SUNCALCULATOR: A program to calculate the angular and spectral distribution of direct and diffuse solar radiation

Understanding the impact of realistic illumination conditions on solar module performance is of utmost importance for manufacturers and customers. Standard testing conditions define only one spectrum with normal incidence to measure the performance of solar modules and thus have only limited value for comparing different solar modules. Considering the angular and spectral distribution of photons over a year is vital for improving modules under more realistic conditions, e.g. by using ray tracing to optimize design and materials. Furthermore, the angular and spectral distribution of solar irradiance directly impacts on optimizing the module orientation of fixed solar systems. In most cases, however, measured data on irradiance is constraint to integral values of global, direct and diffuse radiation. In this work, we calculate the angular and spectral distribution of direct and diffuse irradiation based on measured time-resolved integral values for diffuse and direct irradiation. The procedure is implemented in the SUNCALCULATOR software.