Panel mix and match - diodes?

catkin · May 2008

Hello :-)

Thanks so much for the in-depth info here, especially the panel mixing guidance "two basic rules. One is that the PV modules wired in series have substantially the same Imp specs. The other rule is that the PV strings wired in parallel have substantially the same Vmp specs.". I've been trawling the internet for ages trying to find something like that.

What about the need for blocking and/or bypass diodes with a mix of panel types and ages?

Here's the situation ...

I've inherited a real mish-mash of panels, created by cannibalising two old 12 V systems and one old 24 V system then building the parts into a 48 V, 60 A, 220 V, 2.5 kVA system.

Unfortunately it was done the easy way, taking whole frames of panels from the old systems and throwing them together in ways that add up to 48 V. There are 64 panels in all, ranging from 35 W to 75 W, from 1 year old to more than 12 years old. No documentation. I cant find specs for some of the panels, especially the BP-Tata models. The ID stickers on the oldest ones are so burned they are illegible. Some of the panel types/ages mixed together on single frames suggest warranty replacements so further failures are likely if they haven't happened already.

Presumably the sanest re-arrangement is 16 strings. With so many panels we could easily have faulty ones and not know. Would it be a good idea to fit blocking and bypass diodes to minimise the effect of faulty panels?

mike95490 · May 2008

Re: Panel mix and match - diodes?

Something like that situation, I'd spend a day getting all the wires accessible, so that on a nice sunny day, you can zip thru your array with a good DVM, and measure Open circuit voltage on each panel, and then zip thru again, and measure short circuit amps for each panel.
For every diode you install, you loose a volt across it, so any diodes should be installed with forthought, not just shotgunned in.

Mike

niel · May 2008

Re: Panel mix and match - diodes?

catkin,
as was mentioned you need to match things up somewhat to make it work its best. if you have that many in the mix, it may not be possible to match them up very well. a mismatch will present losses, but may be better than omitting them from the mix.
be aware that some pvs already contain built in blocking diodes and many have built in bypass diodes as well. it is a waste to have more than 1 blocking diode per string as this is meant to prevent the backflow of current into the pvs when the pvs are not producing, but you can add bypass diodes across all pvs if you wish to. be sure the diodes are large enough to handle the current the string will present to them.

catkin · May 2008

Re: Panel mix and match - diodes?

mike90045 wrote: »

... measure Open circuit voltage on each panel, and then zip thru again, and measure short circuit amps for each panel.

Thanks Mike

That makes a lot of sense -- identifying any defective panels before planning a re-arrangement.

mike90045 wrote: »

For every diode you install, you loose a volt across it, so any diodes should be installed with forthought, not just shotgunned in.

That's a useful guideline "loose a volt". The danger is not lack of forethought, it's "analysis paralysis"

!!!

I want to thoroughly understand the issues before deciding what to do. One of the issues is panel degradation. In my (limited -- less than 2 years) experience we've had 3 warranty replacements and had another 2 refused because panels were out of warranty period. Many of the older panels have turned brown. Perhaps we've been unlucky but experience says significant onging degradation is characteristic of our panel population.

So what I'm thinking (please tell me if I'm wrong) is

even if these panels could be matched up perfectly today, they would not stay matched as differential degradation changes their matched characteristics.
blocking diodes will prevent reverse current flowing in "bad" strings. As well as degraded panels, "bad" strings includes fault conditions that result in destructive reverse currents.
bypass diodes will prevent low current panels throttling the current from good panels in the same string.

If I got these right, the guiding design question is "Do these benefits justify the loss caused by dropping 1 V across each diode?" and I don't have data to answer it.

mike95490 · May 2008

Re: Panel mix and match - diodes?

How old are panels that have expired warrentys ??

It may not be worth all the trouble to select and re-wire, if it's going to continue to get failures.

catkin · May 2008

Re: Panel mix and match - diodes?

~12 years. The warranty was 10 years.

BB. · May 2008

Re: Panel mix and match - diodes?

You can use Schottky diodes--they run around 0.2 volt drop. And you would only need to put one in each series string--not one per panel. That will reduce your power losses to an acceptable level.

I was going to say you may not even need a blocking diode (blocking diode typically used to prevent a low current leak at night from the battery back driving the panel, but a solar charger prevents this--But if you have any shading, I am not sure how well a block panel does with 36+ volts back driving it (from the other panels in the string).

The bypass diodes--I am not sure I would even bother with that with so many panels. Too many splices where water could get in or wiring to break. And, I am not sure you would see any useful power "saved" by having a bypass diode.

And, you should test every panel for Voc and Isc... Good Poly/Mono Crystalline panels should not degrade in 20 years more than 20% (and should be less). If you have a degraded panel (say a 30 watt degraded to "15 watt specifications")--don't bother to move it to a "15 watt string", it will continue to quickly degrade and kill the string's output.

Also, you should probably test panels into a 12 volt battery as a load--there have been a few reports of panels that test good with Voc and Isc testing, but do not output rated current at rated voltage.

Lastly, if you can, mount the Schottky diodes in one place on a heat sink where you can easily get to them with an accurate digital volt meter... If the temperature of the diodes is all the same, you can use a DVM to measure the voltage drop across the diodes and make sure each string is performing correct. The voltage drop is non-linear with current (sort of exponential, as I recall--as a non-linear shunt)--but you can log the voltage drops when you first install and check every few months or years, and make sure that each string is still functional.

If you find a good deal--try and get an "active" DC current probe so you can measure the currents in your Solar Array without disconnecting the wires. (may not be worth the cost for your one system--but if you get into the "solar business", it will be helpful).

And, don't forget fuses... If you create a Diode box--this would also be a good place to install fuses and holders. Because you don't have specification sheets for all of the panels, probably just use a fuse that rated 1.5 to 2 times your short circuit current (just get the next larger standard fuse size). You can also experiment and use your DVM to measure the voltage drop across your fuses--they will give more linear voltage drop readings vs current (if your DVM is accurate enough at low voltages).

If you have the chance to find/purchase more solar panels--try to stay away from the thin film if you can. The Crystalline panels usually have better life. And stick with glass panels--try to stay away from the plastic and flexible panels (more chance for water/air to get in and cause the panels to fail).

And try to keep wiring and boxes well sealed against rain and for "boxes", use "drip loops" (where the wire goes down and back up before entering a box or a splice), and drill small "weep holes" in the lowest corners if any "sealed boxes" to let any water/condensation that does get in to drain out (are you near Chennai / Madras India--lots of rain?).

Make sure that wiring cannot flex at joints, boxes, panels, in wind (flexing will cause insulation failure). If you are not using UV/outdoor rated wire, cover the wire/use conduit to protect from sun weather.

If you find any "burned/brown/black" areas in a panel (especially where the metal that joins cells are)--first try to clean the glass and verify if the brown/black is under the glass rather than dirt on the surface. Any panels that are obviously failing (or are cracked, etc.), will probably not last very long... And usually are not repairable (although, it might be worth trying to go through the backing and repairing the rosin based solder, cleaning, drying, and resealing--you have nothing to lose).

About all I can think of at this time...

-Bill

PS: Don't know about India, but from what I have seen, 10 year warranties are typically for thin film panels--generally, they don't seem to last near as long as "brand name" mono/poly crystalline panels (typically 20+ year warranties--and last well beyond that).

catkin · May 2008

Re: Panel mix and match - diodes?

niel wrote: »

.... if you have that many in the mix, it may not be possible to match them up very well. a mismatch will present losses, but may be better than omitting them from the mix.

be aware that some pvs already contain built in blocking diodes and many have built in bypass diodes as well. it is a waste to have more than 1 blocking diode per string as this is meant to prevent the backflow of current into the pvs when the pvs are not producing, but you can add bypass diodes across all pvs if you wish to. be sure the diodes are large enough to handle the current the string will present to them.

Thanks Neil

Like you say, even if I can't match the panels up very well any contribution is better than nothing; we already have the modules, frames, wiring boxes etc so the cost of using them is small.

Understood that some modules include blocking diodes. Regards the ones I can't find any specifications for, is there any way to find out by inspection?

That's a helpful guideline - the only cost of bypass diodes is financial. Now all I have to do is work out whether probable output gains justify that cost ...

catkin · May 2008

Re: Panel mix and match - diodes?

Wow, Bill

That's a full and informative reply. Thanks a lot. I feel the fog of ignorance dispelling, not just with theory (I've been picking up bits here and there) but with real-world experience.

The panels are not partially shaded except by passing clouds.

Noted, what you say about bypass diodes not being justified by probable energy loss reductions.

Helpful, too, about panels going downhill fast after losing half their capacity (that's difficult information to find on the 'net and I would have found out the hard way by keeping them and watching them go down fast).

What a neat idea -- recording the "healthy" voltage drop across each string's diode and using them as a trouble tell-tale. It's almost worth fitting diodes just for that

. It allows ignoring your advice about getting an active DC current probe, though! With the biggest modules at 75 W the maximum current from each string is well under 10 A so within the capacity of our (relatively cheap) DVM and if we only had to open up the suspect ones ...

Fuses could definitely do with re-engineering (like, er, having some) but I was inclining to circuit breakers. As you noticed this installation is in Tamil Nadu, south India. Our rainfall isn't as much as you might imagine because we're on the east coast (far enough back not to have salt in the air) but it is hot and damp -- ideal for corrosion. When it does rain it's quite heavy (but not a patch on the Nigerian south coast!) and lightning can be severe. So far, experience with breakers has been encouraging with the only failure caused by a lightning strike that also cracked the wall the breaker was mounted on. Excused! Likely corrosion at fuse contacts (am I being pessimistic?) would devalue their usefulness to measure current by small voltage drop, even if our cheapo DVM was accurate at such low voltages.

Regards crystalline/glass panels, that's helpful to know. All the seriously degraded modules I know about are BP-Tata panels and those are the ones I've not been able to find any specifications for. We're being offered "Premier Solar" 75 W panels now (to upgrade another system). They are described as "MONO / MULTICRYSTALLINE SOLAR CELLS" with "High Transmission Glass 3.2mm Tempered". Good.

"Drip loops" and "weep holes" make lots of sense. Apart from occasional severe cyclones, wind speeds are low, especially when it's raining so wind-driven rain is not a problem and we can keep stuff dry enough by mounting it under panels.

Regards UV, I don't know the cable specifications but colours are fading where the conduit is missing or loose so there's another bit of fixing to do.

We have very few cracked panels but some of what look like "hot spots" - indicating conducting joints have developed resistance? There's a lot of ingenuity and determination available from workshops hereabouts. People repair computer motherboards! So repair may well be an option, especially with so little to loose.

As for "About all I can think of at this time...", it seemed PDF to me and thanks again for responding so generously!

BB. · May 2008

Re: Panel mix and match - diodes?

Yes, you can see round(ish) "hot spots" light brown at edges to black at center---You can post a picture or two and let the experts here see them...

I have had brown areas on my panels--but, so far, those are probably the result of tree leaves that leave brown stains (especially near the lower edge of panels)... A little soap and water + scrubbing, the the stains went away.

Others here, like Wind-Sun and Solar Guppy have seen more failed panels than I will every see good--so they can probably better describe failure sites.

But, in general, if you see something wrong (coloring, water, mineral deposits, etc.) under a portion of the glass that looks "different" from everything else--it is probably a warning.

Normally, I would recommend breakers for just about everything--but that may be overkill here. Breakers are large and expensive--and have more parts to corrode/fail. Fuses are simpler... Perhaps, if you have a marine shop near by (or some access to a few boats)--you can see what they did to make a reliable installation (or even get parts from a "Junk Yard / Wrecker / Recycler) that is made for damp environments.

Fuses and fuse holders do fail too... Taking your DVM and measuring the drop across the "whole" circuit path (diode/fuse voltage drop) will quickly tell you if you have a poor connection (or, just measuring the diode drop may be enough).

-Bill

niel · May 2008

Re: Panel mix and match - diodes?

catkin,
yes, you can determine if there's a blocking diode present by allowing the battery to pass some residual current back to the pv when the pv is darkened (light is totally blocked). it will be a small amount of current due to the resistance the pvs themselves would impress upon the battery. when you have a blocking diode present, the current won't pass.

System2 · June 2008

Re: Panel mix and match - diodes?

Do you guys ever use mosfets as diodes?

mike95490 · June 2008

Re: Panel mix and match - diodes?

Here's a photo of a "brown spot" one of my 22 panels developed 6 months into use. Others all looked fine. Installer replaced it, no charge. Being in a corner, it had no effect on output that I could tell. I sort of think it was related to a weak contact, and the contact heated and burned, and the actual brown was some sort of chemical reaction. the 3 months from detection to replacement, there was no change in size of the spot. The panels have textured glass, that is the surface finish texture you can see.

catkin · June 2008

Re: Panel mix and match - diodes?

Hi all

I've been holding off saying thanks until there was an update but other works have needed doing and time is slipping by ...

Thanks!

Panel mix and match - diodes?

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