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Title: A proposed working standard for the measurement of diffuse horizontal shortwave irradiance

Abstract

Atmospheric radiative transfer model estimates of diffuse horizontal broadband shortwave (solar) irradiance have historically been larger than measurements from a shaded pyranometer. A reference standard for the diffuse horizontal shortwave irradiance does not exist. There are no current efforts to develop an absolute standard that are known to the authors. This paper presents the case for a working standard for this measurement. Four well-behaved pyranometers from two previous intensive observation periods (IOP) were chosen for this study. The instruments were characterized for spectral and angular response before the IOP and calibrated during the IOP using a shade/unshade technique with reference direct irradiance from an absolute cavity radiometer. The results of the comparison and detailed analyses to explain the differences suggest selecting three of the four for the working standard. The 95% confidence uncertainty in this standard is estimated at 2.2% of reading + 0.2 W/m 2. In lieu of a comparison to this trio, a procedure for obtaining low-uncertainty diffuse horizontal shortwave irradiance is suggested.

Authors:
; ; ; ; ;
Publication Date:
Research Org.:
National Renewable Energy Lab. (NREL), Golden, CO (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1344972
Report Number(s):
NREL/JA-581-42489
Journal ID: ISSN 0148-0227; JGREA2
DOE Contract Number:
AC36-08GO28308
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Geophysical Research; Journal Volume: 112; Journal Issue: D16
Country of Publication:
United States
Language:
English
Subject:
14 SOLAR ENERGY; 47 OTHER INSTRUMENTATION; 54 ENVIRONMENTAL SCIENCES; diffuse irradiance; working standard; pyranometer

Citation Formats

Michalsky, J. J., Gueymard, C., Kiedron, P., McArthur, L. J. B., Philipona, R., and Stoffel, T. A proposed working standard for the measurement of diffuse horizontal shortwave irradiance. United States: N. p., 2007. Web. doi:10.1029/2007JD008651.
Michalsky, J. J., Gueymard, C., Kiedron, P., McArthur, L. J. B., Philipona, R., & Stoffel, T. A proposed working standard for the measurement of diffuse horizontal shortwave irradiance. United States. doi:10.1029/2007JD008651.
Michalsky, J. J., Gueymard, C., Kiedron, P., McArthur, L. J. B., Philipona, R., and Stoffel, T. Mon . "A proposed working standard for the measurement of diffuse horizontal shortwave irradiance". United States. doi:10.1029/2007JD008651.
@article{osti_1344972,
title = {A proposed working standard for the measurement of diffuse horizontal shortwave irradiance},
author = {Michalsky, J. J. and Gueymard, C. and Kiedron, P. and McArthur, L. J. B. and Philipona, R. and Stoffel, T.},
abstractNote = {Atmospheric radiative transfer model estimates of diffuse horizontal broadband shortwave (solar) irradiance have historically been larger than measurements from a shaded pyranometer. A reference standard for the diffuse horizontal shortwave irradiance does not exist. There are no current efforts to develop an absolute standard that are known to the authors. This paper presents the case for a working standard for this measurement. Four well-behaved pyranometers from two previous intensive observation periods (IOP) were chosen for this study. The instruments were characterized for spectral and angular response before the IOP and calibrated during the IOP using a shade/unshade technique with reference direct irradiance from an absolute cavity radiometer. The results of the comparison and detailed analyses to explain the differences suggest selecting three of the four for the working standard. The 95% confidence uncertainty in this standard is estimated at 2.2% of reading + 0.2 W/m2. In lieu of a comparison to this trio, a procedure for obtaining low-uncertainty diffuse horizontal shortwave irradiance is suggested.},
doi = {10.1029/2007JD008651},
journal = {Journal of Geophysical Research},
number = D16,
volume = 112,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • The first intensive observation period (IOP) to simultaneously measure diffuse horizontal shortwave irradiance (scattered solar radiation that falls on a horizontal surface) with a wide array of shaded pyranometers suggested that a consensus might be reached that would permit the establishment of a standard with a smaller uncertainty than previously achieved. A second IOP has been held to refine the first IOP measurements using a uniform calibration protocol, offset corrections for all instruments and validation of those corrections, improvements in some of the instruments, and better data acquisition. The venue for both IOPs was the Department of Energy's Atmospheric Radiationmore » Measurement (ARM) central facility in northern Oklahoma. The nine days of measurements in October 2003 included a better mixture of clear and overcast conditions than during the first IOP and revealed considerable differences among the instruments responses for different cloud conditions. Four of the 15 instruments were eliminated as candidates to be included in the standard because of noisy signals, inadequate offset correction, or instability with respect to the majority of the measurements. Eight pyranometers agreed to within {+-}2% for clear-sky conditions. Three others have a high bias on clear days relative to these eight, but all eleven agree within {+-}2% on overcast days. The differences and causes of this behavior under clear and cloudy skies are examined.« less
  • The first intensive observation period (IOP) to simultaneously measure diffuse horizontal irradiance in the shortwave with a wide array of shaded pyranometers suggested that a consensus might be reached that would permit the establishment of a standard with a smaller uncertainty than heretofore achieved. A second IOP has been held to refine the first IOP measurements using a uniform calibration protocol, offset corrections for all instruments and validation of those corrections, improvements in some of the instruments, and better data acquisition. The venue for both IOPs was the Department of Energy's Atmospheric Radiation Measurement (ARM) central facility in northern Oklahoma.more » The nine days of measurements in October 2003 included a better mixture of clear and overcast conditions than during the first IOP and revealed considerable differences among the instruments responses for different cloud conditions. Four of the 15 instruments were eliminated as candidates to be included in the standard because of noisy signals, inadequate offset correction, or instability with respect to the majority of the measurements. Eight pyranometers agreed to within ?2% for clear-sky conditions. Three others have a high bias on clear days relative to these eight, but all eleven agree within ?2% on overcast days. The differences and causes of this behavior under clear and cloudy skies will be examined.« less
  • The first intensive observation period (IOP) dedicated exclusively to the measurement of diffuse horizontal shortwave irradiance was held in the Fall 2001 at the central facility of the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site with the cooperation of the Baseline Surface Radiation Network (BSRN) community. Fourteen simultaneous measurements were obtained over a two-week period under mostly clear skies with low to moderate aerosol loading. Overcast data were obtained during the morning of one day. The purpose of the comparison was to assess the level of agreement in diffuse irradiance measurements among most commercial pyranometers and a fewmore » prototypes calibrated independently using current practices. The hope was to achieve a consensus for this measurement with the goal of improving the uncertainty of shortwave diffuse irradiance measurements. All diffuse broadband measurements were made using the same type of two-axis tracker with the direct beam blocked by shading balls. Tracking was very good during the IOP with no outages associated with tracker problems. Five of the measurements are reproducible to about 2 W/m2 at the 95% confidence level. Four more agree with the most consistent group to about 4 W/m2 at the 95% confidence level after correction for thermal offsets. The prototypes agree less well with the most consistent group.« less
  • Significant errors occur in downwelling shortwave irradiance measurements made on moving platforms due to tilt from horizontal because, when the sun is not completely blocked by overhead cloud, the downwelling shortwave irradiance has a prominent directional component from the direct sun. A-priori knowledge of the partitioning between the direct and diffuse components of the total shortwave irradiance is needed to properly apply a correction for tilt. This partitioning information can be adequately provided using a newly available commercial radiometer that produces reasonable measurements of the total and diffuse shortwave irradiance, and by subtraction the direct shortwave irradiance, with no movingmore » parts and regardless of azimuthal orientation. We have developed methodologies for determining the constant pitch and roll offsets of the radiometers for aircraft applications, and for applying a tilt correction to the total shortwave irradiance data. Results suggest that the methodology is for tilt up to +/-10°, with 90% of the data corrected to within 10 Wm-2 at least for clear-sky data. Without a proper tilt correction, even data limited to 5° of tilt as is typical current practice still exhibits large errors, greater than 100 Wm-2 in some cases. Given the low cost, low weight, and low power consumption of the SPN1 total and diffuse radiometer, opportunities previously excluded for moving platform measurements such as small Unmanned Aerial Vehicles and solar powered buoys now become feasible using our methodology. The increase in measurement accuracy is important, given current concerns over long-term climate variability and change especially over the 70% of the Earth’s surface covered by ocean where long-term records of these measurements are sorely needed and must be made on ships and buoys.« less