Panel wiring with shading

[RoadTurtle]

I can get three 200w panels on each side of the RV, for a total of six panels.

Plan is wire 2S 3P. Shading from junk on the centerline is a problem depending on trailer orientation. It will get worse with adding these panels.

Should the panels in series be on either side of the the centerline (2 front, 2 middle, and 2 rear), or on the same side (front and center left, front and center right, two rear), or something else? Worst one of the bunch is right rear, as it's going be tight up against the full size AC.

 

[sunshine_eggo]

To minimize penalty, you want to avoid partial shading of a string. Of each of the positions mentioned, it's best when both panels in the string are experiencing the same conditions, but sometimes you take what is offered as that's your only option. The one by the AC is going to sabotage the one in series with it, but it's better than not having panels.

It's also important to remember how partial shading affects panels. When standing one vertically, shading a single cell in any column kills 1/3 of the panel's output. In the same orientation, shading one full row will kill the entire panel.

Another bit of ugly... When a bypass diode is allowing current to flow around the shaded portion, the voltage of that portion is lost. The Vmp of the panel drops by 1/3 to maintain the Imp. If the loss of that voltage causes that string voltage to drop below the other parallel strings, it can't deliver current any longer. The MPPT will have to figure out that it needs to pull the working voltage down by 1/3 of a panel to get output from all of the panels, which hurts the power from the other panels as it forces all panels to operate at the lower voltage.

 

[RoadTurtle]

Sunshine,

Thank you for the "ugly" email. There's a lot of factors impacting this installation.

Knowing that one panel is going to be a constant challenge unless I'm perfectly orientated, am I better off just going with 5 panels and parallel all of them?

Plan was to raise battery voltage to 24v. Would it make more sense to stay with the original 2S 3P of 24v panels and lower the planned battery voltage to 12v?

 

[fafrd]

Sunshine,

Thank you for the "ugly" email. There's a lot of factors impacting this installation.

Knowing that one panel is going to be a constant challenge unless I'm perfectly orientated, am I better off just going with 5 panels and parallel all of them?

Plan was to raise battery voltage to 24v. Would it make more sense to stay with the original 2S 3P of 24v panels and lower the planned battery voltage to 12v?

I have a DC array suffering from sever morning shading and chose to go with a 1S parallel array to maximize panel output and to minimize the activation of bypass diodes.

I’ve got newer 144-cell half-cut panels which allows any unshaded half-panel to contribute at 50% Imp at full Vmp, so I’m easily able to charge my 24V LiFePO4 battery with whatever solar power is available.

Sounds as though you may already have your panels, but if not, buying more modern half-cut panels and possibly also larger 72/144-cell format if you can fit them, might be better.

6 x 200W is only 1200W of PV and 3 x 450W of half-cut panels would give you the equivalent of 6 x 225W panels that are each only 60% the size.

The bigger bit of ugly is that if you suffer from sharp shading during peak hours, activating bypass diodes at high current will wear them out.

Check your panel warranty - most warranties are voided if the panels are used in the presence of shade.

I’m much happier limiting my panels to either contributing 50% of max output from each unshaded half-panel and having any half-panels impacted by shade sit out the party until they become unshaded…

 

[RoadTurtle]

I’ve got newer 144-cell half-cut panels which allows any unshaded half-panel to contribute at 50% Imp at full Vmp, so I’m easily able to charge my 24V LiFePO4 battery with whatever solar power is available.

Sounds as though you may already have your panels, but if not, buying more modern half-cut panels and possibly also larger 72/144-cell format if you can fit them, might be better.

6 x 200W is only 1200W of PV and 3 x 450W of half-cut panels would give you the equivalent of 6 x 225W panels that are each only 60% the size.

I'm very much space constrained. So much so, I drove to Santan to look at their used commecial panels, and even their 225 panels are too wide for two locations. 400w split panel just ain't going to happen with all the crap on the centerline. While I could put them over sun thingy's I'm also at max legal height and I do like the light coming in too.

 

[fafrd]

I'm very much space constrained. So much so, I drove to Santan to look at their used commecial panels, and even their 225 panels are too wide for two locations. 400w split panel just ain't going to happen with all the crap on the centerline. While I could put them over sun thingy's I'm also at max legal height and I do like the light coming in too.

If 60-cell is all you can fit, 60-cell is all you can fit.

You can still probably find 120-cell half-cut panels that will fi on your RV and have high-enough Vmp to charge your 24V battery.

These Longhi 120-cell panels, for example: https://www.renvu.com/LONGi-345W-120-Half-Cell-Mono-PERC-All-Black-Tier-1-Solar-Panel

Vmp of 31.8V is getting into the range we’re you can just barely charge a 24V LiFePO4 battery and need to minimize wiring losses at 1S, but with Voc of 40.2V, at most you’ll be wasting a pit of potential power as the battery nears full charge and you are going into CV mode anyway…

4 of those 345W 120-cell half-cut panels delivers 1.380W, 15% more power than you’d get from your 6 200W panels and you would be able to avoid the 2 worst-shaded locations to boot…

 

[RoadTurtle]

your linked panel is too wide. I'm extremely constrained on width. Pretty much constrained to 36" including mounting brackets, and even then, some areas are even less.

 

[fafrd]

your linked panel is too wide. I'm extremely constrained on width. Pretty much constrained to 36" including mounting brackets, and even then, some areas are even less.

Got it. Sounds like no half-cut panels will fit. Is the Vmp on your 200A panels high-enough to charge a 24V battery or do you need the 2S string voltage?

 

[RoadTurtle]

24V panels is plenty high enough. Question is will I get better performance out of a 12V battery and 24v panels to help?

 

[fafrd]

24V panels is plenty high enough. Question is will I get better performance out of a 12V battery and 24v panels to help?

I’m not sure what performance you are talking about.

Wiring losses with a 12V battery will be higher than with a 24V battery (both ways). 4 times higher (I^2xR).

Most SCCs have a lower MPPT limits of Vbat + 2V (and the lowest I’ve seen is Vbat + 1V).

So if you are charging a 24V battery up to a Vboost of 28.8V, the SCC needs string voltage of over 30.8V (minus wiring losses between panels and SCC).

I doubt the Vmp of your 200V panels is ‘plenty high enough’ to charge a 24V LiFePO4 battery without paying attention to wiring losses - is your 24V battery Lead-Acid?

 

[RoadTurtle]

24v panels have higher voltage. What impact does shadowing have on output watts going into 12v vs 24v LFP battery bank?

 

[fafrd]

24v panels have higher voltage. What impact does shadowing have on output watts going into 12v vs 24v LFP battery bank?

If you are charging a 24V battery with 24V panels, you will pretty much never be able to activate bypass diodes.

If Vmp is 30V, only unshaded panels will be able to contribute to charge current (assuming a 1S parallel string(.

If hard shade is only impacting a portion of 1/3 of a panel, activating the bypass diode to bypass that 3rd panel would drop string voltage to ~2/3 Vmp - 0.5V, or under 20V for a panel with a Vmp of 30V.

With only 2 panels in parallel, dropping to ~2/3Vmp is not impossible (2/3 Vmp x 2 x Imp = 133% Pmp > (0% + 100%) x Pmp = 100% Pmp.

With 4 panels in parallel out of which 3 are unshaded, you’ll never activate any bypass diodes since 2/3 Vmp x 4 x Imp = 267% Pmp < (0% + 100% + 100% + 100%) x Pmp = 300% Pmp.

Parallel 1S arrays of 4 panels will pretty much prevent activation of bypass diodes when charging either 12V or 24V batteries.

Cabling losses will be lower charging a 24V battery than a 12V battery.

The only thing to be careful about charging a 24V battery with is that Vmp of your panels is above Vmppt_min (while no such concern exists when charging a 12V battery with 60-cell panels with Vmp of over 30VDC…).

 

[chrisski]

24V panels is plenty high enough. Question is will I get better performance out of a 12V battery and 24v panels to help?

IME, the difference is insignificant.

I hear low light and cloudy days, but I had a 12 volt System with “12 volt panels” which still produced over cast days. Perhaps a rainy day it would not produce, but you’re looking at such a small amount lost, perhaps 20wh daily produxtion from a 100 watt panel On there rainy days and who will miss that?

Where a “24 volt panel” is a winner is allows for more parallel production of further down the road you upgrade to a 24 volt battery.
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I am a big believer that smaller 100 watt panels put on a roof with as many in parallel as possible will be best for production. This is not a cheap way to do it. Shading sucks on RVs and the best way to overcome this is parallel With smaller panels.

My rule i go with is a 3% to 10% voltage loss total. If the added amperage goes above 30, y connectors shouldn’t be used and it’s time for a combiner box.

I have panels on two or three sides of my rooftop ACs, but they are paralleled so that when I park with the entrance to the south, that shading losses are minimized. I Have 2S3P for flexible panels. Unless the sun is straight up in the sky, these 3 sets of panels won’t be all on max production, but at least one set will.

 

[RoadTurtle]

Somebody sent me a most interesting panel configuration. Panels are hard mounted (not adjustable) where he lifted the center mounts to the height of the AC and other crap on the roof. Now, this puts one side, as being much lower production than the other side. In the winter, I'd expect very little on the panels angled away from the sun, but there are now shadows on the panels. However, the other panels are better angled towards the low sun. It will also permit much larger panels, solving my panel width issues.

Here is his photo

 

[TX2AKRV]

I am not sure I would be comfortable driving my Super C 65 mph down the hwy with these big panels raised at such heights above the roof? There could be a lot of lift from air flow below the panel.

 

[RoadTurtle]

I am not sure I would be comfortable driving my Super C 65 mph down the hwy with these big panels raised at such heights above the roof? There could be a lot of lift from air flow below the panel.

Air flow dynamics is interesting. If the panel is lifted, you set it up with a slight higher elevation at the rear to create down force like a race car spoiler. The other interesting thing, air hitting the front of the trailer will tend to rise above the panels, never really impacting them. Having said all that, I was uncomfortable raising my panels up, and chose to mount all of them about 1/2" above the roof. They all have 8 lag bolts and dicor holding them down.

 

[TX2AKRV]

I can get also 3x 200W 24V panels on each side of my Super C. Planning to mount them on Struts attached to the Fiberglass roof with VHB Tape (and maybe some screws if needed protected with Sika Flex). Other RV'ers have had good experience mounting huge 460W panels this way on RV roof.

Tilting can be done temporarily once parked.

Also thinking about putting one of 100w panel on each of the 2x AC units

 

[RoadTurtle]

I can get also 3x 200W 24V panels on each side of my Super C. Planning to mount them on Struts attached to the Fiberglass roof with VHB Tape (and maybe some screws if needed protected with Sika Flex). Other RV'ers have had good experience mounting huge 460W panels this way on RV roof.

Tilting can be done temporarily once parked.

Also thinking about putting one of 100w panel on each of the 2x AC units

right on top of the AC unit? For me it would make my trailer too tall, but it's an interesting thought. Are you worried about heat coming off the AC unit?

 

[TX2AKRV]

right on top of the AC unit? For me it would make my trailer too tall, but it's an interesting thought. Are you worried about heat coming off the AC unit?

On hot days, yes the panel could get hot, then I just disconnect them from the array.

On cooler days with the AC not running, I think It would be fine.