Serial vs Parallel Solar Panel Shading

[CA_Traveler]

I was asked if a post in another thread if this "Serial is actually better for shading (than parallel) if that is a concern for you." was a typo. It is not a typo but panel shading is not always understood on this board.

What is overlooked is the bypass diodes and how they operate. All 24V 6x10 or 6x12 cell panels have 3 bypass diodes. Some 12V 4x9 cell panels have 2 bypss diodes. The diodes bypass 2 rows each, ie 2x9, 2x10 or 2x12 cells in what I’ll call a panel section. The diodes are installed to protect the serial panels from shade which can damage them which is why virtually all 24V panels have them. BUT they also allow the panel to produce more output in shaded conditions than parallel panels.

While there are various online articles on the need for bypass diodes to protect the panels (which I’ll skip) and articles on the shading/power effects, I’ll provide my own experience.

My 60 cell panels have 3 diodes each and each one can and does bypass 1/3 of a panel voltage in shading conditions. One key to understanding is that while shading on one section reduces the voltage by 1/3, the amps from the other 2 is the full amps from that panel for the given light conditions. Or to express it another way the panel produces 2/3 of its power. This is a dream made in heaven for MPPT controllers because by design they can harvest that 2/3 power from that panel plus all of the power from other series panels.

The same shading scenario for parallel panels means that the reduced panel voltage doesn’t contribute to the total power that the CC can harvest.

My 3 series connected panel’s means that I have 9 panel sections. Shade on one section means that I have 8/9 of the panel power for the MPPT controller to harvest. If 2 sections are shaded with either the same or different panels means that I now have 7/9 of the panel power. And so forth all the way to 2/9, with 1/9 being about 10V for my panels.

My panels are 30V (rounding) panels so I have 90V in series. Each shaded section that activates a bypass diode reduces that voltage by 10V down to 20V and the MPPT is happy as a clam to continue charging the batteries. Ie At high noon if I had 7 shaded sections the panel output would be 20V at 8A or 160W/14.8V = 10A bulk battery charging at 14.8V.

This picture shows the morning leafy shade 10V voltage changes due to shading. This is a graph of the Morningstar MPPT 60 CC and represents any MPPT controller. The thread is Click.

[brulaz]

BFL13 wrote:

...

I tried the 60 cell panel with the batteries at "24v" as an experiment, and it got them to 30v before stalling out. So it can charge the 24v ok, but not do any sort of overcharge when you need one of those from time to time. For that you do need 72 cell panels with a 24v system.

Or a couple of 36 cell 12V panels in series.

That's what I'm using for my portable setup feeding a 24V battery bank via a 20A Epsolar Tracer mppt.

They're big suckers for 12V, 170W, ~25# and a meter square each. Wouldn't want to wrangle anything much bigger for a portable system.

And yes, 12V panels are stupid expensive for what you get. Think they cost ~1.5x what I paid per watt for the fixed 24V panels on the roof. Guess 12Vs are not used much for the big commercial or household markets, so you don't get the cost reductions associated with such large quantities.

[BFL13]

CA Traveler wrote:
Before purchase I carefully checked out my roof layout with different sized panels (watts and physical size) and the best panel was the 60 cell mounted crosswise. This allowed fore/aft roof access from the rear ladder. The 72 cell panels would have blocked fore/aft access which was not desirable for me.

I like my 60 cell 255w panel. It goes well with the RV 12v system. Goes with a 20 amp MPPT controller. It is also not too big and heavy to use as a portable if you are staying a few days, but I would not like to lug it in and out of the RV twice a day every day. For that kind of RVing, I would go with roof mounted for sure.

I tried the 60 cell panel with the batteries at "24v" as an experiment, and it got them to 30v before stalling out. So it can charge the 24v ok, but not do any sort of overcharge when you need one of those from time to time. For that you do need 72 cell panels with a 24v system.

[CA_Traveler]

Before purchase I carefully checked out my roof layout with different sized panels (watts and physical size) and the best panel was the 60 cell mounted crosswise. This allowed fore/aft roof access from the rear ladder. The 72 cell panels would have blocked fore/aft access which was not desirable for me.

[CA_Traveler]

Food for thought from the Solar Blvd website without shipping.

12V panels: 180W ($179)x4 = 720W @ $716

12V panels: 120W ($109)x6 = 720W @ $654

24V panels: 255W ($114)x3 = 765W @ $324

[CA_Traveler]

duplicate

[CA_Traveler]

Since my 2014 install panels have improved and the prices have gone down. Some controllers haven't changed much in price. Others are newer on the market at lower prices.

In 2014 the solar size choice for a new install was about 500W+ for MPPT and 500W- for PWM. Perhaps different today.

For an RVer upgrading a current solar system would likely yield a different result.

[CA_Traveler]

MrWizard wrote:
The previous drawings provided by the OP showed partial shading on one string of cells in one panel, allowing partial power from that panel
What he did not show was the effects was partial shade across the width of the panel shading a few cells in all strings, totally shutting down that panel
Which is what i am referencing in this statement

I brought that up in a later post. For a fully shaded panel parallel has a little more power based on the bypass diode voltage drop.

A series string with possibly lighter guage wire, might be easier to install, but does the extras cost of the mppt controller warrant the minor gains of ease of install
Over the cost of more connections involved with pwm controller

In an RV install verus fixed location

Look at the total picture:

24V panels cost less, MPPT cost more.

24V panels use less footprint than 12V panels considering watts/sq ft.

12V panels use more mounting hardware and more wiring cost.

In 2014 my 750W 24V panels and MPPT cost was only $100 more than an equivalent 12V panel and PWM.

Maybe I should run that again at today's prices.

[red31]

liborko wrote:
What PWM controller does is switching the current on and off at BATTERY voltage once it starts controlling.

So does MPPT, That is how they LIMIT energy when it is not needed.

As this happens the reported panel voltage rises sine it is an average of battv & Voc or Vmp and Voc. The more the off time is the higher the average voltage reported.

[BFL13]

Always ask , "Percentage of what?" With MPPT, "they" never mention, "Maximum of what?" In fact, the MPPT tracks the maximum of what is **available**, not what the panel is rated at. Panel heating reduces what is available by a bunch.

Another salesman's trick is to show what MPPT can do for amps when the battery is really low in voltage, not when it is between 13.5 and 14.5 in real life as with most of the day while camping.

[liborko]

A glance at any I/V curve of a solar panel will immediately reveal that there is very little difference between Isc and Imp. That is what current sources do-they deliver current into any voltage(within reason).
I agree..PWM and MPP don't make any sense in same sentence

[BFL13]

liborko wrote:
The PWM controller does not operate at Isc. Isc is a short circuit current with zero volts on panel's terminals. What PWM controller does is switching the current on and off at BATTERY voltage once it starts controlling. Since solar panels are rated at their Maximum Power Point, only MPPT controller extracts panel's rated power because it operates at Maximum Power Point and not battery voltage which is lower.
With panels in parallel only panels in shade are affected. With panels in series you have to rely on bypass diodes to get any output which is not what I am prepared to do.
How MPPT controller works depends entirely on the manufacturer of the controller and we all know that there is abundance of bogus claims.

You seem to never have actually measured anything in the field. Try it.

The PWM controller passes amps to the battery at the same value as taking the Isc at that time with the panel disconnected. So I talk as though the batteries receive Isc amps as per the panel rating.

You can go blue in the face saying you can't get Isc because that is when the panel is shorted. Fill your boots. 🙂

The fact is you can check your panel Isc with the panel disconnected at any time and reconnect and you will see the same amps as the Isc you just got going to the battery. Of course most of the day the Isc is less than rated due to sun conditions at the time. other times it is over the rated Isc. Whatever it is at, that's the amps to the battery.

Express that however you wish.

And note that Isc goes up with panel heating, not down like its voltage, so with the MPPT using watts, which has the voltage as the major component, it loses watts, while the PWM using amps does not lose anything. This is how they come out near equal for amps to the battery.

And it is ludicrous to even mention Vmp for a panel when using PWM. That immediately shows an ignorance of what happens with solar. 😞 It is not about the watts; it is about the amps.

For example, my 130w 12v panel rated at 8.2a Isc and 21.9v Voc in full sun would measure 20.2 Voc (panel heating reduction!) and 8.2 or 8.3 Isc. On battery it would then show 8.2 or 8.3a on the Timetric going to the battery. That is how PWM works. You get "Isc amps" to the battery.

With MPPT amps to the battery you get output watts/battery voltage, and the output watts is way less than the panel rated watts. Get used to it 😞

[liborko]

Yes. There are limitations to solar power, heat is one of them. That is why I consider solar power a bonus and don't complain when there is nothing coming from solar panels. We in northwest have limited solar help most of the year. If I had to rely on solar only I would approach the problem differently.

[red31]

when heat causes a 12v panels Vmp to be at or near battery voltage then the full panel power can go the the battery.

not all 12v panels have a Vmp in the upper 18s, some spec at 17v or so, NOTC Vmp of kyocera panel of 16v https://www.solar-electric.com/lib/wind-sun/KD140GX-LFBS.pdf

it gets hot some places!

[liborko]

The PWM controller does not operate at Isc. Isc is a short circuit current with zero volts on panel's terminals. What PWM controller does is switching the current on and off at BATTERY voltage once it starts controlling. Since solar panels are rated at their Maximum Power Point, only MPPT controller extracts panel's rated power because it operates at Maximum Power Point and not battery voltage which is lower.
With panels in parallel only panels in shade are affected. With panels in series you have to rely on bypass diodes to get any output which is not what I am prepared to do.
How MPPT controller works depends entirely on the manufacturer of the controller and we all know that there is abundance of bogus claims.