Solar Spectral Irradiance under Clear and Cloudy Skies: Measurements and a Semiempirical Model

In a combined effort, the Centre for Solar Energy and Hydrogen Research in Germany and the Solar Energy Research Institute in the United States analyzed several thousand measurements of the solar spectral irradiance recorded at four sites. The goal was to develop a semiempirical model that describes the total solar spectral irradiance for clear and cloudy sky conditions based on readily available input data. To investigate how the spectral transmission of clouds deviates from an assumed neutral density filter, the measured spectra are compared with simulated clear-sky spectra. A correlation is established between the cloud thickness and the relatively higher transmission of clouds in the ultraviolet and blue region of the solar spectrum. Using this approach, a semiempirical model is proposed based solely on global and diffuse broadband irradiance measurements, precipitable water-vapor data, and the sun's position. The model, called SEDESI, is applied to calculate spectral irradiance in short time steps (30 or 60 minutes) that are needed for either the design and performance analysis of spectrally selective solar energy conversion systems or to predict daylight availability. The results are also applicable to current climate research areas such as validating radiative transfer codes.