Bonding aluminum angle to fiberglass roof

K1ngN0thing
K1ngN0thing Solar Expert Posts: 27 ✭
in PV Installers Forum - NEC, Wiring, Installation #1
I'm planning on using aluminum angle bonded to my van's roof to avoid drilling holes. What's the most effective yet affordable method for going about this?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #2
    One of the major problems with bonding is differential expansion between materials. Aluminum tends to expand a lot when it gets hot, around 13 micro inch per inch per F:

    http://www.engineeringtoolbox.com/thermal-expansion-metals-d_859.html

    And fiberglass/epoxy is in the range of 20-30 micro inch per inch per F:

    http://wacoboom.com/site/pdfs/coefficient-of-thermal-expansion.pdf

    What it comes down to is that as the roof and supports change temperatures (freezing to full sun), any "hard" type adhesive would probably fail pretty quickly because of thermal cycling (i.e., epoxies).

    So, what is used on large panels such as buses--Is double stick tape (the foam tape allows differential expansion between materials to a degree):

    Double-Sided Bonding Tapes - 3M

    I worked with an mechanical engineer that swore by the stuff... Although, for our application (computer cabinets), I was not too thrilled with it.

    Personally, I would probably suggest using standard hardware (plated steel, brass, mixed stainless+brass nuts, etc.) to hold the panels down. I would hate to see them "blow off" the roof at some point and end up in the windshield of a following car.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • K1ngN0thing
    K1ngN0thing Solar Expert Posts: 27 ✭
    #3
    The tape was my original plan but it's pretty pricey. I'd rather not drill holes and risk leaks though so I guess there's no way around it. Guess I could go with tape for the underside of the aluminum and epoxy around the edges to be doubly sure. From the demonstrations I've seen of this tape, I'm not too worried about it blowing off. The question now is, which VHB tape do I need? Are they all priced so high? I'm looking at 4991
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #4
    You could always go with a good old roof rack...
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • scrubjaysnest
    scrubjaysnest Solar Expert Posts: 175 ✭✭✭
    #5
    The tape needs to be as thin as possible. Those of us on the Trailmanor forums have used 3m VHB 4910 for several years now to hold the solar panel z brackets to the roof of the trailer. That said most are mounted to the back shell which sits about two inches below the front shell plus has the roof top A/C unit to help break up wind. AM Solar also uses the same tape for their rocker mounts. I just used the 4910 tape, and because I also used pop rivets; mine tilt; to prevent leaks I sealed with 3M 5200 caulk.
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  • Iceni John
    Iceni John Solar Expert Posts: 103 ✭✭
    #6
    The aluminum walkway on my bus's roof (from which my PV panels are hinged) expands and contracts enough that it makes popping noises each evening as it cools. Partly for this reason I used EPDM rubber washers as the seals between the walkway's support legs and the roof itself, so they can flex slightly and still remain watertight. I bolted my walkway through the roof's ribs with 3/8" stainless bolts - attaching panels only to the roof skin is fraught with potential risk! 3M 5200 is incredibly strong as a permanent bond, probably stronger than VHB tapes, but any such attachment is only as strong as the substrate's own strength, so I wouldn't recommend using any surface adhesive for attaching PV panels to fiberglass in case the fiberglass itself delaminates.

    John

    40' Crown bus with 2kW of tiltable panels on the roof:

    Eight Sharp 255W, two Morningstar TS-MPPT-60, Magnum MS2000, Champion C46540 generator converted to propane, eight golfcart batteries, and maybe a small Exeltech inverter for the fridger.

    Southern California

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