best type of ballast material for solar PV array?

rollandelliott · October 2011

I'm looking at a Schletter windsafe array. which is 50 feet long (total of 100 linear feet for front and back) and requires about 100 lbs per linear foot (10,000 lbs total)

Density of precast cement is around 140 lbs/square feet, which means I'll need 12x12x8" of cement per linear feet.

This is equivalent to 600 cement squares that are 12x12x2" in size

I know I can go to lowes and pay retail but that would cost a lot of $$$.

This is in Orlando FL area. Anyone know of some precast cement places that would do a small order like this?

It's only 2.75 yards of concrete. Maybe just getting a cement truck to come and pour the cement into the ballast trays would be cheaper, but I like the idea of precast blocks since they would be allow the solar array to be moved latter on..... a lot more easily.

mike95490 · October 2011

Re: best type of ballast material for solar PV array?

What about sand and water, like they use for kids street basket ball backstops.

Pour it full of sand, fill the gaps with water. Not quite as heavy as concrete, but you could drain water and let sand dry, then vacuum out.

But be sure the roof can take all the weight!

rollandelliott · October 2011

Re: best type of ballast material for solar PV array?

it's actually going to be a ground mount system. But the weight is 10,000 pounds so point noted.

sand and water is a good idea, though I would probably use sand bags to make moving easier. Now I just need to figgure out if it will be heavy enough,.....100 pounds per linear foot is a lot.

Solar Guppy · October 2011

Re: best type of ballast material for solar PV array?

10,000 lbs doesn't sound right for the Florida Wind Code ... My system has to resist 600 pounds uplift per panel, as calculated by the PE.

I doubt anything except re-bared concrete footers would be approved, anything else can be moved which your then make the structure a hazard

This is not a choice for you to make ... its up to the building department and your PE with stamped plans for your sisters system.

mike95490 · October 2011

Re: best type of ballast material for solar PV array?

You could use plastic rain gutter as a mold and cast as long as you back holds up mixing concrete and cutting re-bar.

rollandelliott · October 2011

Re: best type of ballast material for solar PV array?

Schletter did the ballast calculations.

Their windsafe system uses a back wind blocker panel which according to them reduces the amount of ballast needed. I'm sure the local codes people will be content with the stamped engineering drawings Schletter provides.

Not sure it is worth my time to fill up 200 sand bags at 50 pounds a piece.....that home depot sells?!? Maybe someone that has done that before can tell me how long it would take with a shovel.

Otherwise I'll just buy some precast concrete from this place I googled
Old castle
690 W Taft Vineland Rd
Orlando, FL 32824-8007
(407) 855-7580

Solar Guppy · October 2011

Re: best type of ballast material for solar PV array?

I looked at the manufactures website, the racking your referring is listed as roof mount, not ground mount.

Also, where did you get the numbers for a ground mounted system meeting the 110 mph wind code?

To meet the building department requirements, it will need a plans with a stamped FLORIDA PE, to assume otherwise will likely cause you delays and these plans will specify the required mounting materials.

I took some time and read thru the design process Schletter website, and it mentions the use of third party PE's when required, which will be a requirement for any system designed for installation in Florida. It will be the job of the PE, to tell you what racking system, what footing and what soil tests would be required, at best the 10,000 lb figure you have now is a guesstimate.

To my knowledge, you can only use racking for what they are approved for, so ballast is out for ground mount. On the ground mount arrays they have, it requires soil testing to determine the footing required ( per there design flow document ) or your going to the big footers are shown in the photos on the ground mounted arrays

No other State has the 110mph requirement, its a very hard thing to design for and I know first hand the building departments take this very seriously. Best case you get Schletter to get a Florida PE, which designs and stamps plans that then have to get stamped and approved by the local building department. Until all that is done, you will not know what the cost or what materials will be used. I'm certian you won't be the one to make the choice.

rollandelliott · October 2011

Re: best type of ballast material for solar PV array?

Schletter told me their wind safe racking can be used on any flat surface, ground, roof, parking lot, etc. they just cautioned me that I would need to make sure weeds do not grow up and shade the bottom row since it is so close to the ground. They said any ballast material that meets the weight requirements would work, though most people use some sort of precast concrete material. I'm assuming they know what they are talking about, but maybe Fl is different.

Doesn't really make any sense that you can have a ballasted system on a roof where winds are more consistent vs on the ground. And 100 pounds per sq ft is so low a load. A normal human walking across their backyard puts that much load on the ground.

Their systems are custom engineered so I'm pretty sure their wind load calculations are better than a guesstimate. Their quote included PE fees.

I do appreciate your concerns and will talk to them about it.

mike95490 · October 2011

Re: best type of ballast material for solar PV array?

PV panels on ground (or roof) in high wind + flying resin chair = trash.

You need to consider debris shielding or quick panel removal.

rollandelliott · October 2011

Re: best type of ballast material for solar PV array?

there will be a 6' tall privacy fence around it, might get some plywood in case of hurricane

ggunn · October 2011

Re: best type of ballast material for solar PV array?

rollandelliott wrote: »

Schletter told me their wind safe racking can be used on any flat surface, ground, roof, parking lot, etc. they just cautioned me that I would need to make sure weeds do not grow up and shade the bottom row since it is so close to the ground. They said any ballast material that meets the weight requirements would work, though most people use some sort of precast concrete material. I'm assuming they know what they are talking about, but maybe Fl is different.

But why a ballasted system instead of concrete footers? It seems to me that footers would use a lot less concrete.

rollandelliott · October 2011

Re: best type of ballast material for solar PV array?

I dont' think burying the concrete 1 or 2 feet underground in sand is going to drastically reduce the amount of concrete needed.

I had them do two different traditional ground mount designs with concrete footers, One was a carport canopy type design that used 4x more concrete probably due to 6' height of array.
2nd was more traditional ground mount height and that used metal posts and 2x more concrete; it was higher and had not wind shield

The ballast design used the least amount of cement and was also the lowest to the ground with a wind shield.

I'm no wind load engineer, but talking to them the crucial factors were angle and height of array, thus favoring the windshield racking system.

Solar Guppy · October 2011

Re: best type of ballast material for solar PV array?

ggunn wrote: »

But why a ballasted system instead of concrete footers? It seems to me that footers would use a lot less concrete.

While footers or other type of restraints could be done with less weight ( think the screw type tie downs used for mobile homes) anything less in weight than the uplift requirement of the wind code would require the soil be tested at the exact installation location points.

One of the benifts of bolting to a roof is the structure already has to meet the uplift, an array adds no additional stress uplift wise than the roof was designed for ( assuming it was constructed when the wind code was in force ). In Florida most homes are concrete block, with rebar from footer to headers, so you have the weight of the walls and foundation to resist upload.

Building a standalone array will require substantial weight or engineered restraints with soil testing. Based on what my PE calculated, an array the size of what the OP is considering would need to resist in excess of 30,000 pounds to meet the uplift generated by a 110mph wind, assuming he is building a 10kw array.

I'm not sure why a wind diverter on one end of the array would make any difference, in Florida, we have no prevailing winds and hurricane force winds can come from any direction. Uplift is generated when the air over the array is slower than the wind under it, with or without a diverter, you have the worse case of the wind coming square to the lowest part of the array. A house as an example is closed on all sides, its still generated low pressure on the roof.

rollandelliott · October 2011

Re: best type of ballast material for solar PV array?

"Based on what my PE calculated, an array the size of what the OP is considering would need to resist in excess of 30,000 pounds to meet the uplift generated by a 110mph wind, assuming he is building a 10kw array"

Obviously your array is a different height and angle.

Like I said I'm not a wind load engineer, but I don't think generically saying one needs 30K lbs for a 10K array without knowing the height and angle is possible. Hence the reason Schletter gave me 3 differnet quotes for the same system with concrete needs ranging from 2.5 to 10 yards.

Solar Guppy · October 2011

Re: best type of ballast material for solar PV array?

My PE and building deparment approved array is on a roof at 22.5 degrees, it doesn't have any additional uplifts to what the structure is subjected too. Being closed on all sides except of the 6" lift off the roof, it would seem to be a best case scenario, I was using the uplift per square-foot and scaled to a 10k array.

Mine numbers are from a Florida PE, when you get the final stamped calculation approved by the Orange County building department, it will be interesting to compare notes.

ggunn · October 2011

Re: best type of ballast material for solar PV array?

rollandelliott wrote: »

I dont' think burying the concrete 1 or 2 feet underground in sand is going to drastically reduce the amount of concrete needed.

I had them do two different traditional ground mount designs with concrete footers, One was a carport canopy type design that used 4x more concrete probably due to 6' height of array.
2nd was more traditional ground mount height and that used metal posts and 2x more concrete; it was higher and had not wind shield

The ballast design used the least amount of cement and was also the lowest to the ground with a wind shield.

I'm no wind load engineer, but talking to them the crucial factors were angle and height of array, thus favoring the windshield racking system.

Have you considered helical pilings?

rollandelliott · October 2011

Re: best type of ballast material for solar PV array?

Not really. I can get the cement patio blocks for $ 1.00 each for $600 total plus $150 delivered. Just renting a big drill would cost $300, plus labor and concrete, rebar, etc.

I did wonder why cement cinder blocks aren't use more in Florida. Seems like making a small 4 sided enclosure would prevent wind from putting a load on the panels.......that would be cheaper and more durable than a ground mount that cost $4,000. Of course I've never been through a Hurricane, maybe cinderblocks crumble. Maybe the labor offsets any material savings.....

solarix · October 2011

Re: best type of ballast material for solar PV array?

My structural PE recently obtained a wind-load analysis software tool and is now telling me we need to up the amount of ballast needed for ground mounts. Tells me we need about 300# per module with our modest 90mph wind spec. I can find several professional ground mount designs on-line that used quite a bit less than that, but got to go with the PE cause his neck is the one on the line if an array blows over.

BB. · October 2011

Re: best type of ballast material for solar PV array?

The problem is, in general engineering, the difference between any two "results" or things that is within a factor of 2x, they are virtually the same.

To be quite different, they should be at least 10x different.

So--to see a 2x or greater difference in ballast calculations would not surprise me. And, frankly, a 2x difference in ballast where wind loading goes with the square of the velocity, there is not that much difference. The difference between ~70 MPH and 100 MPH would be a 2x difference in ballast weight (I am guessing here--I do not and have have not done any calculations since school some 30 years ago). Add different assumptions about terrain and geometries--will give different answers.

Add different assumptions by the people doing the calculations and different computer programs/tables--You are not probably going to get 100% agreement (unless somebody has created "standards" that everyone follows to the letter).

-Bill

PS: One thing to take into account is the cost of failure... If this is in the middle of nowhere--perhaps a failure (covered by insurance?) is not the end of the world. If this were next to a home (get the array wrapped around the home, damage the structural integrity, possible injury to occupants) and/or an off-grid system where this was the only reliable source of power--then perhaps erring on more ballast is not the worst thing in the world.

Solar Guppy · October 2011

Re: best type of ballast material for solar PV array?

BB. wrote: »

a 2x difference in ballast where wind loading goes with the square of the velocity,

Wind energy is cubic BB, so to go from 90 to 110mph is a close to doubling the energy, or in this case for lift.

Which interestingly matchs my PE's number per panel of 600 lbs @ 110mph to solarix PE of 300lbs for 90 mph. rollandelliott quoted number ( not from a Florida PE ) of 200lbs panel is fantasy land

BB. · October 2011

Re: best type of ballast material for solar PV array?

I agree that wind energy goes with the cube of the velocity--but drag pretty much goes with the square as does lift.

There is always the problem with "airfoils" that deform as they near design limits. Start picking up one corner (for whatever reason) and the lift/drag/pucker factors will change dramatically.

In any case, the cost of "failure" will help dictate safety factors and the PE's fudge factors.

For normal structural calculations, a safety factor of 10x is pretty standard (i.e., designing buildings, etc. to account for possible defects in materials and incorrect design assumptions, etc.

-Bill

rollandelliott · October 2011

Re: best type of ballast material for solar PV array?

Specifications for wind safe array
3x16 array, evergreen fas210 panels in portrait orientation
approximate overall size of 50x15'
tilt of 15 degrees.
From Schletter
"The upward force at each of the triangles is 694.4 lbs. There are 8 support frames."

installation manual:

http://www.schletter.us/support/WindSafe%20Installation%20MAnual%20Draft%201-7-10.pdf

I'll be sure to ask Scheletter to double check their wind load calculations. Last thing I want is panels blowing away. Schletter isn't some fly by night company, so I will be surprised if their calculations are fantasy.

Solar Guppy · October 2011

Re: best type of ballast material for solar PV array?

How are you getting 3 rows in that racking, its shows it can do two in portrait orientation?

rollandelliott · October 2011

Re: best type of ballast material for solar PV array?

schletter's manuals are generic, they can custom design anything you want.

ggunn · October 2011

Re: best type of ballast material for solar PV array?

rollandelliott wrote: »

Not really. I can get the cement patio blocks for $ 1.00 each for $600 total plus $150 delivered. Just renting a big drill would cost $300, plus labor and concrete, rebar, etc.

I did wonder why cement cinder blocks aren't use more in Florida. Seems like making a small 4 sided enclosure would prevent wind from putting a load on the panels.......that would be cheaper and more durable than a ground mount that cost $4,000. Of course I've never been through a Hurricane, maybe cinderblocks crumble. Maybe the labor offsets any material savings.....

I've never used them myself, but I was under the impression that helical piers don't use concrete or rebar but are just a big screw with a very wide thread that you drive into the ground. They sometimes use them for tracking arrays that have a very tall profile.

The thing about cinder blocks is that they make them out of very porous concrete/cement; that and the physical design are for the intention of lowering the weight, which is the opposite goal you have when looking for ballast.

rollandelliott · October 2011

Re: best type of ballast material for solar PV array?

i'M not using cinder blocks,
they are steping stones/pavers/solid blocks, at home depot they are $1.40 each, the precast company will sell them direct for $1.00 each.

tecman · October 2011

Re: best type of ballast material for solar PV array?

Check with local redi-mix companies. In our area they cast leftover/returned concrete into a variety of blocks. They sell them cheap.

paul

bmet · October 2011

Re: best type of ballast material for solar PV array?

I have purchased something similar to hold my satellite dish in place. Worked very well as I never lost my picture in 4 years. These blocks weighed in at 93 pounds apiece, but I forget what the price was, not much more than what you mention.

rollandelliott wrote: »

i'M not using cinder blocks,
they are steping stones/pavers/solid blocks, at home depot they are $1.40 each, the precast company will sell them direct for $1.00 each.

rollandelliott · September 2012

Re: best type of ballast material for solar PV array?

Well the array was put up last week.
General contractor had it up pretty quick.

Ballast worked out to be 180 lbs/ft for rear and 150 lbs/ft for front. Local inspections dept. was fine with Scheletter's stamped drawings. There was talk of permanently affixing the block to the metal structure, but since the engineered drawings did not require it, we talked them out of that potentially costly upgrade.

It was about the only solar racking 10KW ground mount system I could find that was 6 feet tall and thus met the Home Owner Associations Demand of not being visible behind a 6' tall fence. The major problem is that the bottom row of panels is only a foot off the ground, so constant grass cutting is necessary to prevent weed shading. We might put some pavers down to mitigate this issue.

Couldn't find any local block companies that sold cheap blocks, and I forgot the source for cheap blocks I found almost a year ago....so just bit the bullet and paid $1.20 per block at Lowes.

Overall I was impressed with the WindSafe, it is quite rigid upon completion and comes partially preassembled for quicker set up. This system was around $5,000. I'm guessing the actual aluminum metal was probably around $1,500 in materials. Not sure how much labor it would take to drill all the holes.

Blocks added another $1100 or so.

Total cost around $0.61 per watt.

CDN_VT · September 2012

Re: best type of ballast material for solar PV array?

Pictures Would be great also !!!
Please

VT

Vic · September 2012

Re: best type of ballast material for solar PV array?

Congratulations Rolland. This stuff takes quite a bit of time and planning.

Turn up the A/C! Have Fun! Vic

best type of ballast material for solar PV array?

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