2).
Window Factor | 1.05655 |
---|---|
%SD = Type A Standard Uncertainty (uA) | 0.0002 |
%Standard Unceratainty (u95,BB) | 0.19 |
%Standard Unceratainty (u95,LW) | 0.21 |
Coverage Factor | 1.96 |
%U95,SW | 0.41 |
- Net longwave irradiance measured by a pyrgeometer, Wnet (W・m−2).
- Reference diffuse irradiance measured by a shaded pyranometer, D (W・m−2).
- The solar zenith angle calculated using the Solar Position Algorithm (SPA), Θ(˚) [
The responsivity of each shortwave radiometer is then calculated using two methods: Using the ACRBB as a reference and using the ACRSW as the reference. The following equation is used to calculate the broadband responsivity (RSBB) [
where Rnet is the pyranometer’s net infrared responsivity calculated using the method described in [
The shortwave responsivity (RSSW) is then calculated using Equation (4) by replacing RSBB and WBB by RSSW and WSW, respectively.
Figures 5-8 show the difference between the RSBB and RSSW for five radiometers, two thermopile pyrheliometers (Eppley Laboratory, Inc., model NIP and Kipp and Zonen model CHP1), two thermopile pyranometers (Eppley Laboratory, Inc., model PSP and Kipp and Zonen model CMP22), and one photodiode pyranometer (Kipp and Zonen model SP Lite2). From the figures, pyrheliometers RSSW is larger than RSBB by at least 0.75%, and for pyranometers RSSW is larger than RSBB by at least 0.6%. Historically, direct solar beam and global irradiance is calculated using the RSBB for all radiometers.
This implies that when the radiometers are deployed in the field after being calibrated using ACRSW, the measured direct broadband shortwave beam solar irradiance might be lower than the historical irradiance by at least 0.75%, and that the global broadband shortwave solar irradiance might be lower than the historical global irradiance by 0.6%.
The same calibration method was repeated at the USA-Department of Energy (DOE), Atmospheric Radiation Measurement (ARM) program at the Southern Great
Plains (SGP) site, which is at an elevation of 318 meters above sea level.
ACRSW, the measured direct beam solar irradiance might be lower than the historical irradiance by at least 1.1%, and the global solar irradiance might be lower than the historical global irradiance by 0.9%.
We find that by using the historical calibration method of broadband shortwave radi-
ometers recommended by ISO 9059:1990 results in an overestimation in the field measurement of the direct broadband shortwave beam solar irradiance by at least 0.75% and the global broadband shortwave solar irradiance by at least 0.6%. This overestimation might exceed 1% based on the atmospheric conditions during the calibration, primarily water vapor and aerosols. Since shortwave radiometers are designed to measure the broadband shortwave solar irradiance in the spectral range from 0.3 µm to 3 µm,
per ISO 9060:1990, the recommended calibration method by ISO 9059-1990 would result in biases in the calibration results. These biases might be significant in atmospheric science and solar energy applications.
While most users of such radiometers are interested in measuring the broadband short- wave irradiance, some users are interested in measuring the broadband solar irradiance (WBB). For the latter, it is recommended that radiometers be designed with domes or windows that transmit the broadband spectral range to avoid such calibration discrepancy. The purpose of this article is to raise the awareness of the calibration discrepancy to the users of such radiometers, and open a discussion within the solar and atmospheric science community to define their expectation from such radiometers to the radiometers’ manufacturers and calibration providers.
We would like to thank NREL’s solar program staff, NREL’s Quality Management Systems and Assurance Office, NREL Metrology Laboratory, and DOE-Atmospheric Radiation Measurement (ARM) program for providing the funds for this publication. We extend special appreciation to Martina Newman for providing the superb administrative support by taking care of all the logistics associated with this effort.
Reda, I., Andreas, A., Dooraghi, M., Sengupta, M., Habte, A. and Kutchenreiter, M. (2017) Reducing Broadband Shortwave Radiometer Calibration-Bias Caused by Longwave Irradiance in the Reference Direct Beam. Atmospheric and Climate Sciences, 7, 36-47. http://dx.doi.org/10.4236/acs.2017.71004