Flexible Solar Panel mounting - Magnets?

[booster]

Just as a perspective point of reference.


A 20X40 inch 100 watt solar panel has 800 square inches of area.


Even 1/2 psi on that panel more on the bottom than top would put 400# of force on the panel upwards.


Most humans can blow about 1-2 psi with their own lungs.


The four magnets of 26# would have a hold down power of 104#.

[GeorgeRa]

Originally Posted by booster

Just as a perspective point of reference.


A 20X40 inch 100 watt solar panel has 800 square inches of area.


Even 1/2 psi on that panel more on the bottom than top would put 400# of force on the panel upwards.


Most humans can blow about 1-2 psi with their own lungs.


The four magnets of 26# would have a hold down power of 104#.

I agree, potent forces can be generated by air during driving. I installed solar panels on my Bigfoot trailer using large pads with good VHB with limited success, if I remember correctly 2, perhaps 3 out of 8 pads failed.

On the Sprinter I decided to use factory roof rails with 80/20 crossbars, bulletproof.

If would mount flex panels on the roof I would most likely use VHB on perimeter fully sealed with Dicor, no air gap and less chances for liftoff.

Another issue with any standoff mounts magnets or others with flex panels would be panel flexing in the variable and turbulent air flow below and above. These panels were not designed for constant flexing, they can get “tired”.

[booster]

Originally Posted by GeorgeRa

Another issue with any standoff mounts magnets or others with flex panels would be panel flexing in the variable and turbulent air flow below and above. These panels were not designed for constant flexing, they can get “tired”.

Yep, as mentioned earlier, flutter is a killer in general with flexible stuff.


It makes me recall the times we have been behind a semi giving particularly violent turbulence. It would make the closed fan lid bounce and rattle and make the tv antenna thump up and down onto the roof (pivot down batwing Windgard).

[@Michael]

It's been one year and about 8000 miles since I attached a solar panel to my roof using magnets. The panel hasn't moved a millimeter.

I ended up using 4 x 26lb magnets and 2 x 60lb magnets. I used adhesive backed foam tape on the back of the panel to dampen any fluttering, maintain a constant spacing above the roof, and minimize the amount of air that could get under the panel. The thickness of the foam is about the same as the standoff of the magnets.





The magnets will corrode/rust, so this isn't really a permanent solution.

--Mike

[dmb496]

I own a 2020 Galleria that was originally equipped with 3 flexible solar panels that were applied by Coachmen using Sikaflex. When I removed those 3 panels they were 26 months old and showing surface deterioration.

I installed 4 hard panels and attached them to the roof with aluminum angle and 3M VHB 5952 tape. The average panel hold down strength is about 4k pounds. The VHB is very strong. I deliberately built the system with security, integrity and the safety of others I'd be sharing the roads with.

That being said, I consider your magnetic fasteners to be a shortcut and that luck has been your friend.

[@Michael]

Originally Posted by Ferdekalen

Using neodymium magnets with a bolt and rubber coating to secure it without adhesive or roof holes is a clever idea.

I ended up using two 60lb and four 17lb magnets. It didn't move a millimeter in two years and 20,000 miles.

[booster]

One thing to remember is that magnets will be able slide at a much, much lower force than a straight vertical pull spec would indicate, especially if they are sitting on a slippery surface like a plastic solar panel on a painted surface.


Magnets are also heavy for their size and if they do come loose for whatever reason, even something like a bird strike for instance, they could be deadly projectile that would likely penetrate a windshield.


The risks are too high for my liking, even if they are quite small which they probably are. I do also feel that way about the adhesive only installs of rigid panels, as I am risk very aversive in things like this.


Our rigid panels are attached with screws into soft material pop rivet style threaded inserts that expand to about double the hole size on the inside of the fiberglass roof and also are sealed at held down on the feet with urethane adhesive/sealer.

[dmb496]

I installed 4 rigid solar panels and used VHB double-side adhesive. Based on the performance provided by 3M, each panel has approximately 450 pounds of vertical hold. One year after the installation I did an informal holding power test; I used 2 hands and tried to lift one corner of a panel. The roof of my Sprinter-based Galleria started oil-canning and the panel remained securely adhered to the van. I also added an air dam forward of the front solar panel.

I didn't consider magnets.

[dmb496]

I installed 4 rigid solar panels and used 3M VHB double-side adhesive. Based on the performance data provided by 3M, each panel has approximately 450 pounds of vertical hold. One year after the installation I did an informal holding power test, I used 2 hands and tried to lift one corner of a panel. The roof of my Sprinter-based Galleria started oil-canning and as the panel remained securely adhered to the van. I also added an air dam forward of the front solar panel.

Prior to the build I dismissed the idea of using magnets.

[booster]

Originally Posted by dmb496

I installed 4 rigid solar panels and used VHB double-side adhesive. Based on the performance provided by 3M, each panel has approximately 450 pounds of vertical hold. One year after the installation I did an informal holding power test; I used 2 hands and tried to lift one corner of a panel. The roof of my Sprinter-based Galleria started oil-canning and the panel remained securely adhered to the van. I also added an air dam forward of the front solar panel.

I didn't consider magnets.

The air dam is a good idea as air underneath can generate quite a bit of force.


I did the calcs (simple) to find out how much air pressure it would take to get to 450 # force on the underside of the panel on a 21X61" panel like we have on ours. 450/21x61 = .3513psi. That is about the pressure you get from a small squirrel cage blower so not very much.

[dmb496]

My memory is at fault. Here's a redo on the math. Each connection point uses a 4"L x 3"W aluminum angle. The 3M VHB is rated at 75 pounds of vertical hold-down force per 1 square inch.

3 of the 4 panels are held in place with (4) 4" x 3" aluminum angle. That's 12 sq. in. of surface x 75 pounds per sq. in. of vertical hold down force per angle pad, totaling 900 lb. per pad, 3600 lb. per panel.

The front panel is held in place with (2) 4" x 3" aluminum angles at its rear, and (4) 4" x 3" aluminum angle pieces along the forward edge. The math for the forward panel should equal 5,400 lb. of vertical hold down force.

[booster]

Originally Posted by dmb496

My memory is at fault. Here's a redo on the math. Each connection point uses a 4"L x 3"W aluminum angle. The 3M VHB is rated at 75 pounds of vertical hold-down force per 1 square inch.

3 of the 4 panels are held in place with (4) 4" x 3" aluminum angle. That's 12 sq. in. of surface x 75 pounds per sq. in. of vertical hold down force per angle pad, totaling 900 lb. per pad, 3600 lb. per panel.

The front panel is held in place with (2) 4" x 3" aluminum angles at its rear, and (4) 4" x 3" aluminum angle pieces along the forward edge. The math for the forward panel should equal 5,400 lb. of vertical hold down force.

Those are quite large feet, and I assume flat. Is the roof equally flat where they sit?


We have heard of the solar panels falling off several new vans lately, maybe Thor?


On thing I do remember the 3m reps telling us to do in industrial use of their tapes for mountings was that they recommend that the edges be caulked with urethane sealant to protect the tape and adhesive from the elements. That might explain the common use of Sikaflex over the installation even when there are no fasteners.

[dmb496]

Enlarge the photo and specifically look at each of the 4 front alloy angle pieces. The single center fastening of each alloy angle to the solar panel provides for whatever degree of pivot is needed to allow the alloy angle to remain square with the roof of the van, as the roof does have a slight arc.

[booster]

Originally Posted by dmb496

Enlarge the photo and specifically look at each of the 4 front alloy angle pieces. The single center fastening of each alloy angle to the solar panel provides for whatever degree of pivot is needed to allow the alloy angle to remain square with the roof of the van, as the roof does have a slight arc.

I really don't want to go excessive on this, but a single bolted center like that gives only one axis of adjustability. That would be across the van in this case. The roof of the van appears, at least in the pic, to be at a different slope fore and aft compared to the panel and there is no adjustment for that with a fixed angle like the aluminum angle unless you bend the angle like I had to on our Roadtrek, or bend the flange on the panel which can be done in the middle pretty easily, but tough in the corners. At least the roof looks to be in mostly a single plane which helps if you get both angles correct. Our Roadtrek roof is a slight curve at the mounting points.

[dmb496]

OK, I give up at this point. Some of the members of this group tend to relish over analyzing.

Adios.

[GeorgeRa]

I had similar setup on my fiberglass Bigfoot Trailer, it didn’t work very well hence on the Sprinter I mounted panels to roof rails.

Rigid solar panels are not rigid enough for calculating a lifting force as a total of all mounts, their flex needs to be included in the total mount strength calculation. Not an easy calculation.

I had four feet per each solar panel about 3”x4” each. One of these started to separate forcing the adjacent foot losing its adhesion.

Flying solar panel freebies could be deadly.

[Urlauber]

Just got a recall from Thor for fracturing solar panel mounts (flimsy angles).
Neodymium magnets may be more secure...

[@Michael]

Originally Posted by GeorgeRa

Rigid solar panels are not rigid enough for calculating a lifting force as a total of all mounts, their flex needs to be included in the total mount strength calculation. Not an easy calculation.

AFAIK the entire van flexes and twists. Ridgid panels are likely to expand and contract at a different rate than the van roof. In theory, there's a whole bunch of movement that would have to be accommodated by the mounts.

Since a hailstorm trashed all my panels anyway, my plan is to mount a roof rack using the factory roof attach points that Ford welds into the body, and switch to rigid solar panels.

Also - glue-down flexible factory panels will expand and contract at a significantly different rate than the metal roof, so the adhesive must accommodate that also.

[booster]

Originally Posted by @Michael

AFAIK the entire van flexes and twists. Ridgid panels are likely to expand and contract at a different rate than the van roof. In theory, there's a whole bunch of movement that would have to be accommodated by the mounts.

Since a hailstorm trashed all my panels anyway, my plan is to mount a roof rack using the factory roof attach points that Ford welds into the body, and switch to rigid solar panels.

Also - glue-down flexible factory panels will expand and contract at a significantly different rate than the metal roof, so the adhesive must accommodate that also.

Smart move, using the known to be secure factory roof rack mounts is by far the most secure way to go, assuming good quality hardware is used to that mounting. It should also put the panels up a bit to keep them cooler and let high speed air through so all good.

[GeorgeRa]

Originally Posted by booster

Smart move, using the known to be secure factory roof rack mounts is by far the most secure way to go, assuming good quality hardware is used to that mounting. It should also put the panels up a bit to keep them cooler and let high speed air through so all good.

Agree, the air gap is critical to keep efficiency high and keep roof cool during sunny days.

I made these solar panel mounts 10 years ago and stop thinking about it except occasional cleaning. Hein from the Sprinter Forum sells a production version of theses. https://photos.app.goo.gl/NaDXquJwAigYuTBw1