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KevinH

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Reply with quote  #51 
Rich,
In post #35 you can see the round ducts at the end of each channel.  That is where the measurements would have been taken so that is how the CFM and mph velocity can be the same (each round duct is the same diameter).  You are correct that inside the collector where the rectangular channels have different cross sectional areas, the mph would be different, but that is not where it was measured.

Interesting results Greg.  I would have guessed the smaller gaps would have done better.  Thanks for the testing.

Kevin H
MN

Rick H Parker

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Reply with quote  #52 
Thanks Kevin, I see it now.  Exit cross section is the same for all three channels. That resolves the discrepancies.

Still wondering if the air that does pass through the screen is allowed to exit on top of the screen.

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Rick H Parker
Kansas, USA
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KevinH

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Reply with quote  #53 
Greg will have to answer that one, but I think that only the area between the screens (such as 1.5" x 9") is open at each end and the rest is blocked off.  In the #35 picture you can see the slots where they probably put something to block the air flow above the screen.  That creates a plenum at each end bigger than the screen channel.  The goal with zero-pass (ZP) is to keep most of the air flow between the 2 screens.

Kevin H
MN
Rick H Parker

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Reply with quote  #54 
I would think there would be plenums on both ends above the screens. In that configuration the static pressure on both sides of the screen would reach a state of equilibrium. In a state of static equilibrium, dynamic pressure through the screen would be null.
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Rick H Parker
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toaley76

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Reply with quote  #55 
Newbie question

Like the testing btw


With 2 layers of screen at 1.375" yet a box a lot deeper, did you manage to test the temperatures at the three levels?  above the top screen, in between the 2 screens and below the bottom screen?  

I understand the intent of the ZP but is there any chance that the air is getting trapped below both layers of screen and the 2 layers are creating a barrier between the hot air and the glazing?

like i said newbie question.  I am looking to build one a ZP but with slightly different screens and would appreciate your thoughts.... 

Or am i completely off track???

Andrew from Australia


17806909_10211355678839624_445074815_n.jpg 

gbwillson

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Reply with quote  #56 
Andrew-

Good question. No, we did not measure the temp above and below the screen gap. The temp of the screen gap would be our output temps. I know there is very little air movement above and below the gap, but the fact that there IS movement tells me the air would be drawn back into the screen gap flow at some point. Tiny vortices are created as air passes near the screen layer which helps break up the laminar flow in the screen gap. Kinda hard to imagine no air movement through a screen.

Greg in MN
stmbtwle

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Reply with quote  #57 
The only time I measured the temperature above the screen gap in my ZP ARETHA, the space above the gap was several degrees HOTTER than the gap itself. Seems a bit counterintuitive.

Some baffles or "dams" between the screen and the glazing could stop that airflow, and could also support the screen and/or glazing. But it could be that the screen doesn't work quite as we think, and stopping the airflow is counterproductive. It would be easy for you to test.

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Willie, Tampa Bay
Rick H Parker

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Reply with quote  #58 

"Some baffles or "dams" between the screen and the glazing could stop that airflow, and could also support the screen and/or glazing."

An idea that I have been toying with, is to borrow a page from John Canivan's  Modified Trickle Down (MTD), do a double glaze. The top screen could be attached to the second glaze with an adhesive. If one used a sponge roller lightly loaded with adhesive and lightly rolled on the screen, the adhesive would only be on the peaks of the screen where it would not interfere with anything.

The second glaze would definitely keep all the air flow between the screens, provide support for the top screen, no sagging. Provide a barrier against heat loss by both convection and conduction and still let the solar radiation pass through.

If one could get a good seal between the glazes, one could created a vacuum, which is the best thermal insulator there is. 

These things would add cost to the collector, it might be worth it, it might not. 


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Rick H Parker
Kansas, USA
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gbwillson

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Reply with quote  #59 
I don't think air would get trapped above or below the screen gap. After all, it IS screen. But the air will certainly be moving much slower than air in the gap. In cold climates, the glazing is likely the biggest source of heat loss. Fast moving air minimizes the time air is exposed to the cold.
So I would expect air between the upper screen and the glazing to be the coolest. This is a good thing as there would be less heat loss due to the lower temp difference between the air above the screen and the cold glazing. Any air under the screen will be between insulation and the screens. I would expect it to be warmer than the air passing through the screen gap. Keep in mind, as the air is passing through the collector, it is warming. So if it enters the collector at say, 60˚F, it will leave the collector at 100˚F or more. Air above and below the gap will also warm as it moves through the collector, where at some point it will be pulled into the stream of moving air. 

About 5 years ago I was thinking of double glazing, but not for a ZP-type collector, but a tube/can-type. Since with these, we know and expect the air to be trapped and hot. But this hot trapped air will have a greater heat lose due to the greater temp variance with the cold glazing. BUT, the trapped air will still be much warmer than if you allowed the air above the tubes to vent. Double glazing with twin wall poly might be a wash, as the a 6mm only allows roughly 82% of the light through. Who much would get through with two layers? I suppose you could try it with single layer glazing, but it is often 2-3 times the cost of the twin wall poly.

Greg in MN
gbwillson

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Reply with quote  #60 
Kevin was correct in his answer about the plenums preventing air from bypassing the screen gap. At higher volumes of air, even tiny leaks can allow large amounts of air to bypass the screen gap.

And like Kevin, I was TOTALLY surprised that the larger gaps did better than the smaller 1" gap. I'm still trying to figure that one out! I would think the closer proximity of the air stream to the warmed screens, you would have better transfer of heat when compared with larger gaps with the same flow of air passing by. Maybe the tight fit actually compresses the laminar flow so there is less mingling with the screen layers. Maybe the air is moving too fast for a small gap, but slows when the gap is larger. Maybe the larger gap allows more room for the air to swirl within the air stream. Maybe, maybe, maybe...In any case, a larger gap will also allow more air to pass with less resistance. So as long as you have room in the collector, a larger screen gap is better up to a point. If towards the larger, perhaps a third layer of screen would be effective. Say 3 screens, ½" apart? The third screen would only receive half the sun it gets with only two screens, but their would now be two screen gaps being heated from top and bottom. Hmmmm

It was asked if our test results would be effected by a wider collector, such as a 4x16 or 4x20. I wouldn't think so, since the air flowing through a properly set ZP has even flow from side to side. More airflow from the same fan, or allowing the use of a smaller fan should be possible.

Greg in MN

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