MAILBOX: Smoke & Solar

Intermediate
The two lower rows of modules (one subarray) have been washed; the upper two rows are still covered in ash and dust. The difference in production was nearly 8%.

The summer of 2018 was a bad year for wildfires in the West. Where I live in southern Oregon, we had about six weeks of continuous wildfire smoke from nearby and even not-so-nearby fires. Sometimes, the smoke was so thick that the sun looked like a matte orange rubber ball in the dark gray sky; during those times, air quality here was reported to be the worst in the world.

My estimates are that my PV system’s production was reduced 15% to 30% during those smoky days; smoke, like clouds, prevents sunlight getting to the PV array. This is a rough estimate, since I’ve only had the array for two summers—and we had some smoke last summer, too. Peak sun-hours in June sometimes reach 40 kWh. In August during the fire season, we had just over 20 kWh. A 30% loss may be conservative.

Besides the smoke itself blocking out the sun, it left particulate matter on the array (and cars, and pool toys, and plants). Little ashy-dust particles covered everything and turned into a sticky, brown, sweet-smelling film if it got damp. For sure, this too affected my PV system’s production.

On September 1, when the smoke had cleared, I compared the output of my two identical series strings. At 4:33 p.m., they were producing 1,317 and 1,313 W, respectively—a 0.3% difference. I gave the bottom subarray a good washing. The next day, near noon, I again compared the two subarrays. The still-dirty string was producing 2,056 W, but the freshly cleaned string was now producing 2,232 W—almost an 8% difference.

Assuming that the performance reduction from the ash buildup increased over those weeks, we can speculate that dirty modules were ultimately contributing 25% to 50% of the losses. To be fair, we often don’t receive any rain in the summer. Plus, we’re located near farms, so dust buildup is typical. But as wildfire smoke and ash become more typical, here’s a reminder to wash your array. It’s more important than washing your car!

Benjamin Root • Home Power art director

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