BACK PAGE BASICS: Clean to Be Greener

Cleaning a large array
Cleaning the carport PV array at Googleplex in Mountain View, California.
Cleaning a large array

Dirt and dust buildup on the modules can reduce your PV array’s energy output, so implementing a regular cleaning protocol can be important.

The newly updated PV energy calculator PVWatts assumes that module soiling can cause a 2% reduction in array energy output—making losses over the array’s lifetime add up. In some locations that are particularly dusty and have little rainfall, losses in dusty or heavily industrial areas could easily be 25% to 30% of the array’s potential output. On the other hand, if you have no local industrial polluters, but have regular rainfall—­and your PV modules are at a steep-enough angle to allow rain to wash down the glass­—the effects of soiling on the system’s output could be minimal, and not worth worrying about.

Determining if it is worthwhile to spend time cleaning a PV array is a value judgment that calls for more information than just the local soiling and rainfall conditions. For example, those additional kWh may not be worth trying to capture if the array is on a roof that is difficult or dangerous to access, or if getting running water to the roof is troublesome. A ground-mounted array is likely to be easy to clean, and there’s little reason not to do it at least seasonally.

Commercial window-washing has brought telescopic soft-fiber brushes to the market that are appropriate for washing PV modules. Some can even be coupled to a hose, bringing water right to the bristles. (Shop for “boat-cleaning” brushes—about $35.)

Some PV array washing companies use purified water to avoid hard-water stains and spots, but usually it’s sufficient to scrub—or even just rinse—them with tap water. However, water alone might not remove grime. In that case, use a mild detergent that won’t damage aluminum—usually something suited for washing your car will be acceptable. Follow the suds with a water-only rinse.

For time or safety reasons, you may consider hiring out the array-washing task. In metropolitan areas, there are businesses that specialize in cleaning arrays. Check with local installers—they will likely know of cleaning companies or may even be able to offer you a maintenance contract that includes periodic cleaning.

Not everyone agrees that it makes economic sense to regularly wash a home-scale PV array. Differences in regional circumstances aside, a study published by the University of California at San Diego showed that California arrays that had not been rained on or washed during 145 days of summer drought conditions lost only 7.4% of their efficiency, which equated to about $20 worth of energy in a 5 kW array. It would likely be impossible to hire someone to do the job for that small amount of money.

Automated washing systems are becoming available for home-scale rooftop PV systems. While I cannot vouch for their effectiveness, one washing system can mimic a rainstorm by spraying from sprinkler heads permanently installed between the rows of modules. For a periodic deeper washing, they can also spray a detergent mixture and then follow with rinse water. Another system under development uses a solar-powered robot, similar to a household robotic vacuum cleaner, that activates during rainstorms to scrub the array.

Comments (7)

Paul Hancock_2's picture

To further clarify why I started this thread...My array is located in northern Manitoba, pole mounted and tilted at about a 65 degree angle in winter. Temps at this time of year are typically -15c to -30c. Snow buildup is typically not an issue given the tilt. However in certain conditions a light snowfall on the panels can melt in mid day sun and then freeze as temps fall that night producing a thin rough layer of ice over the panels. A snowfall that night will cling to the panels and if there's a lot of snow coming down it will build up to the point it will not melt the following day even if the sun is out. In certain conditions snow can remain on the panels for several weeks. Given I'm not always at this location it can be problematic for my batteries.

I've often thought there must be something out there similar to silicone spray that could be applied to the panels alleviate this issue but haven't found anything as of yet that is deemed safe for pv systems.

Ed Mahoney's picture

I read bayntons snow mail. In the 1970's many of the applications for PV powered systems were for mountaintop 2-way radio/tv, etc. Many were for US forrestry/BLM/National Parks, etc. It's humerous in that on of these early systems were used for a taxi cab company in Bozeman, MT for their radio communications.
These systems almost always had snow condition locations. We found that the array surface is smooth, the snow is typically translucent and the cells very inefficient reflection for the < 20 % efficient cells were given off in heat. Also, the tilt angle of the array toward a virtical (greater tilt than latitude).
This resulted in a water inrerface on the modules which aided immensely in having the snow slide off the array's surface, hence., NO snow problem!!!!

bayntons's picture

To further pursue the issue that SteveR brings up, what if any things are designers doing/can designers do so that roof-mounted PV or solar thermal systems are accessible for snow removal? To what extent are owners clearing snow? Are people using the weather forecast to decide whether to take the trouble? The impact of snow can be a lot more than the impact of dust.

Greg Smith_4's picture

A quick Google search will reveal several methods for removing snow.

SteveR's picture

Any suggestions for clearing 2+ feet of snow off an array without risking damage to the modules? Here in New England the snow keeps coming and we have not seen temperatures above freezing since 2014! My February production will likely be zero this year, whereas I usually generate 500 kWh. I've used a roof rake in the past, but I worry about damaging the modules.

kenbell48's picture

I've tried RainX with no discernible effect. It's nice to know the 7% number for 145 days. So I'm basically just entertaining myself cleaning the panels every month or so since I have to dump excess power anyway.

Paul Hancock_2's picture

I've often wondered about efficiency loss due to dust and dirt.

However the biggest issue I face is the thin layer of ice that forms on my panels after a sunny winter day and causes subsequent snowfalls to stick to the almost vertical panels.

Does anyone know a product that would prevent this ice buildup? Something like silicone or rainX possibly may work.

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