Proposed Reference Spectral Irradiance Standards to Improve Concentrating Photovoltaic System Design and Performance Evaluation: Preprint

The American Society for Testing and Materials (ASTM), the International Electrotechnical Commission (IEC), and the International Standards Organization (ISO) standard solar terrestrial spectra (ASTM G-159, IEC-9043, ISO 9845-1) provide standard spectra for photovoltaic performance applications. Modern terrestrial spectral radiation models and knowledge of atmospheric physics are applied to develop suggested revisions to update the reference spectra. We use a moderately complex radiative transfer model (SMARTS2) to produce the revised spectra. SMARTS2 has been validated against the complex MODTRAN radiative transfer code and spectral measurements. The model is proposed as an adjunct standard to reproduce the reference spectra. The proposed spectra represent typical clear sky spectral conditions associated with sites representing reasonable photovoltaic energy production and weathering and durability climates. The proposed spectra are under consideration by ASTM. INTRODUCTION Standard reporting conditions for photovoltaic (PV) device performance specify the standard reference spectrum, American Society for Testing and Materials (ASTM) G-159-98 [1,2]. The references contain direct beam and hemispherical (180° field of view, sky dome plus ground reflected) spectral distributions. The hemispherical spectrum is incident on a 37° tilted surface facing south. These spectra were developed in 1982,and revised in 1987. They are based on the work of Bird, Hulstrom, and Lewis [3]. The spectra were computed using the BRITE Monte-Carlo model for 122 individual wavelengths from 305 nm to 2500 nm [4]. The spectra were later extended to 4045 nm using an undocumented simple spectral model. It is not possible to reproduce these spectra, as the BRITE algorithm used seven binary coded data tapes specific to the mainframe computer used for the computations. Other issues with the current reference spectra have arisen as the terrestrial PV community have come to depend upon these spectra for evaluating technologies. The spectral data is recorded at only 122 irregularly spaced intervals. The equivalent spectral resolution is unclear. There are insufficient data below 350 nm (and none below 305 nm) to characterize the ultraviolet (UV) component, important to materials degradation. The aerosol optical depth chosen appears to be to high to represent conditions prevalent where PV, especially concentrating systems, are most likely to be deployed [5,6]. In the years since these spectra were last revised, more accurate and up-to-date knowledge of atmospheric physics and aerosol properties, and improved radiative transfer models have been developed. These models, and in particular the SMARTS2 model of Gueymard [7], can be used to address the issues above. PRESENT SPECTRAL STANDARDS The present spectral reference standard, ASTM G159-98 is based upon the 1976 U.S. Standard Atmosphere (USSA) profiles of temperature, pressure, air density, and molecular species density specified in 33 layers starting from sea level [8]. Other specified parameters are: Absolute air mass (AM) specified AM 1.5 (solar zenith angle 48.19°) at sea level Receiving surface tilted 37° from horizontal, “sun facing” (i.e., south in the northern hemisphere) Aerosol optical depth, AOD, or “turbidity” of 0.27 at 500 nm to “correspond to a sea-level visibility of 23 km”, based on the 1975 work of Shettle and Fenn [9] Constant surface albedo (reflectivity) of 0.2, assuming Lambertian reflectivity Total precipitable water vapor content = 1.42 cm Total ozone content = 0.34 atm-cm. Water vapor and ozone content of the USSA were derived by integrating the 33 layers to produce the total equivalent amounts of these constituents. These conditions in conjunction with the AM0 spectrum of Wehrli and the BRITE Monte Carlo radiative transfer model produced direct normal and total hemispherical spectral irradiance distributions tabulated in the G-159 standard. The hemispherical spectrum is widely used by the flat-plate PV community for rating modules. The integral of the hemispherical spectrum is 963 W/m rather than 1000 W/m. The value of the integrated spectral curves depends on integration technique and can vary by