Objective and Background
The PerduS project aims on the forecast of photovoltatic energy yield reduction due to Saharan dust events over Germany. Soiling of solar panels contributes to reduction of incoming solar radiation and currently cannot be forecasted by NWP models.
A soiling station together with several particle measuring devices and a disdrometer was set up at Campus North of Karlsruhe Institute of Technology. The soiling station consists of two frameless CdTe solar panels deployed with an elevation angle of 16 degree with an uncertainty of 0.1 degree. The panel backside temperatures for both panels were measured with a temperature sensor. One of the panels was cleaned daily around noon and served as reference. The measurement error of the soiling ratio was estimated with 0.001 (0.1%).
The particle mass concentration for large particles up to 80 µm was estimated with a Sigma-2. The temporal resolution was 1 week. Smaller particles up to 32 µm were measured with a Grimm-OPC and a temporal resolution of 1 min. The total particle mass amount was probed weekly by a wet/dry sampler and analyzed in the lab gravimetrically. Rain events were measured with the disdrometer Parsivel-2. Due to its optical principle it can detect even the weakest rain events which cannot be detected by common tipping bucket rain gauges.
Measurements over approximately one year were performed in 2018. In general for typical conditions, the daily change of the soiling ratio was 0.001 which is about the same as the measurement error. Regular rain events cleaned the solar panels often preventing the accumulation of dust. Hence even for 1 week of dry weather the reduction of incoming radiation due to soiling was less than 1%. However during the pollen season of tree pollen this reduction increased up to 0.5% per day which could accumulate up to 3 - 3.5 % within a week.
The mass concentration of large particles shows an annual cycle with values below 5 µgm-3 in winter and between 10 and 15 µgm-3 in summer. During the tree pollen season the values increased to 20 µg m-3 which corresponds to the double value of overall conditions during spring time (10 µgm-3). PM10 mass concentrations are typical below 20 µgm-3. They show a slight annual cycle with lower values in winter and higher values in summer. PM32 mass concentration includes only some pollen due to its size. For instance birch tree pollen have a diameter of ~22 µm and pine tree pollen ~65 µm. During the season of the tree pollen, the PM32 mass concentration increased up to 120 µgm-3. This increase is well correlated with increasing soiling of the solar panels and hence stronger reduction of solar power yield.
Most rain events were sufficiently intense and long and hence the solar panels were cleaned completely within on day. A few very small rain events could be observed which lead only to a partial cleaning. Furthermore several dew-fog events and events were the panels showed frost patterns were observed.