• Energy Research

The accumulation of soiling on photovoltaic modules and on the mirrors of concentrating solar power systems causes non-negligible energy losses with economic consequences. These challenges can be mitigated, or even prevented, through appropriate actions if the magnitude of soiling is known. Particle counting analysis is a common procedure to characterize soiling, as it can be easily performed on micrographs of glass coupons or solar devices that have been exposed to the environment. Particle counting does not, however, yield invariant results across institutions. The particle size distribution analysis is affected by the operator of the image analysis software and the methodology utilized. The results of a round-robin study are presented in this work to explore and elucidate the uncertainty related to particle counting and its effect on the characterization of the soiling of glass surfaces used in solar energy conversion systems. An international group of soiling experts analysed the same 8 micrographs using the same open-source ImageJ software package. The variation in the particle analyses results were investigated to identify specimen characteristics with the lowest coefficient of variation (CV) and the least uncertainty among the various operators. The mean particle diameter showed the lowest CV among the investigated characteristics, whereas the number of particles exhibited the largest CV. Additional parameters, such as the fractional area coverage by particles and parameters related to the distribution's shape yielded intermediate CV values. These results can provide insights on the magnitude inter-lab variability and uncertainty for optical and microscope-based soiling monitoring and characterization.

The use of image analysis has often been suggested as a practical way to monitor the soiling accumulated on the surfaces of solar energy conversion devices. Indeed, the deposited soiling particles can be counted and characterized to calculate the area they cover, and this area can be converted into an energy loss. However, several particle counting methodologies exist and can lead to dissimilar results. This work focuses on the role of thresholding, an essential step where particles are distinguished from a background based on the pixel brightness. Sixteen automatic thresholding methods are assessed using 13 200 micrographs of glass coupons soiled at nine locations globally. In low‐to‐intermediate soiling conditions, the “Triangle” method is found to return the minimum coefficient of variation and a mean deviation closer to zero. On the other hand, methods assuming a bimodal distribution of pixel brightness underestimate the area coverage. In addition, since soiling can be unevenly distributed over a surface, different loss estimations can be returned when the same image analysis process is used on different spots on a sample's surface. For these reasons, image analysis should be repeated at multiple locations on each investigated surface.

Soiling of photovoltaic (PV) modules can significantly reduce their energy yield by reflecting or absorbing the incident light and is of great importance for operation and maintenance of PV systems in dusty environments. In this paper, we examine the influence of dew—which is formed on PV modules during night and early morning hours—on soiling processes. Outdoor experiments were performed in desert conditions in Qatar using unheated and heated glass samples supplied with different heating powers, as well as outdoor microscopy studies. In addition, laboratory soiling experiments were performed using different dust types, controlled dust removal by centrifugal forces at different relative humidity levels, and defined dew cycles. Results from both the field tests and laboratory soiling experiments showed a significant impact of dew on particle adhesion, removal, and corresponding soiling rates, which was independent of the dust type. Microstructural investigations using electron microscopy were performed to determine the cause of increased particle adhesion. Heating of glass surfaces during the night can significantly reduce the processes of cementation, particle caking, and capillary aging and, thus, reduce soiling. This study indicates that the prevention of condensation can be a method to mitigate soiling.

AbstractHigh levels of airborne dust, frequent dust storms and infrequent rain events are some of the reasons why soiling can drastically reduce the energy yield of photovoltaic modules in desert areas. There are ongoing and increasing efforts to identify appropriate and economically feasible strategies that can be used to mitigate soiling in deserts. Both innovative tracking with adapted resting positions during night and anti‐soiling coatings (ASCs) are considered as potential solutions to reduce soiling. In this study, the individual mitigation potential of both ASC and tracking routines as well as the combination of the two approaches are investigated. For this, outdoor exposure tests were carried out in desert region of Saudi Arabia. Coated and uncoated glass samples were tested in different tilt configurations: fixed, 1‐axis tracking with horizontal stowage (facing the sky) and 1‐axis tracking with vertical stowage during the night. Both methods indicate significant soiling reductions, especially for the combined solution of ASC and tracking with vertical night stowage, where soiling losses can be reduced by up to 85%. In addition, it has been shown that the relative ASC performance can be improved when using vertical night stowage compared to fixed tilt or standard 1‐axis tracking scenarios.

Abstract Soiling can significantly reduce the energy yield of PV modules, especially in desert climates due to high airborne dust concentration, frequent dust storms, and rare rain events. The current vast deployment of PV in deserts requires adequate and economically feasible strategies for the mitigation of soiling. In this regard, anti-soiling coatings (ASCs) could be a promising solution as they could significantly reduce the accumulation of dust and resultant soiling losses. To assess the outdoor performance of different ASCs, an outdoor benchmark test setup was designed and installed at eight different locations in Saudi Arabia. The soiling losses have been determined by measuring the transmittance of the samples before and after outdoor exposure for monthly (2 locations) and quarterly (6 locations) exposure periods throughout one full year. Within this study, detailed results are presented for an uncoated reference glass and 3 out of 15 coatings, which have been tested. In addition, a general coating ranking based on average monthly and quarterly soiling losses of the full set of ASCs is also presented. The results indicate huge variations of anti-soiling functionality of the coatings depending on exposure period and location. Overall, both improved and decreased anti-soiling performances have been observed for coatings compared with an uncoated reference glass. An investigation on the correlation between the daily soiling loss and weather parameters indicates that besides precipitation and particulate concentration in the air, also dew and its interaction with the coatings is an important influencing parameter strongly affecting soiling rates. Looking at the two most promising coatings with very low-adhesive properties, the overall performance benefit is higher for a hydrophilic surface in high humid areas (coastal) and a hydrophobic surface in dry areas (central desert).