Cloud Edge Effect- why does it happen and how big an effect?

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Cloud Edge Effect- why does it happen and how big an effect?

Postby Gordon-Loomberah » Fri Aug 13, 2010 12:58 pm

I vaguely recall someone recently asking about how big an increase in output you can get from "edge effect". Today during yet another mostly cloudy (but gradually clearing :) ) morning there was a small break in the clouds and I recorded a nice short-lived increase in ouput, 2630W from a rated 2190W. That is 1.2 times the rated output of the panels. The Outback FM-80 MPPT charging regulator is supposedly limited to 80A, but it is slow to clip to this level, and short overcurrent peaks do regularly occur. I once saw 100A out of it at ~25.5V.

The air temperature was 8.5C, and I measured the cell temps to be ~21C shortly afterwards. Larger effects would be expected at colder temps. The peak in the solar radiation at 10:33:30am was a bit too brief to show on the Pyranometer recording, which integrates over about 1 minute, but there was a peak of 896W/m^2 (on a flat surface) at 9:50am, which is ~1600W/m^2 on my tracked panels facing the sun (34deg above the horizon). The array showed a few peaks around 2300W then.

Cloud edge effect is caused by reflection of light from clouds and forward scattering of light within the cloud, which adds to the radiation coming directly from the sun. It is often said to be caused by refraction, but IMO this is incorrect. There is no giant lens in the sky with a focal lenth of a kilometre or two to concentrate the light on your panels! Certainly water droplets are always refracting incident light, but with a focal length better measured in microns than kilometres, that light is dispersed up at cloud level. Some hits other droplets and is reflected, some is refracted and further dispersed, well up in the atmosphere.

EdgeEffect.gif
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Re: Cloud Edge Effect- why does it happen and how big an effect?

Postby Helipos » Fri Aug 13, 2010 9:17 pm

I too would tend not to believe in edge effect but you could do a test to see what it might be.
Next time the sun is shining quite strongly get a hose and water your panels preferably from the underside, but on the glass should also work. This will see if its just extra output from cold panels. This personally I believe is what the effect is down too, or possibly a combination of cold panels and cool equipment.

Another thing anyone could do is to see if there is actually more light. One of two methods, by using a camera and mounting it on a tripod checking what the desired shutter speed would be at a fixed apeture. The other method would bet one of the old school light meters used for photography and see what it does because they are quite fast acting.

Ive got some good cameras so I might give it a whirl when the weather is just right.

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Re: Cloud Edge Effect- why does it happen and how big an effect?

Postby Gordon-Loomberah » Sat Aug 14, 2010 12:07 am

Helipos wrote:...Next time the sun is shining quite strongly get a hose and water your panels preferably from the underside, but on the glass should also work. This will see if its just extra output from cold panels.


Done that plenty of times over the years, that certainly does increase the output, but *never* to more than rated, usually only up to 80% or a bit more of rated. The Edge effect from bright clouds is real, as I have seen over 2.5kW from this array in early Feb on a hot day when the cells were 45C or more. Cold weather does make it much more likely though, I see greater than rated output quite often in cooler weather, pretty much on a daily basis when there are a few clouds around and the batteries are in the bulk charging stage.

I have a Lunasix F light meter that will read incident or reflected light, but of course I cant easily plot the readings. The pyranometer output shows there is a lot more radiation though, and for grazing edge effects (when the bright cloud edge lasts long enough for an integration cycle) the output is consistently high... such as around the previously mentioned 9:50am this morning- peaking at over 2300W.

I'll bore you with another graph :lol:

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Re: Cloud Edge Effect- why does it happen and how big an effect?

Postby westy » Mon Aug 16, 2010 12:31 pm

Can you measure direct and diffuse irradiation simulataneously? If so you could plot both and see if one or the other peaks significantly during the effect.

Alternatively if your sensor is tracking the sun, you could block the direct and see if the diffuse increases during these times. You will probably need to be creative in how you block the direct.

Would it be helpful to know where the extra irradiation is coming from?
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Re: Cloud Edge Effect- why does it happen and how big an effect?

Postby Gordon-Loomberah » Mon Aug 16, 2010 1:02 pm

westy wrote:Can you measure direct and diffuse irradiation simulataneously? If so you could plot both and see if one or the other peaks significantly during the effect.
Alternatively if your sensor is tracking the sun, you could block the direct and see if the diffuse increases during these times. You will probably need to be creative in how you block the direct.


Not very easily, the sensor is a pyranometer, which is fixed and facing straight up and is part of my Davis VP2 weather station.

Would it be helpful to know where the extra irradiation is coming from?


Its clearly coming from the very bright clouds in the vicinity of the sun in the sky, the sun certainly isnt varying in brightness coincidentally with passing clouds ;) It is simple to demonstrate that the clouds very near to the sun in the sky are much brighter than clouds in other parts of the sky with a camera's light meter through a telephoto lens. I know this from photographing plenty of irridescent clouds and other solar halo phenomena over the years.

Fixed arrays of panels, such as people have on their rooves, are not pointed anywhere near the sun much of the time, so will miss most of the larger edge effect peaks. I saw a nice 2.57kW peak (117% of rated) at 9:51am this morning, if I had fixed panels on a 20degree pitch roof, the panels would have been pointed about 40 degrees away from the sun, and the peak would have been under 2kW (~90% of rated power), and not notable at all.
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Re: Cloud Edge Effect- why does it happen and how big an effect?

Postby Russell Moore » Wed Aug 18, 2010 1:07 pm

What I find is that you can get long bursts of extra power when the sky is clear in the suns direction, but you have cloud directly overhead. This is especilly noticible with high cloud.
My best guess is that you are getting a form of refraction from the overhead cloud down onto your panels, as well as the normal radiation from the sun.
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Re: Cloud Edge Effect- why does it happen and how big an effect?

Postby Inspector » Thu Aug 19, 2010 12:10 am

My system has been connected for three months (almost to the day), and today was the first time I've seen the results of edge effect where the panel's rated output has been exceeded. It occurred right on midday, which has already been mentioned that without a tracked array, is the only time it's going to create the situation I've seen on my graphs today.

The results were as follows:

House array = 6x185w panels=1110W yet edge-effect produced a peak of 1279W (calculated at 15% above panel rating!).

Garage array = 9x185w panels=1665W and edge effect peak=1829W (10% above panel rating).

Even though it was for a few seconds (there were other peaks above panel's rated output, but these values were the highest), that is pretty impressive in my opinion!
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Re: Cloud Edge Effect- why does it happen and how big an effect?

Postby westy » Thu Aug 19, 2010 12:02 pm

However it happens, it just does. Thats why we have safety factors built in to the standards.
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Re: Cloud Edge Effect- why does it happen and how big an effect?

Postby Helipos » Sat Aug 21, 2010 12:27 am

I was doing a bit of thinking today (day dreaming while the trainee was doing some work) of a good theory for this observation.

We all know that light will reflect off water. The more shallow the angle of incidence the more light that is reflected. Now if we had a solar panel on a blanket overcast day. We get our cloud buster and punch a hole right through the layer of cloud, the light hitting the panel is normal output plus any light reflected from the edges of the cloud.

If we were to have a hole of a reasonable depth I'm thinking several kilometers thick. Plus a hole quite large in the scheme of things, we should get the light direct from the sun plus a lot of the reflected light off the sides of the hole would end up falling on average in the middle of the target. Thus creating a direct + reflected light energy for every area of ground near the centre of the target.

Granted this picture is way out of scale but you can get a simplistic idea from it.
There will be some convoluted relationship between cloud depth, hole size and power out of your panels. This reflection plus cold panels is what I'd suggest is the source.

Question for you Gordon how long have you ever seen an edge effect last?

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Re: Cloud Edge Effect- why does it happen and how big an effect?

Postby Gordon-Loomberah » Sat Aug 21, 2010 2:28 pm

Helipos wrote:Question for you Gordon how long have you ever seen an edge effect last?


Several minutes when there is a bright thin cloud edge passing near the sun. Whilst there is some reflected light contribution to the peaks in PV output, I think forward scattering plays a larger role, where a fairly thin cloud effectively becomes another light source in the sky, like a weaker sun next to the real sun. Clouds that are bright due to forward scattering are much brighter than clouds reflecting sunlight. The type of forward scattering is Mie Scattering, due to larger size particles relative to the wavelength of light, such as the small water droplets that make up clouds. It increases to a peak around the direction of the sun. Forward scattering is also why comets have a great increase in brighness when seen in the near-sun direction- sometimes bright enough to be seen in daylight right next to the sun, such as Comet Ikeya-Seki in 1965 and C/McNaught in 2007 to a lesser extent. Raleigh scattering spreads light, predominantly shorter wavelengths, around fairly uniformly in all directions and is due to smaller particles such as the molecules that make up the air- its why we see a blue sky overhead.
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