Let the light shine through

Several experts agree that for the average homeowner in an urban area not close to a major pollution source, rain, wind and snow are enough to keep the panels clean. Daniela Dirnberger of the ISE Fraunhofer in Freiburg oversaw a study on urban solar panel pollution that showed the economic loss from a reduction in performance due to soiled panels was less than the cost of getting the panels cleaned professionally. The study was conducted over three years and involved panels that were between five and 12 years old. None of the panels had been cleaned previously.
During two summer months this summer, half of the panels were cleaned and the others weren’t. The panels were cleaned with a brush and de-mineralized water. “The result was a loss of between one and 1.5 percent in energy output.” She said if you take into account that professional cleaning costs are at least 1.15 euros per square meter, then “it’s not worth it to get the panels cleaned.” And she noted 1.15 euros is an exceptionally low price. Most firms charge more, with average cleaning costs closer to two to three euros per square meter.
But Dirnberger cautioned that this is true for modules in places like Freiburg, Stuttgart or Karlsruhe, where the study took place, as these cities are not close to farms. The panel manufacturer Schott Solar agrees with this assessment. It said panels placed at an angle of 10 degrees or more can effectively be cleaned via rain and wind. “Experts estimate that the performance in this case is reduced by a maximum of three percent, so that the cleaning costs don’t outweigh the benefits,” the company said in information provided to pv magazine .
That’s not true for panels close to farms, manufacturing plants, railroads or other pollution sources. And it’s especially not true for PV installations in desert areas, where experts say sand is a major problem.
“In the desert you can lose up to 80 percent of performance,” said Professor Heinrich Häberlin of the PV Laboratory of the University of Engineering and Architecture in Bern, Switzerland (Berner Fachhochschule, Hochschule für Technik und Architektur HTA Burgdorf, Labor für Photovoltaik).
Häberlin is the co-author of a study of a PV installation near the main train route between Bern and Zurich. These modules developed a permanent strip of dirt on the lower lip of the panel, which impeded performance.
At first researchers thought the train routes close proximity to the panels was behind the problem, but they also inspected panels further away from the rail line and found they too had a permanent layer of dirt, albeit somewhat less than the panels close to the train tracks.
After cleaning the panels, the researchers determined that panel productivity was reduced by 3.1 to 13.8 percent per panel due to the dirt. The average reduction in performance was 7.6 percent, they said. The study was originally published in 1998, but reaffirmed in other investigations done in 2007 and 2008. Professor Häberlin included the information in a book that was published in German in 2007 and will be available in English next year under the title: “Photovoltaics System Design and Practice.” Häberlin concluded that although many believe that in Central Europe, rain and snow can naturally clean PV panels, in cases where a permanent layer of dirt develops, the panel output can affect performance and should be professionally cleaned. The authors did not make a recommendation as to cleaning methods.

The cleaning options

For areas where pollution becomes a major issue and severely impedes solar energy performance, such as in a desert, the question is then: How should you clean panels? There are some technologies in the works and many already on the market.
The Mars technology: Arguably one of the more unusual solutions not yet available commercially, is the use of a self-cleaning technology that scientists working for NASA originally developed for use in lunar and Mars missions.
Conditions on Mars are dry and dusty too and “solar panels powering rovers and future manned and robotic missions must not succumb to dust deposition,” said Malay K. Mazumder, a research professor in electrophysics at Boston University.
Mazumder initially worked on this technology when he was at the University of Arkansas and brought it to BU when he took a position there, said Professor Mark Horenstein, a professor of applied electromagnetics in BU’s Department of Electrical and Computer Engineering, who is working on the technology with Dr. Mazumder.
The technology works by lifting the dust particles off the screen and pushing them laterally across the surface to the edge, where they fall off. Particles often become electrostatically charged in nature; and also when they impinge on the photovoltaic module, they typically acquire a charge due to contact electrification, Dr. Horenstein explained in an interview.
The cleaning works by embedding very thin or transparent electrodes in the transparent cover over the solar panel, forming what is known as an electrodynamic screen. A set of cyclical voltages applied to the wires creates an electrostatic force wave that travels across to the surface of the panel, entraining the particles and driving them to the panels’ edges, where they fall off.
Within two minutes, the process removes about 90 percent of the dust deposited on a solar panel. The system requires only a small amount of power, generally taken from the panel itself, for the cleaning operation.
The BU scientists have been working since about 2009 on the technology and their research was presented at a meeting of the American Chemical Society in August, 2010. They are now exploring ways to commercialize the technology.
Currently the team is also working to develop a sensor that will trigger the cleaning. The sensor is supposed to tell the system when energy output is reduced by a given amount so the cleaning will kick in. But this isn’t an easy task, Horenstein said, because researchers have to overcome the nighttime effect, since evening conditions and clouds cover reduce solar output for reasons that have nothing to do with dirt.
Keeping costs down and finding a commercial partner are also on the list. At the moment the technology is targeted to cost 100 U.S. dollars per panel. The researchers’ goal is to eventually have a technology that can be incorporated into the manufacturing process, so a screen does not need to be retrofitted. Thus, in the future, the technology could be built into the solar cells from the get-go.
Horenstein admitted that for scientists, developing a technology is one thing, but finding a commercial partner and dealing with the business end of a new technology is quite another.
The BU researchers are being funded in part by the United States-Israel Binational Science Foundation, in partnership with the Arava Institute for Environmental Studies and Tel Aviv University. The Arava Institute is based in Hevel Eilot in southern Israel and is affiliated with Arava Power, which describes itself as Israeli’s leading solar power company.
Robots: A few suppliers offer robots to clean photovoltaic panels, among others the Swiss firm Serbot AG. Their system can work in both a vertical and horizontal way and has a remote control that is operated by specially designed software. The unit was developed by Niederberger-Engineering and is built and sold by Serbot. The technology works on a vacuum principle via a feed-in tube. It has a performance level of 350 to 400 square meters per hour and uses 0.5 to 1.2 liters of water per minute. The empty weight of the unit is 54 kilos.
The robot, part of the Gekko family of machines, generally works via a remote control, but can also be programmed to work on its own. The remote can be controlled from as far away as 300 meters. The model used for PV cleaning is the Gekko G3. It sports a number of soft-bristle brushes attached to vacuums that suck up the dirt.
Hansjörg Schindler of Serbot AG said the performance of up to 400 square meters per hour compares to about 40 to 50 square meters per hour that one can achieve when doing it by hand. Also with a robot you eliminate risks to workers climbing up onto roofs to clean the panels. Schindler said the robots are mainly used for middle to large installations and the company’s customers have generally been professional solar panel cleaning firms. The robots can also be outfitted with additional technology so that they can test the efficiency of the solar panels, he said.
Serbot AG has offered the robot to Middle East customers, but has not yet sold one there. The company began three years ago, though the first robot came out in 2004. The robots cost 98,000 euros a piece, including the remote control.
Cleaning solutions: There are of course less high-tech ways of cleaning panels and a variety of cleaning solutions are onthe market. The cleaning firm Hortega GmbH offers a special PV panel cleaner that it claims also protects the panels from accumulating additional dirt. Ingo Bross, a regional manager with the company, said part of the problem is that some dirt that lands on panels contains fat and that isn’t easily gotten rid of with just a water cleaning. Also elements like bird droppings often don’t wash off with the rain. The company maintains that panel performance is reduced by up to 30 percent over time if the panels are not cleaned. The company’s PV cleaner goes for 16.90 euros for a liter of concentrate that can wash up to 100 square meters of PV panels.

Panel maker recommendations

For panels installed near farms, manufacturing or industrial plants, Schott Solar recommends getting its panels professionally cleaned. For those who prefer to do it themselves the company warns owners never to stand on the panels or put any weight on them and says to avoid pressure washing – only soft brushes and sponges should be used.
Schott said professional cleaning costs generally run between two and three euros per square meter for the first cleaning and half of that for follow-ups. Sometimes travel costs are added. Based on expected costs and the return on productivity, panel owners should be able to determine if a professional cleaning is worth it to them. The company said what is important to note, regardless of which company makes your panels, is that panel owners need to follow the panel maker’s cleaning and care recommendation – otherwise one risks losing guarantee coverage, should something go wrong.