Direct Beam and Diffuse Spectral Irradiance Measurements in a Nordic Country Analyzed With the Average Photon Energy Parameter

One year of spectrally resolved direct normal irradiance (DNI), diffuse horizontal irradiance (DfHI) and global horizontal irradiance (GHI) data measured in Roskilde, Denmark are analyzed in terms of the average photon energy (APE). We show that the APE of the GHI component is characterized by spectral distributions with low standard deviations, which is consistent with the findings of previous authors. In contrast, the APE of the DfHI and DNI components show spectral distributions with higher standard deviations, which suggests that a given APE value for DNI or DfHI can represent a wider array of spectral distributions. Finally, it is shown that the DfHI APE is characterized by high energy spectral distributions, which has significance in the case of building integrated photovoltaic (BIPV) applications where the glass superstrate is colored or textured in a way that it reflects blue light.