Soiling of solar collector surfaces affects the efficiency of solar power plants. Dust, sand and other dirt particles that adhere to the mirrors reduce the global power generation of solar systems by at least 3%-4%, despite regular cleaning. Depending on the region, the influence can be significantly stronger and it is estimated that global losses will increase to 4-7% by 2023. This development is related to the growing number of PV power plants in countries like India and China, which are even more affected by soiling. Even in Germany, an average yield reduction of around 4% is to be expected, among other things due to a larger proportion of PV systems on roofs, that are cleaned less frequently and subject to urban soiling influences such as pollen or exhaust gases.
At DLR, scientists from the Solar Energy Meteorology group are developing methods to measure soiling for CSP and PV collectors and receivers. In addition, our scientists develop models to derive soiling and dew formation from other meteorological data. Soiling data is necessary to determine how often and in what way the collectors should be cleaned in order to optimize cleaning strategies, where the basic principle is to clean as often as necessary and as little as possible. The methods and models serve to keep the efficiency of the mirrors as high as possible in all soiling conditions. The DLR has access to various measurement locations in representative climates, including the Plataforma Solar de Almería (PSA; owned by the Spanish research center CIEMAT), the Jülich solar tower and a network of meteorological measurement stations in the MENA region (Middle East and North Africa). The stations of the enerMENA network are operated by DLR in cooperation with local partners.
In order to measure the soiling of PV modules, CSP mirrors and absorber tubes, the DLR develops, characterizes and tests methods and devices. For example, with the help of the TraCS measuring device, the degree of soiling of four CSP test mirrors (e.g. equipped with different anti-soiling coating candidates) can be compared in high time resolution under natural soiling conditions. Additionally, laboratory and hand-held devices are available for reference measurements and tailor-made measurement campaigns for CSP reflectors and absorbers.
For PV-specific investigations, reference cells, modules and commercial sensors are used to measure soiling. Panel temperatures, irradiance components, environmental parameters and the solar spectrum are continuously measured at PSA. Among other things, a tiltable test bench for PV modules of different technologies is available on CIEMAT's PSA. There, I-V curves as well as the power outputs of cleaned and identical, soiled modules are measured for up to 12 individual modules to facilitate in-depth soiling and performance modelling activities.
In addition, DLR is working on automated methods for airborne, soiling measurements of entire PV systems and CSP solar fields with the help of drones. Knowing the local soiling distribution can help streamline cleaning instructions and enable more accurate performance modeling. The use of drones to measure soiling reduces the workload in power plant operation and at the same time increases the measurement coverage. A drone-based soiling measurement can also improve IR defect detection in PV power plants.
The methods described are used, among other things, as a database for modeling activities, to test anti-soiling coatings in outdoor conditions or to validate new types of soiling sensors.
In order to derive the soiling rate (accumulation of soiling over time) on the basis of more broadly available parameters, a historic database of soiling rate and relevant meteorological parameters (aerosol concentration, wind, soil moisture, sand traps, dew, aerosol optical depth (AOD) and many others), , is used. The aim of the physical model is to be able to derive soiling characteristics for locations without direct soiling measurements from meteorological data. The coupling of the model to numerical weather and dust prediction models enables the the development of soiling forecasts and -maps on a more global scale.
In order to develop and optimize cleaning strategies, an environment for simulating power plant performance in dependency on cleaning processes and soiling data is available at DLR. These simulations are also used to realistically consider soiling in yield analysis studies.
DLR is active in measurement and knowledge exchange networks for the topic of soiling in photovoltaics and CSP (PVQAT Task Group 12, PV CAMPER, SolarPACES Task III, IEA PVPS Task 16 "Solar Resources") and participates in the development of guidelines and standards for soiling measurement.