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Solar Panel Putzfimmel

August 19, 2013

It seems as if every other advertisement on television is for one cleaning product or another. Without these essential products, "as seen on TV," it's a wonder that humans survived for so many millennia.

Much of this marketing relies on the psychological urge of many people to clean to excess. As usual, the German language has a colorful word for this, "Putzfimmel," meaning an obsession for cleaning.
Figure caption

An illustration by Charles Keene (1823–1891), from a book by Douglas William Jerrold, "Mrs Caudle's curtain lectures," page 193.

This was a comic strip originally published in Punch magazine.

(Modified Wikimedia Commons image.)

If you haven't noticed, the Earth is a dirty place. We should be thankful for that in one sense, since all that dirt (technically, regolith) became a host for the vegetation that sustained life. The Earth is still collecting regolith material from the infall of countless micrometeorites. Since most of these are magnetic, it's possible to harvest them, along with other bits of magnetic debris precipitating from the atmosphere.[1-2]

Micrometeorites are just a small part of the natural atmospheric particulates in Earth's atmosphere. Other sources are volcanoes, dust storms, forest fires and grassland fires, and pollen from vegetation. Because of our technology, the atmosphere is also laden with artificial atmospheric particulates that add to the natural load. Fuel burning generates large quantities of carbon soot, which I wrote about in a previous article (Paint it Black, February 13, 2013).

Humans put about seven trillion tons of black carbon annually into the atmosphere.[3] One surprisingly active source is kerosene lamps, which generate about 270 billion tons, or seven percent of the total.[3] Ninety percent of black carbon comes from diesel-fueled vehicles, biomass for household cooking and heating, small kilns and industrial boilers, and open burning, such as fires for forest clearing.[3]

Human activities such as overgrazing, agriculture, and cutting down forests also cause soil particles to enter the atmosphere. About half of atmospheric dust is likely caused by human activity.[4] The following figure summarizes atmospheric particulates and their particle sizes.

Atmospheric particulates and their sizes Atmospheric particulates and their sizes.

Click for a larger image.

(Wikimedia Commons image, modified for clarity.)

At present, solar panels coexist with such atmospheric pollutants, so the question arises as to how much this dirt blocks solar radiation when it soils the panels, and how often you need to wash your panels clean. That's the question that two engineers at the University of California, San Diego, Department of Mechanical and Aerospace Engineering, addressed in a recent paper in the journal, Solar Energy.[5-6]

The UCSD research team studied 186 photovoltaic sites during 2010. They found that losses from soiling of the panels averaged about 0.051% per day, although a quarter of the sites had loses greater than 0.1% per day. Not surprisingly, panels with shallow tilt angles, defined as less than five degrees, showed larger loses from soiling.[5]

Since nature does a reasonable job of rinsing solar panels with rain, the UCSD research team looked at the loss encountered for a 145 day summer's drought in California. This was only about 7.4%. They calculated that for a typical five kilowatt residential system this loss translated to a monetary loss of just twenty dollars; so, economically, it doesn't pay to wash your panels. This assessment is true even for larger commercial rooftop systems, although cleaning may be warranted for very large installations.[6]

A dirty photovoltaic panelA dirty photovoltaic panel.

(Photograph: Jacobs School of Engineering/UC San Diego.)[6]

Jan Kleissl, a professor of mechanical and aerospace engineering at UC San Diego and principal investigator on the study, says
"You definitely wouldn't get your money back after hiring someone to wash your rooftop panels."[6]
The study was limited to residential and commercial sites in the region from the San Francisco Bay Area to the United States-Mexico border. After the extended drought, just a tenth of an inch of rainfall was sufficient to clean the panels.[6] The study authors explain that pollution and dust levels in California are fairly representative for the United States; and, except for things like dust storms in Arizona, can be considered a worst case.[6] They did find that panels located in the Los Angeles basin and the Central Valley were dirtier.[6]

There are cases for which cleaning is economically warranted. Heavy accumulations of bird droppings should be washed, since these completely block solar radiation. Also, sites near or downwind from a highway, factory or agricultural field may accumulate dirt at a more rapid rate, so they should be cleaned to maintain efficiency.[6]

The next research effort will be to identify the sources of dirt at various locations, and assess materials and washing systems that might economically keep panels clean.[6]

References:

  1. J. Witzel, Micrometeorites, IEEE Instrumentation & Measurement Magazine, vol. 6, no. 1 (March, 2003), pp. 55-56, DOI:10.1109/MIM.2003.1184295. This article is pay walled with a rather hefty fee ($31), but an Internet search will yield equivalent information, such as ref. 2, below.
  2. How to collect space dust!! (micrometeorites), YouTube Video.
  3. Pete Spotts, "Soot is No. 2 global-warming culprit, study finds," Christian Science Monitor, January 15, 2013.
  4. Ron Miller and Ina Tegen, "Desert Dust, Dust Storms and Climate," National Aeronautics and Space Administration, Goddard Institute for Space Studies Web Site, April 1997
  5. Felipe A. Mejia and Jan Kleissl, "Soiling losses for solar photovoltaic systems in California," Solar Energy, vol. 95 (September, 2013), pp. 357-363.
  6. Ioana Patringenaru, "Cleaning Solar Panels Often Not Worth the Cost, Engineers at UC San Diego Find," University of California, San Diego, Press Release, August 1, 2013.