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Scott Davis

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My friend Gary Reysa, creator of the website Mecca for all things solar, http://www.builditsolar.com and I set out to do some side by side testing to determine which hot air collectors perform best.  We learned many things, mostly that there are still tons of good materials left to try, but right now, the double layer screen wins for best performance, ease of build, quickest build and lowest pressure drop (resistance to air).

Here are the construction steps on Gary's site.  Note that this page shows an absorber with 3 layers, but Gary notes that we later determined that a double layer screen with the screens separated was the sensible approach for our reference standard:

http://www.builditsolar.com/Experimental/AirColTesting/ScreenCollector/Building.htm

Here is a video showing how I built the double layer screen absorber.  You don't have to watch the whole video to get the idea, but it proves you can literally build the absorber in 10 minutes!



It's cheap, easy and works great!

We haven't tested the screen in horizontal configurations extensively yet, but we think it will work very well that way too.

Have you built a screen solar hot air collector? Please reply and tell us about it here! Please be sure to include pictures if you can!

Do you have questions about the screen solar hot air collector? Please reply, this is the place to ask about it! We will do our best to help!

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jpeg BasicScreenLayout.jpg (38.01 KB, 1319 views)


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taoswheat

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I have been running my single 4X8 two screen collector for a few weeks now and it does put out the heat. The collector was made with 6" metal studs and 1" foam backing and 1" foam in the studs. The black aluminum screen is glued to 3/4" EMT conduit (use wood next time). Glazing is Polycarbonate double wall. It is just providing garage heat as an experiment. Still do not have it automated so I have to turn the blower on in the morning and off in the afternoon. It produces useful heat from about 7:45AM until 4:40PM. Today at mid-day, the collector temperature went up to 100F with the blower on full speed. It has 6" inlet and outlet and a 430 cfm rated blower. Mid day I measured about 350cfm and 35F delta for a heat output of about 9600BTU/hr using:
H=0.79 X (cfm) X (Tout-Tin)- I am at 7200ft. Early in the morning it was producing about 2700BTU/hr.
Just thought someone would be interested. It surprised me to find the screen worked so well- there is magic after all and the garage is warmer.
JohnW
Taos, NM
Buffalobillpatrick

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Does 3 layers of screen work any better than 2? OR 4?

Has anyone run tests comparing reflective foil insulation vs it painted flat black?

JohnW

Your air weight is about .058 lb/ft3 mine is about .054 @ 8,800'


http://www.engineeringtoolbox.com/air-altitude-density-volume-d_195.html

And hot air even less

http://www.engineeringtoolbox.com/air-desity-specific-weight-d_600.html
Scott Davis

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Hi BuffaloBill.  More side by side testing is needed to be sure, but preliminary results seen to favor painting everything flat black, so no light is reflected all the way out of the collector.  Any light that escapes is lost heat.
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solardan1959

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Buffalobill,
   I did some test doing what you asked for and posted the results here:
http://simplysolar.websitetoolbox.com/post/Side-By-Side-Testing-for-Hot-Air-Panels-6086998

   Like Scott says, it to early to really say, but I felt the results were pretty good for reflective foil with at least three layers.  With two layers it was not as good as a black body collector.
Dan
netttech

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Reply with quote  #6 
Quote:
Originally Posted by Scott Davis
Hi BuffaloBill.  More side by side testing is needed to be sure, but preliminary results seen to favor painting everything flat black, so no light is reflected all the way out of the collector.  Any light that escapes is lost heat.


Interesting post on performance. Well, I finally completed my parabolic hot water panel, installing it Saturday.

On the other site there wasa discussion between 2 or 4 layers. Once I clean up from this panel I plan to make another window unit with 4 layers to compare to my 2 layer panel. I remeber reading a post last last the 4 layer seemed to have airflow restriction comapred to the 2 layer.

This panel will have 2 separate dual screen layers with space separating the 2 individual layers. With the two layers separated, it may resolve the airflow issue, but still capture any sun bounce back from the backing (foil poly iso). I included a Paint image so you can better understand what I am talking about.

Jeff
Central IL

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solardan1959

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Jeff,
   Good luck with that, seems complicated to build and I am having a hard time visualizing the airflow.  I have not gotten to four layers yet, and remember somebody making that statement about restricting flow.  I think they were talking about on passive systems though.  To me it seems like four layers would completely block the light from getting back out of the box.  It also blocks enough light to the back so that it would not really heat up also.  My thinking lately is we do not want the back to collect heat we want the screen to absorb all the heat.  Then when the air flows through the screen it better collects the heat than it does from the back wall and can better extract all the heat in the box quicker.  The fan will overcome any resistance four layers of screen will have.  I will get there but have not had enough sun and it will be worse for the next month.  The down side is Scott is talking proven methods where mine is still in the wishfull thinking/unproven area, well lets say unverified by others area. (as I feel my three layer foil is at least as good as my black body 2 layer).
Dan

netttech

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Reply with quote  #8 
It's a standard back-pass design. It's also not diffcult to build. I'll post pictures during the build process.

Since the backing is foil poly-iso there really shouldn't be much heat absorbtion. What bounces back should be caught by the 2 separate dual layer screens (at least in theory). 

Last year I compared the dual screen to down-spouts, so I needed something to compare this year.

The other option I considered was suspending black aluminum flashing under dual screen layer, but above the backing. I did that to my dual screen 8' x 3' panel. I don't think I have enough flashing left-over from a previous panel to do that though.

However this should be an interesting comparison. I'm also constructing the panel with only 1/2" foam so I will be able to compare how well it survives to the 3/4" foam panel.

The window units are hanging out from the 2nd story windows, so it has to be light-weight.

Jeff
Central IL.
solardan1959

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Reply with quote  #9 
By difficult I meant the screen layout, and by flow I was concerned the air may bypass a lot of the screen going up next to the glazing then just through the last two layer bit by the top.  Only the air knows what it will do. Rather than the flashing why not add another layer of screen.  I have one more "space blanket" left that Greg Willson gave me.  If you want to try some reflective mylar, it reflects a lot better than the foil faced poly,  send me an address and i'll pop it in the mail.
If you did not see the photos they are here:
http://simplysolar.websitetoolbox.com/post/Side-By-Side-Testing-for-Hot-Air-Panels-6086998

By the way I saw an interesting heater at Wal Mart today.  I looked just like one of your reflective water collectors but worked the same way in reverse.  The hot water pipe was run down the center of a curved reflector and the water heat was radiated by the curved reflector down towards the ground.  It really put out quite a bit of heat and I did not see any fans.
Dan

netttech

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Reply with quote  #10 
Hmm, at Walmart? It would be interesting to see that. Next time I'm at walmart I wander around in that area to see if they have them here.

I try to use (& re-use) materials I have on-hand. I built a big (16' x 5') hot air panel 4-5 yrs ago & have been re-using the black aluminum flashing since I took it apart. I cut smaller pieces, formed them into smaller troughs for the pex tubing under the floor.

Each of the parabolic troughs is flashing from that panel. I have mirror mylar left over from making the parabolic panel.

We will see how the new panel performs compared to my dual-screen 

The screen lay-out is also what I was referring too on the difficulty....it's not. Of course, I've done it once already & know how to do it. My screen comes pretty close (1/4-1/2") to the glazing & the backing (3/4") on my original panel & suspect this panel will too.

I tried to perform a smoke test on my window unit this fall. It didn't work very well because the smoke dispersed quite at bit coming down thru the back-pass. Since the smoke was widely dispersed, it was hard to see behind the twin-wall glazing. My digital recorder barely picked it up. I will see if I still have that video & try posting it on You-tube.

Jeff
Central IL
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