Vaccuums and roof pitches...

Photowhit
Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
in Advanced Solar Electric Technical Forum #1
While at home visiting I got into a discussion about my plans to put a solar array above the porch of my house and about a foot above the roof. I was talking with some pretty bright guys (and one out of place awesome lady) and they felt I had no issues with wind from behind since wind coming from the roof side would create a vacuum under the array and actually pull the array toward the ground.

So, now I've decided to increase the pitch to one greater than the roof line. The pitch of the roof is very shallow, perhaps 2 feet over 8 feet (3:12) I've decided to run the array at closer to 6:12, I guess I loose that vacuum effect? Particularly since I want to increase the height at the array to about 3 feet above the bottom of the roof. So now do I have a kite? I do have trees behind the home, so I'm not expecting huge forces. just getting nervous. The porch is a floating porch and I will put in at least one anchor, but I have a water line coming in somewhere around there...

Sorry if the placement of this post is a bit odd, I didn't know where to post it. Mods feel free to change it...
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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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Vaccuums and roof pitches...

    Don't bet on there being any vacuum effect regardless of roof pitch and/or panel angle.
    Turbulence, yes.

    Wind does not always come from the same direction, so it doesn't always apply force the same way.

    If you lift the whole of the array you get a 'sail'. If you change the angle of the array in respect to the roof angle you get changes in the air pressure between the two surfaces.

    Mostly it is not a concern unless you experience hurricane force winds, in which case all bets are off and you'll probably get a tree branch blown through a panel anyway.
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #3
    Re: Vaccuums and roof pitches...

    Cariboocoot did an excellent job of putting my thoughts into words. Yes, in wind tunnel experiments with everything arranged just so, it would definitely be possible to have angles etc that create a reduced pressure between the panels and the roof. But in nature? In real life? I wouldn't give it a value of more than 2 cents.
    As Coot suggested, in real life you'll see lots of turbulence, and at times lots of "lift". Located where I am on the East Coast, we have to pay extra attention to such things because of tropical storms and hurricanes that come up the coast every year. So far, so good, and thankfully we haven't had a direct hit with a really bad hurricane in a number of years. Hoping it stays that way!
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #4
    Re: Vaccuums and roof pitches...
    Cariboocoot wrote: »
    Mostly it is not a concern unless you experience hurricane force winds, in which case all bets are off and you'll probably get a tree branch blown through a panel anyway.

    Tornados are more likely here in Missouri...lol.

    Wayne, I understand what you are saying, and I'm changing from the original design so all bets are off. As to the original design, It was a bar room discussion, but with people who you would trust with a 'back of envelope' design or at least I would, professors, engineers and a builder(bridges?) who they deferred to. I likely didn't describe it very well, as it is only over the roof for a couple feet, mostly over the porch, and originally at least a foot below the roof peak. Also perhaps 10 feet from either end of the 'Tin Can'/home.
    Cariboocoot wrote: »
    If you lift the whole of the array you get a 'sail'. If you change the angle of the array in respect to the roof angle you get changes in the air pressure between the two surfaces.
    Yep, kinda what I'm worried about, I'm creating a big lever, only 8 feet across the porch and a single anchor in the back might not cut it. It will become nearly14 feet tall next to the house. I guess I could tie it into the Mobile frame across the wooden piers and under the mobile. means cutting the skirting and scrabbling under the tin can, things I never liked doing less so as I age.
    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.
    · Share on Twitter

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