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Jimmy

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Reply with quote  #31 
I think I have to choose the best available screen material.
Unfortunately I cannot find solar screen material nearbay. What I have now is the normal mosquito-net or window screen. And I think it is not effective. It is not dense enough. At least for the lower layer I'd choose something more heat absorbing. I have a black sun shade sail in my garage which is not in use. So I'm wondering if it wouldn't be a better choice. 
Today there was not so many sun but I have put a piece of mosquito-net and this sail next to each other and as I've measured the sail heated up much quicker. So maybe I give it a try.
In my current setup this lower absorber screen is missing so I might put this sail in.
What do you think?

gbwillson

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Reply with quote  #32 
Jimmy-

Your collector is not so big that you couldn't paint it inside to paint, if needed. The darkened surfaces help warm the unit faster in the morning before the fan kicks in. And if you can see the reflective surface through whatever screen you choose, you are losing solar energy as it reflects back out through the glazing. Aesthetically, I like the look of black, or should I say I don't like the idea of being able to read the name of the insulation manufacturer. 

I've always used insect screen for all layers. But for the back, I double up the layers. As you said, insect screen isn't normally very dense, but it allows some light to pass through to the second screen layer. So both screen layers are heating air as it passes by. Even the relatively open screen still keeps the moving air between the screen layers, as long as the air velocity is high enough. I'd also consider giving the screen a light coating of the flat black paint. Most of the screens I've seen have a satin, almost shiny surface. The paint will noticeably reduce the sheen. 

Greg in MN
Jimmy

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Reply with quote  #33 
No, it's not the size of the collector it is the outside temperature which is not good for painting. [smile]

My thoughts are to have the upper layer the insect screen and for the lower I'd give a try to this sun shade sail. (Or other possibility to have the lower layer double as you've mentioned.)
gbwillson

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Reply with quote  #34 
Before you use the sunscreen inside your collector you may want to check if it can handle the heat. If doesn't take long for a collector to get very hot inside if the fan were to lose power. If you know what the sunscreen is made of, you can check online to see the temp it starts to melt or distort. Most insect screen is made from either aluminum or fiberglas. Otherwise, take a scrap and put it in the oven at about 200˚F for an hour or so to see if it shrinks or shrivels. Make sure you have good ventilation as it might start to stink too. 

And be careful about leaving your collector exposed to the sun, even if the two entry ducts are fully open. I found that out the hard way. I wasn't ready for the ducts to be connected to a fan, so I stuck a small fan in one of the openings to move a bit of air through and cool things down. Without air movement, it can still get pretty warm near the top. Stick your hand inside near the top or better yet, hook up your thermometer to give you an accurate reading.

Greg in MN
Jimmy

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Reply with quote  #35 
Late. It's in. [smile]
I've replaced the missing lower screen with that.
But you are right, I'll take a small piece and test it with a heat gun.
Anyway it has been used for a summer to give shade. That time the temperature was about 100F and it was on direct sunlight for weeks.
Till that I have my temp sensor in and the arduino controls and logs everything.

I just want to have some info about it comparing 2 similar days. I can see from the logs I have that removing the lower screen some weeks before (even if it was touching the back) was a wrong decision.
stmbtwle

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Reply with quote  #36 
Why is that? Were you getting better heat capture with the screen?
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Willie, Tampa Bay
solardan1959

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Reply with quote  #37 
Willie,
  His output dropped to about 30C from earlier readings of 40C.  I originally thought it was from too large of a gap as he had a foil type covering on the back of the collector, very similar to a design I was talking about recently.

  Still, this use of sail cloth is interesting, this also is something I have talked about a long time ago.  I have made a long narrow collector with solar screen as the back layer and it performed very well.  I did not have the upper layer screen channel so mine was not a zero pass.  There are all types of shade cloth which is about the same thing as his sail cloth I think.  You can pick anything from about 35 to 95 percent blockage of light.  Landscape fabric will also work well but I have tried several types and had them melt due to poor flow in that back and forth channel design.  But with a straight flow like this that will flow even if the fan breaks, that should not be a problem.
Dan
stmbtwle

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Reply with quote  #38 
Was wondering about that. I've about decided that my next ARETHA will have 'solar" shade screen against the back, probably without the rear air gap as I'm still not convinced it's necessary. I'll use a standard front screen so it will be a zero pass. While the cinefoil dual pass worked well, cinefoil is expensive and I don't think it will scale up that well.

What was meant by "sail cloth"? Modern sail cloth is darned expensive and being quite smooth (and usually white) probably wouldn't work all that well anyway. So what WAS used?

Ok I found it, it was a sunshade... not a 'sail' in the nautical sense. Shade cloth would work well.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
Jimmy

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Reply with quote  #39 
So the first test failed. This day was not as sunny as the days before.
And I have to quickly double check what I did yesterday with a single led light on my head. [smile]

I will be away for the rest of the week but I'll be able to read the logs.

Dan,
 the 1st readings with 40C increase was with the first collector I made 2-3 years ago. But it was rather restrictive.
Than I've tried the ZP but as the lower screen was touching the back I thought I'd better remove that.
Onestly I haven't seen that much of a difference after removing that.
Later Wilson explained me that the lower layer should be the absorber so I've installed this sunshade yesterday.
But now I'm afraid that the air gap of my design could be too narrow. (about 1cm) 
Let's see what happens. Maybe I'll rework it with bigger air gaps.
Can I reduce the air gap between the reflector and the lower layer to have a bigger gap between the 2 screens?
solardan1959

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Reply with quote  #40 
Jimmy,
Define "reflector"

I think you are basically saying what Willy said, lay the back layer right on the back of the panel.  Yes, if you lay screen on the back you can open up the gap, I still think you should be shooting for at least 3/4 of an inch in you present collector.  It sounds like the collector is kind of thin, can you move the screen closest to the glazing closer?  1/2 inch is probably plenty far enough from the glazing though I would prefer about 3/4 of an inch.

Quote:

Than I've tried the ZP but as the lower screen was touching the back I thought I'd better remove that.
honestly I haven't seen that much of a difference after removing that.


Since your back was a foil like material, the gap still kind of narrow, and the layer of screen closer to the glazing, you effectively had a zero pass with you foil being the second layer..  I am not sure you will get much of an increase over that but maybe the air can pick up more heat off the rougher surface of the shade cloth or screen.

30C is a good output with good flow on your size of a collector, ideally you would try to maintain that output and increase your flow as the collector gets bigger.  The higher flow will better circulate the air in the garage and produce BTUs more efficiently.
Dan
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