Fraunhofer Institute for Microstructure of Materials and Systems IMWSEvery year, the soiling of solar modules and the resulting reduction in power production causes revenue losses amounting to at least three billion euros in the solar industry. Researchers are constantly intensifying their efforts to find out how this soiling occurs and what can be done to prevent it.
Photovoltaics constitutes a growth market, with installed capacity predicted to exceed 3 TW by 2040. A significant portion of this growth will take place in countries such as China and India. Many new solar parks are also being built in desert regions, thus bringing an issue with major consequences for the efficiency of photovoltaic systems to the fore: the accumulation of contaminants on solar modules. Known as soiling, this phenomenon can drastically reduce power production.
Together with its partners, the Fraunhofer Center for Silicon Photovoltaics CSP has developed a process that can be used to simulate soiling in a laboratory. This is an important requirement for optimizing the materials used and so also the power yield, as soiling processes are physically complex and have yet to be fully analyzed and understood. The results of this process indicate that at present, soiling is reducing worldwide solar power production by at least three to four percent, which corresponds to annual revenue losses of at least three to five billion euros.
For certain regions, the focus on the development of successful countermeasures offers far more potential than exclusively concentrating on further optimizing the efficiency of crystalline solar cells — especially when you consider that the enhancements achieved in that area over the last 20 years are close to the physical optimum.
Due to the many influencing factors involved, such as unique weather, location and system specifications and surface nanoproperties, and the way these factors vary over time (e.g.
weather changes over the course of a day or year), the physics behind dust deposition and adhesion is highly complex. This problem is being systematically investigated at Fraunhofer CSP by means of open-area and laboratory tests and simulations, right down to microstructural material characterization.
So far, results have shown that dust concentration in the air is the most important influencing factor for soiling, together with rainfall frequency, as rain can clean soiled modules very effectively. Other important parameters include wind speed (which affects particle deposition mechanisms) and the photovoltaic module’s angle of inclination (soiling rates are higher on flatter surfaces). Relative humidity and dew play a particularly prominent role, as both increase dust adhesion on surfaces due to capillary forces and cementation processes. Another factor affecting deserts is that the photovoltaic modules’ glass surfaces cool down overnight and become even colder than the ambient air due to radiative cooling through the night sky. This regularly leads to the formation of dew on the module surfaces. The combination of soiling and humidity can permanently reduce the photovoltaic capacity.
Even today, there are multiple solutions for countering soiling. Soiled modules are primarily cleaned by mechanical means, for example, through manual, semi-automated or fully automated wiping or sweeping. However, this can result in scratching or abrasion that can damage the anti-reflective coating (ARC) typically used on solar modules, which negatively impacts their efficiency. Other possible consequences include corrosion or thermal shocks, because when cold water meets the hot modules, it can cause the solar cells or their glass covers to break. It can also result in the widening of microcracks.
Consequently, optimized module surfaces that do not allow dust and sand to adhere strongly to them in the first place would be a better solution. Developing these anti-soiling coatings (ASC) is another key research area at Fraunhofer CSP. In ideal circumstances, the coatings would be highly transparent, anti-reflective, long-lasting, non-toxic, suitable for application at industrial scale, cost-effective and of course, selfcleaning. In some studies, the coating reduced the soiling effect by more than 80 percent. When viewed over a longer time period, the average values currently amount to a reduction in soiling rates of between 20 to 50 percent.
Solar park operators have other options for reducing the negative effects of soiling apart from protective coatings, such as changing the modules’ angle of inclination for example. In addition, it has been shown that soiling is particularly severe at night. That means solar trackers, i.e. motorized modules that automatically align themselves with the sun, could be positioned vertically or flipped over at night in order to reduce soiling.
Another solution involves heating surfaces to prevent dew from forming. Dew formation is especially common in the period before dawn, when the relative humidity is high and the temperature of the photovoltaic modules is lower than the ambient air temperature. If the modules are heated during this period, by means of a controlled power supply to the solar cells for example, it can reduce condensation. And last, but not least, the design of and the materials used in the photovoltaic modules can also be optimized. Examples include using half-cells, which reduce the effects of partial shading on the overall performance of the module, or frameless modules, which helps to prevent the accumulation of grime at the edges.
The selection of the location also plays a decisive role. The combination of precise knowledge of meteorological data and site-specific soiling risks enables the development of optimized cleaning solutions, taking into account soiling types and deposition rates, water availability, location and system configuration.