El Phoenix Sun

Commercial solar cells with dust shields in NASA lab (Photo by Dr. Carlos Calle)

Following up on Monday’s story on self-cleaning solar panels, I contacted Dr. Carlos Calle, senior research scientist at NASA’s Electrostatics and Surface Physics Laboratory at the Kennedy Space Center, where the work originated. Calle was lead author of a fascinating 2009 technical article ($) on adapting the extra-terrestrial dust removal technology for solar panels here on Earth (and on the moon which has extremely fine dust particles).

The technology uses strategically placed electrodes to generate a electromagnetic wave that first lifts sunlight-blocking dust particles off the surface of the solar panel and then carries them away from the panel.

NASA's Dr. Carlos Calle

Calle’s lab made a variety dust shields, some rigid and opaque, some flexible and transparent. These were placed on top of off-the-shelf solar panels squares measuring two inches on each side. The test square were then covered with fine dust particles (roughly the size of the width of a human hair). In the journal article, Calle reports that “the transparent dust shields applied to commercial solar panels operate successfully under high vacuum even under extreme dust loading conditions that caused the solar cell performance to drop to 11-23%” of its normal output.

In fact, only the dustiest test cell (11% of normal performance) failed to reach 98.4% of normal output after the dust shield was activated. One panel (20.3% of normal) regained 99.4% of its electrical output after the dust curtain was energized.

I asked Calle if he thought water would still be needed to augment the dust shields. He was confident that the technology would “eliminate the need for water cleaning of solar panels.”

At this stage, Calle isn’t ready to speculate on how much the dust shields will add to the cost of a PV panel. However, given the simplicity of the design, the fact that so little power is needed to remove nearly all the dust, and the money saved by not using water in a desert, the dust shields will likely be attractive to manufactures, rooftop installers and utilities building large-scale solar PV projects in the desert.

Even clean energy can get dirty

Dust: it’s enemy number one for solar photovoltaic panels (PV) in the sunny, warm areas with the most potential for solar power. That’s because it takes less than a tablespoon of dust per square meter to reduce the electrical output of a typical PV panel by 40 percent.

“In Arizona,” says Professor Malay Mazumder of Boston University, “dust is deposited each month at about four times that amount. Deposition rates are even higher in the Middle East, Australia and India.”

Few home owners in the Southwest want to climb up on their roof several times a month to hose off the light-blocking dust. Utility-sized PV installations are hand-washed or use mechanical sprayers — but either way is costly. In the desert there’s the additional problem of increasing water use in an arid land — one that is likely to grow drier as the climate changes.

The solution to this problem (or at least a solution) comes from the U.S. space program — which is fitting, given that PV panels were pioneered by NASA in the 1960s and ’70s to power satellites and, most recently, rovers on Mars.

At the 240th National Meeting of the American Chemical Society (ACS) last weekend, Dr. Mazumder reported on advances in bringing the same technology used to clean dust from the Mars rovers down to Earth.

The trick to cleaning PV panels without water is to incorporate an Electrodynamic Screen (EDS) like the one on the Mars rovers. An EDS is a thin, electrically sensitive layer on the surface of the panel. When enough dust accumulates on the EDS, a sensor triggers a small electric pulse which repels the dust.

NASA first developed the idea for an “electric curtain” in 1967. In 2003, NASA’s Electrostatics and Surface Physics Laboratory (ESPL) worked with researchers at the University of Arkansas at Little Rock (where Dr. Mazumder then taught) to design and build an EDS for the Mars rovers. (The ESPL website has a short video showing test modules working under space conditions.)

Mazumder reports that the EDS developed by his lab can remove 90 percent of dust particles from a square meter of PV paneling in two minutes using just 10 watts.

Mazunder said the Earth-version of the EDS should be commercially available within a year.

Not all PV panels may require EDS technology, says Alan Bernheimer, a spokesman for First Solar, the world’s largest manufacturer of thin-film PV.

“Theoretically it would be possible to apply this technology to thin film solar modules,” Bernheimer wrote in an Email. “First Solar’s advanced thin film technology, however, is productive in diffuse and lower light conditions, such as those caused by dust.”

While even thin-film panels eventually need to be cleaned, Bernheimer said First Solar has no plans to adopt the EDS technology.

For traditional silicon-based PV manufacturers, however, the development of waterless cleaning technologies is likely to be seen as a milestone on the road to renewable, sustainable, energy.

You can read a 2008 paper about EDS technology co-written by Prof. Mazumder here (PDF).

Ohio's new Wyandot Solar Farm

The second largest utility-scale solar plant east of the Mississippi River opened Thursday and is now generating enough electricity to power an estimated 1,500 homes in north-central Ohio. The 12MW Wyandot Solar Farm uses 160,000 thin-film solar panels set on 84-acres of former farmland.

The panels were produced by First Solar at its Perrysburg, Ohio, manufacturing plant, which was recently expanded.

The thin-film technology is cheaper to produce than silicon-crystal based photovoltaic panels, but is less efficient at converting sunlight into electricity. PSEG Solar Source, the Newark, NJ-based company that owns the solar plant, decided the trade-off was worthwhile, according to spokesman, Curt Judy.

Location of new Ohio power plant

“The thin-film technology is also very well suited at capturing lower radiance or low sunlight which we tend to see in the winter months,” Judy told local television station NBC channel 4 out of Columbus.

Ohio’s Renewable Portfolio Standard (RPS) requires utilities to obtain a minimum of 12.5 percent of electrical generation from renewable sources, including solar, by 2025.