Irradiance maps applied for the performance assessment of pv systems: A case study for the German Federal State of Saxony

For the estimation of the expected annual energy yield and the month by month check of a PV system’s performance, methods based on irradiance maps published by weather services, both general or dedicated to solar energy application, are in use. Examples for these types of information for Germany are the annual and monthly radiation maps as published by the German Weather Service DWD or the data bank of hourly irradiance data with continuous spatial coverage prepared by the University of Oldenburg. To assess the validity of these data sets for the aforementioned tasks, a case study for a region covering the German federal state of Saxony is performed using data for the year 2005. For this region sets of measured irradiance data from stations operated by the German Weather Service and a state-owned agro-meteorological network are available. An assessment of the end use accuracy of the irradiance data is done by a set of monthly energy yield data of grid connected PV systems. The comparison of information on radiation sums is on one hand performed by the monthly analysis of the bias and the RMS-error for the data bank versus the ground station data. For an additional inter- comparison of the different data sources, the annual maps presenting the estimations of the irradiance sums for 2005 are analyzed, giving information of data accuracy with respect of the spatial structure of the irradiance field. The assessment of the end use accuracy of the data is - for the data generated in Oldenburg - done via the estimation of the system yield using a system simulation as developed within the PVSAT-2 project.