DISCUSSION: PV Series vs. Parallel. Is parallel REALLY better for partial shading situations?

[Will Prowse]

I actually tested this years back on my RV roof when I had shading issues in some parking spots, and parallel configuration produced more energy over the course of a day than if I put them all in series. Great discussion above and I figured series would be better, but parallel was actually superior. There was always a noticable increase.

It does depend on the wiring configuration, SCC being used, how fast the mppt can track and change when shading does occur (some are very slow), nominal voltage of battery bank and how well it's matched to the panels, and the converters input to output differential.

Victrons excel with shading because they can track multiple power points and track very quickly.

But overall, I found parallel did produce more. It was quite interesting. I always figured the bypass diode would only drop the voltage a little bit, but it seems to really throw something off and reduce output.

 

[zanydroid]

If parallel was amazing in a lot of cases, surely we would see more MPPTs optimized for parallel operation? As things stand, the vast majority of MPPTs that can boost are in microinverters. If parallel was so good, wouldn't we see MPPTs that incorporate boost so that they can charge 48V batteries etc?

(Throwing out a strawman. I appreciate that series has a lot of other design advantages)

 

[Will Prowse]

If parallel was amazing in a lot of cases, surely we would see more MPPTs optimized for parallel operation? As things stand, the vast majority of MPPTs that can boost are in microinverters. If parallel was so good, wouldn't we see MPPTs that incorporate boost so that they can charge 48V batteries etc?

(Throwing out a strawman. I appreciate that series has a lot of other design advantages)

Gosh it really depends on the components and voltages being used. In low voltage with a 12V battery and 20VOC panels, parallel does offer an advantage. Especially if the shading is covering a few panels. If all the panels were in series and feeding a higher voltage battery or the grid, the string voltage may decrease to a point that the mppt can not operate. The voltage drop across bypass diode and shaded cells can drop the voltage significantly.

 

[timselectric]

I think that's the situation.
At lower voltage, parallel is better. But at higher voltages, a small loss in series is negligible.

 

[zanydroid]

As things stand, the vast majority of MPPTs that can boost are in microinverters.

And while I'm taking a hit off this pipe, why can't we have MLPE that are microinverters with the inverter, grid tie, and anti-islanding chunk deleted.

Parallel me the 340VDC rail directly onto the branch circuit as a voltage source with no weird logic nonsense.

Keep some RSD functionality.

 

[octal_ip]

I actually tested this years back on my RV roof when I had shading issues in some parking spots, and parallel configuration produced more energy over the course of a day than if I put them all in series. Great discussion above and I figured series would be better, but parallel was actually superior. There was always a noticable increase.

It does depend on the wiring configuration, SCC being used, how fast the mppt can track and change when shading does occur (some are very slow), nominal voltage of battery bank and how well it's matched to the panels, and the converters input to output differential.

I'm the same - I've done the tests both on the ground and on the roof, and at least for my systems, parallel produced more power by quite a large margin.
While it's an interesting thought experiment, there's too many variables at play to isolate a single factor and draw conclusions around the behaviour of an entire system, particularly when there's not linear or binary relationships between these variables and conditions constantly vary.

 

[Shimmy]

I understand that you've simplified this for the sake of the example, however it's important to remember that a shaded cell won't reduce its Voc to 0v. In practice the Voc change during shading can be surprisingly small - the change in ability to source current though, is huge.

This is what I see as well-- most of my shading does not activate the bypass diode as the voltage drop is minimal. System configuration really has to be looked at holistically.

 

[Hedges]

Shade sucks no matter how the panels are wired. I assume it is possible to cherry pick the shade scenario where either could show better or worse.

The goal is to have a system where shaded panel(s) or potions of a panel cause no more loss than the diode-bypassed portion.
What we don't want is for one shaded panel to prevent others from contributing.

Optimizers might do this, if they can boost sufficiently. Microinverters should do this, with the reduced efficiency of having to boost voltage so much (about 6x). I think MPPT per series string does it (except if you're trying to capture power from a panel with 50% reduced current.)

 

[Hedges]

I re-ran my old test of shading one panel in a series/parallel array.

"SunPower" branded Sunny Boy 8000US
SunPower SPR-327NE-WHT-D panels, 8s2p array, 5232W STC

First data with unshaded panel was out of family from the others, probably more clouds.
The two tests with "B" string having one panel shaded had very similar results, which I averaged.
The unshaded test which I used (didn't discard) occurred between those two.

Would like shading one panel out of 16 to produce 93.8% as much power, but it was 90.8%; a 3% penalty for both strings being a bit off their Vmp.
The string with one panel shaded ideally would have been at 7/8 of Vmp for an 8-panel string, 87.5%, but it was at 90.8% of that voltage. The unshaded string would ideally have been at 100% of voltage, but was also at 90.8%. (The power vs. voltage curve falls off steeper on the right, so Vmp for the two parallel strings is closer to Vmp of a 7s string.)

Operating a bit above its Vmp, the string with a shaded panel delivered 83.1% of Imp. Operating 9.2% below Vmp, the unshaded string delivered 97.3% of Imp.

	"A" Amps	"B" Amps	Vmp	Power
Unshaded	4.11​	4.44​	392​	3722​	<-- discard (low light?)
"B" Shaded	5.36​	4.42​	356​	3687​
Unshaded	5.53​	5.44​	392​	4059​
"B" Shaded	5.40​	4.62​	356​	3683​
"B" average	5.38​	4.52​	356.00​	3685.00​
"B"/unshaded	97.3%​	83.1%​	90.8%​	90.8%​
		7/8		15/16
		87.5%​		93.8%​

It seems we each reconfirm what we found worked best.
Some arrays and shading conditions drive the outcome, but also MPPT algorithm of the SCC.

 

[Hedges]

Is that a vent pipe poking through a panel on the right side?

... or are you just happy to see the sun? ?

 

[octal_ip]

Good on you for actually doing the test @Hedges, well done.

I think it's important to remember the difference between shading, and completely blocking all light to a cell. Under normal shading the cell will still have a relatively high Voc due to ambient light alone, whereas when there's no light hitting the cell at all due to direct opaque covering, it will have a very low (0 or nearly 0) Voc. This will change the outcome of the tests.

Both situations are worth understanding. In one case a cell might be directly covered by fallen leaves, bird droppings, etc. The alternative being shading from clouds, trees or more distant objects which still allows normal scattered ambient light to hit the cell.

 

[Hedges]

Yes, I was thinking about that earlier comment on reduced light vs. hard shade, after I took this data.
Maybe tomorrow I'll check that too.
But I'm confident these Sunny Boys will pick the voltage which produces most power, not stop at first local maxima they find. Other MPPT will do that.

Maximum power point tracking - Wikipedia

en.wikipedia.org

Have you thought through or tested half-cut cells?
I have some now (REC 325W), will be used in my next system.
With distinct 50% reduction in current, I expect them to trip up dumb SCC. I also think their parallel-series internal wiring is sub optimum, prevents the reduced losses I reported for my series-parallel array.

 

[zanydroid]

I was reading up on this topic, and found out that Aurora claims to incorporate information about whether the inverters selected for modeling have local or global MPPT search, and model the output from shading accordingly.

 

[sirslayer]

Parallel is better, I’ve have done multiple configurations with 8 x 250 watts panels and testing between 12 volts and 24 volts batteries modes, parallel of the solar panels always yield more total wattage.. 1500 watts at its best in series vs 1800 watts average on parallel solar panels. Maybe it depends on the solar charger and could yield different results with other brands .

 

[sunshine_eggo]

Whelp, the next time I'm up North with my 3S3P array, I'll do some testing to include blackout/actual partial shading. I think the conclusion to my example of a blacked out cell is correct, but real-world rarely (if ever) involves blackout shading.

 

[time2roll]

Whelp, the next time I'm up North with my 3S3P array, I'll do some testing to include blackout/actual partial shading. I think the conclusion to my example of a blacked out cell is correct, but real-world rarely (if ever) involves blackout shading.

Yes park under a random tree or power pole that swings shade across the panels. Shade moves.

 

[sunshine_eggo]

Yes park under a random tree or power pole that swings shade across the panels. Shade moves.

My shipping container has been parked in the same spot for over three years...

 

[timselectric]

The tires are probably flat, by now. lol

 

[Offgrid LiFePO4]

It is extremely hard to gain a true understanding of all these issues from personal anecdotes and experimental data where all the variables are not known.

Here is a very good article explaining very clearly the various factors at play here:

Do Solar Panels Work In The Shade? A Complete Guide To Solar Panel Shading, Its Effect, And Its Solutions | RenewableWise

If a solar panel is completely under shade, power production will be very low, . If the solar panel is only partially shaded, depending on which cells are shaded and if the solar panel has working bypass diodes, it might still work.

www.renewablewise.com

 

[octal_ip]

It is extremely hard to gain a true understanding of all these issues from personal anecdotes and experimental data where all the variables are not known.

Here is a very good article explaining very clearly the various factors at play here:

Do Solar Panels Work In The Shade? A Complete Guide To Solar Panel Shading, Its Effect, And Its Solutions | RenewableWise

If a solar panel is completely under shade, power production will be very low, . If the solar panel is only partially shaded, depending on which cells are shaded and if the solar panel has working bypass diodes, it might still work.

www.renewablewise.com

Fantastic article, thanks for sharing.