Wind load effect on the PV system

Lege
Lege Registered Users Posts: 3
Hi,
I'm a structural design engineer, and I'm new to the solar energy world,
it's required from me to design a solar mounting system resisting a wind speed of 150 kph,
In the attachments you can find a sketch of the required structure, and a picture taken from AutoCAD file showing the deatails of the solar system,

what I really need to know is the wind load affect (pressure) on my structure and PV panels???
and what is the main part of the whole system should prevent the uplifting forces acting on the panels???

It'll be open from all sides and the roof is only covered by PV's which are placed flatly,

I really need your help, I have no one to help me here and finally I could find a solar energy forums,
I appreciate your fast replies for me,
thanks a lot in advance :)

Comments

  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Wind load effect on the PV system

    welcome to the forum,
    i can't say that i am qualified to answer your questions, but if the pvs are flat the wind will have almost negligible effect on the pvs barring some of those crazy winds from derechos, hurricanes, and tornadoes. i would think there would be more pressure upon the supporting structures as the winds attempt to pass under the pvs. i am in a similar situation as i would need to design a horizontal structure (pipe) needing to span over 20ft and hold 4 large pvs while free standing from 2 vertical side supports by about 15ft (also pipes), but my pvs will not be flat and will have a greater push/pull from the wind. we have similar problems even though yours is much broader in scope.

    if anyone does help you with your needs i'd then like mine addressed either after yours or do mine in a separate thread. i have an ahj that would be all over me without facts and specs to back me up. i guess the base will also need addressed in both of our cases with mine needing to be anchored in the ground with concrete. a roof would be trickier as it is not as solid of a base for such a large array and there would need to be huge structural reinforcements if not a total redo.
  • Lege
    Lege Registered Users Posts: 3
    Re: Wind load effect on the PV system

    Thanks Neil,
    but I couldn't understand what do you mean of (if the pvs are flat the wind will have almost negligible effect on the pvs)????
    since it's open structure, 3 meters high from the ground, I think the wind will try to uplift the PV's, so how it's negligible???

    through my search in the internet, I could know that since the Aluminum rail which will the PV's be attached to make a gap between the panels and the steel structure, then it'll be a matter of fluid mechanics,
    and this what increased the complexity in my mind, I feel like that I'm really lost :cry:

    and what do you mean of (ahj) ???
  • peakbagger
    peakbagger Solar Expert Posts: 341 ✭✭✭
    Re: Wind load effect on the PV system

    I know enough to admit that I dont know enough about wind loads on roofs. Here is what I think I know.

    Wind up lift on the edge of roofs is usually the worse. Its somewhat easier to visually if you think of the wind as flowing water. If you stick a vertical wall in stream of water with the bottom of the wall stuck in the bottom channel, the water has to go somewhere. It cannot go down and cant go forward so it has to go up. Water and air dont turn corners very well so what happens is the water go up over the wall and tries to smooth out of the edge and then tries to smooth out vertically but it cant make the turn quick enough and it forms circular eddies on the roof that pull up and push down on the roofing. This is made far worse by any overhangs out over the side of the roof as the upwards pressure and eddying is even worse. Therefore the solar panels should not overhang out past the vertical walls. Eventually the flow straightens out on the roof but in certain circumstances the terrain and wind can create a vertical uplift on the panels. The interaction of the panel and the underlying roof can create two air flows on top and on bottom of the panels. This acts likes a primitive wing and causes uplift.

    Many roof are designed for downward load but not upwards loads, so a review of the structure needs to be made to make sure that the structural is adequately anchored for vertical uplift.

    These effects can be made even worse by adjacent buildings or terrain features. Most building codes require the a strip along the edge of the roof usually 10 to 20 feet has additional bracing and additional fastener density to deal with the possible wind uplift and turbulence so this is an area best avoided by solar panels.

    In general if you have a reliable prevailing wind direction you can normally get away using wider mount spacing down wind and tighter on the upwind side.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Wind load effect on the PV system

    Iron Ridge has some engineering design information--It has been years since I last looked, but some of it was pretty detailed about the effects of wind:

    http://www.ironridge.com/products/groundmounting/systemsupport

    Google also has some pretty good search results on wind loading:

    https://www.google.com/search?q=solar+array+wind+load

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Organic Farmer
    Organic Farmer Solar Expert Posts: 128 ✭✭
    Re: Wind load effect on the PV system

    Last year, I mounted a row of eleven 6X8 posts in the ground. 2foot below grade, 8foot above. Set into concrete sonotubes. Each tube reached 4foot below grade [as per frost-line code]. Each sonotube was 4 foot long, with the top 2 foot holding one end of a 6X8. Each post was 10foot long, 2foot below grade in a tube, and 8foot above grade. The 6X8s were pressure-treated lumber. Across the top of the 6X8s I mounted a 1inch blackpipe, running the length of the row of posts, 70foot. Onto this blackpipe I mounted solar panels, in pairs, 6foot tall by 6 foot wide. Each panel is roughly 3foot by 6foot, placing them in pairs made for 6foot by 6foot pairs. When the array was completed the top of the panels were 11foot above grade, and the bottom of the panels were 5 foot above grade. There were a total of 10 pairs of panels in the array [20 panels].

    During 'Sandy'; all posts snapped the sonotubes below grade. The entire array held together and laid down onto the ground. Where I was able to disassemble and recover all parts.

    I live in North-Central Maine. We did not feel the main force of Sandy.
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: Wind load effect on the PV system
    Last year, I mounted a row of eleven 6X8 posts in the ground. 2foot below grade, 8foot above. Set into concrete sonotubes. Each tube reached 4foot below grade [as per frost-line code]. Each sonotube was 4 foot long, with the top 2 foot holding one end of a 6X8. Each post was 10foot long, 2foot below grade in a tube, and 8foot above grade. The 6X8s were pressure-treated lumber. Across the top of the 6X8s I mounted a 1inch blackpipe, running the length of the row of posts, 70foot. Onto this blackpipe I mounted solar panels, in pairs, 6foot tall by 6 foot wide. Each panel is roughly 3foot by 6foot, placing them in pairs made for 6foot by 6foot pairs. When the array was completed the top of the panels were 11foot above grade, and the bottom of the panels were 5 foot above grade. There were a total of 10 pairs of panels in the array [20 panels].

    During 'Sandy'; all posts snapped the sonotubes below grade. The entire array held together and laid down onto the ground. Where I was able to disassemble and recover all parts.

    I live in North-Central Maine. We did not feel the main force of Sandy.

    I'm sorry to hear that. Did the glass on the panels survived the fall?

    My 6x6 posts go 6 ft into the ground. At the bottom they have 1/2 lag bolts screwed on the sides, then concrete poured around the base of each post. With my structure being 18ft high, this is barely enough to keep it from tilting over in 70 mph wind (by my calculations anyway). I can only hope we won't get it.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Wind load effect on the PV system

    Another big help is cross bracing. A 4x4 or larger post in the ground sticking many feet in the air is actually not very strong against lateral loads at the free end (once all the "worst case" analysis is done and the 10x fudge factors are taken into account).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Organic Farmer
    Organic Farmer Solar Expert Posts: 128 ✭✭
    Re: Wind load effect on the PV system
    NorthGuy wrote: »
    I'm sorry to hear that. Did the glass on the panels survived the fall?

    Yes the panels are okay.

    They laid down very gently.


    ... My 6x6 posts go 6 ft into the ground. At the bottom they have 1/2 lag bolts screwed on the sides, then concrete poured around the base of each post. With my structure being 18ft high, this is barely enough to keep it from tilting over in 70 mph wind (by my calculations anyway). I can only hope we won't get it.

    You 6X6 posts go 6foot below grade, and then are screwed into a concrete slab poured 6foot below grade?

    Or is your concrete slab poured at grade, in which case I missed what the lag bolts anchor into?

    18foot high is very high.



    I had wanted the bottom of my panels to be at least 5foot high, so I can easily drive my tractor around them to blow snow away.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Wind load effect on the PV system
    Lege wrote: »
    and what do you mean of (ahj) ???

    AHJ - Authority Having Jurisdiction:
    The (often unqualified) civil employee who has final say on what is allowed for any given installation/construction.
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: Wind load effect on the PV system
    Yes the panels are okay.

    They laid down very gently.

    That's a good luck!
    You 6X6 posts go 6foot below grade, and then are screwed into a concrete slab poured 6foot below grade?

    Or is your concrete slab poured at grade, in which case I missed what the lag bolts anchor into?

    I dug a hole, 18" in diameter or so and 6'+ deep. I made it a little bit wider at the bottom. I prepared the bottom of the hole. Then I screwed 1/2 lag bolts 6" or so from the bottom of the post, so that they stick out 2-3". Then I put the post into the hole, aligned it and braced it. Then I poured concrete into the hole, so it would fill the widened space at the bottom and cover the screws. The concrete was may be 1' deep. Then I backfilled the holes.
  • solarix
    solarix Solar Expert Posts: 713 ✭✭
    Re: Wind load effect on the PV system

    In my area with 90mph design windspeeds, my wind analysis gives 9.4psf, normal to the panels. My county of course, doesn't take my word for anything, so have to pay a structural PE to analyze any non-manufactured design. He uses StruCalc software and manages to come up with pressure numbers about double my 9.4psf. The last pole mounted job we did with 12 modules had to have 4 yards of concrete.
  • gww1
    gww1 Solar Expert Posts: 963 ✭✭
    Re: Wind load effect on the PV system

    Listening to you guys I am going to be buying a lot of new panels.

    Attachment not found.

    Just 4x4s sunk 18" in ground with dry pre-mix pored in the hole. 12'x40' 45 degree angle. 2"x6" boards.

    I am not done with the bracing yet.

    Hope my panels land softly.
    Cheers
    gww
  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    Re: Wind load effect on the PV system

    The AHJ here would probably have issue with the possibility of the structure racking. My neighbor decided to put up a gazebo/outdoor kitchen without permits, full tile roof and 4 box posts attached to uni-pour slab with integral footings. With no walls or cross bracing they would not issue a permit to get him legal, unless he could find an engineer to sign off the drawings. He never did. He sold the place and I guess the new owner has no clue as to the lack of permit structure on his property. That was 15 years ago and it still stands today.

    But if that building blows over the insurance company will surely check the permitting before issuing a check..
  • sunbunny
    sunbunny Solar Expert Posts: 59 ✭✭✭✭
    Re: Wind load effect on the PV system

    Here is a link to some UK calculations on verticle surfaces. Won't help much for flat panels or uplift.
    Says 32 pounds per sq. foot at 90 mph.

    http://www.sussex.ac.uk/weatherstation/technical/Windforce.html
  • gww1
    gww1 Solar Expert Posts: 963 ✭✭
    Re: Wind load effect on the PV system

    On the other hand, I replaced my roof on my house and found out later that the insurance probly would have covered it. I live in a zone four county and have used the same insurance company for 30+years. They might pay or they might cancel me but they won't rely on a AHJ cause I don't think we have one. I don't want it to fall though, so I will work to srengthen it. I have had big 12" trees broke 15' up with strait line winds and there would be nothing stand up to that. There is a lot to learn when you try to put a system together and start by knowing nothing. I been reading nothing else for a couple years and still make bonehead moves all the time.

    Cheers
    gww
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Wind load effect on the PV system

    lege,
    i'm not as qualified to say about these things as you are, but what i was saying is that a surface will have a minimal impact when there's the least area in resistance to it. as such i was thinking this would have less of an impact than tilted pvs that would catch more of the wind. i don't know of the effects the building would have on the winds though.

    as i was saying about the contact area with the wind, the pvs would present less area than the supporting structures, but would add to the overall resistance to the wind on top of your building. you (and i) need to address this simple pushing power first and then the possible up and down forces that were mentioned. if the pv structure is then calculated to be good for its straight resistance and can accommodate updrafts and downdrafts then how will your building itself react to the stresses the pvs will put on it? as i said mine will mount into the soil with concrete, but yours will attach to the building's roof. the stresses could be as bad as adding another story to it.

    i might not be as knowledgeable as you on the structural stuff and i could benefit from some of your wisdom here, but you've got more to deal with than my situation too as i am not going to anchor mine to a building even though it will be in close vicinity to one.

    i don't know if the link sunbunny has made is enough for you to gain your answers or not.
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Wind load effect on the PV system
    Lege wrote: »
    Hi,
    I'm a structural design engineer, and I'm new to the solar energy world,
    it's required from me to design a solar mounting system resisting a wind speed of 150 kph,
    In the attachments you can find a sketch of the required structure, and a picture taken from AutoCAD file showing the deatails of the solar system,

    what I really need to know is the wind load affect (pressure) on my structure and PV panels???
    and what is the main part of the whole system should prevent the uplifting forces acting on the panels???

    It'll be open from all sides and the roof is only covered by PV's which are placed flatly,

    I really need your help, I have no one to help me here and finally I could find a solar energy forums,
    I appreciate your fast replies for me,
    thanks a lot in advance :)
    As one engineer to another - I would not advise you to take advice on engineering issues, especially in designing structures for deployment in a hurricane zone for which you would be liable if something goes wrong, from forums like this. Mike Holt's forum would be somewhat better.

    That's not to cast aspersions toward anyone; there is a wealth of knowledge and experience among the regulars in here. But this is a hurricane zone if wind speed of 150mph is a design criterion, and serious liabilities are at stake.

    It puzzles me, though; if you are indeed a degreed structural engineer, didn't you study wind loading in school? Are you a PE? Is this a commercial structure which will require a stamp and seal?