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Unistrut Installation - How Much Spacing Between Roof and back of Solar Panel?

TedH

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Jun 5, 2020
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I am in process of purchasing four, 10-foot sections of unistrut channel. My plan is to secure two pairs of two, 10-foot rails in the 20-foot space between the rear and front access hatches on my bus (MCI 102AW3, 40-foot bus).

I have a set of six (6) 3x6 SanTan Solar 270watt (250 effective given aging) polycrystalline panels with cabling.

The unistrut rails will come with 8-12 short lengths (I will call them 'feet') that I can use to create/add space between the roof and the back of the panels.

My question: What is the optimal spacing? 2-3"? 3-5"? More?

With the short lengths that will be attached to the bus at the roof bow rivet locations, the 10' rails will attach to them. Then, the panels would rest on the rails and be attached with end and mid-point clamps.

Before I install the feet, do I really need them OR is it sufficient to attach the unistrut rails to the roof and then attach the panels to the rails? I hope to position the panels across the 9.5" roof so the panels do not come in contact with the roof (slightly bowed/curved roof) and they do not protrude with the 6' length to either side of the bus (unless I have to work around the black water vent and refrigerator vent that are on the roof currently).
 
While I haven't done an install with Unistrut I have looked into it for my RV. The strut is bolted right to the vehicle and then the panels go right on top of the strut. There are special clamps that hold down the panels from the top of the panel's frame. With Unistrut you don't bolt through the solar panel's frame.

The Z brackets I'll be using say that they raise the panel only 3/4" off the surface. I'm throwing that out for comparison.
 
It really depends on you roof shape. You need more distance when roof is flat and less distance when you angle the panels.
Hot air raises, when one end of the panel is higher then other you can get by with about an inch. if you roof is completely flat I would go for 2 inch.

The more distance you got the greater the force on the mounts when you are driving.


Any airgap will do
When you go from 1 inch to 4 inches you increase your efficiency by 2.5% (two percent)
After 4 inches there is no statistical improvement anymore.

I'm more concerned about the mechanical structure and the wind forces then peak efficiency. I've mounted my panels as flat as possible, which is about 1,5 inches with my fairly curved roof.
 
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Right now, with the unistrut feet (2-3" short sections of rail at 8-12 points down the length of a 20' configuration of 4, 10' rails) and rails, I will have 3-4" of air gap between the roof and the fiberglass backing of the solar panels. I am planning to use industrial rivet nuts drilled thru 4-6 of the roof ribs (2 feet per rib) of the roof. Then, bolt into the rivet nuts with locking washers and sealant under, between and around all hardware touching the roof. I am using grade 5, 1/4-20 fasteners.
 
Right now, with the unistrut feet (2-3" short sections of rail at 8-12 points down the length of a 20' configuration of 4, 10' rails) and rails, I will have 3-4" of air gap between the roof and the fiberglass backing of the solar panels. I am planning to use industrial rivet nuts drilled thru 4-6 of the roof ribs (2 feet per rib) of the roof. Then, bolt into the rivet nuts with locking washers and sealant under, between and around all hardware touching the roof. I am using grade 5, 1/4-20 fasteners.

you are right in the perfect range for panels distance. Never used rivet nuts, but heard many good stories about them.

I'm not sure about locking washer on rivets since they are apply constant force. I'm using Loctite thread locker?

Everything automotive usually uses thread locker or locking nuts. While I would trust a locking washer on a house roof. I'm would no use them in a high vibration environment.
 
Good idea on the thread locker. Will consider. Yes he rivet nuts are extra long and zinc coated steel. I bought a tool that attaches to high torque reversible drill.
 
Thread locker or a nylon locking nut. I prefer the nylon locking nut, but only because I have a tendency to make a huge mess with liquids.

Doubling up nuts is also an option, but it takes longer than the other two options, in my opinion.

I created a trommel this summer. I used all three nut options. Using nothing, not even a lock washer, was definitely a mistake. However, a trommel is an extreme example that involves a lot of vibration. A bus roof has a lot less vibration. I'll be using nylon lock nuts on my RV roof install. The mounting brackets I bought came with lock washers, which will go in my spare parts bin.
 
A bus roof has a lot less vibration.

Wind produces vibration, while driving down the highway. Ever had one of those window flags? That same flapping back and forth happens to a panel just at a much smaller scale.
 
Wind produces vibration, while driving down the highway. Ever had one of those window flags? That same flapping back and forth happens to a panel just at a much smaller scale.

Have you ever run a trommel? My "much less" comment was in the context of comparing a bus roof to a trommel.
 
Thread locker or a nylon locking nut. I prefer the nylon locking nut, but only because I have a tendency to make a huge mess with liquids.

Doubling up nuts is also an option, but it takes longer than the other two options, in my opinion.

I created a trommel this summer. I used all three nut options. Using nothing, not even a lock washer, was definitely a mistake. However, a trommel is an extreme example that involves a lot of vibration. A bus roof has a lot less vibration. I'll be using nylon lock nuts on my RV roof install. The mounting brackets I bought came with lock washers, which will go in my spare parts bin.
Funny... I'm looking at this post as I'm getting ready to put a system on a 43' toy hauler and was considering using unistrut... and noticed your trommel comment, do you mean like this one, this kind of trommel? You never know who you'll bump into on these crazy forums...

516B1C23-DC19-4DAD-83F7-22422B14E13F.jpeg
6A10E0D6-06BC-4855-9D8F-79367DD207C7_1_105_c.jpeg
 
Yeah, mine is just like that. Only smaller. And made out of wood. Natural wood color too, not green. The screen section is 6' long and it's powered by an old garage door opener.
 
Yeah, mine is just like that. Only smaller. And made out of wood. Natural wood color too, not green. The screen section is 6' long and it's powered by an old garage door opener.
So in other words, not like mine at all.. LOL (kidding).
 
Before I install the feet, do I really need them OR is it sufficient to attach the unistrut rails to the roof and then attach the panels to the rails? I hope to position the panels across the 9.5" roof so the panels do not come in contact with the roof (slightly bowed/curved roof) and they do not protrude with the 6' length to either side of the bus (unless I have to work around the black water vent and refrigerator vent that are on the roof currently).
 
Funny... I'm looking at this post as I'm getting ready to put a system on a 43' toy hauler and was considering using unistrut... and noticed your trommel comment, do you mean like this one, this kind of trommel? You never know who you'll bump into on these crazy forums...

View attachment 54218
View attachment 54219
That will take some serious solar for processing thru that! :cool:

Have you done well with it?
 
That will take some serious solar for processing thru that! :cool:

Have you done well with it?
I'm still stockpiling panels to power it... LOL... joking... it's three phase and uses a 15kw starting 12kw running Champion genie via a pair of Variable Frequency Drives CFW10 Single Phase - Three Phase Converters (one for the trommel barrel and one for the vibrators... HEEEEEY NOW.... for the screen deck... come on now!! ? LOL)... But yea... I do ok on the gold recovery.... check this out... ??? "Don't walk away Renee"..... lalalalala la la laaaaaa ????


Jen
 
I'm still stockpiling panels to power it... LOL... joking... it's three phase and uses a 15kw starting 12kw running Champion genie via a pair of Variable Frequency Drives CFW10 Single Phase - Three Phase Converters (one for the trommel barrel and one for the vibrators... HEEEEEY NOW.... for the screen deck... come on now!! ? LOL)... But yea... I do ok on the gold recovery.... check this out... ??? "Don't walk away Renee"..... lalalalala la la laaaaaa ????


Jen
Pretty neat, would love to come and watch your operation in action. Are you doing the tours yet?

I've always been a gold bug, invested in miners and did well. Always thought the prospecting for placer would be fun.
 
Pretty neat, would love to come and watch your operation in action. Are you doing the tours yet?

I've always been a gold bug, invested in miners and did well. Always thought the prospecting for placer would be fun.
Thanks Zwy.... I actually live about 20 minutes from the Grand Canyon Skywalk and bought 65 acres, I'm setting up an AirBNB / Campsite operation here, 100% off grid etc and will be inviting select gold prospectors and solar friends who want to fly into Vegas (I'm very close to Vegas so if we ever do a meetup, that would be cool eh) and come out, there's all sort of fun out here..... so stay tuned...

Jen

 
Thanks Zwy.... I actually live about 20 minutes from the Grand Canyon Skywalk and bought 65 acres, I'm setting up an AirBNB / Campsite operation here, 100% off grid etc and will be inviting select gold prospectors and solar friends who want to fly into Vegas (I'm very close to Vegas so if we ever do a meetup, that would be cool eh) and come out, there's all sort of fun out here..... so stay tuned...

Lots and lots of ATV trails out there too. I was there recently. It seemed like around every bend in the trail there was an abandoned mine.
 
Sorry Ted, it appears I kind of hijacked your posting, it's just that this group is full of some pretty awesome guys and gals with wonderful and diverse hobbies and backgrounds.... If we continue the chat we'll take it to a unrelated post, in fact I'll go create one now so we can get this one back on track cus I too am interested in this exact topic.....

To continue this topic, gold, trommels, Arizona trails, Vegas area (home of Will and Jen and probably more), please go here...: https://diysolarforum.com/threads/t...up-one-day-and-unrelated-to-strut-talk.24951/

One of the great things about this board is the wonderfully diverse interests we all have but nothings more frustrating than asking a question as you try to do a project and have all the answers be unrelated...sorry Ted..., so let's take it over to the link above and bring this back on topic. :)

Jen
 
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