Proposal and Evaluation of Subordinate Standard Solar Irradiance Spectra with a Focus on Air Mass Effects
Abstract
This paper introduces a concept for global tilted irradiance (GTI) subordinate standard spectra to supplement the current standard spectra used in solar photovoltaic applications, as defined in ASTM G173 and IEC60904. The proposed subordinate standard spectra correspond to atmospheric conditions and tilt angles that depart significantly from the main standard spectrum, and can be used to more accurately represent various local conditions. For the definition of subordinate standard spectra cases with an elevation of 1.5 km above sea level the question arises whether the air mass should be calculated including a pressure correction or not. This study focuses on the impact of air mass used in standard spectra, and uses data from 29 locations to examine which air mass is most appropriate for GTI and direct normal irradiance (DNI) spectra. Overall, it is found that the pressure corrected air mass of 1.5 is most appropriate for DNI spectra. For GTI a non-pressure-corrected air mass of 1.5 was found to be more appropriate. The suitability of this selection for a given site is best for mid latitudes (~35°) and elevations below 1 km. It also depends on the vertical distributions of aerosols and water vapor. Keywords: solar spectra, efficiency rating, spectral response, standardization, air mass