August 22, 2010 | 8
The best places to collect solar energy are also some of the dustiest on Earth and beyond, a quandary that leads to inefficiencies in how well the cells are able to convert strong sunlight into renewable electricity. The solution, according to new research, is to coat solar cells with material that enables them to chase away dirt particles on their own with the help of dust-repelling electrical charges.
A dust layer of 4 grams per square meter can decrease solar power conversion by 40 percent, says Malay Mazumder, a research professor in Boston University’s Department of Electrical and Computer Engineering. To put this in perspective, dust deposition in Arizona is about 17 grams per square meter per month, and the situation is worse in many other solar-friendly sites, including the Middle East, Australia and India. Mazumder, who led the study, presented the results Sunday at the 240th National Meeting of the American Chemical Society (ACS).
The electrodynamic transparent screen developed by Mazumder and his colleagues is made by depositing a transparent, electrically sensitive material—indium tin oxide (ITO)—on glass or a clear plastic sheet covering the solar panels. When energized, the electrodes produce a traveling wave of electrostatic and dielectrophoretic forces that lift dust particles from the surface and transport them to the screen’s edges. The researchers found that 90 percent of deposited dust can be removed by the transparent screen in fewer than 60 seconds.
This works in part because many solar panels are positioned at an angle—the raised dust would simply fall off. Whereas solar panels are generally placed in dry, open spaces, the researchers are hoping to make their technique and technology also work to keep raindrops and mud from adhering to solar panel surfaces as well.
Mazumder and his colleagues had been researching the problem in 2004 for the U.S. Department of Energy when NASA came calling. The Jet Propulsion Laboratory (JPL) was looking for an answer to at least part of the dust problem it had encountered on the moon and Mars, where robots and other machinery must contend with dirt and debris that coat their sensors and solar panels. The researchers are also looking at more terrestrial applications.
Future generations of dust-proof solar panels are likely to have electrodes embedded in their glass surfaces. These electrodes, like the original filters, would be powered by the solar cell itself, so they would have to operate using very little energy. Sensors would monitor dust levels on the surface of the panel and energize the surface when dust concentration reached a critical level.
This dust-busting technology may have applications outside the renewable energy market as well, Mazumder says. Helicopters taking off and landing in dry, dusty areas could benefit from self-cleaning windshields, as could delicate lenses used in laboratories and telescopes.
The Boston University research is just the latest attempt at self-cleaning technology. With inspiration from the sacred lotus (Nelumbo nucifera), which remains pristine despite growing in muddy waters, a "revolution in self-cleaning surfaces is under way," Peter Forbes wrote in the August 2008 issue of Scientific American.
Several other approaches rely on nanotechnology. U.K.-based glass manufacturer Pilkington has since 2001 sold its ActivGlass, which features a nanocoating of transparent titania. And Michael Rubner and Robert Cohen of the Massachusetts Institute of Technology (M.I.T.) are working with industrial partners to commercialize glass surfaces (mirrors and windshields, in particular) coated with nanoparticles that resist fogging.
Image of solar panel installation in rural Mongolia courtesy of Chinneeb, via Wikimedia Commons