On the uncertainty of energetic impact on the yield of different PV technologies due to varying spectral irradiance

Varying spectral irradiance affects performance of Photovoltaic (PV) modules depending on their band gap and spectral response. Therefore, energy rating or yield prediction procedures should take into account the irradiance that is effectively available for a specific PV technology. This effective irradiance differs from broadband solar resource information first due to the different spectral sensitivity of PV devices and pyranometers, and second due to the difference between real and reference spectral distribution. The combined difference, also referred to as 'spectral impact', is to date considered in form of fixed spectral gain or loss factors applied to the broadband solar resource - if it is considered at all. Values for the monthly or annual spectral impact have been reported for several locations; however without estimates of their uncertainty. This article addresses the question of how large the uncertainty of the spectral impact is. Contributions to the uncertainty of the spectral impact are discussed, with focus on the method of determining the spectral impact by means of measured spectral irradiance and calculated spectral mismatch factor (MM). Using a simplified procedure, the uncertainty of spectral impact was estimated from the average spectral mismatch uncertainty. It was found to be of the same magnitude as the spectral impact itself, and dependent on the PV technology. For five single-junction PV technologies, amorphous silicon, cadmium telluride, standard crystalline silicon, high-efficiency crystalline silicon and a wide-band-gap CIGS technology, the estimated uncertainty values were 1.8%, 1.4%, 0.9%, 1.0% and 1.2%, respectively. The results were obtained from a specific measurement campaign conducted from 01.06.2010 to 31.12.2013 in Freiburg im Breisgau, Germany, but the order of magnitude is considered to be typical. In order to validate the simplified uncertainty estimation method, a sensitivity analysis was carried out to evaluate the influences of calibration and drift of the spectroradiometer, high-angle-of-incidence conditions, the available wavelength range of the spectroradiometer and results obtained with different spectroradiometers. With the smaller wavelength range of 350-1050 nm, which is a typical range for various instruments used in many scientific publications, the calculated spectral impact was significantly smaller. The other influences were found to affect the result only within the estimated uncertainty limits. The uncertainty values presented here should be considered lower limits until they are backed up by further analysis