Deserts are ideal locations for solar farms - they get a lot of sunshine and the land is relatively inexpensive. On the other hand, their arid nature means that a lot of sand flies around, and sand on a photovoltaic panel blocks valuable sunlight, decreasing its output. Depending on location and weather conditions, dusty panels could decrease solar farm production by 3% to 15% in a given month. Periodic cleaning ensures that the panels get as much light as possible, but it’s a labor intensive process that may not pay for itself.
Ecoppia, an upstart company out of Israel, has an innovative, water-free solution: the E4 robotic PV cleaning system that dusts the panels using a self-cleaning microfiber cloth.
Image courtesy of Ecoppia
One Robot per Row
Each row of a solar farm has one E4 robot attached to it. The E4 is battery-powered; the battery is recharged by its own solar panel, shown at the bottom of the photo above. The entire rack moves horizontally, making a complete pass over the row each day. Here it is in action:
Video courtesy of Ecoppia
The E4 has five motors per unit: two for horizontal movement, two for vertical movement, and one to spin the microfiber brush. The vertical motors serve a dual purpose. First they pull the cleaning unit up to the top of the panel. Gravity lets the unit descend, and the motors become generators, converting that gravitational motion into electricity to recharge the batteries - just like regenerative braking in an electric or hybrid car. The unit was designed to operate in desert conditions, and has been stress-tested at temperatures of 66oC (150oF).
The microfiber cloth cleans itself at the bottom of each pass. After completing its work for the day, the robot returns to its docking station, where it recharges its batteries.
Impact on the Panels
Conventional wisdom says that solar panels shouldn’t be cleaned while dry. That’s certainly true if you’re applying pressure to the panel, as you would if you were using a squeegee, but the E4 uses a soft microfiber cloth to lightly brush away the dirt. Accelerated testing by the Berlin Photovoltaic Institute determined that the E4 caused no damage to PV panels, even after 20 years of daily use.
Limited to Fixed-Tilt Arrays
As you may have noticed, the E4 is designed for fixed-tilt arrays. If the panels had single or dual-axis tracking, they would require a complete E4 per panel, which would not be cost-effective. That made me wonder: when is it cost-effective to mount panels on a tracking system? Well, like many engineering questions, the answer is, “It depends on several variables.” Let’s look at a few.
Fixed Tilt vs. Tracking
For a single solar panel, a tracking system will provide more energy than a fixed-mount panel. Your mileage may vary, but using average NREL numbers for my location, I found that the production of a PV panel would be about 25% higher with a single-axis (east-west) tracking system, and 30% higher with a dual-axis tracking system. But does that justify the added cost? In some cases, a tracking system nearly doubles the cost of the array, so it would be more cost-effective to simply add more panels and install them at a fixed angle.
That’s fine if you have the extra space, but what about solar farms? Land isn’t cheap, even in the desert, so PV farms need to maximize the energy density for a given area, which makes a tracking system a logical choice, right? Not so fast. In the photo below, you can see that as a panel tracks the afternoon sun, it casts a shadow to its side. Designers must leave empty space to prevent shading of neighboring panels, because even partial shade on a panel can drastically reduce its power output. This, coupled with the added cost of a tracking system, is why many solar farms use a fixed-tilt mount.
To Clean or Not To Clean?
Is it cost-effective to clean the panels? Again, it depends. A rainy day does a nice job of cleaning PV panels at no cost, but the desert doesn’t get much rain. A study conducted at the University of California at San Diego concluded that an automated panel washing system could improve PV production by nearly 10%. A similar study conducted in Arizona found that periodic cleaning could increase production by 7% to 15%.
Calculate it For Yourself
If you're designing a solar farm and you'd like to know if you should consider the E4, Ecoppia has a handy calculator that enables you to estimate your return on investment based on your conditions. Here’s a screen capture:
Image courtesy of Ecoppia
Ecoppia has only been in business since 2013 and currently their only installation is on the Ketura Sun 5MW array in Israel. The solar farm was built in 2011 and prior to the addition of the E4 robots in 2013, the panels were cleaned manually, ten times per year. I’m looking forward to seeing the difference in performance between manual cleaning less than once a month and automatic cleaning every day. Will a robotic cleaning system become a regular feature of solar farms in the desert or is it just a cool piece of automated technology?