Dry and sunny locations are highly favorable for cost effective energy generation from PV modules. However, with few exceptions, these locations expose the module glass surface to dry dust and soil, which can accumulate on the surface and reduce light transmission and power output. A number of researchers have attempted to quantify the effects of this soiling. One study found that efficiency for a PV system declined by approximately 0.16% per day between significant rainfalls, resulting in an annual energy loss ranging from 2.5 to 6%. Another researcher calculated that soiling causes annual system losses of 5%. Specifically in the southwest region of the United States, soiling rates have been measured at 1% per month.
In considering how to address this issue, utilities and project owner-operators have had to make careful calculations to arrive at the optimal frequency for cleaning PV modules; the costs for field labor and water delivered to site have a significant impact on economics of cleaning. The calculation is even more difficult given the unpredictability of rainfall, which in sufficient amounts can remove soiling almost as effectively as manual cleaning. However, in the arid locations where PV modules are most effective, infrequent rainfall inhibits cleaning.
A new coating solution
Now, however, system owners have a new tool to help solar modules in arid, dusty environments stay cleaner, longer. A new coating solution, 3M Anti-Soiling Liquid, can significantly reduce dry dust accumulated on PV modules and CSP mirrors. The product works by forming a hydrophilic coating on bare glass, which creates resistance to dry-dust soiling. With this coating, light transmission to PV cells is increased and overall power output is improved. Testing has demonstrated that modules coated with the liquid may generate significantly more energy than uncoated modules, highlighting the potential return on investment made possible with this new technology.
The coating is designed for aftermarket use in the field, and can be applied during regular cleaning and maintenance. The water-based coating is safe for the environment, and can be easily applied with hand tools, making it simple and safe for workers to use. To apply, workers simply clean the glass surface and remove any stuck-on debris, then rinse the surface and squeegee off excess water. The coating can then be rolled on with a paint roller. Two passes are recommended to ensure complete coverage. After 30 seconds, any excess coating is removed with a squeegee before the liquid dries.
After one application, the product can continue working effectively for up to three years in arid climates, as shown by accelerated weathering data. This lifespan gives system operators years of improved performance from their PV modules with just one straightforward application. The coating has been tested in both the lab and the field, and has been shown to deliver improved results in real-world conditions.
The photo shows the results of dust adhesion testing on a glass mirror panel tested in a controlled chamber at 70°F and 10 percent relative humidity. Excess ASTM Fine test dust was applied over the face of the panel, which was coated on one section with Anti-Soiling Liquid. After the panel was gently tapped in a vertical position, the coated section exhibited significantly less adhesion of dust particles than the uncoated section.
PV module soiling has been a longstanding issue for system owners, resulting in frustrating and unpredictable losses in efficiency. With this new technology, however, the industry now has an opportunity to combat the problem of soiling and fully maximize power output.
Written by Dr. Dan Chen, Business Development Manager at 3M Renewable Energy Division.