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ASTM Proposing New Reference Spectra for Irradiance Standards

Absorptance, reflectance, and transmittance of solar energy, factor into the performance of photovoltaic systems and other applications, according to Radiometry Subcommittee G03.09, part of ASTM Committee G03 on Weathering and Durability. These properties require that the spectral distribution of the solar flux be known before the solar-weighted property is calculated to judge product performance. To help establish uniform calculations, the subcommittee developed standard reference solar spectral irradiance distribution in 1987.

A subcommittee task group is proposing new reference spectra for:

• ASTM E 1036, Standard Test Methods for Electrical Performance of Nonconcentrator Terrestrial Photovoltaic Modules and Arrays Using Reference Cells, (1998); and
• ASTM G 159, Standard Tables for Reference Solar Spectral Irradiance at Air Mass 1.5: Direct Normal and Hemispherical for a 37° Tilted Surface, (1999).

“Terrestrial solar spectral distributions are important for testing weathering and durability of materials, solar-energy system components, and the optical and thermal properties of windows,” says task-group member Daryl R. Myers, senior scientist II, National Renewable Energy Laboratory, Golden, Colo. “The solar spectrum at the earth’s surface varies considerably depending on atmospheric conditions and time of day.”

The task group developing the spectra represents manufacturers and testers of accelerated-weathering and solar-energy systems. “The U.S. Department of Energy Renewable Energy Laboratory (NREL, Golden, Colo.) and Florida Solar Energy Research Center (FSEC, University of Central Florida, Coca) have contributed to the research leading to this activity,” says Myers.

ASTM standard solar-terrestrial spectra provides fixed reference spectra developed over 15 years ago by R.E. Bird, and the task group is proposing new spectra based on research done in 2000 by Myers and S. Kurtz, et al. “The proposed spectra represent typical clear sky spectral conditions associated with sites representing reasonable photovoltaic energy production and weathering and durability climates,” says Myers. “Modern terrestrial spectral radiation models and knowledge of atmospheric physics are being applied to develop suggested revisions to update the reference spectra. A moderately complex radiative transfer model, SMARTS2 [devised by C. Gueymard, Ph.D.] is used to produce the revised spectra using the same standard atmosphere, but a different concentration of aerosol absorbers [referenced in the research of E.P. Shettle and R. W. Fenn].

“These changes reflect a better understanding of the aerosol climate and atmospheric constituents resulting in much more realistic conditions prevalent for materials durability, renewable energy production, and window performance evaluation,” says Myers. “SMARTS2 has been validated against the complex MODTRAN4 [refer to the description in G.P. Andersen, et al.] radiative transfer code and spectral measurements from around the globe.”

According to Myers, the proposed changes result in slight changes to the existing hemispherical spectrum on a tilted surface, and dramatic changes to the direct beam spectrum. The graph shows the proposed (SMARTS) revised spectra for hemispherical tilted (HT) and direct normal plus circumsolar (DNI + CS) reference spectra, compared with the existing direct normal (DNI G159) and hemispherical (normalized to 1000 W/m_ integrated power: Norm HT G159). The much reduced present DNI G159 spectrum is due to an assumption of very high aerosol absorption assumed as an average for the United States in the 1982-1987 period. Analysis of recent broadband and spectral measured data in the period 1980-2002 has indicated much lower aerosol concentrations are appropriate for representative conditions [based on research by Kurtz, Myers, K. Emery, and Gueymard et al.]. Thus the much higher amplitude (and more correct) proposed direct normal spectrum, Myers says.

For further technical information, contact Daryl Myers at the National Renewable Energy Laboratory, Golden, Colo. (phone: 303/384-6768). Committee G03 meets Jan. 12-15 in Ft. Lauderdale. For meeting or membership information, contact Tim Brooke, director, ASTM Technical Committee Operations (phone: 610/832-9729). //

Copyright 2002, ASTM