Solar Radiation

In this chapter the physical processes and the composition of the sun leading to the sun as radiation source will be described. From that the properties of extra-terrestrial irradiation close to the earth are derived. Depending on the orientation of a surface different amounts of irradiation is received (horizontal, tilted, tracked). The atmosphere is interacting with this radiation through absorption and scattering depending on a number of parameters (aerosols, clouds) and modifies the incoming terrestrial radiation on the surface of the earth. For collectors not only the beam radiation directly from the sun, but also diffuse components from sky and ground reflected albedo are important. Short-wave diffuse sky radiation depends on the clearness of the sky and sun position and is in general anisotropic. Models for the angular distribution of diffuse irradiation describe this behaviour. Statistical methods exist to derive the diffuse fraction of global irradiation based on the clearness index. The basic methods for measuring solar radiation for different locations are presented, with terrestrial methods and satellite measurements. Using long-term time series representative meteorological data sets for planning purposes can be constructed.