KillaWhat
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Snow-Solar Panels-microinverters and weird production values

Sun Feb 22, 2015 2:23 pm

So it’s been snowing.

This is the first winter with my little PV Array.

I installed Black on Black panels in the hope that they would handle the snow better, and they do a pretty good job.

90% of the snow melted off quickly, leaving a little bit at the bottom of a couple of panels.
I figured with about 5% of the panel still covered, I would have 95% output, but this is not the case.

Production is off by about 90% for these panels, which didn’t make much sense to me.

My theory is that since the individual cells in each panel are wired in series instead of parallel, then we are seeing the production of the worst case cells.

This is a great real world demonstration of why you don’t want to daisy chain the panels for high voltage DC using a single inverter.

With a single inverter, we see the output of the lowest producing panel as the output, but with micro inverters, each panel is a stand-alone- self-contained power producer.

If anybody has a better explanation as to why these panels are not producing, please correct me.
Thanks.

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RegGuheert
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Re: Snow-Solar Panels-microinverters and weird production va

Sun Feb 22, 2015 3:30 pm

KillaWhat wrote:My theory is that since the individual cells in each panel are wired in series instead of parallel, then we are seeing the production of the worst case cells.
Basically. Here is an NREL presentation which goes into much more detail.
KillaWhat wrote:This is a great real world demonstration of why you don’t want to daisy chain the panels for high voltage DC using a single inverter.
It sure is.
KillaWhat wrote:With a single inverter, we see the output of the lowest producing panel as the output, but with micro inverters, each panel is a stand-alone- self-contained power producer.
Exactly.

I have a fake dormer on my roof below a couple rows of panels. You can see it in the pictures at the link in my signature. Normally it doesn't cast much of a shadow until the sun is very low in the sky, but when it snows, that thing keeps a couple of panels from shedding often for days after the others have been fully clear. In fact, it happened again today. The other 52 panels have shed their snow, but those two are unable to because of this adornment. I'm inclined to remove the darned thing and put more panels there instead.
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smkettner
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Re: Snow-Solar Panels-microinverters and weird production va

Sun Feb 22, 2015 4:59 pm

The panels have bypass diodes so if a panel is blocked from shade the others in series push the power through the bypass diodes.
It is not all or nothing by putting the panels in series with a central inverter. Shade does have significant issues with either set up.
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DNAinaGoodWay
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Re: Snow-Solar Panels-microinverters and weird production va

Sun Feb 22, 2015 8:30 pm

I modified a roof rake with a soft brush head and pull snow off my panels when needed. Your shed roof doesn't look that high, you might want to try something similar.
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BrockWI
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Re: Snow-Solar Panels-microinverters and weird production va

Mon Feb 23, 2015 7:06 am

I also get impatient waiting for the snow to slide off. I use a 20 foot pool brush / broom. It seems like every time it snows a lot we have lots of clear cold sun the next day :) most of our panels are tilted at a 45* but the last set is only about 30*, the 30* ones definitely take longer to clear themselves off. Also I am not using micro-inverters, they are all feeding a charge controller, so a bit of shading on on of the four 250w panels means I loose 1000w.
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essaunders
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Re: Snow-Solar Panels-microinverters and weird production va

Mon Feb 23, 2015 7:51 am

that's an awesome array! I almost want to build a shed/barn so I can self-install an array like that (as opposed to putting the panels up on my main house)!

KillaWhat
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Re: Snow-Solar Panels-microinverters and weird production va

Mon Feb 23, 2015 9:03 am

essaunders wrote:that's an awesome array! I almost want to build a shed/barn so I can self-install an array like that (as opposed to putting the panels up on my main house)!


Thank you.
I keep feeling like I should be able to put the whole thing on a big lazy Susan or rollers , and make it track :D
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NeilBlanchard
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Re: Snow-Solar Panels-microinverters and weird production va

Mon Feb 23, 2015 10:30 am

Once you lose a certain threshold to a shadow/cover percentage on a panel, you lose the output for that panel. The two low output panels are the same ones that have some snow on them in the photo.

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dgpcolorado
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Re: Snow-Solar Panels-microinverters and weird production va

Mon Feb 23, 2015 3:57 pm

DNAinaGoodWay wrote:I modified a roof rake with a soft brush head and pull snow off my panels when needed. Your shed roof doesn't look that high, you might want to try something similar.
I use a window squeegee on a long painter's pole (16 feet fully extended) to pull snow off of mine.
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Re: Snow-Solar Panels-microinverters and weird production va

Mon Feb 23, 2015 7:45 pm

Kudos for thinking about the color of your panels/frames re snow clearance, and I assume you figured you'd gain more in winter than you'd lose in summer. As others have mentioned, with such a low tilt angle you're going to need to use a brush to clean snow off the panels if you want max. output with minimal waiting time. And clean them, too; you may find dirt accumulating along the bottom corners/edge of the modules, which can have a considerable effect on output. But do your cleaning early in the day when the modules are cool, to minimize thermal stress.

On the plus side, with a string inverter you're much better set up to go off-grid at some point down the road (if you aren't already) than if you used micro-inverters.
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