Wind Protection for Ground Mount PV Panels

[bookish]

I'm looking for thoughts and/or experiences protecting a PV ground mounted array from wind.

I just built this array in the picture, and want to protect it from wind damage. We do get very high winds there in SE Arizona.

I'm considering building a brick wall on the north side, from the same adobes the building behind is made from. If I were to do that, I think some sort of curve in the wall would help from the standpoint of structural integrity as well as airflow characteristics. Anybody versed in fluid dynamics and such who could give advise? Any other way of doing it I'd also like to consider. I don't want to just attach a wall to the north-side uprights because then the stresses would just fall on that side of the frame instead of being taken off the frame, and I'm nervous about those footings being less than adequate.

Attaching a drawing of my idea for a possible wall footprint.

 

[DThames]

I can't answer your question but how deep and how large are your post holes? How much concrete is underground on each post? Is the structure wood?

 

[bookish]

I can't answer your question but how deep and how large are your post holes? How much concrete is underground on each post? Is the structure wood?

It's PT wood. The post holes are only 12" deep. The south three have extra concrete around the tubes but not so much on the northern three.

 

[DThames]

It's PT wood. The post holes are only 12" deep. The south three have extra concrete around the tubes but not so much on the northern three.

The mount structure in the attached image would shake a little side to side until I put those diagonal braces on it. They stiffened it up a lot. I would think that if the wind could shake and buffet it, it would be likely to loosen it up a bit, where if it were stiffer, that would be better. If you can shake it side to side with a jolt from your hand, then maybe stiffening it up would be worthwhile. That might help as a little protection at least from being weakened by constantly being twisted back and forth.

 

[Wellbuilt]

o boy , that looks like you need angle bracing.
I would add bracing side to side like the pic above , then from the top of the short wall to the bottom of the long wall
front to back .
Then add some dirt around the bottom to cover the concrete footing .
I would just close in around the sides with siding like you have on the peek of the house .
If the wind can’t get under it to lift it up , it will stay put .
If you add the wall in the pic it looks lik it could funnel wind agents the house and up under the panels .

 

[Diysolar123]

lets do some math!!
first, for wind pressure loading we will use the simple cheat formular of around: pounds/sqft = 0.00256*(mph*mph)
so 90mph wind results in a force of 21lbs/sqft (its double for a cat5 hurricane hehe).
For a 4'x8' solar panel you have 32sqft * 21 = 672lbs force per panel.
you have 3 of those panels so thats a total of 2016lbs or 1ton of shearing force in total.
lumber has a variable strength based on lots of factors beyond what type of wood(knots in wood, is it wet, etc.) so its tough to figure that out without knowing more; that said, it does not hold as much as you think.

keep in mind what you have made is basically a lever with your post so...perhaps more critical than the shear strength is weakness of the soil the post is mounted in.
lets just do a simple one post calculation.
You have a 6-8ft wooden lever buried 1ft in ground and you are pushing on it with a force of around 600+pounds, ahhh, not very good.
using a lever arm force calculation lets go with a 7ft out of the ground, 1ft in ground or a force multiplier of 7x.
That means the force to push over your post that is buried just 1ft is 7x what is being applied at the top end (ignoring lots of other things related to distribution of force).

so a single panel is applying a torsional force of 7*672 = 4704 lbs of force!!!

You may want your post hole at LEAST 36" deep...48 would be better; set in concrete is also a good idea OR you can just use concrete forms which are sold in most hardware stored and make a rebar enforced concrete cylinder.

trying to just shield the panels from wind involves some interesting fluid dynamics you probably do not want to do (wind can come from any direction, even straight down, depending on your structure you could even cause an "uplift" direction, hello wings, hehe)

 

[svetz]

Anybody versed in fluid dynamics and such who could give advise? That's not me, but the Engineering toolbox can tell you the force on the panels. Force (N) = area in m² ×(1.229 kg/m³) × (air-speed in m/s)² A 100 mph wind speed is about plus .18 psi (and this is why my panels are flat, have to engineer for 180 mph windspeeds); and at 30° tilt, that's about 200 lbf per panel, not including any levering. I'm looking for thoughts ... protecting a PV ground mounted array from wind. Ideally, the windward posts are heavy enough to overcome the lift. Anchored guy wires?

How did you connect the panels to the boards (#1), the boards to the cross-boards (#2), and the cross-boards to the joists (#3) as marked in the image. "Pull out" strength on screws/lags are far weaker than orthogonal attachments. Hopefully, #1/2 are bolts with fender-washers. #3 is using some type of strapped hanger?

 

[pvdude]

I am planning on using some of these to help hold the ground mount down when hurricanes approach.

https://www.tractorsupply.com/tsc/product/36-in-earth-anchor-with-auger-1181625

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The rear concrete pilings of our PV array are 5’ deep, fronts are 30”. Lots of sail area!

 

[Wellbuilt]

DIY , nice math .
It looks to me that the solar panels in the rack above are 40x66’ panels so its really a lot worse then your math is showing .
I don’t deal with 90mph winds but 60mph happens at my place every year .
I build structures like the solar mount all the time .
Free standing gazebos , pergolas of course they get 4‘ deep footings for frost
I’ve often thought they would just twist and fall over in the ? wind But every plan I look at dosent seam to worry about any of that ?
The roof and deck in front of the trailer is 10x16 and gets hit with 60mph winds every year.
The wind hits from left to right and the top is 30” above the trailer , the 4 legs just sit on ledge rock sticking out of the ground .
I thought it would of blown away long ago .?‍
That being said what option could be offered to help ?
I’m sure he could add 2’ of dirt under the structure to hold it down ?
I’m sure 6 tractor supply screw anchors one on each pole could hold it down ?
I would not want to recommend a full blown do over .

 

[Hedges]

so a single panel is applying a torsional force of 7*672 = 4704 lbs of force!!!

That is the concern I express when I see a pole-top mount.

With front and rear legs, I usually wouldn't worry about the torsion. A triangle is formed, and rather trying to tilt the post/footing, wind has to pull it out based on the lever created by spacing of legs.
I would look at weight of footings, which might be too little given enough wind from behind. or even from in front.
If deep enough in soil, friction on sidewalls holds drilled piers (it isn't bearing on the bottom.) Top one foot may not even be same material as deeper, probably mixed with organic matter. Also not compacted the same.

But the structure shown isn't just a triangle. It is a triangle on top of a rectangle 30" tall, which will allow tilting as a parallelogram.
I would want the horizontal member to go from top of front leg to bottom of rear leg.
That would improve the wood portion, but if the footings are 2' above the soil and only 1' into the soil, they could still tilt.

I also agree with diagonal bracing for left/right. Mine has that for earthquake resistance. It is possible wind would cause array to move that way.

 

[Rider]

What I'm seeing are all these panels on supports, completely open to the wind. They have to be reinforced so the wind getting under them doesn't make sails.

Just talking out loud here without thinking it thru totally, what if you were to build a walled structure around the base of the panels so the wind can't get under them, and there would be no 'wind friction' to speak of across the exposed outward face of the panels. IE, box the underside of the panels. The walls would take the brunt of the wind.

 

[Wellbuilt]

I’ve been considering mounting 15 panels on these concrete blocks one row 36” off the ground in the front , and 9’ in the rear .
‘the panels are around 300sq foot 40”x66x 15 panels .
The blocks are 1800lbs each 5400 lbs
I’m sure panels and racking would be 600 so total of 6000lbs
Dose any one think this will stay put ?
I don’t think this could be blown over but don’t want to end up having a do over .?

 

[svetz]

I wonder if you could plant trees or something to act as a wind-break?

 

[Wellbuilt]

i think trees would help 15 years from now .
The op panels look like they could fly off any day .
I think he need to do some thing now

 

[bookish]

That's not me, but the Engineering toolbox can tell you the force on the panels. Force (N) = area in m² ×(1.229 kg/m³) × (air-speed in m/s)² A 100 mph wind speed is about plus .18 psi (and this is why my panels are flat, have to engineer for 180 mph windspeeds); and at 30° tilt, that's about 200 lbf per panel, not including any levering. Ideally, the windward posts are heavy enough to overcome the lift. Anchored guy wires?

View attachment 50100

How did you connect the panels to the boards (#1), the boards to the cross-boards (#2), and the cross-boards to the joists (#3) as marked in the image. "Pull out" strength on screws/lags are far weaker than orthogonal attachments. Hopefully, #1/2 are bolts with fender-washers. #3 is using some type of strapped hanger?

All the connections are simple wood screws. That would be simple to retrofit with lag bolts + washers. Would some 90 deg. steel brackets help a bit, d'you think?

 

[Sanwizard]

They make ground mounts for a reason. Why only 12 inches deep for the posts?
You could augment the post anchors by going wide also, like a pad of cement.

 

[bookish]

Here's what I ended up doing for now. The intention is that after I add more slats a strong north gust's lifting force will be mitigated.
From comments made here I conclude I should add concrete in the footings, and cross-brace. I can also replace the wood screws in the z-brackets with lag bolts/washers, and put the same where lumber joins lumber. Could add angle iron at some joints also.

It's stood now for a couple of months. The most strong winds come from the west and northwest, which impacts the edge of the panels or the building. But my little setup shown, even if it is effective for north wind, doesn't do much about northeast wind. And what someone said is right, that wind can come from any old crazy direction. Once I had a popup shade thing setup on a calm day and without warning a very strong dust devil materialized and made a beeline for it, ripping it out from its stakes.

 

[Sanwizard]

Damn nice view on your property!

 

[bookish]

Damn nice view on your property!

Thanks. It's near Portal, Arizona, with the Chiricahua mountains to the west.

 

[Wellbuilt]

I would get some cross bracing on the tall panel support , you are defying every building rule not having it .