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Daox 06-13-11 01:45 PM

Desigining & building a solar hot water panel rack
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I've started designing the rack for mounting my solar hot water panels I picked up with Bennelson last fall. I've tenatively set it up at 70 degree tilt which is optimized for winter collection since I plan on using the panels for space heating as well as domestic hot water heating. I am also designing the racks about 5ft in the air. This improves winter heating since my house will block the sun in the morning during the colder months. I've also left 6 inches between the panels for making connections. I'm not sure how normal collectors are put together, but I think they're just soldered directly to each other. Rubber hose with clamps seems like it might be easier to do.

One problem I am seeing is that at 70 degree tilt, the spacing between my risers is quite short. The distance from the one end to the other is only about two feet. This leaves me with some reservations about wind loads and the panels. However, I am going through the calculations and I'll see what is needed to make this happen.

Here is an image of what I have drawn up so far. The blue top plane represents ground level. There is still lots to do with it.

AC_Hacker 06-13-11 03:23 PM

Wind Load?

Originally Posted by Daox (Post 14056)
Here is an image of what I have drawn up so far...

Does it get very windy there where you live?

That much area standing up that tall could create a large wind load...


Daox 06-13-11 03:46 PM

I have yet to look up maximum wind speeds for my area. However, it seems like 80mph would be a safe number to use, especially considering the rack will be surrounded on 3 sides by woods. Using 80 mph as a baseline, I'm looking at a ~6600lb side load on the structure. I'll be running it through finite element analysis (FEA) since we have a program here at work that'll work very nicely to show the stress distribution through the rack. I'll reinforce as necessary after running it through that program.

Xringer 06-13-11 08:50 PM

I liked this design for PV. Seems like a way to keep them up out of the snow pack.
It looks pretty solid, but at a 70deg angle, the front and rear legs might be too close together.

Maybe using an A-frame design on top of the base section would work better.?.

Then you could set a narrow angle and still have good front and rear leg width..

Daox 06-14-11 12:28 PM

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Well, it looks like the rack you see above will do amost fine. I added one brace between the two legs and that really reduced the peak stress (which will cause a failure at that point). The max stress you can see on the graph is 3,060 psi. The max stress 'northern white pine' can handle is 3,400 psi. So, it looks like I'm in the clear for 80 mph winds.

Here is a picture of the stress distribution. You can see the one brace that was added in an effort to reduce the stress on the junction of the shorter leg and bottom solar panel mounting 2x4. It worked out great and reduced the peak stress by half. I'm still going to do a little tweaking to see if I can get the stress down a bit more. But, I think I'm pretty much in the clear.

raydias 06-14-11 12:49 PM

Have you considered metal vs wood? like the home depot Superstrut 12 guage strut channel
Product Information Page

Daox 06-14-11 12:52 PM

No, I'm pretty set on wood. Its much easier to work with, holds up better (rust wise), is plenty strong and much cheaper. Those calculations above are for 80 mph winds! At even 60 mph the load on the structure is halfed. I don't know if Wisconsin has ever seen sustained 80 mph winds. :)

Daox 06-14-11 01:05 PM

Here is some info I found on peak wind speeds near Milwaukee, Wisconsin. The record wind speed is 81 mph.

Wisonsin Wind Data - Milwaukee

Daox 06-14-11 03:30 PM

So, moving on to the next phase now that the design is sound. This phase is making a list of what I need. So, how do you go about ordering concrete? I looked up the calculator on quikrete's website and it says I'll need ~75 bags of 80lb concrete! Thats ~1.75 yards, or $260 worth of quikrete. The wood and everything else needed to make the rack is only about $250. So, I'm thinking it will be more cost effective to order some on a truck? I've never done this so I'm not even sure where to look.

AC_Hacker 06-14-11 07:40 PM


Originally Posted by Daox (Post 14077)
I don't know if Wisconsin has ever seen sustained 80 mph winds.

If anything was gonna get you it would probably not be sustained load, but rather any kind of gusting condition that might cause the 1250 pounds of collectors (educated guess, since I helped carry ten of them into my back yard) to start rocking. A couple of gusts that were close to the natural frequency of your array could cause sudden additional stress, above static wind loading. Does your modeling program have a way to account for such a condition?

Some guy wires (cables) strategically fastened from the top of your array to the mounting platform, or anchored to the ground, would be cheap, strong and prevent movement.

I think we all really want you to succeed.


Xringer 06-14-11 09:02 PM

I have a lot of shielding trees north of my tracking array and
a sudden gust from the west took it right over. I heard a roaring noise and look up
to see it lifting up and tipping over..

I had planned on waiting a little while longer before driving in some anchors,
bad mistake! Could have cost me much more than some bent aluminum..

Daox 06-15-11 07:20 AM

Well, the record GUST is 81 mph, so I'm really not worried. The panels will be in a little alcove that I'll cut out into the treeline in my yard, so there will be trees on three sides plus my house and garage to the south and east, and another tree line beyond them. It'll be pretty protected.

Daox 06-15-11 09:32 AM

So, I got my first quote back on 2 yards of concrete for $276. Sounds a lot better than mixing it all myself. I'm still not sure I really need that much though. The quikrete site says if you have a 4" pole, you need a 12" hole. I'm not sure what it means when you have a 2x6. I also don't think I'll be sinking the 2x6 into the concrete. Talking with some of the guys here at work they think it would be better if I sunk some steel into the concrete and bolted to that. That would make the wood last longer since it won't be exposed to as much moisture. This sounds good to me since it will be easier to make the holes, pour the concrete, and line things up afterwards instead of making some framing to hold everything in place while I pour. Any recommendations? I'm completely new to this.

Xringer 06-15-11 12:04 PM

One of these things?

Or, if you had some very large angle iron. Put some bolts or steel rods
under the surface to hold-down, and 3" lag bolts up top..

Or, a pair of angle irons cross bolted together (with a 4" spacing)
under the concrete, so the 4x4s would fit between them, up top.
Use 3" lag bolts or drill the 4x4s and use 3/8" or 1/2" carriage bolts with washers & nuts.

Daox 06-15-11 12:16 PM

Yeah, I was thinking of going with jusT a single piece of angle iron. It would bolt to the 2x6s with two bolts.

Xringer 06-15-11 01:40 PM

Here's what happened before.

I put 4 steel rods into the ground at an angle, to hold down the legs of my tracker mount.
But I was still very worried during the reason tornadoes were had around here.
The winds were wicked strong in the back yard. The old tracker was swaying big-time.

Daox 06-15-11 01:57 PM

How far did your rods go into the ground? The front of my posts will be 40 inches deep, and the rear ones will be 56 inches deep. That is what is recommended (1/3rd the full height of the post should be underground).

Xringer 06-15-11 02:11 PM

My setup is not going to hold up to 80 mph winds. The 4 foot rods (5/8" steel) were driven in at
45 degrees and are not deep enough to hold up in serious winds.

But, they did keep the array from flipping over during the tornado weather we had.
Plus the winter winds we had this past January and March.
It's not what I should be using, but it's 90% better than nothing.

bennelson 07-11-11 08:24 AM

I like what Xringer stated in post 14.

If you ever need to make any changes to the system, it would be a little easier.

Should make it a little less work to set everything up.

When are you inviting me over to get this set up? I refuse to dig holes, but I'll help with the woodwork! :D

Daox 10-04-11 01:03 PM

I'll be back working on this project in a few weeks. The father in law is coming down and wants to help with something. I still really want to get these things up before winter, but I'm not going to be able to do it without help. So, I'm finalizing the rack plans. There are a few changes since my last update. I changed the vertical risers to 2X6s vs 2x4s for some extra strength and only about $30 extra in material cost.

I am still debating weather to sink the wood risers directly into the cement, or to bolt them on. The father in law seems to think that sinking them in will be easier. I'm not so sure. Anyone know the pros/cons to using metal in the cement vs just putting the wood right in?

Xringer 10-04-11 01:42 PM

Cement is porous and water will wick into any wood in contact with it.

If you place your 4x4 mailbox post in the ground, it will slowly rot away.
If you put it in cement, it will rot away faster, since water will run down
the post and into the seam, where the wood can soak it up. & rot.

Found some tips at How To Set a Post in Concrete - Learn How at ACME

"Setting the Post

The extra six inches in the depth of the hole is for gravel. If a wood post, even treated wood, touches the soil or is completely encased in concrete, it will prematurely rot. Metal posts will rust. Setting the post onto 6" of gravel allows the water to drain away instead of ruining the post. Adding gravel will add a lot of time to the life span of a wooden or metal post. Tamp down the sides and bottom of the hole and then add the gravel."

Daox 10-12-11 09:06 PM

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Alright, I'm moving forward with this and am running into a big issue. Basically, I believe the grade of the ground away from the house isn't going to allow me to bury all the pipe underground. The picture below basically illustrates the problem I think I"m going to have. The only difference being that there is going to be a fair amount of pipe sticking above ground. I'm thinking I could fix this by bringing in fill and basically having a mound that follows the pipe out to the panels. I'm not worried about freezing as the pipes will be insulated, more so just about the look of the yard, mowing, etc.

I'll be measuring everything this weekend to see how bad the problem really is.

So, I'm looking for any ideas on how to fix the problem. I know I could go with a glycol system, but I really don't want to have to. Its more expensive and more of a pain.

Xringer 10-12-11 09:33 PM

I take it this will be a drain-down system with a vacuum breaker on top of the highest panel plumbing?

If the lines are really long in the down hill part, once the pump shuts down and the vacuum breaker opens air into the loop,
the water in the down hill section will really be pulling a good suction on all the water,
so even a couple gallons stuck in a little dip in the lines might sucked right back into the tank..
You might need to test the idea to see how it works.

It all depend on how low the tank is and how long the down-hill section of line is..

My neighbor across the street uses a long down-hill run on his sump pump hose..
Many times, 10 minutes after his pump turns off, the long hose is still siphoning
water out of his sump hole. It's got to lift that water up about 8 feet out of the basement..

Daox 10-12-11 09:43 PM

Yes, it will be a drain back system. The panel rack will be roughly 80 feet from the tank. That means it needs a 20" incline over the 80 feet to meet the 1/4" per 1ft rule.

ThomSjay 10-12-11 10:38 PM

If you were to add fill in the "problem" area, you could accomplish two things. One is that the pipe is underground; the other is that more of the leg/cement column would be covered and thus add more support against wind.

bennelson 10-13-11 10:42 AM

I think some fill is a good idea.

It will look better, be easier to mow around, etc.

Try to do some volume calculations, so you have a good guess as to how much fill you need.

Daox 10-15-11 03:51 PM

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Well, I measured things today. It looks like I'm going to need quite a bit of fill just as I thought. The edge of my yard is 3.5 ft from the basement floor as shown in the picture below. I had planned on the tank being about 4.5ft tall. Then, you have to add the slope for the drain back piping on top of that. 80ft at .25 inches per foot gives us 20 additional inches. So, it looks like I'm short about 2.5feet at the edge of my lawn. Plus, you need to have a few inches of dirt over that, so lets just say 3 feet. That sounds like a LOT of fill...

Daox 10-22-11 09:26 PM

After calculating things and remeasuring I found out I would need bring the ground level up at least 2.5ft to bury the pipe. So, I've decided to go with a closed loop glycol system.

Other than that, we started building the rack today. After some talking with my father in law, we changed the design of the solar rack and also increased the height of it.

In preparation for digging the trench to the rack, we dug out the flower bed around my porch. Digging under the porch will be the fun task for tomorrow.

This is the location the solar rack will sit. We had to cut down a few trees that were in the way.

In the morning we went to the equipment rental place and got us one of these. I gotta say, this thing works like a dream. We had to drill five 10" holes four feet deep. With this machine it didn't even take two hours. Well worth the ~$75 it cost to rent.

Here are all the holes dug, its hard to see but there are five holes dug over roughly 35 feet.

These are the posts we used. We nailed and screwed a few 2x6s to create a 6x6 20 feet long. This thing was heavy! We made and used five of them.

As you can see, its no short piece of lumber, and that is with 4 feet in the ground. :)

After getting the first post up we noticed we needed to do a bit more tree trimming.

And here is the far post put up. We got all 5 posts put in the ground and cemented in. However, it was too dark to take pictures by then. I'll snap a few in the morning.

Daox 10-23-11 07:59 AM

Here is a shot of the rack with the posts up.

Xringer 10-23-11 08:45 AM

Sweet, the sun is already on one of the posts!! Looking good!

That 7Up can is still on the clothesline post. ;)
Or is that Dew?

bennelson 10-23-11 10:04 AM

Wow, looks like the rack is bigger and farther away than I first would have imagined!

Got any rental time left on that post hole digger? We could come and do my post holes next!

Daox 10-24-11 07:17 AM


Originally Posted by Xringer (Post 16963)
Sweet, the sun is already on one of the posts!! Looking good!

That 7Up can is still on the clothesline post. ;)
Or is that Dew?

Yeah that was not too long after the sun came up. I'm glad we got the higher posts as it'll really help things out in winter with the low sun.

That is Dew, the Mrs Daox drink of choice. :)


Wow, looks like the rack is bigger and farther away than I first would have imagined!

Hot any rental time left on that post hole digger? We could come and do my post holes next!
Its bigger than I imagined too. Once we started clearing things we moved the rack back more than I thought too. Between the height increase and moving it back farther I'm hoping to have much better winter solar gain.

I'm also thinking about thinning out some of my trees that will block the collector a bit in winter. But, we'll probably see how it works before doing that.

zick 02-01-12 07:59 PM

Seriously, what is the deal with this thread?
It just keeps attracking these single posters from apparently all over the world.

Xringer 02-01-12 08:23 PM


Originally Posted by zick (Post 19502)
Seriously, what is the deal with this thread?
It just keeps attracking these single posters from apparently all over the world.

I think we have to face facts! The world has lost interest in tracking mounts.
Everyone is starting to become fans of Fixed mount arrays.. :p

And, I like em too!! :D

Daox 02-02-12 06:07 AM

I'm not sure, but its pretty annoying.

Xringer 02-02-12 07:48 AM

Those tags, are pretty general.. "design, panel, rack, solar"..
Will any one of those cause hits?

Maybe they should be edited? Maybe to term like "Solar mount design" ??

Yeah, it gets About 4,960 results (0.15 seconds)
Whereas the word 'Solar' gets about a zillion hits..

Daox 02-02-12 08:05 AM

Hits are good! Spammers are not.

Xringer 02-02-12 08:21 AM

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Maybe new posters should be required to take some extra steps for their first 10 posts?

"As a final step before posting your comment,
enter the letters and numbers you see in the image below"???

I was on one site where they wanted me to enter the sum of 3 and 7.. :)

Daox 02-02-12 08:40 AM

We already have bot screening filters on the registration pages. Whoever gets through is human (and obviously has too much time on their hands). This is why we restrict posting any links for X number of posts.

Xringer 02-02-12 10:00 AM

Yeah, a lot of them do look human in origin.. :rolleyes:

It must be a real PITB to have to read those posts and start deleting..

Saw this..
Official Google Webmaster Central Blog: Keeping comment spam off your site and away from users

Maybe limiting their post count to 2 per day for the first 10 will slow the spam flow.?.
Cut back your work-load a bit.. :)

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