Blocking Diodes on each Panel in a Series String – Partial Shading

[Crashcourse]

Hello folks, I’m wondering if it is feasible (or stupid) to put blocking diodes (I have a bunch of 20A 50V) on each panel positive output in a series string if the panels experience partial (often one part panel at a time) shading due to neighboring trees? Currently, one array consists of 9 100W 21.6 Voc panels in a 3S3P configuration paralleled in a combiner box (no diodes in this particular combiner) mounted on an awning that experiences the mentioned partial shading. I also have a separate 6 panel 3S2P array into a separate CC that doesn’t get much shading until mid winter but if blocking diodes would help…
Also have some bigger panels to get up and running once the snow is gone.

I’m aware there will be a slight voltage loss across the diodes that I’m willing to live with, just want to know if this is a dumb idea.

Bonus Question:
Concerning cold temperature voltage increase, it gets COLD here, often down to -35C (-31F), -45C (-49F) windchill (I’ve personally witnessed -38C (-36F), -51C (-59F) windchill).
On the coldest sunny day (so far) this winter I’ve noted the CCs reporting a max of 72V @ -34C, -46 windchill from the 3S arrays.
That’s 24V per panel. I’d like to go 4S, I have 100V max input CCs.

Is this also a dumb idea pushing the CCs to their max? I could go to 150V CCs but if this could work..?

Thanks for any help!

 

[rmaddy]

Do your panels already have bypass (not blocking) diodes? If they do then a partially shaded panel will already still allow the rest of it to work at a diminished power.

What is the actual temperature coefficient for Voc for your panels? With that you can calculate the exact possible Voc your panels will reach at a given temperature. Many panels are at about -0.3%/ºC which would put your panels at 101.7V at -34ºC. If the coefficient for your panels is a little higher then you might just stay under 100V.

 

[Crashcourse]

Thank you for the reply!
Yes they do already have bypass diodes, I was under the impression that one shaded panel in a series string would pull down *all* panels in that string. Is that not correct?

 

[chrisski]

Thank you for the reply!
Yes they do already have bypass diodes, I was under the impression that one shaded panel in a series string would pull down *all* panels in that string. Is that not correct?

I tried measuring this once, and the added panels did not pull down the panels in full sunshine.

I had partially shaded panels In a 3S3P setup where they were oriented East South and West. I measured the output of each string by shutting the other two circuit breakers off. To my surprise, the total output was close to the sum of the three stings individually. Of course I did not write down the results, but it varied between 30 watts and 300 watts for the three strings. I consider this here say (not the right word) but the math did not validate that.

What I read about blocking diodes is they should of shut down the lower wattage strings to maximize output, but they did not. I’ve been meaning to record my results after trying a couple of times but have not gotten to it yet. The blocking diode should have shut off the 30 watt panel to let the 300 watt panel deliver fully, instead it without a blocing diode the total wattage wasString 1 + String 2 + String 3 = total wattage.

If when I repeat my results, I get the same thing, means blocking diodes are not quite what they’re supposed tobe. Will does say in one of his videos there is loss, but what he expects is they are insignificant like 2 watts per diode.

 

[rmaddy]

Yes they do already have bypass diodes, I was under the impression that one shaded panel in a series string would pull down *all* panels in that string. Is that not correct?

My understanding is that if the panels have no bypass diodes then one partially shaded panel will bring down the whole string.

But with bypass diodes, only the shaded panel is affected. The other (unshaded) panels in the string will still produce the same voltage as if the partially shaded panel wasn't shaded. Of course the string will be reduced some due to the loss in the shaded panel.

Let's say the panels with a Voc of 22V have 2 bypass diodes which splits each panel into thirds. 4 in series with no shading can produce 88V. If one panel has shading only on 1 of the 3 sections then that one panel will be reduced by 33% down to about 14V but the other unshaded panels will each still be 22V. So the whole string will be reduced to about 81V instead of 88V.

 

[mikefitz]

Is this also a dumb idea pushing the CCs to their max?

yes especially with Epever controllers, components don't seem have enough overhead to tolerate higher voltages than spec, and there are reports of systems behaving strangely when operating near the maximum.

I’ve noted the CCs reporting a max of 72V @ -34C

this is the loaded voltage of the panels, the OC voltage ( that may be experienced whilst the MPP is being determined) will be higher . A nominal 22v OC panel would be higher than 25 volts at that temperature. A 4S string would easily exceed the 100 volts.

but if blocking diodes would help…

blocking diodes will do nothing to reduce shading effects.

Mike

 

[upnorthandpersonal]

The only real solution to partial shading when you can't move the panels or remove the thing causing shade are optimizer. I don't think they're worth looking at for 100W panels though, they probably cost more than the panel, if you can even find them for those sizes.

 

[acdoctor]

You could reconfigure both SCC to 2s. You would loose one panel. You didn’t say what battery voltage. This would work for 12 or 24 volts.

 

[sunshine]

If when I repeat my results, I get the same thing, means blocking diodes are not quite what they’re supposed tobe.

Initially I put blocking diodes on my 12v setup because that was the recommended thing to do when parallel panels have different orientations.
As with your results I found they were not needed and have been removed.
Probably because when disconnected even a shaded unloaded panel's V would exceed that of a panel in the sun under load.

 

[Crashcourse]

yes especially with Epever controllers, components don't seem have enough overhead to tolerate higher voltages than spec, and there are reports of systems behaving strangely when operating near the maximum.

this is the loaded voltage of the panels, the OC voltage ( that may be experienced whilst the MPP is being determined) will be higher . A nominal 22v OC panel would be higher than 25 volts at that temperature. A 4S string would easily exceed the 100 volts.


blocking diodes will do nothing to reduce shading effects.

Mike

Excellent, thanks Mike! With no disrespect to others who commented (I appreciate them) this is the type of answer I was looking for!
Thanks again so much for setting me straight.

 

[Crashcourse]

You could reconfigure both SCC to 2s. You would loose one panel. You didn’t say what battery voltage. This would work for 12 or 24 volts.

24V battery voltage presently, I expect to be stepping up to 48V in the future. I will be investing in some 150V CCs because I would like to keep the voltage on the higher side.
Thanks for the reply!