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 July 2007
Feature
DARYL R. MYERS has a B.S. in applied mathematics from the University of Colorado. He leads a Solar Radiometry and Metrology Task Group, coordinating solar radiation research at the U.S. Department of Energy’s National Renewable Energy Laboratory in Golden, Co., where he has worked since 1978.

Solar Radiation Metrology for Solar Energy Systems

The growing national and international concerns over the source and impact of global warming and the limited nature of traditional fossil fuels are increasing the momentum and prospects for deploying alternative renewable energy systems. Renewable systems deriving their fuel from sunlight have been considered since the oil embargo years of the 1970s and even earlier.

Solar renewable systems convert sunlight to electricity or generate thermal energy. The design and performance of these systems depend on the available magnitude and quality of solar radiation, or solar resource. The reliability and durability of solar conversion systems deployed in the environment depends on exposure to the elements and damaging ultraviolet radiation.

Two ASTM committees — E44 on Solar, Geothermal and Other Alternative Energy Sources and G03 on Weathering and Durability — work with industry to develop standards that assure quality measurements of solar radiation resource and exposure. Unique measurement quantities, instruments and techniques are used to validate solar conversion system test performance, evaluate durability and model solar resources for designing systems for deployment. Standardized solar radiation calibrations and measurements developed by ASTM International provide measurements of known quality for these applications.

Measurement of Sunlight

Solar radiation is made up of photons from the solar disk that pass through the atmosphere, are roughly parallel and cast shadows (so-called “direct” solar radiation). It is also made up of photons scattered by the atmosphere, creating the diffuse sky radiation from the entire sky dome.

Measuring the magnitude, intensity and quality (i.e., the distribution of energy with wavelength) of these components is an important aspect of solar energy conversion system design, performance testing, durability and deployment decisions. Pyranometers measure the total sky radiation (combined direct and diffuse) and pyrheliometers measure the direct radiation within a small solid angle about the sun. These detectors are generally based on thermopiles, which are multiple junctions of dissimilar metals that generate an electrical signal when heat flows through the junctions.

Other sensor designs are based on solid-state devices that convert solar photons to electrons and an electrical current, or photovoltaic devices. Although many optical detectors can be characterized and calibrated in the laboratory, the magnitude and wavelength distribution of sunlight is difficult to achieve in the laboratory. Therefore, the world-recognized reference for the calibration of these detectors is a group of specially designed and carefully characterized absolute-cavity pyrheliometers maintained by the World Meteorological Organization at the World Radiation Center in Davos, Switzerland. This World Standard Group defines the World Radiometric Reference for solar radiometry.1 Instrument manufacturers and government and industry research laboratories strive to relate their solar measurements to the WRR reference scale.

With the advent of government-sponsored R&D for solar alternative energy sources, three key issues have been addressed by activities within ASTM International committees. The first issue is: how much solar energy is available? The second is: how well do solar conversion systems perform? And the third issue is: will materials in solar energy conversion systems degrade? The first and third issues are addressed by Subcommittee G03.09 on Radiometry. The second issue is primarily the concern of Committee E44, particularly Subcommittee E44.09 on Photovoltaic Electric Power Conversion.

Solar Radiometer Calibrations and Applications

Subcommittee G03.09 has developed solar radiometer calibration standards that provide up-to-date methods for calibrating radiometers that measure direct-beam and total-sky radiation, as well as various subsets of the total solar spectrum, such as ultraviolet and visible radiation.

This map shows estimates of annual available solar energy (darker colors are higher values) for concentrating solar collectors based on earth orbiting satellite data. Accurate ground-based solar radiation measurements calibrate and validate the algorithms for converting the satellite data to solar radiation fluxes.

Reference instrument and field-measurement instrument calibrations are described in ASTM E 816, Test Method for Calibration of Pyrheliometers by Comparison to Reference Pyrheliometers; G 167, Test Method for Calibration of a Pyranometer Using a Pyrheliometer; and E 824, Test Method for Transfer of Calibration from Reference to Field Radiometers. These methods result in calibrated instruments for assessing the available solar radiation resource, the efficiency and performance of solar conversion systems, or total radiant exposure in degradation studies. A standard describing the proper installation and maintenance of radiometers in the field is also available, G 183, Practice for Field Use of Pyranometers, Pyrheliometers and UV Radiometers.

For applications such as accelerated indoor testing or exposure to natural ultraviolet radiation, ASTM G 130, Test Method for Calibration of Narrow- and Broad-Band Ultraviolet Radiometers Using a Spectroradiometer, describes the calibration of instrumentation for measuring UV radiation and radiation in any spectral band, including visible radiation measured by instruments emulating the response of the human eye.

Because many solar conversion systems and technologies have greatly differing responses to various parts of the solar spectrum, measurements of the solar (and also artificial light source) distribution of power or energy with respect to wavelength are important. ASTM G 138, Test Method for Calibration of a Spectroradiometer Using a Standard Source of Irradiance, describes the proper procedures for calibrating spectral distribution measurement equipment. Solar spectral distributions vary widely with atmospheric conditions, so these measurements are important in evaluating how changing natural conditions affect the efficiency of solar conversion systems.

Standard Test Conditions for Solar Conversion Systems

To complement the measurements discussed above, standard reference conditions, including standard reference spectra, are required to relate system performance for widely varying technologies to a common baseline. Reference spectral standards have been in place since 1982. However, these standards were becoming dated with respect to the state of the art by 2000. In that year, Committee G03.09 undertook an extensive revision of the existing reference spectra, including the addition of a computational model permitting a user to produce the reference spectra tables (and many other realistic spectral distributions) as needed.2,3

The present version of these reference spectra are described in G 173, Tables for Reference Solar Spectral Irradiances: Direct Normal and Hemispherical on 37° Tilted Surface, and the CD-ROM adjunct to G 173, Simple Model for Atmospheric Transmission of Sunshine (SMARTS2 Version 2.9.2). These reference spectra are also used to evaluate the performance of artificial sources meant to simulate sunlight. Many other important measurement parameters and test procedures are required to evaluate the performance of solar energy systems as described in the article by Keith Emery in this issue.

Weathering and Durability

The main focus of Committee G03 is on weathering and durability and the service lifetime of materials systems. Because solar conversion systems are deployed outdoors, the performance over time of component materials systems, including polymer encapsulants, mirrors and coatings, is important. Many of the system components are tested using accelerated weathering systems in the laboratory or by natural exposure techniques outdoors.

One of the important radiometric standards in this regard is a maximum ultraviolet reference spectrum, G 177, Tables for Reference Solar Ultraviolet Spectral Distributions: Hemispherical on 37° Tilted Surface, computed using the same model as the general solar reference spectra.

Another extensive set of standards describing testing procedures, exposure cycles, analysis of exposure degradation data and service lifetime predictions are maintained by Subcommittees G03.02 on Natural and Environmental Exposure Tests, G03.03 on Simulated and Controlled Exposure Tests, G03.08 on Service Life Prediction and G03.93 on Statistics. Many of these standards are supplementary to and referenced by the testing standards for evaluating the performance of photovoltaic modules developed by Committee E44.

Impact of ASTM Standards on the Solar Energy Industry

With the development of the standards described above, the solar energy conversion industry has worked to develop fair and accurate means of measuring the performance of their systems, progress in improving the efficiency and durability of their products, and bolster consumer confidence in the quality of their products. In particular, radiometric standards contribute to a better understanding of the resources available to conversion systems and to developing and improving computer models of the geographical distribution of solar resources for areas where measured solar data are not available. ASTM standards support the present and future deployment of solar conversion systems, which help to reduce global warming, foster independence from fossil fuels and establish a sustainable energy economy for the long-term good of the planet. //

References
1. See http://www.pmodwrc.ch/pmod.php?topic=ipc10.
2. C. Gueymard, “Parameterized transmittance model for direct beam and circumsolar spectral irradiance.” Solar Energy 71(5),
p. 325–346 (2001).
3. C. Gueymard, D. Myers, and K. Emery, “Proposed reference irradiance spectra for solar energy systems testing.” Solar Energy 73(6),
p. 443–467 (2002).