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Anyone care to share their passive solar air heater data???? No fan, convection only please!  RSS feed

 
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****After my discussions with C. Letellier, I've heavily edited this original post. He brought a number of things to my attention which are reflected in my edits*****

Please Post Your:
1- outside Ambient temperature during winter operation
2- air input temperature
3- air output temperature
4-anemometer speed reading of the very center of your air intake
5-anemomter speed readings of the very center of your air output
6-the diameter of your air intake
7-the diameter of you air output
8-time it takes for you to reach maximum room temperature during cold sunny days in winter.

I put in a passive solar air heater (no fan, convection only).

After reading all the suggested info C. Letellier, I am convinced the optimal design with a fan will not be the same as the optimal design without a fan using convection flow only (a fan can push air thru a screen or a soffet).

I will post a link to a google doc here later with everyone's data.

I am really happy with my SAH under cold full sun conditions in winter!
  Outside temperatures: 34 to 37F
  Full sun attained at 9:20 am
  after about 1 hr 40 minutes of full sun
  Inside temperatures: 72 to 75F
Cubic Footage of air in dwelling: 1,440 ft^3

I have found that not all solar air heaters are equal so I want to see:
          -what the highest temperatures are and WHY those temperatures are higher;
          -why other solar air heater are getting lower temperatures;
          -pictures to help troubleshoot or improve by.
         
Please share your data and I'll send you pictures of my naked wife (oh wait, do I have a wife???, uh-oh am I the wife???).

It's build is here (the build of my solar air heater silly):
Twobirdstone's SAH build (solar air heater)
sahBeforeCnx_20181117_164617.jpg
[Thumbnail for sahBeforeCnx_20181117_164617.jpg]
 
master steward
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Location: Northern WI (zone 4)
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I merged your stuff with the following thread. I hope that is okay by you.
 
pollinator
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Hi Orin, my friend came back to me, he told me that they are on now, end march, (meaning the fan) and that temperature entering the house is 39 degree celcius 102 F in an area with an annual solar radiance average of 1200kWh/m2. Boxed in windows, 2meter bij 40 cm, 6 and a half foot bij 1.2 foot.
 
Posts: 291
Location: Greybull WY north central WY zone 4 bordering on 3
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Here is the info on mine.  I had hoped for convection but I can see I will want it running under fan.

my collector

One comment.  Max temperatur isn't necessarily the goal.  The goal should be maximum heat which is air flow and temperature.  A cooler temperature with higher air flow can give you more heat than max heat with low air flow.
 
Orin Raichart
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C. Letellier wrote:
One comment.  Max temperatur isn't necessarily the goal.  The goal should be maximum heat which is air flow and temperature.  A cooler temperature with higher air flow can give you more heat than max heat with low air flow.



thank you for your post and it's technical content. I am still ruminating everything you presented before I ask all the question I have. I agree that air flow is just as important as temperature:I don't have an anemometer but I do have a IR temp gun. Right now, I use incense to get an idea of how fast the air is flowing and how the inside of the building feels after the 1.5 hours of full sun. By the way, you'll see I found the same results that winter full sun is about 5 hours long per day.

The back of my solar air heater is a sandwich of 2"foam sided by two layers of 3/4" plywood. The sides are true 2x6 cut on a woodmizer. I didn't post theses facts because I don't like the idea of using foam for a number of reasons: I used a bunch of scrap that was just lying around and would've rotted (such is my financial status).

Also, thank you for doing youtube research: would you please post the youtube user(s) you felt did the most unbiased work so I can examine their methods for their proof that window screen is the best solution?

With your post, I will create a post on this thread with data from your project and mine if you give me the go ahead. My intention is to start a research tab here on permies.com where things are peer reviewed so a person can more quickly decide (rather than hours of redoing experiments or watching hours of youtube), what their best options are.


 
C. Letellier
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Location: Greybull WY north central WY zone 4 bordering on 3
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I have a bit of a problem because this started out as a read for fun not planning to do anything so I didn't create a bookmark research file to begin with.  It was purely intellectual entertainment.  Then I encountered a video of a collector on a garage that was doing a measured 123 CFM  on pure convection and it played into other thinking.  Suddenly I was seeing a fan rather than a collector.  As I had been looking at earth banking green houses now suddenly I was seeing a completely passive fan to pull air down into the earth storage tubes for the green house heat storage.  Now I suddenly got more serious about this.  Then things stalled because I wasn't ready to do the greenhouse yet but wanted proof of concept.  I was looking at how to modify the house to test it.  Finally I woke up to the fact that I could come out the top of the basement bedroom escape hatch and go back in the upper window without making any permanent mods to the house beyond a few screw holes, replace a door and a bit of caulk mess.  All easily fixed if this didn't work.  So I don't have a huge file of all the things I looked at.  Here are some of the main ones I have found again in the time since.

Here is one.  It has a 4 video links with one being a good primer for teaching the beginning stuff as well as comparison data.  It is a good starting point.  If you follow all the links including webpage links some of them go 4 layers deep so be sure you hit all the links and dig to the bottom.  You will find their work is a major reference point for most people going forward in the nearly decade since this work was done.

collector comparison 1

And here is a pop can vs window screen comparison.  Notice they put the window screen in on the wrong diagonal so still less effective than it could be.

collector comparison 2

If you dig more you will find dozens, possibly hundreds of comparison videos.  In every side by side comparison I looked at they always showed the screen collector winning although sometimes not by much.  Most though it was a solid win of 10% or so.  Being way easier to build, having really good air flow and still being really cheap to build it was to me at least giving a clear win to

The main ones I need to find a way back to were 2 forums for solar that I got most of the newer rest of the info out of.  There were 3 issues covered here that not much mention is made of elsewhere.  1.  Metal screen vs fiber glass screen, 2.  black box liner vs aluminum foil liner.  3.  diagonal screens vs parallel screens and which diagonal they belong on.  I will quickly summarize what I remember for now and add a bit of my own thoughts.

1.  metal vs fiberglass.  The intial people doing screen collectors talk about metal being better because of its conductivity and its mass plus it is durable.  Thus most of the early stuff on screen collectors has metal winning.  Later people tackle these and counter.  On conductivity the argument goes that it doesn't matter because the goal is screen to air conduction and that how well the screen conducts doesn't matter and what matters  is basically the same because of surface area is the same.  On mass they point out that screen weighs so little it stores almost nothing so there is no real gain.  One of the discussions actually argued that metal slightly hurt because it started transferring heat into the building a few minutes later and getting off to a fast start was important because typically morning was when you most wanted the heat and any late day storage of a few minutes had a lesser value.  Finally comes durability.  Unprotected fiberglass screen apparently begins to get fuzzy and gray in this application but some of the early users had spray painted theirs for a better black for better absorption and theirs wasn't deteriorating.  So there seems to be a current consensus that fiber glass painted with black bbq paint is as good or better than metal and is a bit cheaper.  Now I will add a personal argument to this.  Metal window screen is more porous and has less absorption area for a given grid size and thus you will need more layers to achieve the same absorber area increasing cost.

2.  The black box liner vs foil liner was found because someone got in a hurry and was out of paint and failed to paint the foil on the insulation in a panel.  They found that panel ran just a bit hotter on an equivalent fan.  Other comparison since were showing 3% to 5% gains vs a black box.  Not huge but still small improvement.  My argument in favor of this would be that by reflecting the light back out we are actually heating both sides of the screen.(more important with fiberglass and its low conductivity)  But looking at the light reflected off the foil in my collector, I am going to say that the 2 or 3 layer screen stuff needs to be jazzed up to at least 4 layers.  So any light escaping actually has to go through 8 layers of screen.(4 in and 4 back out.)

3.  Skipping this one as I can't summarize it quickly or well.  Personally I believed the angling to the back wall at the top over the angling away from the back wall at the top.  But so far this one had no good data with it.




 
pollinator
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C. Letellier wrote:
And here is a pop can vs window screen comparison.  Notice they put the window screen in on the wrong diagonal so still less effective than it could be.

collector comparison 2

If you dig more you will find dozens, possibly hundreds of comparison videos.  In every side by side comparison I looked at they always showed the screen collector winning although sometimes not by much.  Most though it was a solid win of 10% or so.  Being way easier to build, having really good air flow and still being really cheap to build it was to me at least giving a clear win to

The main ones I need to find a way back to were 2 forums for solar that I got most of the newer rest of the info out of.  There were 3 issues covered here that not much mention is made of elsewhere.  1.  Metal screen vs fiber glass screen, 2.  black box liner vs aluminum foil liner.  3.  diagonal screens vs parallel screens and which diagonal they belong on.  I will quickly summarize what I remember for now and add a bit of my own thoughts.

3.  Skipping this one as I can't summarize it quickly or well.  Personally I believed the angling to the back wall at the top over the angling away from the back wall at the top.  But so far this one had no good data with it.



Hi C., I've spent a bunch of time poring over stuff on BuildItSolar, and have considered building these collectors. I think Gary has the angle right in his screen collector (and I thought he had shared his reasoning).
The cold air is entering at the bottom/against the glass, and exiting at the top/behind the screen. Some heat will be lost through the glass, so having the cold air get a little colder is more desirable than losing it from the heated air (going from hot to warm....)
(higher delta T = higher losses, so less lossy for cold air on glass; and higher delta T works the other way too, for more efficient transfer of heat from the screen)
As for the angled screens vs. parallel screens, there are two reasons, one is to balance the flow through the entire screen rather than a short-circuit between the openings, the second would be if one were to make the openings in the sides of the collector, you'd need it to be twice as deep as the ducts and then some...
 
C. Letellier
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There is some secondary arguments that go with it about layering the screens like shingles in a taller collector and using the wedging action to pull the heat away from the glass and another discussion air flow and stratification.  And there was something about absorption angle especially in foil lined boxes.

It went something like this.  In a convection collector chimney effect matters and the lower in the box you can start the air warming the stronger up draft will be.  If you look at leaning the screen out at the top basically half the air is only heated in the top half of the collector reducing flow.  But if instead of doing a single long screen top to bottom you put in a series of screen louveres that lean to the back of the box it will pull the heat to the back of the box and heat more of the air lower in the box.  That much of the discussion I understood and went with.  Now there was also part of the discussion about sun angle and the foot print it makes on the screen.  That by aiming the screen closer to perpendicular to the screen more of the light makes it thru the first layer and the hottest part of the screen being shielded so it radiates more of the heat in a trapped location rather than  right back out the glass.  The numbers they had were close but the little they had to show seemed to indicate that stronger air flow and more heat transfer happened with the screens leaning in rather than out.  In both cases they end up with colder air next to the glass.  The first way gives colder air next to the glass but higher radiant losses and the second loses more to the glass from conduction and has lower radiant losses.  The one that made the most sense to me of the second version they actually brought the cold air down between the glazing and a heat shrink plastic in front of the collector.  The side view thermal images had me mostly convinced of it.   More of the box was warm lower so there should be better stack effect.

 
Orin Raichart
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After reading all the suggested info C. Letellier provided, I am convinced the optimal design with a fan will not be the same as the optimal design without a fan using convection flow only (a fan can push air thru a screen or a soffet easier than convection flow). Please disagree with me and explain why if you know.

Here are the items I agree with from BuildItSolar:
-keep the air flow away from the outside glazing
-air temp and air flow are equally important

If I remember laminar flow fluid dynamics correctly P1V1=p2v2 and A1V1=a2v2....this would imply for convection flow a pressure drop means an increase in velocity and so would a cross sectional area change in the input and output feeds. so for fans, a low pressure drop would be acceptable, but for convection only, a high pressure drop might mean a higher fluid velocity.  Please disagree with me and explain why if you know.

Here are somethings I believe might make a difference for a convection flow only sah:
-an empty box with two layers of glazing and evacuate the space between the two layers of glazing (this keeps the air flow away from the outside glazing)
-a metal backing, such as copper,  to keep a sudden drop of air flow happening when a cloud passed
- the geometry of opposite side entries for input and output
- the geometry of tall instead of long for convection flow only (mine is long and probably would output a higher cfm if it were tall instead)
-the height difference inside the connected room from the bottom of the intake to the top of the output
-the height difference inside the sah itself

I really wish buiditsolar had done pure convection flow tests....but then determining air flow accurately might have been proved impossible

why do I want convection flow only maximized??? because when my power goes out, my sah, on a cold sunny day will heat me where as one designed for a fan might not do so well.  My philosophy is, maximize design with no electricity then add electricity for luxury....if my power goes out, I'm still comfortable shelter wise.

one way to gauge air flow would be to tell how long it takes to heat the connected room in full sun on a cold winter day (same temp, same humidity)....
 
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