Researchers in India found that dust during robotic cleaning can dramatically accelerate the degradation of hydrophobic antisoiling coatings on PV modules. Softer brushes, higher cleaning speeds, optimized cleaning intervals, and appropriate brush rotation can significantly extend coating lifetime.
Researchers from the Indian Institute of Technology Bombay (ITT Bombay) have investigated how robotic cleaning affects antisoiling coatings used on solar modules and found that the presence of dust during cleaning cycles can significantly reduce coating lifetime.
“Using an accelerated cleaning testbed that emulates relevant field conditions, we studied the effects of dust during cleaning, brush material, brush rotation direction, and horizontal cleaning velocity on four commercial hydrophobic coatings,” corresponding author Sonali Bhaduri told pv magazine.
“We found that dust during cleaning was the most damaging stressor, reducing coating life to approximately 1/82 of that observed when cleaning without dust,” Bhaduri said. “Among the other operating parameters, using a harder brush material reduced coating life to approximately one-third. Similarly, rotating the brush in the direction of travel reduced coating life to approximately one-third compared with the other brush-rotation configurations. Reducing the brush travel speed from 0.4 m/s to 0.1 m/s also decreased coating life to approximately one-third.”
The scientists evaluated four commercial hydrophobic antisoiling coatings, labeled A, B, C, and D. Coatings A, B, and D were fluoropolymer-based, while coating C was phenylsilicone-based. None exhibited antireflective properties, and their initial solar-weighted transmittance was comparable to that of uncoated glass.
The researchers developed an indoor abrasion testbed to reproduce key stressors affecting PV modules, including thermal cycling, dew formation, dust deposition, and brush cleaning. They simulated field-like conditions by cooling the samples to 21 C, applying a fog-like water mist, depositing dust, and then heating them to 65 C. A dust loading of 0.2 mg/cm² represented approximately two weeks of natural soiling at IIT Bombay, with controlled deposition uniformity.
The team used three exposure protocols to assess the effects of different stressor combinations: only-clean, dew-dry-clean, and dew-dust-dry-clean cycles. Identical brush parameters, primarily involving a Nylon 6,12 rotary brush, enabled consistent comparisons of coating durability under repeated abrasion.
The researchers assessed coating performance by measuring contact and roll-off angles, surface roughness, weighted average transmittance, and tapping-mode atomic force microscopy (TM-AFM) phase imaging. Additional analyses quantified coating area coverage, dust properties, brush damage, and cleaning efficacy. They defined coating failure as a decline in contact angle below 90 degrees.
The analysis showed that dust was the dominant stressor in the degradation of the antisoiling coatings, followed by dew. After 120 test cycles, all coatings showed significant changes in roll-off angle, while coatings A and C also experienced significant reductions in weighted average transmittance. Coating C was particularly vulnerable and was completely removed after 120 cycles, while the Nylon 6,12 brush itself suffered chemical, mechanical, and morphological damage.
The scientists said brush rotation strongly influenced abrasion. When the brush rotated toward the direction of travel, it dragged dust particles across the surface, causing severe scratches. Coatings exposed to this configuration consequently exhibited shorter lifetimes than those subjected to rotation opposite to the direction of travel or conventional clockwise rotation. After 550 abrasion cycles, antisoiling performance declined under all rotation conditions, and coating C was completely removed regardless of the direction of rotation.
Cleaning speed also played a significant role. Reducing the horizontal brush velocity from 0.4 m/s to 0.1 m/s increased bristle-surface contact and accelerated coating degradation. At 0.1 m/s, 10 rows of bristle tufts contacted the glass during each pass, compared with four rows at 0.4 m/s, resulting in greater abrasion.
After 3,700 cycles, coating C had lost about 50% of its surface coverage at both cleaning velocities, while slower brushing resulted in a greater reduction in bristle hardness. Brush material also affected coating durability, although all four brushes tested provided similar cleaning efficacy.
Overall, the results indicate that coating durability strongly depends on environmental stressors and cleaning conditions, with dust, harder brushes, rotation toward the direction of travel, and slower brush movement causing the most severe degradation.
Based on the findings, the researchers proposed several measures to minimize abrasion damage during waterless cleaning of PV modules. They recommended optimizing cleaning intervals to prevent excessive dust accumulation, as greater quantities of deposited dust can intensify abrasion during cleaning.
They also recommended using brushes with softer bristles that exert minimal pressure on the module surface, such as the microfibre cloth brush tested in the study. In addition, they advised against rotating brushes toward the direction of travel, as this configuration increases interactions among dust particles, brush bristles, and the module surface. The researchers also recommended avoiding low horizontal brush velocities, as slower movement increases bristle-surface interaction and accelerates coating degradation.
The research was presented in “Factors Influencing the Abrasion Damage to Antisoiling Coatings on Photovoltaic Modules,” published in Progress in Photovoltaics.
The same research team recently investigated the impact of rain on antisoiling coatings for solar panels, finding that coating lifetime can vary significantly depending on local climate and installation conditions.
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