Dust And Tilt: The Impact On Solar Panel Performance

Dust can have a negative impact on solar panel performance. But module tilt angle also plays a role in how much the impact will be, as this study carried out in an extreme environment indicates.

It’s now been 3 weeks since any rain fell in my neck of the woods in Adelaide. This shot of my solar panels yesterday shows them looking rather dusty.

But according to SolarAnalytics¹, the system has been performing within the normal range expected from an installation like mine:

It’s one of the advantages of having an “oversized” system; where solar panel capacity (10kW in my case) is greater than inverter capacity (8kW). Oversizing is a very common and desirable practice.

Our roof’s pitch is 22.5 degrees²; so even just a few millimetres of rain within a reasonably short timeframe should give them a decent clean. But what happens if this dry spell went on for months and months? What might be the impact?

Researchers from Saudi Arabia and the UK sought to help answer this by studying power yields of clean and dusty PV modules measured over a year in desert conditions.

They installed seven pairs of 10 Watt solar panels on the rooftop of the Faculty of Engineering at King Abdulaziz University (KAU) in Jeddah City. The region experiences high temperatures, low rainfall, drought, and dust and sand storms. While Adelaide can be a dry and dusty place, we don’t see sand storms and only the rare dust storm.

The panels were installed at angles of 0° (flat), 15°, 25°, 45°, 60°, 70°, and 90°³; facing south. One of each pair was cleaned daily and the other allowed to accumulate dust. The duration of the dry and “rainy” periods was 183 and 182 days, and output power of each pair was recorded daily at one-minute intervals between 9am and 4pm.

During the “dry” period (which included a dust storm), each uncleaned module experienced a significant decline in performance. This was in addition to heat impacts; heat being the enemy of solar panel efficiency. After 182 days of dust accumulation without enough rain to wash it off, the power reduction for the uncleaned 0°, 15°, and 25° modules reached 80.4%, 75.6%, and 60.2% respectively.

The normalized monthly average power output across different tilt angles for the uncleaned solar panels during the year was as follows, with 1 being the scale’s maximum:

• 0°: 0.50
• 15°: 0.61
• 25°: 0.73
• 45°: 0.65
• 60°: 0.48
• 70°: 0.37

And the clean panels for comparison:

• 0°: 0.69
• 15°: 0.79
• 25°: 0.84
• 45°: 0.83
• 60°: 0.59
• 70°: 0.43

And graphing the above:

Little Rain Worse Than No Rain For Flat Panels

Following a light rain event (0.8 mm) during the dry period, performance of the dusty 45° tilt module improved, approaching that of its initial clean state. But the module installed flat retained a significant quantity of water on its surface, which combined with dust caused cementation as it dried that resulted in decreased performance. Here’s what that solar panel pair looked like, with labels included by the helpful researchers in case we couldn’t tell which is the dirty one.

Given the areas of dried gunk, this could also lead to “hot spots” if not dealt with. But a 6mm rain event occurred 16 days after the dust storm, significantly improving performance of all modules; particularly those at smaller angles.

After “heavy” rain events (7 mm and 9 mm back-to-back on two days) as the wet season came into play, another significant performance boost was noted. Those at the 45° and 25° tilt angles yielded the highest power output, and maintained it consistently until the end of the experiment.

25° The Sweet Spot

“A 25° angle demonstrated the best performance during the dry period, balancing effectively the reduction of dust accumulation and maximum capture of the solar irradiance,” state the researchers. “During the rainy period, the optimal angle shifted to 45°, which enhanced the module performance due to natural cleaning by rain. Over the entire year, modules with a 25° angle consistently performed best, even without regular cleaning.”

It’s important to note that generally speaking, optimal solar panel angle/tilt and orientation/direction will depend on what part of the world you’re in.

The full study report, titled “Impact of dust and tilt angle on the photovoltaic performance in a desert environment” has been published in the journal Solar Energy.

On a related note, we reported last week on a Finnish study that suggested using dishwashing detergent for cleaning solar panels should be avoided – find out why. By the way, you can pick up some tips on how to clean solar panels here, and read the debate on whether it’s worth it here.