Shading Tests - Serial vs Parallel Panels

CATraveler

Are there any tests on the boards comparing serial vs parallel connection of 2 panels as it relates to shading? ie A direct comparison of the MPPT controller output for various shading with the panels connected in parallel then serial.

My interest is for a mobile RV/boat application where shading is a given at times. Plus my interest is in 200W+ panels in the 30V range with bypass diodes.

For example the following tests:
1. Shade a single cell.
2. Shade 2 or more cells in the same cell group (as determined by bypass diodes).
3. Shade 2 or more cells in 2 (or more) cell groups.
4. Shade one of the 2 panels.

I'm guessing that different panel mfgs and different controllers could yield different results.

Actually I’m also interested in a 4 panel test that would include a series/parallel test. But a basic comparison of 2 panels would be very helpul in deciding how to wire the panels.

lamplight

Re: Shading Tests - Serial vs Parallel Panels

CATraveler wrote: »

I'm guessing that different panel mfgs and different controllers could yield different results.

definitely. it depends on the type of panel and the controller.. testing my setup will likely only help determine my setup's results. I did this a few years ago... found that my sharp nt-175u parallel panels were significantly more KwH's over the whole day in my location (vs having them in series) - as i have significant midday shade in winter. recently switched back to 48v/series panel strings due to wire issues.

some panels have more bypass diodes than others, ive read that poly panels do better in shady conditions.. im sure that any results will only help those with the same panel type and wiring.

mike95490

Re: Shading Tests - Serial vs Parallel Panels

Shade, willl cut out about half of a panel's power, unless it's enough to cut out all the power, and both bypass diodes activate.

In a series array, you loose half the voltage of 1 panel in the array, so if you had 2ea, 100W, 18V panels, that normally produce 36V @ 5.5A, you would loose 9V, resulting in 27v @ 5.5A If the 27V is enough to keep your gear going, great. If not, you are idle.

If both panels were parallel, your 18V would remain, but your current (was 11A) would drop in half, as one panel shuts off, and you can still charge, but at a reduced rate.

lamplight

Re: Shading Tests - Serial vs Parallel Panels

isn't there more to it than that mike (assuming there's only two bypass diodes in a panel? I though for sure I read about a (some?) panels having more bypass diodes.. I was looking up my sharps and can't find anything other than "72 cells in series".. on the new sun tech panels I got for cells is says "6 x 32". not sure what to make of that...

I think I do recall not losing full power on a panel during partial shade with the sharps but it was a lot of drop even if I shaded just a couple cells.. maybe you're right (shrug)

CATraveler

Re: Shading Tests - Serial vs Parallel Panels

The number of bypass diodes seems to follow the cell layout: ie a 6x10 would have 3 which would use the cell wiring located at the external wire end. Sometimes a 6x10 would have 6 diodes but that seems to be related to 2 parallel diodes with the second diode designed to protect the first from transients, etc.

But the mfg information tends to be vague on bypass diodes.

niel

Re: Shading Tests - Serial vs Parallel Panels

catraveler,
that's not quite how it is as they usually try for one bypassed section for every 6v vmp. the paralleled diodes are because it is cheaper for them to parallel 2 or more smaller current valued diodes than one of the proper current value.

gdoty

Re: Shading Tests - Serial vs Parallel Panels

mike90045 wrote: »

Shade, willl cut out about half of a panel's power, unless it's enough to cut out all the power, and both bypass diodes activate.

In a series array, you loose half the voltage of 1 panel in the array, so if you had 2ea, 100W, 18V panels, that normally produce 36V @ 5.5A, you would loose 9V, resulting in 27v @ 5.5A If the 27V is enough to keep your gear going, great. If not, you are idle.

If both panels were parallel, your 18V would remain, but your current (was 11A) would drop in half, as one panel shuts off, and you can still charge, but at a reduced rate.

So my question is; if you run a MPPT controller that's going to drop the voltage down to 14.6 or so anyway to charge your 12v bats, (depending on the bat specs), if your run the array in series and you lose voltage from say 36 volts down to 27v, you're still at the same amps and the MPPT will still drop the volts down to 14.6. So wouldn't setting up the panel in series be better for a low (12v) system?

CATraveler

Re: Shading Tests - Serial vs Parallel Panels

niel wrote: »

catraveler,
that's not quite how it is as they usually try for one bypassed section for every 6v vmp. the paralleled diodes are because it is cheaper for them to parallel 2 or more smaller current valued diodes than one of the proper current value.

That's one application for parallel diodes.

The second that I read about is one Schottky diode for the current bypass. The second bypass diode can be used to provide better transient protection (than a Schottky) for power lines or lightning, etc. In other words the second diode protects the first diode. I did a quick search but couldn't find the reference this morning.

CATraveler

Re: Shading Tests - Serial vs Parallel Panels

gdoty wrote: »

So my question is; if you run a MPPT controller that's going to drop the voltage down to 14.6 or so anyway to charge your 12v bats, (depending on the bat specs), if your run the array in series and you lose voltage from say 36 volts down to 27v, you're still at the same amps and the MPPT will still drop the volts down to 14.6. So wouldn't setting up the panel in series be better for a low (12v) system?

Yes in the example given you would harvest more panel power with bypass diodes, serial connection and mppt controller.

But the exact effects may differ between different panels, different controllers and just what is shade anyway? Shade from a tree, pole, cloud, etc.

BB.

Re: Shading Tests - Serial vs Parallel Panels

Diodes are "tricky" animals. The have voltage drop (from ~0.2 for a Schottky to ~0.7-1.0+ volts for a standard). Low power diodes are cased in plastic and only the leads can dissipate the heat. So, every series diode added will cause more losses (bypass diodes that are conducting add voltage drop to the system and excess heat/losses).

Higher powered diodes are in metal cases, that in this case, is not floating or at ground potential, so any heat sinks need to be isolated from the metal tabs or the entire heat sink needs to be isolated electrically to avoid short circuits and shocks (especially if you are going to run them >~20 volts or so to avoid electrical shocks/burns).

And, diodes are highly non-linear and very heat dependent. The hotter they get, the lower their forward voltage is... So if you have two diodes in parallel, the hotter one will attempt to carry more current--possibly to destruction (which may fail open or shorted).

In general, I would personally try to avoid adding diodes to a solar system unless they are really needed (blocking, AC to DC rectification for alternators, etc.).

-Bill

niel

Re: Shading Tests - Serial vs Parallel Panels

BB. wrote: »

Diodes are "tricky" animals. The have voltage drop (from ~0.2 for a Schottky to ~0.7-1.0+ volts for a standard). Low power diodes are cased in plastic and only the leads can dissipate the heat. So, every series diode added will cause more losses (bypass diodes that are conducting add voltage drop to the system and excess heat/losses).

Higher powered diodes are in metal cases, that in this case, is not floating or at ground potential, so any heat sinks need to be isolated from the metal tabs or the entire heat sink needs to be isolated electrically to avoid short circuits and shocks (especially if you are going to run them >~20 volts or so to avoid electrical shocks/burns).

And, diodes are highly non-linear and very heat dependent. The hotter they get, the lower their forward voltage is... So if you have two diodes in parallel, the hotter one will attempt to carry more current--possibly to destruction (which may fail open or shorted).

In general, I would personally try to avoid adding diodes to a solar system unless they are really needed (blocking, AC to DC rectification for alternators, etc.).

-Bill

i pretty much agree with you bill, but the bypass diodes are an integral part of most pvs and nobody was really talking of adding them as they are already there. they often do parallel diodes as we've seen in many instances of people reporting their diodes have blown and it isn't good to parallel them as you've pointed out, but it's how some of the manufacturers do it. it is a cost cutting maneuver on their part to run 2 or more paralleled diodes that are cheaper than 1 of the proper specifications. as many in electronics know this can be done if a balancing resistor is utilized, but they don't do this because the resistor would in turn add cost to the pv again. they are just as cheap for the consumer to replace too, but really is a pain as these pvs are usually mounted high or aren't readily accessible.

jagec

Re: Shading Tests - Serial vs Parallel Panels

gdoty wrote: »

So my question is; if you run a MPPT controller that's going to drop the voltage down to 14.6 or so anyway to charge your 12v bats, (depending on the bat specs), if your run the array in series and you lose voltage from say 36 volts down to 27v, you're still at the same amps and the MPPT will still drop the volts down to 14.6. So wouldn't setting up the panel in series be better for a low (12v) system?

MPPT controllers use DC-DC conversion to wring every last bit of power out of the array. Therefore, if the voltage drops, even though the current remains the same, less power will reach the batteries.

Example: 36V, 10A array in full sun. A 100% efficient MPPT controller will downconvert that 36V, and supply 24.7A @14.6V.
Shade the array so that it is now 27V, still 10A, the MPPT controller would supply 18.5A @ 14.6V

So you've lost some charge current, even though the array current is the same.

With a PWM controller, however, 10A would be going to the batteries in both cases, and shading wouldn't matter. But you'd be giving up a LOT of potential power by using an array that has a much higher voltage than your battery bank.

Cariboocoot

Re: Shading Tests - Serial vs Parallel Panels

Actually solar panels are current sources. When shade hits them the first thing that happens is the current goes down. It is possible for a panel to put out quite high Voltage and no significant current. In fact it happens every morning when the first rays of light cause the panel to hit Voc @ 0 Amps.

The MPPT power advantage is found at maximum power demand (hence the name) which will be at the lowest battery Voltage (discounting load draw), not the Absorb level. It is found in the difference between that battery Voltage and the panel's Vmp potential. With a PWM controller that extra power (which exists as panel Voltage less battery Voltage times panel current) is lost as they only pass the current (which can remain maximum right down to zero Volts).

So with a PWM controller passing 7 Amps @ 12 Volts you get 84 Watts, even though the panel's Vmp is 17.5 (122 Watts). It would pass the same maximum current if the battery had been drained to 10 Volts: 70 Watts. Since battery Voltage will never be as high as panel Vmp you will never see full power potential from a PWM controller.
The MPPT will take the available difference between the battery Voltage and the panel's Vmp (+/- depending on the controller's programming) and convert it into extra charge current. You also will never see full power potential from an MPPT controller (aside from the occasional aberration due to temps of edge-of-cloud events) but you will see more than from the PWM. Typically the panel derating plus the controller's consumption comes to 77% of the nameplate rating, and so we use the formula: Watts * 0.77 / system Voltage = peak potential charge current (it is fairly accurate). Or with the above theoretical panel: 122 * 0.77 = 94 / 12 = 7.8 Amps. That is an 11% increase over the panel's spec'd Imp.