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ASTM G173 – 03

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ASTM G173 – 03 Designation: G 173 – 03 Standard Tables for Reference Solar Spectral Irradiances: Direct Normal and Hemispherical on 37° Tilted Surface1 This standard is issued under the fixed designation G 173; the number immediately following the designation indicates the year...
ASTM G173 – 03
Designation: G 173 – 03 Standard Tables for Reference Solar Spectral Irradiances: Direct Normal and Hemispherical on 37° Tilted Surface1 This standard is issued under the fixed designation G 173; the number immediately following the designation indicates the year of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscript epsilon (e) indicates an editorial change since the last revision or reapproval. INTRODUCTION A wide variety of solar spectral energy distributions occur in the natural environment and are simulated by artificial sources during product, material, or component testing. To compare the relative optical performance of spectrally sensitive products a reference standard solar spectral distribution is required. These tables replace ASTM standard G 159, which has been withdrawn. The solar spectral energy distribution presented in this standard are not intended as a benchmark for ultraviolet radiation in weathering exposure testing of materials. The spectra are based on version 2.9.2 of the Simple Model of the Atmospheric Radiative Transfer of Sunshine (SMARTS) atmospheric transmission code (1,2).2 SMARTS uses empirical parameterizations of version 4.0 of the Air Force Geophysical Laboratory (AFGL) Moderate Resolution Transmission model, MODTRAN (3,4) for some gaseous absorption processes, and recent spectroscopic data for others. An extraterrestrial spectrum differing only slightly from the extraterrestrial spectrum in ASTM E 490 is used to calculate the resultant spectra (5). The hemispherical tilted spectrum is similar to the hemispherical spectrum in use since 1987, but differs from it because: (1) the wavelength range for the current spectrum has been extended deeper into the ultraviolet; (2) uniform wavelength intervals are now used; (3) more representative atmospheric conditions are represented,; and (4) SMARTS Version 2.9.2 has been used as the generating model. For the same reasons, and particularly the adoption of a remarkably less turbid atmosphere than before, significant differences exist in the reference direct normal spectrum compared to previous versions of this standard. The input parameters used in conjunction with SMARTS for the selected atmospheric conditions are tabulated. The SMARTS model and documentation are available as an adjunct to this standard. 1. Scope 1.1 These tables contain terrestrial solar spectral irradiance distributions for use in terrestrial applications that require a standard reference spectral irradiance for hemispherical solar irradiance (consisting of both direct and diffuse components) incident on a sun-facing, 37° tilted surface or the direct normal spectral irradiance. The data contained in these tables reflect reference spectra with uniform wavelength interval (0.5 na- nometer (nm) below 400 nm, 1 nm between 400 and 1700 nm, an intermediate wavelength at 1702 nm, and 5 nm intervals from 1705 to 4000 nm). The data tables represent reasonable cloudless atmospheric conditions favorable for photovoltaic (PV) energy production, as well as weathering and durability exposure applications. 1.2 The 37° slope of the sun-facing tilted surface was chosen to represent the average latitude of the 48 contiguous United States. A wide variety of orientations is possible for exposed surfaces. The availability of the SMARTS model (as an adjunct to this standard) used to generate the standard spectra allows users to evaluate differences relative to the surface specified here. 1.3 The air mass and atmospheric extinction parameters are chosen to provide (1) historical continuity with respect to previous standard spectra, (2) reasonable cloudless atmo- spheric conditions favorable for photovoltaic (PV) energy production or weathering and durability exposure, based upon modern broadband solar radiation data, atmospheric profiles, and improved knowledge of aerosol optical depth profiles. In nature, an extremely large range of atmospheric conditions can be encountered even under cloudless skies. Considerable departure from the reference spectra may be observed depend- ing on time of day, geographical location, and changing atmospheric conditions. The availability of the SMARTS model (as an adjunct to this standard) used to generate the 1 These tables are under the jurisdiction of ASTM Committee G03 on Weathering and Durability and is the direct responsibility of Subcommittee G03.09 on Radiometry. Current edition approved Jan. 10, 2003. Published April 2003. 2 The boldface numbers in parentheses refer to the list of references at the end of this standard. 1 Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States. standard spectra allows users to evaluate spectral differences relative to the spectra specified here. 2. Referenced Documents 2.1 ASTM Standards: E 490 Solar Constant and Air Mass Zero Solar Spectral Irradiance Tables3 E 772 Terminology Relating to Solar Energy Conversion4 G 113 Terminology Relating to Natural and Artificial Weathering Tests of Nonmetallic Materials5 3. Terminology 3.1 Definitions—Definitions of most terms used in this specification may be found in Terminology E 772. 3.2 The following definition differs from that in Terminol- ogy E 772, representing information current as of this revision. 3.2.1 solar constant—the total solar irradiance at normal incidence on a surface in free space at the earth’s mean distance from the sun. (1 astronomical unit, or AU = 1.496 3 1011 m). 3.2.1.1 Discussion—The solar constant is now known within about 61.5 W·m-2. Its current accepted values are 1366.1 W·m-2 (ASTM E 490) or 1367.0 W·m-2 (World Meteo- rological Organization, WMO), and are subject to change. Due to the eccentricity of the earth’s orbit, the actual extraterrestrial solar irradiance varies by 63.4 % about the solar constant as the earth-sun distance varies through the year. Throughout this standard the solar constant is defined as 1367.0 W·m-2. 3.3 Definitions of Terms Specific to This Standard: 3.3.1 aerosol optical depth (AOD)—the wavelength- dependent total extinction (scattering and absorption) by aero- sols in the atmosphere. This optical depth (also called “optical thickness”) is defined here at 500 nm. 3.3.1.1 Discussion—See Appendix X1. 3.3.2 air mass zero (AM0)—describes solar radiation quan- tities outside the Earth’s atmosphere at the mean Earth-Sun distance (1 Astronomical Unit). See ASTM E 490. 3.3.3 integrated irradiance El1-l2—spectral irradiance inte- grated over a specific wavelength interval from l1 to l2, measured in W·m-2; mathematically: El12l2 5 * l1 l2 El dl (1) 3.3.4 solar irradiance, hemispherical EH—on a given plane, the solar radiant flux received from within the 2p steradian field of view of a tilted plane from the portion of the sky dome and the foreground included in the plane’s field of view, including both diffuse and direct solar radiation. 3.3.4.1 Discussion—For the special condition of a horizon- tal plane the hemispherical solar irradiance is properly termed global solar irradiance, EG. Incorrectly, global tilted, or total global irradiance is often used to indicate hemispherical irradiance for a tilted plane. In case of a sun-tracking receiver, this hemispherical irradiance is commonly called global nor- mal irradiance. The adjective global should refer only to hemispherical solar radiation on a horizontal, not a tilted, surface. 3.3.5 solar irradiance, spectral El—solar irradiance E per unit wavelength interval at a given wavelength l (unit: Watts per square meter per nanometer, W·m-2·nm-1): El 5 dE dl (2) 3.3.6 spectral interval—the distance in wavelength units between adjacent spectral irradiance data points. 3.3.7 spectral passband—the effective wavelength interval within which spectral irradiance is allowed to pass, as through a filter or monochromator. The convolution integral of the spectral passband (normalized to unity at maximum) and the incident spectral irradiance produces the effective transmitted irradiance. 3.3.7.1 Discussion—Spectral passband may also be referred to as the spectral bandwidth of a filter or device. Passbands are usually specified as the interval between wavelengths at which one half of the maximum transmission of the filter or device occurs, or as full-width at half-maximum, FWHM. 3.3.8 spectral resolution—the minimum wavelength differ- ence between two wavelengths that can be identified unam- biguously. 3.3.8.1 Discussion—In the context of this standard, the spectral resolution is simply the interval, Dl, between spectral data points, or the spectral interval. 3.3.9 total ozone—the depth of a column of pure ozone equivalent to the total of the ozone in a vertical column from the ground to the top of the atmosphere (unit: atmosphere-cm or atm-cm). 3.3.10 total precipitable water—the depth of a column of water (with a section of 1 cm2) equivalent to the condensed water vapor in a vertical column from the ground to the top of the atmosphere (unit: cm or g/cm2). 3.3.11 wavenumber—a unit of frequency, n, in units of reciprocal centimeters (symbol cm-1) commonly used in place of wavelength, l (units of length, typically nanometers). To convert wavenumber to nanometers, l nm = 1 · 107/ n cm-1. See X1.2. 4. Significance and Use 4.1 Absorptance, reflectance, and transmittance of solar energy are important factors in material degradation studies, solar thermal system performance, solar photovoltaic system performance, biological studies, and solar simulation activities. These optical properties are normally functions of wavelength, which require the spectral distribution of the solar flux be known before the solar-weighted property can be calculated. To compare the relative performance of competitive products, or to compare the performance of products before and after being subjected to weathering or other exposure conditions, a reference standard solar spectral distribution is desirable. 4.2 These tables provide appropriate standard spectral irra- diance distributions for determining the relative optical perfor- mance of materials, solar thermal, solar photovoltaic, and other systems. The tables may be used to evaluate components and materials for the purpose of solar simulation where either the 3 Annual Book of ASTM Standards, Vol 15.03. 4 Annual Book of ASTM Standards, Vol 12.02. 5 National Renewable Energy Lab., 1617 Cole Blvd., MS-3411, Golden, CO 80401. G 173 – 03 2 direct or the hemispherical (that is, direct beam plus diffuse sky) spectral solar irradiance is desired. However, these tables are not intended to be used as a benchmark for ultraviolet radiation used in indoor exposure testing of materials using manufactured light sources. 4.3 The total integrated irradiances for the direct and hemi- spherical tilted spectra are 900.1 W·m-2 and 1000.4 W·m-2, respectively. Note that, in PV applications, no amplitude adjustments are required to match standard reporting condition irradiances of 1000 W·m-2 for hemispherical irradiance. 4.4 Previously defined global hemispherical reference spec- trum (G 159) for a sun-facing 37°-tilted surface served well to meet the needs of the flat plate photovoltaic research, devel- opment, and industrial community. Investigation of prevailing conditions and measured spectra shows that this global hemi- spherical reference spectrum can be attained in practice under a variety of conditions, and that these conditions can be interpreted as representative for many combinations of atmo- spheric parameters. Earlier global hemispherical reference spectrum may be closely, but not exactly, reproduced with improved spectral wavelength range, uniform spectral interval, and spectral resolution equivalent to the spectral interval, using inputs in X1.4. 4.5 Reference spectra generated by the SMARTS Version 2.9.2 model for the indicated conditions are shown in Fig. 1. The exact input file structure required to generate the reference spectra is shown in Table 1. 4.6 The availability of the adjunct standard computer soft- ware for SMARTS allows one to (1) reproduce the reference spectra, using the above input parameters; (2) compute test spectra to attempt to match measured data at a specified FWHM, and evaluate atmospheric conditions; and (3) compute test spectra representing specific conditions for analysis vis-à- vis any one or all of the reference spectra. 4.7 Differences from the previous standard spectra (G 159) can be summarized as follows: 4.7.1 Extended spectral interval in the ultraviolet (down to 280 nm, rather than 305 nm), 4.7.2 Better resolution (2002 wavelengths, as compared to 120), 4.7.3 Constant intervals (0.5 nm below 400 nm, 1 nm between 400 and 1700 nm, and 5 nm above), 4.7.4 Better definition of atmospheric scattering and gas- eous absorption, with more species considered, 4.7.5 Better defined extraterrestrial spectrum, 4.7.6 More realistic spectral ground reflectance, and 4.7.7 Lower aerosol optical depth, yielding significantly larger direct normal irradiance. 5. Technical Bases for the Tables 5.1 These tables are modeled data generated using an air mass zero (AM0) spectrum based in part on the extraterrestrial spectrum of Kurucz (5), the 1976 U.S. Standard Atmosphere (6), the Shettle and Fenn Rural Aerosol Profile (7), the SMARTS radiative transfer code, version 2.9.2, and associated input data files. 5.2 In order to provide spectral data with a uniform spectral step size and improved spectral resolution, the AM0 spectrum FIG. 1 Plot of Direct Normal Spectral Irradiance (Solid Line) and Hemispherical Spectral Irradiance on 37° Tilted Sun-Facing Surface (Dotted Line) Computed Using Smarts Version 2.9.2 Model With Input File in Table 1 G 173 – 03 3 used in conjunction with SMARTS to generate the terrestrial spectrum is slightly different from the ASTM extraterrestrial spectrum, ASTM E 490. Because ASTM E 490 and SMARTS both use the Kurucz data, the SMARTS and E 490 spectra are in good agreement though they do not have the same spectral interval step sizes, spectral interval centers, or spectral resolu- tion. 5.3 The 1976 U.S. Standard Atmosphere (USSA) is used to provide documented atmospheric properties and concentra- tions of absorbers. However, some newly documented (and relatively minor) absorbers are taken into consideration in the present standard spectra. See X1.3. 5.4 The SMARTS model code and documentation is avail- able from the NREL (National Renewable Energy Lab) website (www.nrel.gov). 5.5 These terrestrial solar spectral data are based on the work of Gueymard (1,2) and Gueymard et al. (8). Previously defined reference spectra were based on the work of Bird, Hulstrom, and Lewis (9). The current spectra reflect current (as of 2002) improved knowledge of gaseous absorption, atmo- spheric aerosol optical properties, transmission properties, and radiative transfer modeling. 5.6 The terrestrial solar spectra in the tables have been computed with a spectral resolution equivalent to that of the wavelength interval. Parameterizations in the SMARTS2 model are based on high resolution (2 cm-1) MODTRAN (2,3,10,11) results subsequently “degraded” or smoothed to the SMARTS2 model wavelength interval. 5.6.1 Discussion—This approach emulates the procedure of measuring spectral data with a monochromator by using the wavelength interval equivalent to the spectral passband of the instrument. 5.7 To represent favorable conditions for PV energy pro- duction and exposure conditions for weathering and durability testing, sites in the National Solar Radiation Data Base (13) with annual daily average direct normal solar radiation exceed- ing 6 kWh·m-2 (or 21.6 MJ·m-2) per day were analyzed. The mean aerosol optical depth at 500 nm for these sites was determined to be 0.085. A very slightly smaller AOD of 0.084 results in a hemispherical tilted spectrum integrating to 1000.4 W/m2, nearly exactly the irradiance used in photovoltaic standard reporting conditions. See X1.2. 5.8 Previous reference spectra were generated using a wavelength-independent albedo of 0.2. The present standards utilize measured wavelength-dependent reflectance data, rep- resenting light sandy soil of the southwest U.S. See Fig. X1.1. 5.9 The direct normal spectrum includes the circumsolar spectral irradiance that would be measured with a collimated spectroradiometer or pyrheliometer with a 5.8° field of view (aperture half-angle of 2.9°) representing common commer- cially available radiometers. TABLE 1 SMARTS Version 2.9.2 Input File to Generate the Reference Spectra Card ID Value Parameter/Description/Variable Name 1 ’ASTM_G 173_Std_Spectra’ Header 2 1 Pressure input mode (1 = pressure and altitude): ISPR 2a 1013.25 0. Station Pressure (mb) and altitude (km): SPR, ALT 3 1 Standard Atmosphere Profile Selection (1 = use default atmosphere): IATM1 3a ’USSA’ Default Standard Atmosphere Profile: ATM 4 1 Water Vapor Input (1 = default from Atmospheric Profile): IH2O 5 1 Ozone Calculation (1 = default from Atmospheric Profile): IO3 6 1 Pollution level mode (1 = standard conditions/no pollution): IGAS (see X1.3) 7 370 Carbon Dioxide volume mixing ratio (ppm): qCO2 (see X1.3) 7a 1 Extraterrestrial Spectrum (1 = SMARTS/Gueymard): ISPCTR 8 ’S&F_RURAL’ Aerosol Profile to Use: AEROS 9 0 Specification for aerosol optical depth/turbidity input (0 = AOD at 500 nm): ITURB 9a 0.084 Aerosol Optical Depth at 500 nm: TAU5 10 38 Far field Spectral Albedo file to use (38= Light Sandy Soil): IALBDX 10b 1 Specify tilt calculation (1 = yes): ITILT 10c 38 37 180 Albedo and Tilt variables-Albedo file to use for near field, Tilt, and Azimuth: IALBDG, TILT, WAZIM 11 280 4000 1.0 1367.0 Wavelength Range-start, stop, mean radius vector correction, integrated solar spectrum irradiance: WLMN, WLMX, SUNCOR, SOLARC 12 2 Separate spectral output file print mode (2 = yes): IPRT 12a 280 4000 .5 Output file wavelength-Print limits, start, stop, minimum step size: WPMN, WPMX, INTVL 12b 2 Number of output variables to print: IOTOT 12c 8 9 Code relating output variables to print (8 = Hemispherical tilt, 9 = direct normal + circumsolar): OUT(8), OUT(9) 13 1 Circumsolar calculation mode (1 = yes): ICIRC 13a 0 2.9 0 Receiver geometry-Slope, View, Limit half angles: SLOPE, APERT, LIMIT 14 0 Smooth function mode (0 = none): ISCAN 15 0 Illuminance calculation mode (0 = none): ILLUM 16 0 UV calculation mode (0 = none): IUV 17 2 Solar Geometry mode (2 = Air Mass): IMASS 17a 1.5 Air mass value: AMASS G 173 – 03 4 5.10 The profile for the United States Standard Atmosphere of 1976 results in a carbon dioxide volume mixing ratio of 330 ppm. It’s current (2001) measured value is about 370 ppm. The latter is the value used in the computation of the reference spectra, as noted in Table 1. 5.11 The selected air mass value of 1.5 for a plane parallel atmosphere above a flat earth corresponds to a zenith angle of 48.19°. The SMARTS2 computation of air mass accounts for atmospheric curvature and the vertical density profile of molecules, which results in a solar zenith angle of 48.236°, or an equivalent plane parallel atmosphere air mass of 1.50136. The angle of incidence computed by SMARTS2 for the direct beam irradiance incident on a 37°-tilted plane facing the sun is thus 11.236°. 6. Solar Spectral Irradiance 6.1 Table 2 presents the reference spectral irradiance data for direct normal spectral irradiance within a 5.8° field of view centered on the sun; and hemispherical spectral solar irradiance on a plane tilted at 37° toward the sun, for the conditions specified in Table 1. 6.2 The spectral table contains: 6.2.1 Direct normal spectral irradiance in the wavelength range 280 to 4000 nm. 6.2.2 Hemispherical solar spectral irradiance incident on an sun-facing plane tilted to 37° from the horizontal in t
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