Proposed reference irradiance spectra for solar energy systems testing

In 1982, the American Society for Testing and Materials (ASTM) adopted consensus standard solar terrestrial spectra (ASTM E891-82, E892-82) to provide standard spectra for photovoltaic (PV) performance applications. These spectra have been also used for other applications such as solar energy systems, fenestration, and materials degradation. These reference spectra were recomputed and the standards revised in 1987. The International Standards Organization (ISO) and International Electrotechnical Commission (IEC) adopted these spectra into spectral standards ISO 9845-1 and IEC 60904-3. These reference spectra are current as of 2002, even though they are based upon spectral solar radiation models and information on atmospheric attenuation from the 1980s. We summarize important issues concerning the definition of atmospheric parameters, spectral range, accuracy and resolution, and documentation of the standards. We suggest substantial improvements to meet the current and future needs of the various technologies using the reference spectra. Modern terrestrial spectral radiation models and better knowledge of atmospheric physics and prevailing radiometric quantities in the natural environment are used to develop suggested revisions to update the reference spectra. These revisions extend and improve the documentation of the hemispherical ('global') tilted reference spectrum with minor modifications. They also provide a more realistic clear sky direct normal spectrum for the intended applications. The revised reference spectra would include more detailed and reliable spectral information from the UV (280 nm) to the near infrared (4000 nm). The amplitude of the proposed hemispherical tilted reference spectrum differs very little from the current standard hemispherical tilted spectrum. Conversely, the proposed direct normal reference spectrum is substantially more energetic than the current standard spectrum, and is typical of a cleaner atmosphere where solar applications can be deployed more advantageously.