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Scott Davis

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Reply with quote  #1 
Here are the testing concepts that Gary and I used for comparing hot air collectors.  I can't emphasize enough that BOTH temperature rise and airflow must be considered when comparing the performance of collectors.  Collectors must be compared under identical conditions too!

Here is Gary's page:

http://www.builditsolar.com/Experimental/AirColTesting/Index.htm

Here is the page I set up:

http://www.n3fjp.com/solar/comparisonhotair/comparisonhotair.htm

And this video summarizes the testing as well as the concepts for designing a high performance solar hot air collector:


Here are more details on how you can help with side by side testing along with us! 

The Great Simply Solar Hot Air Collector Absorber Challenge!

Have you done any side by side testing of solar hot air panels?  What materials / designs are you interested in testing?  What questions do you have about the test procedure?  Please reply and share your thoughts and questions.  This is the place to ask and we would very much love to have more folks helping with side by side testing!

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solardan1959

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This is my "pole barn heater test bed", The obvious use was to heat my pole barn but I got sidetracked when we started discussing differences between types of collectors, black or reflective boxes, etc.  I decided to make the collector easy to modify so I could remove the glazing and make changes fairly quickly and easily.  The right bay is my two layer, black backed, standard collector, the middle is a reflective screen collector and the left is a downspout collector of my design.  I did test the middle and right collectors as empty boxes first and the results are in the "empty box collector" section.  The black box did do very well on it's own and slightly better than the foil backed when the screens were added. (5 or six degrees hotter but less under different conditions)  Both run about 10 degrees hotter than the downspout collector.  All have about 96 cubic feet per minute flow with the fan sucking air from the panels.  My next test is to add a more reflective mylar backing and another layer of screen to the middle collector.  I think the additional layers will increase the output by capturing more of the reflecting rays in the box.  If it outperforms the standard I will add another layer to the standard to see what it does.
Dan

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Scott Davis

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Reply with quote  #3 
Thanks for the update Dan!   I'm eager to hear what develops!

Scott

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solardan1959

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With some sun I finally got to my next stage, But first a recap.  My right bay is my standard two layer screen and a flat back background, currently the best performer.  My middle bay is the same thing with the foil face poly unpainted and reflective, it currently is a few degrees behind, generally 3 to 5 degrees.  My left bay is a downspout/backpass of my own design, usually about 10 degrees behind the others.
   Today I removed the poly and screen and installed some thin film mylar thanks to Greg Willson and then added another layer of screen to this collector only.  I have attached several pictures, one shows the difference between the mylar and the foil faced poly, the next shows the mylar covering the whole panel, the next shows the screen installed, thae last shows the twin wall poly back on.  Notice the time on the last two, I can install the poly and molding in about 5 minutes, less than 15 to remove it, take out the screen and replace it back to normal.  Also notice for the most part the screen hides the mylar fairly well and it's almost as black as the black painted.  The only thing I see is the wrinkles reflecting sun faintly.  Should have been a two man job to really get it on right but the spray adhesive did a great job, again thanks Greg!
   It's to early to make any claims but as a first look it does appear to be outperforming the 2 layer screen test standard.  I does not lag at all and even seems to be a couple of degrees higher most of the time.  I do think it will outperform the two layer but also think one more layer would totally capture all the light and be even better.
   Before anybody ask, the center reflective collector does have a slight bit more flow already so it was already at a disadvantage over the standard.
   I also plan on adding a layer to the standard to keep it fair and see what it does but only after a new baseline is established with the reflective panel.
   Another thing I noticed with the old two screen foil collector was it did seem to follow the standard pretty close in less than full sun conditions almost as if the foil was bouncing more light around and helping.  May be wishful thinking though.
Dan



 


Scott Davis

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Reply with quote  #5 
Thanks for the post and update Dan!.  I love being able to see the pictures along with your narrative here on the forum!

Take care,
Scott
MD

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DANSAGE

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SCOTT,

I BELIEVE I WROTE TO GARY ON THE BUILD IT WEBSITE.  I DESIGNED AND HELPED BUILD A 3,540 SQ.FT. THREE STORY HOUSE IN CENTRAL OREGON.  ONE OF THE DESIGN GOALS WAS TO HAVE ONE OF THE METHODS OF HEATING IT, TO BE AN ACTIVE SOLAR HOT AIR COLLECTOR MOUNTED ON MY 60 DEGREE SLANTED TWO STORY HIGH SOUTH WALL (COLLECTOR SIZE ~432 SQ.FT. (6 RUNS OF 4' W X 18' H)) FED BY A PLENUM AND COLLECTED IN A PLENUM AT THE TOP OF THE COLLECTOR, WITH 2 EA. 636 CU.FT. DUCT FANS COLLECTING THE HOT AIR AND DUCTING IT INTO THE HEAT DISTRIBUTION SYSTEM IN THE HOUSE, WHICH INCLUDES A HOT AIR RADIANT HEATED GROUND FLOOR AND A THERMAL MASS COLUMN, WHICH EXTENDS FROM THE FIRST FLOOR UP TO THE BOTTOM OF THE THIRD FLOOR ROOF (3' X 10' FOOTPRINT X ~ 33' AVG. HT.).  I WAS GOING TO USE THE BACKPASS DESIGN FOUND IN THE COMPLETE HANDBOOK OF SOLAR AIR HEATING SYSTEMS TO BUILD THE SOLAR COLLECTOR NEXT SUMMER, BUT THE VENTED SOFFIT COLLECTOR DESCRIBED ON THE BUILD IT WEBSITE SOUNDS INTERESTING. 

IT APPEARS FROM YOUR VIDEO ON YOU TUBE THAT YOUR TEST COLLECTOR USED THE SOFFIT PIECES LENGTH WISE INSTEAD OF CUTING THEM IN 4' LENGTHS TO GET THE BEST FLOW THROUGH AND CREATE THE MOST TURBULANCE IN THE AIR FLOW FROM THE RIDGES.  IS THIS CORRECT?  IT APPEARS THAT GARY'S VERSION DID HAVE THE "BETTER???" ORIENTATION ON THE BUILD IT WEBSITE.  THE WEBSITE  SAYS, THAT YOU BOTH CHANGED YOUR BACKPASS REFERENCE TO YOUR SCREEN COLLECTOR AS A REFERENCE DURING THE TESTS, BUT I COULD FIND NO EVIDENCE IN THE TEST RESULTS, THAT THIS WAS SO?  THE RESULTS ALL APPEARED TO BE REFERENCED TO THE BACKPASS COLLECTOR.  DID I MISS SOMETHING? 

MR. KORNHER SEEMS TO BE VERY CONVINCED IN HIS BOOK THAT THE BACKPASS COLLECTOR IS THE WAY TO GO.  LOOKING AT THE DESIGN AND THE RESULTS ON THE BUILD IT WEBSITE, I HAVE A COUPLE OF POSSIBLE ISSUES WITH YOUR TEST VERSION.  FIRST OF ALL IN ORDER TO GET THE NECESSARY LENGTH YOU NEEDED OUT OF IT, YOU INSTALLED A LOT OF BAFFLES.  I THINK THIS MAY HAVE INTRODUCED A LOT OF FLOW RESTRICTION AND THEREFORE INCREASED THE PRESSURE DROP THROUGH THE COLLECTOR.  THIS ALSO DECREASED THE CHANNEL WIDTH AND THEREFORE MAY HAVE RESTRICTED THE VOLUME OF AIR ALLOWED TO PASS, MAYBE ADDING TO THE PRESSURE DROP?  AM I ALL WET IN THESE SUPPOSITIONS???  I AM SURE THE SIZE OF THE VENTS/HOLES IN THE SOFFIT COLLECTOR, ALSO ADDS TO THE PRESSURE DROP IN IT, BUT I HAVE NO GUT FEELING ON HOW THIS COMPARES TO THE BACKPASS RESTRICTIONS. 

I WAS THINKING ABOUT MAKING A COUPLE OF 18' LONG TEST COLLECTORS, ONE BACKPASS AND ONE SOFFIT COLLECTOR, AND SEE WHAT THE RESULTS WERE FOR THEM.  THE BACKPASS COLLECTOR WOULD FALL INTO MR. KORNHER'S RECOMMENDED SIZE WITHOUT ANY CHANGES IN THE DIRECTION OF THE AIRFLOW.  IF BOTH YOU AND GARY ARE CORRECT, THEN THE SOFFIT COLLECTOR SHOULD BE BETTER DUE TO THE TURBULANCE CREATED BY THE FLOW OF AIR THROUGH THE ABSORBER PLATE.  (I BELIEVE THAT DUCTING AIR THROUGH HOLES IN AN AIRPLANE WING WAS ONE WAY TO STOP THE WING FROM STALLING, SO IT SHOULD BE INTERESTING.)  TO THAT END, DO YOU HAVE ANY ADVICE ON WHAT THE TILT ANGLE AND TOP AND BOTTOM SPACING SHOULD BE ON AN ~18' LONG ABSORPER PLATE FOR THE BEST RESULTS?  I HAD THOUGHT ABOUT BREAKING THE PLATE UP INTO TWO OR THREE SECTIONS SLANTED FROM THE BACKPLANE UP TO THE GLAZING INSIDE THE COLLECTOR.  DO YOU THINK THAT THERE IS ANY ADVANTAGE TO THAT KIND OF CONFIGURATION, OR DO I JUST NEED TO TRY IT AS A POSSIBLE CONFIGURATION???  THAT SHOULD BE THE SAME AS 2 EA. 9' OR 3 EA. 6' COLLECTORS HOOKED UP END TO END, WITH NO FLOW RESTRICTIONS DUE TO THE HOOK UP. 

BY THE WAY DO YOU KNOW WHERE YOU CAN GET BLACK ALUMINUM SCREEN WIRE MESH.  THE FIBERGLASS SCREENING YOU USED IN YOUR EXPERIMENTS WILL EVENTUALLY DETERIORATE DUE TO U.V., UNLESS THE GLAZING YOU ARE USING FILTERS THE U.V. OUT.  ALSO, HAS ANYONE GIVEN ANY THOUGHT TO THE POSSIBLE LOSS OF TRANSMITTANCE THROUGH THE GLAZING DUE TO DIRT ACCUMULATION DUE TO DIRTY AIR PASSING THROUGH THE COLLECTOR OR JUST HAVING THE GLAZING AQUIRE A STATIC ELECTRIC CHARGE OVER TIME, WHICH WOULD ATTRACT ANY AIRBORN IMPURITIES.  IT WOULD SURE BE THE PITS TO HAVE TO UNSEAL ALL THE GLAZING EVERY FEW YEARS JUST TO CLEAN OFF THE INSIDE SURFACE.

ANY ADVICE, YOU OR ANYONE ELSE, COULD GIVE WOULD BE APPRECIATED.

TAKE CARE,

DAN SAGE

Scott Davis

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Reply with quote  #7 
Hi Dan,

Wow, you covered a lot of ground in one post!  Let me try to answer as succinctly as I can. 

-  Yes, the collector I built ran the soffit from top to bottom instead of from side to side as Gary did.  Gary and I had similar results, so it didn't appear to matter much.  The soffit and screen collectors both performed very well, just about the same.

-  Yes, our reference standard going forward is the double layer screen.  It is the best performer, cheapest and easiest to build and most importantly of all for side by side testing from folks around the world, easiest to duplicate.  You'll find more detail on that here.  http://www.n3fjp.com/solar/comparisonhotair/comparisonhotair.htm

-  The backpass as we built definitely underperformed and had a higher pressure drop.  That's not to say that the backpass designs, or all these designs for that matter, couldn't be enhanced and improved.

-  Doing some side by side testing would be fantastic!  Especially 18' long, horizontal collectors as you describe.  Please consider including screen with one of your collectors as again, that is the reference standard we are comparing all other options against. 

-  Home Depot has both fiberglass screen (which should last under UV protected glazing such as SunTuf) and aluminum screen inexpensively in rolls in Charcoal Black.  The fiberglass has slightly narrower openings and may have performed a wee bit better, but both are fine.

-  I may move this thread to our DIY Solar Experimentation & Performance page is I think it is more pertinent there.

Take care,
Scott


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solardan1959

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Reply with quote  #8 
Dan,
   I do think your cleaning argument is one of the best for a backpass.  A downspout would also have the same benefits but is much harder and more expensive and more so on a collector the size you are talking about.  In my opinion the pressure drop would not be an issue with a large collector like you are proposing because you have no need to add baffles on a collector of that size.  What gap behind the metal plate does the plans that Mr Kornher recommend?  We have several times discussed the backpass test bed and think it could perform better if the gap behind the absorber was a little bigger, (maybe 1/2 inches and the baffle was a little shorter by a few inches).  But in a two story run with an 1 1/2 behind it would be the equivilent of a 432 square inch vent.  Lots of air flow with that though I do not know how much resistance the narrow duct,(216" X 432"),  will have.  My guess is that will heat a lot of house but you have a lot of house to heat.
   A screen or soffit collector would put out more heat, probably quite a bit but even with filters there will always be the cleaning issue.  I'm seeing about 10 more degrees on two different screen 4X8 panels than a 4X8 downspout on my side by side test bed, if it were 4X16 would that be 20 degrees better?  So which is better, higher performance or lower maintenance?
Dan
  
DANSAGE

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Reply with quote  #9 
STEVE KORNHER AND ANDY ZAUGG RECOMMENDED A MAXIMUM 1" DEEP AIR CHANNEL BEHIND THE ABSORBER PLATE IN THEIR BOOK, SINCE THE ABSORBER SURFACE IS FLAT, WITH NO IRREGULARITIES FOR THE CREATION OF TURBULANCE.  THEY CLAIM YOU NEED A SHALLOW CHANNEL TO GET THE MAXIMUM HEAT TRANSFER.  I DON'T KNOW IF A LAYER OF SCREEN WIRE SANDWICHED ON THE BACK SIDE OF THE PLATE WOULD ADD THE NEEDED TURBULANCE AND THEREFORE ALLOW A DEEPER AIR CHANNEL.  SOMETHING FOR ME TO THINK ABOUT, OR MAYBE TEST, WHEN I GET THE TIME. 

TAKE CARE,

DAN SAGE
DRDean

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Reply with quote  #10 
For cleaning the inside surface of the hot air collector, would it not be in the best interest to design the construction of the panel with that in mind?  Could dividing the long 18ft panel into (4) 4.5ft x 4ft panels with hinges on one side be an option?  Weather stripping along the contact edges could form the seal.  Using hinged lever action clasps would ensure the hinged panel maintained a tight fit.  Food for thought.
Dave
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