Mismatched solar panels (really different)

Venezuela Solar · Sep 17, 2020

I have a client that has (1) 300w solar panel with this spec's

300wp
33.24v Vmp
9.03 Imp
40.3 Voc
9.70 ISC

And (4)100w solar panels with this spec's

100wp
18.2v Vmp
5.5 Imp
21.4 Voc
5.7 ISC

And want to improve the output, i know that this modules are very different, but i still want to try to help him, i have some ideas that i plan to test

1) get rid of the 300w and only use the 4 units of 100w in series (the MPPT supports up to 145VOC

2) wire the 100w modules in two groups of 2S and wire the 300w in parallel too, it would be 200w@24v + 200@24v + 300w@24v, all in parallel

3) wire all in series (I think this doesn't make sense)

4) 3S 100w and parallel with the 300w, discard the extra 100w for other use

Does any idea makes sense? Any safety issue with any of the options?

snoobler · Sep 17, 2020

#2 is optimal. You'll lose a about 10-15% with 2S2P 100W panels paralleled to the 300W.

#3 would limit the current from the 300W to 5.5A nearly cutting its output in half, and you'd be too close to the 145V limit.

#4 wouldn't work at all. way too much voltage disparity between the types.

With #2, all your panels are Vmp at 33-36V

Venezuela Solar · Sep 18, 2020

snoobler said:
#2 is optimal. You'll lose a about 10-15% with 2S2P 100W panels paralleled to the 300W.

#3 would limit the current from the 300W to 5.5A nearly cutting its output in half, and you'd be too close to the 145V limit.

#4 wouldn't work at all. way too much voltage disparity between the types.

With #2, all your panels are Vmp at 33-36V

Thanks!

spelbynder · Sep 18, 2020

I have a similar question. I just bought a house with a 24 volt system and 2 panels that I have no specs for since they are attached to the roof already. Before buying this house I bought 3 solar panels with the following specs, intending to build a system at my previous place.

Pmax 345W
Vmp 38.5V
Imp 8.96A
Voc 46.7V
Isc 9,55A
Max series fuse 15A
Power Selection 0~5W
Module application Class A
Maximum System Voltage IEC1500V/UL1500V

My inverter is a Magnum RD1824
Input V nom 24VDC
Input V range 21.6-31.0VDC
Power rating 1800W
Surge rating 3600W
Rated power 95A
Full V 0.750A

I'm a noob so I don't know if I can add my new panels to this older system, or maybe replace the older panels with my newer ones. Can you help? Thanks.

snoobler · Sep 18, 2020

Without knowing your old panel specs, it's impossible to know for sure. Either dismount to get specs or measure a single panel's Voc and Isc.

GXMnow · Sep 18, 2020

The best way to join panels with different specs is to use separate charge controllers, but you can pretty much parallel any group that has the same number of series cells. So in the first example, I agree that two of the 100 watt panels in series looks pretty close to the 300 watt panel voltage, so doing the "Option 2" works fairly well. Can you count the number of cells in each panel? You will get a voltage close to the VMP of the weakest panel, maybe a little higher, but below the best panel. And the current of each panel string will add up. So voltage will be about 33 and current will be a little less than 5.5 + 5.5 + 9.03 = 20 amps in perfect sun. So about 600 watts peak, more realistic is about 75-80% of that at Solar noon. Figure real 500 watts.

If you series panels, then you want to match the current at maximum power. So you might get away with two pairs of the 100 watt panels in parallel, and then series those two pairs with the 300 watt panel also in series, but the parallel option 2 is probably still better. You will get the current of the weakest set in the series, and the voltages will add. In this case, you get about 9 amps limited by the 300 watt panel, and 18+18+33 = 69 volts. Again, in perfect sun, maybe 600 watts again, and 80% = a little under 500 watts.

Either case could make about 3 KWH on a good sun day.

On the second example, You do not mention a charge controller, and you don't know anything about the old panels.

Does the existing system have a charge controller? Can you measure the current and voltage of the existing panels?

If the current 2 panel system has a 24 volt charge controller, just get another charge controller for the new panels, and they can both go to your battery bank and their charge currents will add together. With your 3 panels, I would suggest a controller that can handle 150 volts and run your 3 new panels all in series. You will want a charge controller rated at 40-50 amps to handle a peak of almost 1,000 watts into your 24 volt battery. In most cases, you will likely only get about 800 watts. Depending on where you are and the angle of the panels, you should average 5 "sun hours" per day. The 3 new panels could produce about 5 KWH on a good sun day. The two old panels won't hurt you, but if they are just 100 watt panels, they will only give another 1 KWH per day with their own charge controller. Trying to combine on the same controller would only be worth it if you can get close to the same voltage, but to do that, you would have to run the new 3 in parallel, and that is much higher current and will need heavier wire etc.

Venezuela Solar · Sep 18, 2020

GXMnow said:
The best way to join panels with different specs is to use separate charge controllers, but you can pretty much parallel any group that has the same number of series cells. So in the first example, I agree that two of the 100 watt panels in series looks pretty close to the 300 watt panel voltage, so doing the "Option 2" works fairly well. Can you count the number of cells in each panel? You will get a voltage close to the VMP of the weakest panel, maybe a little higher, but below the best panel. And the current of each panel string will add up. So voltage will be about 33 and current will be a little less than 5.5 + 5.5 + 9.03 = 20 amps in perfect sun. So about 600 watts peak, more realistic is about 75-80% of that at Solar noon. Figure real 500 watts.

If you series panels, then you want to match the current at maximum power. So you might get away with two pairs of the 100 watt panels in parallel, and then series those two pairs with the 300 watt panel also in series, but the parallel option 2 is probably still better. You will get the current of the weakest set in the series, and the voltages will add. In this case, you get about 9 amps limited by the 300 watt panel, and 18+18+33 = 69 volts. Again, in perfect sun, maybe 600 watts again, and 80% = a little under 500 watts.

Either case could make about 3 KWH on a good sun day.

On the second example, You do not mention a charge controller, and you don't know anything about the old panels.

Does the existing system have a charge controller? Can you measure the current and voltage of the existing panels?

If the current 2 panel system has a 24 volt charge controller, just get another charge controller for the new panels, and they can both go to your battery bank and their charge currents will add together. With your 3 panels, I would suggest a controller that can handle 150 volts and run your 3 new panels all in series. You will want a charge controller rated at 40-50 amps to handle a peak of almost 1,000 watts into your 24 volt battery. In most cases, you will likely only get about 800 watts. Depending on where you are and the angle of the panels, you should average 5 "sun hours" per day. The 3 new panels could produce about 5 KWH on a good sun day. The two old panels won't hurt you, but if they are just 100 watt panels, they will only give another 1 KWH per day with their own charge controller. Trying to combine on the same controller would only be worth it if you can get close to the same voltage, but to do that, you would have to run the new 3 in parallel, and that is much higher current and will need heavier wire etc.

Thanks, that's pretty much what i guess it would produce, will test both ways and see

snoobler · Sep 18, 2020

I assumed the goal was to take what was given and make it work. A good MPPT charge controller is spendy, and it might not be worth the extra cost to capture the 10-15% loss you would see with option #2.

Granted, the most efficient way to connect different panels is via a separate charge controller for each type of panel.

In that spirit, the 4X 100W are 12V panels, so putting them on a separate PWM controller is an option, and PWM is cheap, BUT you still might not recover the 10-15% loss from option #2.

Venezuela Solar · Sep 23, 2020

snoobler said:
#2 is optimal. You'll lose a about 10-15% with 2S2P 100W panels paralleled to the 300W.

#3 would limit the current from the 300W to 5.5A nearly cutting its output in half, and you'd be too close to the 145V limit.

#4 wouldn't work at all. way too much voltage disparity between the types.

With #2, all your panels are Vmp at 33-36V

Would it be wrong to arrange 2P2S and that array with the 300w in series? the 2P would take the 5.5A to 11A, and the 2S would take the voltage close to the 300w module, it would be like a 400w 11A @36vmp module and a 300w 9.5A @33Vmp, so best case scenario would be 9.5x33 like 470w? I'm more concerned about safety issues and having voltage higher than 24, in this case every watt that i can take, it counts

Thanks.

gnubie · Sep 23, 2020

One string will have a lower voltage than the other and will draw power from the other trying to reduce it down to its voltage. On paper you can think of it as being 11A x 33V and 9.5A x 33V but it's not that simple since the SCC will be loading the panels well below Voc anyway and be presenting a lower impedance load to the panels than they do to each other, ie most current flows towards the SCC. Once you go to the right hand side of the peak of the I/V curve current quickly falls off so the SCC will naturally try to move to the left/peak if the load can take all the current. As it pulls down past 36V the power from the 36V panels reduce but the power from the 33V panels quickly rises up. Where exactly the SCC decides to sit depends on the quality of its algo and the present amp capabilities of each string (sun exposure etc)

Things change if the SCC lowers its draw.

You can go deep into this sort of thing and put blocking diodes in so that the arrays can only provide current into the common wiring never draw it out. The SCC will pull down past each array's Vmp as it performs a MPPT scan so it will still find both and settle at Vmp for the array still. That might find Vmp sitting between 33 and 36V but where ever it is, the SCC will find it. For such a small difference I wouldn't do the diodes personally.

Smith · Sep 23, 2020

You only need to be within the specs required by your controller.. within its limites, in a reasonable manner.
Utilizing it's range of it's abilities.. but not overwhelming it in any fashion..

Option #2.. sounds like you could use all panel's with out waste..

I'd also think that whatever a panel specs are, they will fluctuate thru out the day, depending on clouds and actual sunlight hitting them, so voltages are changing and fluctuations are made, even in the best senerios...

A controller should be able to handle the changes, within it's designed abilities with out damages...

snoobler · Sep 23, 2020

Venezuela Solar said:
Would it be wrong to arrange 2P2S and that array with the 300w in series? the 2P would take the 5.5A to 11A, and the 2S would take the voltage close to the 300w module, it would be like a 400w 11A @36vmp module and a 300w 9.5A @33Vmp, so best case scenario would be 9.5x33 like 470w? I'm more concerned about safety issues and having voltage higher than 24, in this case every watt that i can take, it counts

Thanks.

So the 2S2P would behave like a 36.4V/11A panel (400W)

Being in series with the 300W panel would restrict the current of the 2S2P to 9A, so you're losing 18% of your potential from that 400W panel being in series with the lower current 300W panel.

With the 2S2P 100W array in parallel with the 300W panel, you have NO restriction on amps, and a 33-36V sweet spot for Vmp, so more like a 4=5% penalty assuming you lose a little from each working in the middle.

Summary:

2S2P in parallel with 300W = 33-36V sweet spot with a ~5% penalty on either group assuming one extreme or the other.
2S2P in series with the 300W = ? sweet spot with a guaranteed 18% penalty on the 400W group due to the current restriction.

Mismatched solar panels (really different)

Venezuela Solar

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snoobler

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spelbynder

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GXMnow

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gnubie

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