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Cargo Trailer Solar Panel Roof Rack Design Review

rmaddy

Full-time Solar-powered Trailer Life
Joined
Nov 16, 2019
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Location
USA
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 each. The three panels just fit (1" to spare) between two roof fans.

I've modeled most of the trailer frame and my rack plan using SketchUp. The attached screenshots are from the model.

The first picture shows a view from above. The back of the trailer is to the left. The "floating" fan to the right is in the V-nose which isn't shown in the model. The panels are shifted to line up with the left side of the trailer because the right edge of the roof will be used for something else.

View_ISO.png

As you can see the rack is comprised of four rails made from 1.5" aluminum extrusion. The panels will be secured at the 4 provided mounting holes in their frames. I'm going to make custom feet for the rails using 2" leg, 1/4" thick aluminum angle. This setup lets me deal with the roof curvature and the fact that the roof beams don't line up with any of the mounting holes in the panels. I'm using four shorter rails instead of two full-length rails to save a bit of weight (and a few $$). This setup also allows the panels to be roughly 3.5" - 4" off of the roof for good ventilation as recommended by the manufacturer.

The second picture is a view from the right side of the trailer looking toward the left side.
View_Side.png

The third picture is a view from the front looking to the rear.
View_Front.png

The fourth picture is a close-up of one of the rails and the custom feet.
View_Close.png


1. My main question is this a viable, safe design? In other words, is there any risk of this ripping off of the trailer while driving down the highway at 70mph? Each panel will be mounted to the rails with four 1/4" stainless steel bolts using the panel's designated mounting holes. Each custom foot is attached to the rail with two 1/4" screws. The two halves of each custom foot are held together with two 1/4" bolts. And each foot will be attached to the roof beam using 1/4" bolts screwed into drilled and tapped holes in the beam. Each rail will be secured with three feet on three beams.

2. While the panels will be mounted at their designed mounting spots, how well will the panels survive the rigors of being on the roof of a moving vehicle over time? The panel frames are sufficiently secured to the rails but I'm worried about those 41lb panels bouncing and vibrating with no support to the glass itself except around their perimeter.

3. Grounding - My thought is that I don't need any grounding with this setup since the metal solar panel frames are bolted to metal rails which are bolted to metal feet that are bolted into the steel beams of the trailer. The rails are anodized so I'll use exterior tooth lock washers in the mix to help ensure the anodizing is broken at the connection points. Is this valid logic or should I still attach grounding wires to each panel and each rail and connect those wires directly to the steel roof beams?

Any feedback is appreciated. Thanks.
 
Looks like those panels are 40 Voc


If wired in parallel, not a particularly dangerous voltage. In series, more so.
So if in series I'd be more inclined to ensure positive grounding of frame. That way if a fault caused PV voltage to reach the frame, it would be grounded to trailer chassis.

You have 4 short rails, one panel spanning all. If flexing spread rails apart, could pull frames off glass panel.
Maybe not a great likelihood, but how about tying the pairs of rails together with a rod under tension?
(I have a concept planned for rearranging my array that will have a similar issue, so plan to tether the rails to keep frames under compression.)

Flat orientation of panels will produce much less power on a clear winter's day. Can you come up with a secure tilt mount?

We have contemplated that large a sheet of glass bouncing, thought gluing stiffener rails on underside might help.

That large a panel might get three rails in a high wind area. Possible way to make it more secure.

"drilled and tapped holes in the beam"
How thick is material of beam? I would guess very few threads, will end up looking more like a Tinnerman clip than a nut.
Sometimes (e.g. Square D boxes) holes in sheetmetal are made by punching not drilling, so thread length can be much longer.
Drilling all the way through to interior and adding a washer would be more secure. I did that to crudely mount a rack over the cab of my pickup.

1/4" bolts aren't that much. Stainless is maybe half the strength of regular steel. Could up-size them a bit. Use a suitable locking mechanism due to vibration, maybe nylon lock on nuts would be good.

Rails look like a type I've seen for assembling structures.
If you don't already have it, also look into Unirac and Iron Ridge.
 
Looks like those panels are 40 Voc


If wired in parallel, not a particularly dangerous voltage. In series, more so.
So if in series I'd be more inclined to ensure positive grounding of frame. That way if a fault caused PV voltage to reach the frame, it would be grounded to trailer chassis.

I will have them in series. They are the SIL-300BL (not ML) which has an even higher Voc of 42.1 at STC. So that makes your point even more. Good point to consider.

You have 4 short rails, one panel spanning all. If flexing spread rails apart, could pull frames off glass panel.
Maybe not a great likelihood, but how about tying the pairs of rails together with a rod under tension?
(I have a concept planned for rearranging my array that will have a similar issue, so plan to tether the rails to keep frames under compression.)

I hadn't considered the possible tug of war that might happen on the center panel. I need to rethink that gap.

Flat orientation of panels will produce much less power on a clear winter's day. Can you come up with a secure tilt mount?

Ideally it would be nice if the panels could tilt. But with 123 pounds of panels plus the weight of the rack, adding the ability to tilt is asking a lot. Due to the slight curve of the roof I could get the panels set with about 2º but that's about it. I'm worried that if they are fixed with that slight tilt, I always have to be sure to park facing west or I'll get even less output. With them flat, it doesn't matter which way the trailer faces.

We have contemplated that large a sheet of glass bouncing, thought gluing stiffener rails on underside might help.

Interesting idea.

That large a panel might get three rails in a high wind area. Possible way to make it more secure.

I originally planned on a third rail between the two I showed above. But the extra cost and weight started adding up plus it will be really difficult reaching that part of the roof to do the work. Being a cargo trailer with a very thin aluminum skin you can't get on the roof at all. At best you can lean over the edge while standing on a ladder.

"drilled and tapped holes in the beam"
How thick is material of beam? I would guess very few threads, will end up looking more like a Tinnerman clip than a nut.
Sometimes (e.g. Square D boxes) holes in sheetmetal are made by punching not drilling, so thread length can be much longer.
Drilling all the way through to interior and adding a washer would be more secure. I did that to crudely mount a rack over the cab of my pickup.

I believe the beams are 1/8" steel, maybe 3/32". I could tap for 1/4"-28 instead of 1/4"-20. That would give me 3.5 threads instead of 2.5. While through-bolts would be much stronger, I have a lot of the interior done already so I have no access to the beams from the inside of the trailer. I should have built the rack much earlier in the build I suppose. Too late now.

1/4" bolts aren't that much. Stainless is maybe half the strength of regular steel. Could up-size them a bit. Use a suitable locking mechanism due to vibration, maybe nylon lock on nuts would be good.

I didn't know stainless steel bolts were so much weaker. I realize I could use regular bolts for the feet into the roof beams since those bolts will be sealed with Dicor. I will be using nylon lock nuts for the bolts that hold the two halves of the feet together. And I will be using split washers and lock washers in the other places to deal with vibration. Some Loctite would be good too.

Rails look like a type I've seen for assembling structures.
If you don't already have it, also look into Unirac and Iron Ridge.

The rails are standard aluminum extrusion from 80/20 (https://8020.net/1504-ls.html). I'll look into the others you mention.

Thanks
 
The clearance to your front vent may be an issue, if you are using a fantastic fan or similar you would have shading. Putting a fairing on the front panel has been discussed to reduce lift at highway speeds. I would be concerned with the large panels mounted flat on a bouncy trailer, would like to reinforce the glass as Hedges suggested.
 
The clearance to your front vent may be an issue, if you are using a fantastic fan or similar you would have shading. Putting a fairing on the front panel has been discussed to reduce lift at highway speeds. I would be concerned with the large panels mounted flat on a bouncy trailer, would like to reinforce the glass as Hedges suggested.
Adding a fairing to the front sounds like a great idea. And avoiding I-80 in Wyoming most days.

I have been worried about shading from the fans. But that's what I have to work with. I thought I would have more space. The rear fan was supposed to go further back but for some mysterious reason the aft-most roof beam is only 14.5" from the next beam instead of 16" and the fan wouldn't fit. So I lost that much roof space moving the fan forward one set of beams. But hopefully shading from the fans will only be an issue early/late in the day when the sun angle is low giving poor results anyway. And I chose this particular model of panel because they use 120 half-cells which supposedly deal with partial shading better.
 
Here are the items I saw:
I would use a single beam of the 8020 rather than the two for each side of the rail. Or at least add a beam between the two for stiffness - connected to both beams. This will also add one more support onto another roof rib.

Assembly and disassembly... You will need to assemble and disassemble (someday your vent will die somehow and will need to be replaced). Is the (what looks to me to be) 3" enough space for assembly and disassembly?

Add some VHB tape to the bottom of the bracket against the roof, then screws, then covered in Dicor.

One thought about completely flat panels - when it rains and the pollen and pollution cover everything, completely flat panels won't tend to shed the dirty water, some slope may be good.

I like your rail system using the 8020.

Good Luck!
 
I'm updating my plan to make the beams full length.

The panels get to about 1" from the fans but they shouldn't cause any issue if I need to remove a fan in the future.

What does adding VHB tape between the feet and roof skin do? Add strength? Help with vibration? I suppose it can only help.

Good points about flat panels.
 
I think the vhb tape would add a lot of strength. If one of your screws starts to come lose the tape should keep the bracket held down, thus slowing down how fast is loosens and maybe preventing it from letting go. Also the tape by itself should hold the panels down (don’t need the screws - but I would use them).
 
Yeah, I wouldn't trust the panels being held down just with VHB tape. But using that with the screws sounds good.
 
Just let us all know when you're leaving so we can give you a head start and not be behind you. :)
 
Interesting timing. I was actually just outside installing the first main support to the trailer roof. Since I last posted on this thread I've completely changed my design for how it is all mounted to the roof. I'll post an update later.
 
Interesting timing. I was actually just outside installing the first main support to the trailer roof. Since I last posted on this thread I've completely changed my design for how it is all mounted to the roof. I'll post an update later.
What time are you leaving? LOL... just kidding.
 
Interesting timing. I was actually just outside installing the first main support to the trailer roof. Since I last posted on this thread I've completely changed my design for how it is all mounted to the roof. I'll post an update later.
got some pictures? ?
 
Here's a final update. I ended up completely changing my rack design. The feet I was originally planning were going to be 1) a giant PITA and 2) far from ideal.

My finished rack was much easier and it is much stronger and safer. But it is heavier. It consists of two main Superstrut (Unistrut) rails down each side of the trailer roof. These are 10' long each and bolted every 12" into the trailer frame. Then there are four Superstrut rails running perpendicular to the two side rails. Then there are two 10' long 80/20 aluminum extrusions running fore/aft across the 4 cross beams. The 3 solar panels are bolted (using their standard mounting holes) to the 80/20 extrusions. To help with vibration I added additional short lengths of 80/20 extrusions under the edges of the solar panels and then used IronRidge panel mounts. So each panel is held down by 4 bolts and 6 panel mounts.

Here are some screenshots from my Sketchup model.

This is an overview of all of the rack pieces:
Rack2.png

And with the panels in place:
Rack1.png

Here's a different angle of the rack parts:
Rack3.png

In the end I have 1/2" between the rear panel and the rear fan and 1/2" between the front panel and the front fan. The panels have a 1/2" gap between them for airflow. I have no problem with shading from the fans at all. All of the rails and panels are grounded together with one final bolt going into the trailer frame.

Here are a few pictures of the actual setup:
IMG_2723.JPG
IMG_3117.JPG
IMG_3118.JPG
IMG_3119.JPG
IMG_3298.JPG
 
Here's a final update. I ended up completely changing my rack design. The feet I was originally planning were going to be 1) a giant PITA and 2) far from ideal.

My finished rack was much easier and it is much stronger and safer. But it is heavier. It consists of two main Superstrut (Unistrut) rails down each side of the trailer roof. These are 10' long each and bolted every 12" into the trailer frame. Then there are four Superstrut rails running perpendicular to the two side rails. Then there are two 10' long 80/20 aluminum extrusions running fore/aft across the 4 cross beams. The 3 solar panels are bolted (using their standard mounting holes) to the 80/20 extrusions. To help with vibration I added additional short lengths of 80/20 extrusions under the edges of the solar panels and then used IronRidge panel mounts. So each panel is held down by 4 bolts and 6 panel mounts.

Here are some screenshots from my Sketchup model.

This is an overview of all of the rack pieces:
View attachment 64312

And with the panels in place:
View attachment 64313

Here's a different angle of the rack parts:
View attachment 64314

In the end I have 1/2" between the rear panel and the rear fan and 1/2" between the front panel and the front fan. The panels have a 1/2" gap between them for airflow. I have no problem with shading from the fans at all. All of the rails and panels are grounded together with one final bolt going into the trailer frame.

Here are a few pictures of the actual setup:
View attachment 64316
View attachment 64317
View attachment 64318
View attachment 64319
View attachment 64320
This looks great!
 
Like that all black setup!
Thanks. The panels just happened to be all black. For the 80/20 aluminum extrusion I ordered the black anodized. For the steel Superstrut I bought the plain silver version at Home Depot along with a couple cans of Rustoleum flat back spray paint. For the "U" brackets I did the same. They are actually for Unistrut ordered elsewhere but they were the same silver finish as the Superstrut. The IronRidge panel clamps came in black so they were easy.
 
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