Cloud-Edge Effects on Solar Arrays

On March 23, 2020 my solar array’s output spiked from its rated 3.51 kW to 4.54 kW. The surge only lasted a few seconds, and the system can handle up to 4.0 kW continuous output, so no worries. But that 29% excess output was a surprise because solar electric systems usually perform at less than their rated output. Don’t get me wrong … I was thrilled to get that extra power. I just didn’t understand where it was coming from.

The graphic shows the user interface for my solar electric system. Note the weather banner along the top curiously displaying “cloudy” (more on that later). Note also the arrowed values. My array consists of nine CSUN 390W panels providing a total output of 3.51 kW at STC (standard test conditions). The official STC protocol uses 1000 W/m2 (watts per square meter) of white light on solar panels kept at 25°C (77°F).

1000 W/m2 is the average solar insolation at sea level on a clear day. The STC temperature must also be specified because, as a solar panel’s temperature increases, its energy output decreases. You can see on the weather banner air temperature was only 70°F, so that worked to my advantage.

Also, my array is 670 meters above sea level. Insolation here 5.3% higher than STC because there’s less air between me and the Sun. But there’s no way altitude and air temperature could account for a 29% spike, so I decided to go outside and check the sky. Here’s what I saw:

Note: For the top photo, with the camera pointing directly toward the Sun, I used an exposure of 1/1000 s. That makes the sky look darker than it really was. To my eyes, the sky was the same bright blue you see in the second photo.

Both photos show the Sun near open patches of sky flanked by bright white clouds. My first thought was that any amount of clouds could only reduce the output of a solar array. I was obviously missing something significant. After a bit of online research I found the answer.

The explanation involves something called the cloud-edge effect. Under certain meteorological conditions the solar insolation at ground level can exceed 1000 W/m2. This can happen when sunlight scattered through clouds exceeds the amount of light normally contributed from clear blue sky. You can see in both photos that the cloud edges are brighter than the adjacent sky.

On a clear day, according to NREL, 85% of ground-level insolation comes from direct sunlight and 15% comes from diffuse light scattered by the rest of the sky. That diffuse light is the blue wavelengths we see as the color of the sky. Other colors are scattered less strongly and continue on their original path.

The following graphic shows how fortuitously positioned clouds can increase the insolation at ground level to greater than 1000 W/m2 and cause an array to produce more than its STC rated output. That +290 W/m2 value is estimated from my 4.54 kW spike. Your mileage may vary.

Water droplets in a cloud will reflect the Sun’s rays randomly via Mei scattering. Several outcomes are possible. The yellow rays (left to right) show that light can:

  • reflect off the top of the cloud and back out to space
  • bounce around inside the cloud, then head back out to space
  • bounce around inside the cloud, then head toward the ground
  • bounce around inside the cloud, then head toward the array

Where the cloud is thick, less light makes it through to the ground — that’s why most clouds are darker at the bottom and brighter at the top. When the Sun is shining directly through a hole in the clouds, as shown, one can sometimes get “bonus” energy from light scattered by thin clouds around that hole and elsewhere in the sky.

On partly cloudy days there’s an additional effect that can further boost array output. While the array is shaded by clouds it will be cooler and running at higher efficiency. If the Sun then pokes through a hole in the clouds, the array will continue operating at higher efficiency until its temperature rises. This can add to the cloud-edge effect and, at least for a short period, cause an even higher peak in array output.

It’s a good thing to understand how your technology works and what constitutes “normal” behavior. I had already been surprised that, even on a totally overcast day, the array would produce around 0.5 kW from the dim greyish light. And today I learned that, despite what “common sense” would dictate, partly cloudy days can (briefly) provide more solar energy than clear days.

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2 comments on “Cloud-Edge Effects on Solar Arrays”

  1. Isn’t this kind of like the erroneous thinking that being in the water at the beach protects one’s skin from burning when in fact the water reflects the sun’s rays more intensely causing a faster burn?

    1. Exactly! Anything that redirects sunlight that would otherwise miss you, and causes it to strike you, will increase your UV exposure. Snow cover can do that too. Or an evil giant with a magnifying glass. 🙂

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