Panel Mounting on Flat Roof Aluminum Cargo Trailer

[carbon60]

Hi All,

I'm getting ready to mount 4x 425W rigid panels to a 7x14' enclosed aluminum cargo trailer.

The details:

1. Panels are 208.0 cm (81.9") by 103.0 cm (40.6"), 24.5 kg (54.0 lbs).
2. I want to install four of them flat on the roof.
3. The trailer is a 7' by 14' (Approximately 84" x 168"), with an additional 3.5' "nose" that I can use for wiring ingress, ventilation, antennas, etc.
4. The roof is flat, aluminum skin over 1" aluminum square tube, 12" o.c.
5. The interior of the trailer will be completely unfinished when I do this install.

Questions:

1. The local supplier suggested a rail system under the panels that would lift them up quite a bit. I'm looking for better ideas, anyone have suggestions?
2. Any reason to mount them all transversely across the roof vs two pairs end to end?
3. Since I will often be partially shaded from trees or just have inflexible parking options, should I wire them 2S2P or 4P? (These panels should run @ 40V and I have a 24V battery.)
4. I have previously used simple ABS entry glands adhered to the skin of my other trailer. Is there a better option?
5. All thread links, comments, suggestions, criticisms, and questions are welcome!

Thank you all for your input,

A.

P.S. I'm floored at the quote I received for the panels: CAD$278 (USD$218 at the moment)!

 

[HRTKD]

Those are big panels. I paid about $180 each for 320 watt panels last fall. I tried to get Q cells locally but nobody had them. They were in my top five. If you have to ship them, that's going to be expensive.

Rail systems are nice. They give you a lot of options for how to install the panels. Upgrades to different panels would also be easier with a rail system, but most panels are in place for years, so I discount the upgrade "feature". Rail systems add a lot to the cost of an install.

Since you have an aluminum skin, VHB tape has been suggested for attaching the brackets to the roof. I'm not a fan of VHB tape but there are quite a few people that have it work for them.

The standard Z bracket is wider than 1". That's going to be a problem. One bolt will make it through the 1" roof square tube, but the other bolt would be in just the sheet metal. Either don't use the second bolt or figure out a way to attach a piece of angle aluminum to the square tube to give the second bolt something firm to attach to. For panels that large, consider using six brackets instead of the usual four.

As far as orientation of the panels, whatever works for your roof. My preference is to install the panels parallel to the trailer for less wind resistance. The brackets probably don't add that much resistance, but I do what I can to minimize it.

When figuring out what panels fit on your roof, don't forget to take into account the width added by the Z brackets, if you decide to use those.

The wiring configuration is often dictated by your solar charge controller. 4s may be too many volts for your solar charge controller. 4p may be too many amps for the wires. 2p2s is a good compromise. Modern panels have diodes that can minimize the effect of shading on the panels.

 

[carbon60]

Those are big panels. I paid about $180 each for 320 watt panels last fall. I tried to get Q cells locally but nobody had them. They were in my top five. If you have to ship them, that's going to be expensive.

They are in stock at a local solar retailer, amazingly. Makes life a lot easier.

The standard Z bracket is wider than 1". That's going to be a problem. One bolt will make it through the 1" roof square tube, but the other bolt would be in just the sheet metal. Either don't use the second bolt or figure out a way to attach a piece of angle aluminum to the square tube to give the second bolt something firm to attach to. For panels that large, consider using six brackets instead of the usual four.

Good points. Z-brackets would only work with the panels parallel to the trailer and the fit would be really tight.

I used ABS brackets and VHB to install a couple of 100W panels on my off-road trailer, worked quite well. But since I have access to both sides of the roof in this new trailer, I figure I might as well bolt through. I will think about it, though.

The wiring configuration is often dictated by your solar charge controller. 4s may be too many volts for your solar charge controller. 4p may be too many amps for the wires. 2p2s is a good compromise. Modern panels have diodes that can minimize the effect of shading on the panels.

The diodes don't help with share when the panels are in series, right? How would that work?

 

[HRTKD]

The Z brackets would work really well with the panels perpendicular to the trailer, or to state it a different way, parallel to the roof supports. But that only works if the roof support spacing exactly matches with width of the panels+mounting bracket. Good luck with that.

Diodes help with the panels in series. Diodes isolate sections of a single panel.

 

[carbon60]

But that only works if the roof support spacing exactly matches with width of the panels+mounting bracket. Good luck with that.

Yeah, I discounted that, it won't work.

Diodes help with the panels in series. Diodes isolate sections of a single panel.

Right, so the voltage will just change as shade cross them cells, so no real issue. These panels are Voc of 49V, so I need a controller than can safely handle up to 200V if I want them in series. So likely it's easier to do 2P2S as you suggested.

I'm looking forward to receiving this trailer and getting to work…

 

[charly]

Ive used superstrut / unistrut rails (parallel to long side of trailer) and then mounted panels to them. allows some variation and gets them up off roof providing cooling. Tilt options can built right away or later too.

 

[Hedges]

Yeah, I discounted that, it won't work.



Right, so the voltage will just change as shade cross them cells, so no real issue. These panels are Voc of 49V, so I need a controller than can safely handle up to 200V if I want them in series. So likely it's easier to do 2P2S as you suggested.

I'm looking forward to receiving this trailer and getting to work…

250V.
Voc is increased by record cold temperature and thermal coefficient. +16% for my location and panel specs.
You can get a 250V charge controller from Midnight. 2kW into 24V battery.

Product - MidNite Solar

MidNite Solar Product Page.

www.midnitesolar.com

Series only makes sense if same orientation. If you might tilt in multiple directions, only have a given direction in series.

Panels that large may bounce a lot. (of course, same happens when shipped.) It has been suggested that ribs be added to stiffen the glass for mobile applications.

 

[carbon60]

250V.
Voc is increased by record cold temperature and thermal coefficient. +16% for my location and panel specs.

Right, I had heard that and thought this factored into the specs, thanks for correcting me.

Panels that large may bounce a lot. (of course, same happens when shipped.) It has been suggested that ribs be added to stiffen the glass for mobile applications.

Stiffening the glass directly, like by gluing an extra rib to the back? I was looking at putting two parallel rails down the roof, about 28" apart, then mounting the panels perpendicular to that.

Thanks for the reply,

A.

 

[Hedges]

Stiffening the glass directly, like by gluing an extra rib to the back? I was looking at putting two parallel rails down the roof, about 28" apart, then mounting the panels perpendicular to that.

Yes, gluing rib on bottom of glass was an idea.

Panels across rails is a good way to mount. Usually we have the longer direction of panel crossing rails. 28" between rails sounds rather short distance for 82" panel. I would think of 42" between rails, panel extends 20" out either side, for approximately lowest deflection under force (bouncing in your case, wind loading in mine.)
There are bottom mounts, which I use for ground mounted arrays (easy access underneath) and top-clamps for low on roof.

For something mobile I might prefer bottom clips, through a hole in frame. Some top clamps worry me that a bit of flexing could pop off. Probably depends on how well designed (not just a bolt for last panel in a row, but something that mechanically avoids tilting away.)

Adjustable tilt to could useful. Panel crosswise on roof, hinge on one end, something to mechanically secure flat or tilted on poles.

 

[HRTKD]

While I don't plan to add a tilt mechanism to the panels on my roof, it would be helpful to know how far apart the panels have to be mounted to ensure that one panel doesn't shade the other. My panels are mounted on the rear of my roof, side by side, about 16" apart. That's the only location they fit in. I suspect that given that they are only 16" apart there is a limit to how much they can be tilted before shading occurs. Is there a tilt calculator out there that could be used to optimize the layout and understand maximum tilt before shading?

I realize that mounting the panels in series, parallel to the length of the trailer, would work better for tilting. But not everyone has that option.

 

[Hedges]

One rule of thumb is distance to a higher obstruction should be 2x height.
For moderate latitudes, at least.

I would suggest leaving a clear view at least 45 degrees from panel, however it may be tilted. That way sun starts to be obstructed only when effective area of panel is 0.7x what it presents when facing sun directly. For East/West shading, that means 3 clear hours before/after ideal (noon?) orientation.

For seasonal angle, that may not be good enough. Sun low in winter, if a single PV cell width is shaded, may take out entire panel depending on layout. Then you get nothing all day.

Ideally, angle from bottom of rear panel to top of front panel allows noon sun to reach it, for your latitude and winter tilt of earth.

 

[HRTKD]

One rule of thumb is distance to a higher obstruction should be 2x height.

d = h x 2 or h = d / 2
Where:
d = distance between panels/objects
h = height of tilted panel/object

Taking my install as an example, the front panel (closest to the sun) could be tilted no more than 8" from horizontal. That's not much.

Did I get it right?

 

[carbon60]

I spoke to the manufacturer of my trailer, yesterday, and he can put 2”x2” cross bars, ladder rack style, wherever I would like them.

So now I’m thinking that this may be a smarter approach. I could order 4 cross bars, located in pairs 40” apart. Each pair of panels on a pair of cross bars.

but then now to secure the panels to the cross bars?

 

[HRTKD]

but then now to secure the panels to the cross bars?

u-bolts?

 

[carbon60]

u-bolts?

Be a bit of a bear to reach under and tighten them, but certainly possible.

 

[carbon60]

u-bolts?

Actually, now that I sketch it out, some of the u-bolts would be more than 40" inboard from the edge of the roof and impossible to tighten:

Please excuse my sketch…

 

[Whinny]

The manufacturer would probably weld tabs on the racks if you asked. There is another conversation with the same topic....

Cargo Trailer Solar Panel Roof Rack Design Review

I'm finalizing my rack for solar panels on my 7'x14' V-nose cargo trailer. The trailer's roof frame is curved (roughly 1" taller in the middle than the sides). The steel beams are roughly 16" on center. I am installing three 330W panels (Silfab SIL-300BL) that are 66.9" x 39.4" and 41 pounds...

diysolarforum.com

 

[carbon60]

The manufacturer would probably weld tabs on the racks if you asked. There is another conversation with the same topic....

Cargo Trailer Solar Panel Roof Rack Design Review

I'm finalizing my rack for solar panels on my 7'x14' V-nose cargo trailer. The trailer's roof frame is curved (roughly 1" taller in the middle than the sides). The steel beams are roughly 16" on center. I am installing three 330W panels (Silfab SIL-300BL) that are 66.9" x 39.4" and 41 pounds...

diysolarforum.com

I had seen that, very thorough models! But I'm going to avoid holes in the trailer roof.

Regarding tabs: how do you see that working? I was thinking I could sister an extrusion capable or receiving captive t-nuts a la 80/20 or Kinetic Rapid One solar mounting rail. Doesn't seem very elegant, though.

 

[Whinny]

Tabs on the roof racks to bolt your rails to-instead of the "U" bolts mentioned above....unless I'm misunderstanding something-not the first time that has happened..............

 

[carbon60]

Tabs on the roof racks to bolt your rails to-instead of the "U" bolts mentioned above....unless I'm misunderstanding something-not the first time that has happened..............

Those fasteners need to be blind, since I would not be able to reach underneath to hold a nut. That's why the commercial roof-mounting systems use captive t-nuts or the equivalent.