Researchers from the Indian Institute of Technology Bombay (IIT Bombay) have conducted a study examining the effects of robotic cleaning on antisoiling coatings applied to solar modules. Their findings indicate that the presence of dust during cleaning cycles can drastically shorten the lifespan of these coatings.
Sonali Bhaduri, the corresponding author of the study, explained that an accelerated cleaning testbed was developed to replicate conditions found in the field. The research focused on how dust, brush material, brush rotation direction, and horizontal cleaning speed affected four different commercial hydrophobic coatings.
Bhaduri noted that dust was identified as the most detrimental factor, reducing the coating’s lifespan to about 1/82 of what was observed when cleaning occurred without dust. Other factors also played a role; for instance, using a harder brush material decreased the coating life to around one-third. Additionally, rotating the brush in the direction of travel similarly reduced the lifespan to about one-third compared to other rotation methods. Slowing the brush travel speed from 0.4 m/s to 0.1 m/s also resulted in a one-third reduction in coating life.
The study evaluated four commercial hydrophobic antisoiling coatings, designated A, B, C, and D. Coatings A, B, and D were based on fluoropolymer, while coating C was phenylsilicone-based. None of the coatings displayed antireflective properties, and their initial solar-weighted transmittance was similar to that of uncoated glass.
To simulate the key stressors affecting photovoltaic (PV) modules, the researchers created an indoor abrasion testbed that incorporated thermal cycling, dew formation, dust deposition, and brush cleaning. They mimicked field conditions by cooling samples to 21°C, applying a fog-like mist, depositing dust, and subsequently heating them to 65°C. A dust loading of 0.2 mg/cm² was used to represent approximately two weeks of natural soiling.
The team employed three exposure protocols to evaluate the effects of various stressor combinations: only-clean, dew-dry-clean, and dew-dust-dry-clean cycles. Consistent comparisons of coating durability were achieved by using identical brush parameters, primarily involving a Nylon 6,12 rotary brush.
Coating performance was assessed through measurements of contact and roll-off angles, surface roughness, weighted average transmittance, and tapping-mode atomic force microscopy (TM-AFM) phase imaging. The researchers defined coating failure as a decline in contact angle below 90 degrees.
The analysis revealed that dust was the primary factor contributing to the degradation of the antisoiling coatings, followed by dew. After 120 test cycles, all coatings exhibited significant changes in roll-off angle, with coatings A and C experiencing notable reductions in weighted average transmittance. Coating C was particularly susceptible, being completely removed after 120 cycles, while the Nylon 6,12 brush itself sustained chemical, mechanical, and morphological damage.
The study highlighted that brush rotation had a significant impact on abrasion. When the brush rotated in the direction of travel, it dragged dust particles across the surface, leading to severe scratches. Consequently, coatings subjected to this rotation exhibited shorter lifetimes compared to those where the brush rotated in the opposite direction or in a conventional clockwise manner. After 550 abrasion cycles, all coatings showed a decline in antisoiling performance, with coating C being entirely removed regardless of the rotation direction.
Cleaning speed was also found to be a critical factor. Reducing the horizontal brush velocity from 0.4 m/s to 0.1 m/s increased bristle-surface contact, which accelerated coating degradation. At the slower speed, more bristle tufts contacted the glass during each pass, resulting in greater abrasion.
After 3,700 cycles, coating C had lost around 50% of its surface coverage at both cleaning velocities, with slower brushing leading to a more significant reduction in bristle hardness. While all four brushes tested provided similar cleaning efficacy, the material of the brush did influence coating durability.
The overall findings suggest that the durability of coatings is heavily influenced by environmental stressors and cleaning conditions. Factors such as dust, harder brushes, rotation towards the direction of travel, and slower brush movement were identified as causing the most severe degradation.
To mitigate abrasion damage during waterless cleaning of PV modules, the researchers proposed several strategies. They recommended optimising cleaning intervals to limit excessive dust accumulation, as higher dust levels can exacerbate abrasion during cleaning. Additionally, they suggested using brushes with softer bristles that apply minimal pressure on the module surface, such as the microfibre cloth brush tested in the study. The researchers also advised against rotating brushes in the direction of travel and cautioned against low horizontal brush velocities, as these conditions increase bristle-surface interaction and accelerate coating degradation.
The research findings were published in the article titled “Factors Influencing the Abrasion Damage to Antisoiling Coatings on Photovoltaic Modules” in the journal Progress in Photovoltaics. The same research team has also recently explored the effects of rain on antisoiling coatings for solar panels, noting that coating lifetimes can vary significantly based on local climate and installation conditions.
Source: pv magazine Global
