Calculating minimum ballast for ground mounted pv system

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jngggggggg
jngggggggg Registered Users Posts: 2
in Solar Beginners Corner #1
Hey everyone, i know this question has been asked quite a bit but i figure id re-iterate.

I am looking to determine the minimum weight of ballast necessary to counter slipping, tipping-over and lift off of a ground mounted system.
I saw a fellow post a sample calculation on this forum a while back
(http://forum.solar-electric.com/showthread.php?8639-Does-anyone-trust-anyone-s-ballast-calculations)

But am concerned how accurate these calculations are.
Its hard to find reliable info on this stuff because most private companies choose not to release their calculations, only the results.
I always envisioned that the area of the actual ballast (i.e. if its 50m^2 vs 150m^2), would change the amount of required ballast drastically.

If anyone can provide a more structured approach to the minimum ballast calculations, that would be awesome
thanks
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #2
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    Re: Calculating minimum ballast for ground mounted pv system
    jngggggggg wrote: »
    Hey everyone, i know this question has been asked quite a bit but i figure id re-iterate.

    I am looking to determine the minimum weight of ballast necessary to counter slipping, tipping-over and lift off of a ground mounted system.
    I saw a fellow post a sample calculation on this forum a while back
    (http://forum.solar-electric.com/showthread.php?8639-Does-anyone-trust-anyone-s-ballast-calculations)
    I always envisioned that the area of the actual ballast (i.e. if its 50m^2 vs 150m^2), would change the amount of required ballast drastically.

    The calculations of tipping and restoring moments are pretty simple physics/engineering. Slippage is more of a problem, involving soils, etc. But if the foundation does not penetrate the ground, the friction from a lump of weight versus distributed weight is theoretically identical. As for lift off, again the distribution of the ballast mass is not as important as the location of the center of gravity of each group of equivalent ballast weights. For example, for lift off of the back of the array, you can consider all of the ballast at the back of the structure as having the same effect as if it were concentrated at the center of the back line of the array (as long as you consider the structure to be perfectly rigid. In practice you want to distribute the ballast from end to end, but again the area does not matter. The distance from the front of the structure to the back does.
    SMA SB 3000, old BP panels.
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  • Ralph Day
    Ralph Day Solar Expert Posts: 1,019 ✭✭✭✭
    #3
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    Re: Calculating minimum ballast for ground mounted pv system

    My ballast load for the 10kw array in the signature was/is 55pounds per sq foot of array area. Concrete and steel racking comprise the ballast.

    Ralph
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #4
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    Re: Calculating minimum ballast for ground mounted pv system

    Guy who lives a few miles from me, and who happens to think he's God's gift to the world and therefore the world owes him, didn't use quite enough ballast on his set up. Strong gust of wind struck his panels and they ended up in a pile, upside down on the rocks.
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  • Ralph Day
    Ralph Day Solar Expert Posts: 1,019 ✭✭✭✭
    #5
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    Re: Calculating minimum ballast for ground mounted pv system

    Does the world owe him insurance coverage, or does it come in under the idiot clause? (You're the idiot responsible, it's your responsibility). I try to avoid those kind of claims at all cost. Like the "covered under Ralph Warranty" . Ralph broke it, Ralph fix it (happens more often than I'd like).

    Ralph
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #6
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    Re: Calculating minimum ballast for ground mounted pv system
    waynefromnscanada wrote: »
    Guy who lives a few miles from me, and who happens to think he's God's gift to the world and therefore the world owes him, didn't use quite enough ballast on his set up. Strong gust of wind struck his panels and they ended up in a pile, upside down on the rocks.

    When I designed mine, the worst were these tipping forces because my rack is tall and tilted at 65 degrees. At 70 mph wind to the back of the modules, it's going to be over a ton of force pulling each of the back posts out of the ground. I certainly hope we never see winds like that.
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  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    #7
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    Re: Calculating minimum ballast for ground mounted pv system
    NorthGuy wrote: »
    When I designed mine, the worst were these tipping forces because my rack is tall and tilted at 65 degrees. At 70 mph wind to the back of the modules, it's going to be over a ton of force pulling each of the back posts out of the ground. I certainly hope we never see winds like that.

    That was my concern when I added my second array recently. We do get very high, gusty winds here - sometimes swirling from many directions. So despite my array being backed up to trees, I decided to just build a shed - sort of an asymmetric A-frame with the big side the array. It was more work but I'm glad I did it this way. I didn't want to be stressing about the incalculable forces coming from behind and under my array during a wind storm. Plus - i got a nice shed out of the deal...:cool:
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #8
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    Re: Calculating minimum ballast for ground mounted pv system
    mtdoc wrote: »
    I decided to just build a shed - sort of an asymmetric A-frame with the big side the array. Plus - i got a nice shed out of the deal...:cool:

    More or less what I did. Built the shed with a high, South facing wall. The roof slants away from the top of that wall and the panel rack is hung and hinged from the top of the wall. Living in the snow belt, it works perfect. Rack is let down to the vertical position for Winter, then tilted up from the bottom after the last expected snow. Very happy with it!
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #9
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    Re: Calculating minimum ballast for ground mounted pv system
    mtdoc wrote: »
    That was my concern when I added my second array recently. We do get very high, gusty winds here - sometimes swirling from many directions. So despite my array being backed up to trees, I decided to just build a shed - sort of an asymmetric A-frame with the big side the array. It was more work but I'm glad I did it this way. I didn't want to be stressing about the incalculable forces coming from behind and under my array during a wind storm. Plus - i got a nice shed out of the deal...:cool:
    waynefromnscanada wrote: »
    More or less what I did. Built the shed with a high, South facing wall. The roof slants away from the top of that wall and the panel rack is hung and hinged from the top of the wall. Living in the snow belt, it works perfect. Rack is let down to the vertical position for Winter, then tilted up from the bottom after the last expected snow. Very happy with it!

    That is an interesting solution. May look better too. Do you have pictures of them?
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #10
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    Re: Calculating minimum ballast for ground mounted pv system
    waynefromnscanada wrote: »
    Rack is let down to the vertical position for Winter, then tilted up from the bottom after the last expected snow.
    If there is still unmelted and clean snow on the ground from the last snowfall, you might get a bit more production by leaving the panels down until that snow melts.
    It would take some testing to be sure.
    SMA SB 3000, old BP panels.
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