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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.


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