Cleaning the PV system solar panels generally seems very simple to a lot of solar panel owners due to the apparent reasoning behind it. Dirty panels generally produce more power than clean panels simply because they have less dirt and therefore produce less power than their original condition prior to getting dirty, and so cleaning them using a common household detergent such as dish soap should put the panels back to their original performance levels. However, research conducted at the University of Turku, Finland, has come to an unexpected conclusion concerning the product most people instinctively turn to after cleaning their solar panels; dish soap can actually do just the opposite of what you want to accomplish, and by doing so can further degrade the output.
What the Research Revealed
Researchers Julianna Varjopuro and Kati Miettunen at the University of Turku have conducted a series of tests on photovoltaic glass samples in a Finnish study. The researchers compared dish soap to a variety of other cleaning products including ethanol, acetone, isopropanol, a special detergent made specifically for cleaning solar panels, and a common window cleaner. Two different testing methods were used; the first involved cleaning the samples by soaking them in the various cleaning agents over an extended period of time; the second involved the growth of algae on the glass surface to replicate real life cleaning conditions. Light from an extensive number of wavelength ranges was measured using a spectrophotometer to see how much light was transmitted after each cleaning method. The results were clear-cut.
On clean glass, most products either maintained or slightly improved light transmission. Dish soap, however, reduced transmission by approximately 1%. On glass that had been soiled with algae and then cleaned, the disparity widened: dish soap left a persistent transmission loss of nearly 4% compared to properly cleaned surfaces. This translates directly to electricity output-a 3–4% reduction in energy yield, simply from using the wrong detergent.
Practical Implications for System Owners
As a result of these findings, here are some recommendations for both the residential and commercial PV system owner:
Avoid the use of dish soap entirely. Although solar panels may appear clean post-use with dish soap, the soap is leaving behind an invisible residue, which will continue to inhibit performance until it is removed naturally through weathering (rain, UV and wind exposure). In some areas of the world with minimal rainfall, such as the South Western portion of the U.S., the detergent residue can remain on modules for as long as 6 months.
Use only recommended cleaning products. Cleaning products that contain isopropyl alcohol (typically 70% concentration or higher) or commercially marketed products specifically for cleaning glass but contain no conditioning or wax materials are safe to use. Purpose-built solutions for cleaning solar panels may be higher priced; however, the chemistry in such products has been specifically developed for use on anti-reflective coatings.
Use the highest quality water possible when you do use water alone. Ideally, you should use deionized or distilled water. If you use tap water for cleaning, you will leave mineral residue behind on the solar module and this residue could build up overtime and limit the amount of light transmitted through that area.
Consider mechanical methods carefully. Soft brushes or squeegees are acceptable; abrasive tools can scratch the glass or coating, causing permanent damage unrelated to cleaning chemistry.
