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netttech

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Posts: 720
Reply with quote  #1 
On my post in the Projects section I mentioned my brainstorm idea about making tubes out of screen instead of cinefoil. Screen is easily purchased, handled, & it may perform as well. Someone replied about their attempt at making a screen tube & the issues they encountered.

I tried tonight with some scrap screen pieces. Below are the pictures of two screen tubes. One was made from screen that was already 2 layers, folded on top of each other. It equated to 6 layers of screen in the tube. The 2nd tube, was a single layer of screen, but didn't quite have enough screen to completely make 2 revolutions.

The first tube I rolled by hand & the 2nd was rolled upon 1 1/4" PCV pipe I had. I didn't have any trouble removing the screen from the PVC. It actually helped to fasten screws thru the layers, as it was removed from the pvc.

The only question is whether it will perform.
Jeff
Central IL
Solar air & water

Two layer screen, hand rolled. PA072467.JPG  PA072468.JPG 

Single layer rolled onto pvc pipe.
PA072469.JPG  PA072470.JPG  PA072471.JPG 

KevinH

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Posts: 565
Reply with quote  #2 
That's about how mine ended up looking years ago.   It gets more difficult to roll the tubes the longer they get (may need some extra hands), especially when going counter to the way the screen was originally rolled.  Good idea using the coarse threaded large head screw.

One other thing I thought about trying was having a triangle of screen inside of the tube, but getting precise bends would require a bending tool.  And the screen is already curved in the direction it was rolled when purchased.

My crude test box was setup like a solid tube collector with plenums and round openings for the screen tubes.  Pop cans were used at the plenums.  Another idea was blocking every other tube at each end so air flows in one tube, out of that tube, and into another tube to exit. That would only work if the total opening size was enough to avoid restricting air flow.  It would be interesting to see how the air actually flows with a mesh tube (smoke test).

X  O  X  O  X  O
|| ||  || ||  || ||
O  X  O  X  O  X

A negative to tubes versus regular flat screen is that the tubes lose some optimal collector area to plenums (unless the plenums are built behind the collector).

Unfortunately when playing with these ideas I didn't have the equipment to do any scientific side by side comparisons and have since moved on to using solid tubes.  I think it was way back on the Yahoo group.

Kevin H
MN
gbwillson

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Posts: 2,352
Reply with quote  #3 
I thought about the idea of screen tubes a while ago after Tom built his Cinefoil tubes, but never built a collector. But I like the concept. What about something like this? 


Take a small, flat board
IMG_0862.jpg 

Line up the screen carefully on the slat
IMG_0863.jpg 

Staple the screen along one edge
IMG_0864.jpg 

Flip over the slat to bring the opposite edge of the screen over the first attachment point
IMG_0865.jpg 

Staple the 2nd edge to the slat
IMG_0866.jpg 


Attach the slat to the bottom of the collector and Viola! Easy-Peezy screen tube
IMG_0867.jpg 

I also see no reason you couldn't wrap the screen(and slat) around a tube if needed.

Greg in MN

gbwillson

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Reply with quote  #4 
GH-

I like the twin wall or chloroplast mount idea. Having the ribs running in line with the tubes should also reduce any airflow lost by having something inside the tube. Just make sure you use divergent staples as opposed to flat-ended. I thought the slat, or whatever you attach the screen to, would create more mounting options for the screen tubes. The wider the slat, or chloroplast,  the more elongated and flatter, the tube, which may have some advantages. Someone could play around with fins or turbulators inside the tubes. Simply attach to the slat during assemble of the tubes.

I wonder if this could be used for making Cinefoil tubes? Hmmmmmm

Greg
KevinH

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Posts: 565
Reply with quote  #5 
The slat would also help support a tube using fiberglass screen.

My first collector used aluminum flashing, which was a lot more difficult to roll into a tube than I thought it would be.  To roll it into tubes, I had to use the method Greg showed  (used screws instead of staples).  The wood inside the tube was covered with foil.

Kevin H
MN
gbwillson

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Reply with quote  #6 
Kevin-

My demo tube was made with fiberglas screen. My screen scrap had some minor kinks, but the method should work fine for either screen, fiberglas or perhaps Cinefoil tubes. Obviously using fresh screen keeps the "spring" or integrity of the tube the best.

I wonder too if more than one wrapped layer is needed. Too dense, and the sun won't reach the back of the tube and I don't think the screen(aluminum or fiberglas), would transfer heat to the back of the layer very well. My ZP shows that air will stay between the layers quite well, but that is a flat, narrow passage at high velocity. Perhaps using high velocity in a round tube or oval would work too.

Greg
KevinH

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Posts: 565
Reply with quote  #7 
What I meant was that if you did a long tube, say 7 feet, with fiberglass it might collapse in the middle if you had the tube raised off the back of the collector.  The wood would help support the tube over longer lengths.  Or the wood would be a firm surface to put supports under to keep the tube off the back.

Seems like the ZP concept should work with tubes and high velocity.  Ovals are probably better than round.  Maybe someone will try it some day.  For ease of building, your flat screen method is better and may even be better on efficiency.

You are right about the number of layers being a factor, but I don't know how you could figure that out.  In order for a given spot on the screen to transfer heat to the air, the screen has to be hotter than the air.  It's next to impossible, though, to measure the temp of screen to see how hot the bottom of the screen tube is getting.  I suppose you could see how hot the back surface of the collector gets with different numbers of layers in the tubes.

Kevin H
MN
netttech

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Posts: 720
Reply with quote  #8 
Good ideas floating on the post.

I'm getting the impression a test panel may be in store to prove or disapprove the idea of screen tubes.

[smile]

Jeff
Central IL
Solar air & water.
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