Analysis of spectral irradiance variation in northern Europe using average photon energy distributions

One major factor affecting the energy yield of photovoltaic modules is the spectral distribution of incident solar radiation. As spectral irradiance data is scarce, this study provides further documentation of recorded spectra at tilt angle 30°– 45°over a period from one to several years, with the resulting distributions of average photon energy (APE) in the 350–1050 nm wavelength range, from five locations in northern Europe. The results show a general trend of higher monthly APE values in summer and lower values in winter, with more pronounced APE variation at increasing latitude. Compared to the reference APE value of 1.88 eV, the largest variation in monthly APE is seen for the northernmost location of Grimstad, Norway, ranging from 1.82 eV to 1.93 eV between January and July with an annual average APE of 1.90 eV. The smallest variation is found for Merklingen, Germany, ranging from 1.86 eV to 1.88 eV between March and July, with an annual average APE of 1.86 eV. Comparing the annual average APE values of the various locations, the study shows a slightly blue-shifted spectrum for Berlin, Enschede and Grimstad, whereas Merklingen experiences a slightly red-shifted spectrum and the APE at Utrecht is similar to the standard reference spectrum. The simulations through SMARTS show air mass, water vapor and aerosols as the major parameters affecting the spectrum. During the winter months, distinct contributions from both clear and cloudy sky conditions result in a bi-modal APE distribution for all locations, which is not observed during the summer months. Analysis of APE demonstrates different site-specific behaviors, even though all sites are categorized in the same Köppen–Geiger (KG) climate class. These differences arise mainly due to atmospheric factors, whereas dissimilarity in albedo conditions, plane of tilt and instrumentation also have some contributions.