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Bartman

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Reply with quote  #11 
Quote:
Originally Posted by KevinH
Regarding the original question, I haven't seen any side by side testing that isolates the outside temperature.  For example, compare a screen collector to a tube collector at 40F, 20F, 0F, -20F and plot the efficiencies.  Most testing involving tube collectors like the downspout and pop can have been done at fairly moderate outside temps.  Gary was going to do a cold weather comparison between the pop can and screen collectors, but I don't think it happened (original test was at 80F outside).  I have always thought that the performance would shift towards the tube collector as the outside temperatures drop.  That has been my own experience, but it is subjective and not based on a controlled side by side test.  Both collectors types are losing heat due to radiation out the glazing and conduction with the glazing, but in the tube design you have the dead air space which should act as an insulator (a little like a dual pane window).

G_H,
That was a surprising conclusion.  I would have expected a thicker tube to have more heat transfer resistance, but I haven't looked into the physics.  The study is based on a simulation (not real testing) so it depends on how good their model is.  It is also based on a hot water collector.  It is something that could be tested if the setup was good enough to detect a 15% difference.  Menards, for example, sells two grades of downspout (thick and thin).

Kevin H


Hi Kevin,

I am a big fan of your flex tube collectors. Thanks for your info.

Regarding the thickness... please check #4 of this article... might help future designs...
http://www.hindawi.com/journals/isrn/2012/282538/

Thick is good with higher flow rates.. Thin is good for lower air flow..

Thanks for all the answers guys.

SolarInterested

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Reply with quote  #12 
Quote:
Originally Posted by Bartman

Regarding the thickness... please check #4 of this article... might help future designs...
http://www.hindawi.com/journals/isrn/2012/282538/

Bartman thanks for that link. That's also a unique design in that it draws air in through the glazing which is made of slats of glass. A separate thread about it has been posted for discussion
http://simplysolar.supporttopics.com/post?id=7399523

[image] 

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Both temperature rise and airflow are integral to comparing hot air collectors
jeharms

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Posts: 28
Reply with quote  #13 
Quote:
Originally Posted by Bartman
Hello Everyone,

I like the screen collector but my concern is the inside air coming in contact with the twinwall glazing. I am thinking that the downspout is the better way to go because the air only touches the heated metal and avoids the cold glazing.

I would love to find an answer.

Bart


I have a similar question.  The replies here focus on various designs that keep the airflow away from the cold glass.  But I'm interested in the effect of humidity in the box that manifests itself as frost on the interior of the glazing when the sun isn't shining.

I have built a few popcan heaters and my brother built a double-screen collector for locations north of Winnipeg, Manitoba.  One advantage of the pop can heater is that, even at night, the humid air (which came  from inside the building when the fans were running in the daytime) doesn't come into contact with the cold glazing. Not so with the screen heater.  The difference is clearly visible in the morning.
  • On the popcan heaters, the glazing in front of the cans is clear.  The glazing on my heaters extends over the bottom and top headers/manifolds (which distribute the airflow into/out of the columns of cans). In those header areas, the humid inside air clearly touches the glazing.  In the morning, the glazing on the headers is all frosted up. It clears up after a while of sunshine, but water runs down the inside of the glazing.  I assume that, over the course of a sunny date, it evaporates as the heater warms up.  My 4'x8' heaters typically heat up to 40 to 44 deg. C. on a cold sunny day (with 100 cfm blower fans).
  • On my brother's screen heater, the entire glazing is frosted up in the morning from the humid air that was in the box overnight.  This takes longer to melt and disappear because the frost reflects the sunshine and doesn't let it get to the black absorber screens.
Has anyone found ways to address the issue of frost on the inside of the glazing?  Has anyone developed anything to drain the water from the melting frost in the mornings, or do you just rely it evaporating in the heated air?

Jake
SolarInterested

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Reply with quote  #14 
Jake I don't recall any posts about dealing with the meltwater from frosted glazing. I suspect it's what you thought that it is allowed to evaporate in the heated air. On a similar theme, do you know if there's condensation inside the pop cans where the humid air is in contact with the cold walls of the cans overnight?
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Both temperature rise and airflow are integral to comparing hot air collectors
joebehr

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Posts: 109
Reply with quote  #15 
Has anyone tried a downspout collector without glazing?
gbwillson

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Reply with quote  #16 
Joe-

What would be the reason to forgo glazing, especially in frigid climates? 

Greg in MN
Garage_Hermit

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Reply with quote  #17 
@joe

Quote:
Has anyone tried a downspout collector without glazing?


Not tried one, but I have a design - I thought I had already posted it on here, but can't find it for the moment?  I'll try & dig out the original file, and E-mail it to you...


G_H


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(1)  "Heat goes from hot to cold, there is no directional bias"
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solardan1959

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Reply with quote  #18 
I try not to go negative but in this case I just have to.  Why would you want to try and build a tube collector without glazing.  While the metal may get warm if laying in the sun, this gets harder and harder as the temps drop.  Feel your car when it's zero out, I doubt very much it will feel hot.  The reason is the cold air is extracting more heat than the metal/sun can generate.  Take that same chunk of metal and put it in a sealed box with a glazed cover and point it at the sun and the metal will heat up nicely.
  At 50 degree temps it is feasible but will still perform so much better with glazing.  At zero the covered will produce heat the uncovered is just going to blow cool air, an air conditioner not a heater.
  At -30C, (just realized the topic was Celsius), at -22 Fahrenheit heat starts getting very hard to produce even with glazing.  Obviously can be done you just need a better insulated box and better glazing. 
Dan
gbwillson

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Reply with quote  #19 
The only air heaters that I am aware of without covers are the aspirator-type on the side of a building. This is essentially a flat plate collector with thousands of tiny holes in black metal surface. The primary reason is to preheat incoming air as it passes through the black metal plate for HVAC fresh air exchange. A tube-type collector wouldn't do that...unless there were holes along the tubes. But as Dan said, incoming air would be too cold for heating, unless perhaps, you like in a moderate climate.

Energy efficient and tightly sealed homes don't "breath" like homes of old when gas was cheap so they need a fresh air exchanger. Kevin built a very small "pre-heater" for his fresh air exchanger for that very reason.

Greg in MN


netttech

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Posts: 720
Reply with quote  #20 
I wonder how well a panel would work with one opening, but with glazing? If the bottom of a panel was left open, but with glazing it may work decently if big enough.

Besides always getting fresh air in.....it can't possibly have any other benefit.

Jeff
Central IL
Solar air & water
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