Really goofy shading question

dropkick

Will the shade from the utility's overhead powerline drop to the weatherhead significantly affect a panel underneath it? The shadow is running lengthwise down the whole panel. The line is only about 2' above the roof so it's a good 1" strip of visible shadow. The weatherhead itself is out of the picture. Panels would be in series 3 x 250W so I'm worried about it dumping half the system. There will be another string of 3 next to it and if I shove them all over and invade the 3' set back and leave one panel by itself on the otherside of the shadow it might barely fit (I don't really care about the code setback cause these are just sitting up there in an experimental off-grid system, non penetrating mounted on the flat part of the roof, but *I* still need to be able to walk up there to get the rest of the roof and this area is the easy way up )
Just curious what you think. I'm going to go prop up a panel up and see what a rake handle does to the voltage at different distances and angles...

dropkick

OK so I can answer myself... I didn't plan it, but my test area happened to be under that same powerline, tho 8 feet lower. I tested with a 1/2" EMT conduit shadow maker as well. Looks like Voc drop is about 0.1 - 0.2V with a shadow crossing slightly diagonally "2 columns" of cells over the entire lengthwise run. What was more interesting was moving the panels from a shaded by unconditioned storage area into the sun I could watch the voltage steadily drop 5V (38 to 33) as the panel heated up from 86F ambient to 150F front and 162F underside. They're not lying about temperature effects! I let it cook for 20 minutes before trying the shadow trick again. So 0.2V open voltage is nothing, but I know that's going to be more telling with a real load....

littleharbor2

Your amperage would be more affected by the shadowing. What are the short circuit amperage readings with your test panel?

BB.

Yes, power lines across an array can kill upwards of 50% of the output pretty easily.

Problem is that solar cells only allow/generate current vs solar energy exposure. If a cell is shaded, it stops current flow. Which kills the output of that section of panel.

You need to measure both current and voltage under operating conditions. Voc and Isc are not good enough.

-Bill

inetdog

BB. wrote: »

Voc and Isc are not good enough.

-Bill

Correct, since bypass diodes will prevent shading of one or two sections from affecting Isc or even Imp, but can really affect Vmp

dropkick

So I hooked up one panel (24V 250W) to our r-pod trailer which has an MPPT CC in it. Shut off shore power and turned everything on I could, including the 12V fridge. I've done so much to reduce power in there, it was hard to find extra load... Anyhow I put 196.4W (25.82V 7.61A) of load on one panel randomly leaned up with very little 'tuning'. I tried some 1/2" EMT and some 1" RandomStick. Pretty much regardless of direction it knocked down the power to 165W (25.69V 6.47A). The harder the shadow the more it dropped. The EMT directly on the panel did the least about a 10W drop.
So I can see that on one panel by itself the amperage drop may not make a huge difference however with the plan for 2 or 3 in series this drop would rapidly be magnified (88W in this example). Power line is no bueno!

inetdog

dropkick wrote: »

So I hooked up one panel (24V 250W) to our r-pod trailer which has an MPPT CC in it. Shut off shore power and turned everything on I could, including the 12V fridge. I've done so much to reduce power in there, it was hard to find extra load... Anyhow I put 196.4W (25.82V 7.61A) of load on one panel randomly leaned up with very little 'tuning'. I tried some 1/2" EMT and some 1" RandomStick. Pretty much regardless of direction it knocked down the power to 165W (25.69V 6.47A). The harder the shadow the more it dropped. The EMT directly on the panel did the least about a 10W drop.
So I can see that on one panel by itself the amperage drop may not make a huge difference however with the plan for 2 or 3 in series this drop would rapidly be magnified (88W in this example). Power line is no bueno!

The drop with two or three in series should be no worse than the drop with all panels in parallel. The drop in power from a single panel will show up either as a drop in voltage with roughly the same current (as the bypass diode routes current around the shaded cells) or as a drop in current at roughly the same voltage if the MPPT algorithm is not actually finding the real power peak or the current reduction is small.
If only one panel in a string is partially shaded, you will almost certainly end up in the first condition rather than the second, and the total power will be pretty much the reduced power of the shaded panel plus the full power of the unshaded panels.

If your panels do not have working bypass diodes, then the current will in fact be reduced in the whole string and the shaded cells in the shaded panel way suffer permanent damage.