One shaded solar panel dragging down the system - switch it off?

[AJ452]

I've got a '97 Dodge Phoenix campervan that already has a standard roof vent installed so I'm not really flexible with where that opening is.  It's not moving anywhere! As it is I believe I can fit 4 100 watt flexible solar panels up there with plenty of spacing from the vent so as to not worry about shade from it, even when it's upgraded to a Maxxair swanky motorized model with built-in rainguard.  There is roof space for a 5th solar panel but it will be quite close to the vent and its rainguard extension and will certainly be shaded by it at times.

I understand that even a partially shaded panel can drag down the whole system but I'm not familiar with just to what degree.  Will the shadow from the roof vent cause just a slight hiccup from full performance of the entire system or is that one panel now enough of a liability that removing it from the system would be more beneficial overall?  I think that setting up some switching could possibly bypass that panel entirely when desired.  And then put it back in the loop when sun angles are more favorable.

Or is the partially shaded panel (just the shadow of a roof vent over a 48" x 21" panel) possibly still enough of an asset that it still contributes positively  to the overall power generation and removing it from the loop would be self-defeating?

Thanks for any help!

 

[John61CT]

Not that expensive to just give each panel their own matched controller.

 

[tx2sturgis]

I understand that even a partially shaded panel can drag down the whole system but I'm not familiar with just to what degree.  Will the shadow from the roof vent cause just a slight hiccup from full performance of the entire system or is that one panel now enough of a liability that removing it from the system would be more beneficial overall?  

It depends entirely on whether that shaded panel is wired in series or parallel with one or more other panels.

If wired in series with other panels, such as with an MPPT controller, and that panel gets enough of its surface shaded, then the output from that entire string of panels will be reduced...a lot.

But if the shaded panel is wired in parallel with one or more panels, such as with a PWM controller, then the reduction in total power output will be reduced by about what that one panel produced, and it will have little effect on what the other panels are producing. 

It also depends on how much of each cell gets shaded, and whether that shade is across cells in separate strings within the panel, or cells in the same string within the panel.

I've experimented with the panels I have, and what I can say is, try to avoid mounting panels where they can be shaded for hours by anything more than a whip antenna or very slender flagpole or similar. In my experiments, shading about 10% of the panel sometimes reduced the output by 50% or more, depending on the angle and position of the shade across the panel. That's enough to choke an entire series string of panels down by a large amount.

(my experiments used panels with bypass and blocking diodes, most panels you can buy today have these)

If you must mount a panel in an area that is partially shaded for hours, then make sure that panel is wired in parallel with other panels.

The irony is that in the winter, with shorter days and low sun angles, when you need all the amps you can gather, is exactly when roof A/C units and roof vents will produce the most shade on those panels.

 

[Spaceman Spiff]

tx2sturgis is correct:  
     if panels are wired in series one panel shaded will degrade the output of the other panel(s).
     if panels are wired in parallel one panel shaded will not effect the output of the other panel(s).

Handybob documented how little shading it takes in his opus The RV Battery Charging Puzzle (about 4/5 way down in the text):
https://handybobsolar.wordpress.com/the-rv-battery-charging-puzzle-2/

If you haven't already read him, this article and website has a wealth of information on how to make your system work.  He rants a lot about how bad Quartzite installers are but the information is solid and he has lived for years off grid.

 

[John61CT]

Yes great starting point but some info / rants are now well outdated, e.g. all newer quality controllers are MPPT, and you get the (yes overhyped but real) advantages at a much lower gap in price now compared to quality PWM.

 

[regis101]

Having an MPPT would allow series or parallel . An easy thing to overlook is the capacity ability of CC and panels

 

[tx2sturgis]

Having an MPPT would allow series or parallel .  

True, of course, but the real advantage of MPPT comes when panels are wired in series AND/OR you use higher voltage residential style panels. Neither is the case with normal 100 watt flexible panels referenced in the original post. 

12v (~20 VOC) panels wired in parallel can put out about as much power for $$ spent by using a good PWM controller.

 

[Canine]

Shading a panel that is in parallel will still reduce overall efficiency. You have about four options:

1- Disconnect the shaded panel when it is shaded.
2- Install a diode inline so that it won't drag down the other panels.
3- Connect another charge controller with a paralleled connection.
4- Install another charge controller that is networked. This is the ideal solution.

Also, since you are asking this question, I'm concerned the wires and/or CC would have too many panels running through it.

 

[John61CT]

Victron 75 /15 MPPT are well worth their $120 or so even if using 19Voc panels means a minor efficiency increase.

Custom adjustability, logging, smartphone control via built-in BT, future flexibility and maybe BM integration, Dutch build quality / engineering / design, great support, 5 year warranty. . .

______
No need at all for multiple SCs to be networked

just stack the outputs to the target bank

along with whatever other concurrent disparate charge sources, no problem.

Ideally similar profiles, but long as none are outside the bank charging specs, NP.

If spending lots on a central BMV / BMS coordinating things, you **can** network the SCs, but few small setups want to spend that kind of money.

 

[Spaceman Spiff]

Shading a panel that is in parallel will still reduce overall efficiency . . . 
I haven't seen this and haven't run across it in my reading.  The output of my system is reduced, but only by the amount the shaded panel's output is reduced.

1- Disconnect the shaded panel when it is shaded.
My shaded panels put out something, usually 1 - 3 amps.  So why disconnect?

2- Install a diode inline so that it won't drag down the other panels.
I have not seen this.  When one panel is shaded the other is producing at full power (connected in parallel).
 . . . 

 

[tx2sturgis]

Victron 75 /15 MPPT are well worth their $120 or so even if using 19Voc panels means a minor efficiency increase.

Do a little bit more math with the OP specs and you will see that a 75/15 wont work if he has 4 or 5, 100 watt panels in parallel OR series. They are only rated for 220 PV watts at 12 volts. He might need 2 or 3 of these $120 controllers...and some complex wiring or programming to make them all work correctly.

And a Victron 100/50 or 150/35 (which should safely handle 5 100 watt panels in series OR parallel) is on the high side of $300.

A Morningstar TriStar 45 amp PWM controller that will easily handle his configuration will be about half that cost with virtually the same power produced if those 5 panels are wired in parallel (which is logical with an odd number of panels).

There will of course need to be some fairly fat wires involved.

 

[John61CT]

Morningstar TriStar 45 amp PWM controller that will easily handle his configuration will be about half that cost

Great brand, and yes at $150 that would be good value

Did not realize they were going that cheap

 

[Canine]

Spaceman,

Shading a panel in parallel will reduced the entire output. No where near as much as series, but it still does.

If keeping a shaded panel connected gives more output than disconnecting, then keep it connected.

Installing an inexpensive diode between panels is not uncommon. It's cheap with a small voltage drop; good bang for the buck and easy to do.

 

[tx2sturgis]

Shading a panel in parallel will reduced the entire output. No where near as much as series, but it still does.

In several experiments I have done, only the shaded panel loses output, the other panels in parallel continue to provide their normal output.

If the somewhat depleted battery bank being recharged is placing a high demand on the system, then the remaining panels are 'working harder'...meaning they may show a lower voltage on your controller, but this does not mean that power from those remaining panels has been reduced. Overall power produced, will, of course, be lower due to a panel being shaded.

But I have not seen any reduction of power from the panels still receiving full sunlight.

 

[John61CT]

May vary by panel type / brand, diode built in maybe?

Sheer speculation

 

[bonvanroulez]

“Dutch build quality / engineering“

Lol, I never knew that was a thing. Tulips, beer, hashish, Victron?

But of course you know that Victron products are actually made in India.

 

[tx2sturgis]

“Dutch build quality / engineering“

Lol, I never knew that was a thing. Tulips, beer, hashish, Victron?

Don't forget the wooden shoes.  

 

[Weight]

Lots of . I have 4 Renogy 100W panels connected parallel to 45 amp PWM Morningstar. When one panel is shaded even if it puts out zero, it doesn't 'drag down' the other three panels. Maybe it would be different if I had off branded panels and cheap controller.

 

[Spaceman Spiff]

Shading a panel in parallel will reduced the entire output . . . 

Installing an inexpensive diode between panels is not uncommon . . .

I am having trouble understanding why one panel will affect another when connected in parallel.  The two panels should be independent of each other and are only connected downstream.

As a practical matter, my system (2 X 100W Renogy mono panels) act totally independently when in parallel.  I don't know (didn't look before installation) if there are diodes in the connector box on the back of he panel.  One panel's output is not reduced when other panel is partially or completely shaded.

 

[Canine]

Here is how a shaded panel drains power in a parallel system:

Voltage is what pushes the electricity into the battery. Like a bicycle pump. When a tire is completely empty, the pumping is easy. When the tire nears capacity, the pumping becomes more difficult. If the pump is capable of 35 pounds of pressure, the tire will get to only 35 pounds max with increasing difficulty along the way. Volts act in much the same way.

Now lets use that analogy in regards to the panels. If both panels are getting full sun and the sun is at full brightness, they are able to "pump" voltage into the batteries. The when one panel gets completely shaded, some of the electricity goes into the other panel. The shaded panel doesn't have enough pressure to pump enough voltage to charge the battery, so the voltage from the panel in the sun gets pumped into the panel in the shade.

Why does the voltage go into the shaded panel? Because electricity takes the path of least resistance which is the panel with less voltage (pressure) than the battery. Then when the shaded panel gets "pressurized" with enough voltage from the sunlit panel, the rest of the electricity is then able to go into the battery because that then becomes the path of least resistance.

A big part of a charge controller is to keep electricity from the battery going back into a shaded panel. At night without a diode or a charge controller, the battery will drain to some extent into the panel because the voltage pressure is less in a shaded panel. A shaded panel doesn't care if the electricity originates from a solar panel or if it originates from a battery, electricity will take the path of least resistance and higher voltage will go to lower voltage unless a diode is blocking the electricity or the connection is disconnected.

The parallel panels do share a path because they are connected together. If each panel had it's own path, then shading would not effect the other at all. How do you get each panel to have only one path for the electricity to travel? Use another charge controller or a a diode.

How much shading and where the shading occurs on the panel also makes a difference, but I don't want to get into that.

Here is a YouTube video showing how a diode works. It also specifically shows panels connected in parallel, and she discusses how a shaded panel connected in parallel can suck juice from a sunny panel without additional diodes or charge controllers. 4:57: