Measurement of spectral-spatial distribution of global irradiance

Accurate determination of power output and energy yield is crucial for profitable operation of PV power plants. Usually the level of global irradiance is used to determine resp. actual power output. However, this method does consider neither incidence angle nor spectral effects in detail. Usually, a rather vague "performance ratio" is used instead. In order to reduce that uncertainty, a simulation program to follow every possible ray from the sky sphere through the module encapsulation into the cell has been set-up, considering reflection at every interface, transmittivity, dispersion and angular (resp. spatial) effects. However, this program requires as input the spatial-spectral distribution of irradiance to carry out accurate simulations. To provide that data, the spatial distribution of the spectra and luminescence has been measured in an outdoor laboratory via a sky-scanner and a spectrometer. The measured irradiance distribution over the sky-sphere has been compared to standard models ([3],[4]) including spectral distribution; moreover the measured data has been used to calculate the effective irradiance received from different modules during a period of one year. The calculations of the resulting, power outputs, and energy yield shall be compared to actual measurements. The results of this work will become even more relevant while emerging thin film technologies such as a-Si, CdTe and CIGS have rather different spectral responses which cannot be analyzed and compared using the conventional global irradiance approach.