I have a little metal clad garage 6Mx3.6M with a north facing wall (I'm in the Southern Hemisphere) that I'm looking at building a thermosiphon air panel on to help warm it during the day. I read this article www.motherearthnews.com/Renewable-E...e-Sun.aspx on integrating the wall as the thermosiphon air panel for metal clad buildings.

I'm not that keen on putting all the vent holes in the metal wall and was wondering if it would work just as well having the thermosiphoned air running up the inside of the metal wall and the frame with glazing mounted on the outside to heat the metal wall. The garage is currently unlined so part of this work will be insulating and lining the garage interior. Which makes me think of making the chamber for the thermosiphoned air on the inside by using the exsisting internal wall cavity and adding insulation on to this.

Am I correct in thinking that it is the heat transfer from the metal wall to the air that that creates the major heating effect rather than the direct heating of the air in the camber from the sun? If this is the case then it shouldn't mater if the air is on the inside of the wall or outside of the wall. I'm guessing there may be some slight loss in efficiency as there is some direct heating from the sun however the most will be absorption and radiating from the metal wall to heat the air. I've searched and haven't been able to find anything that that goes against the idea, then I haven't found anything that supports it either.

To me if it was a truly integrated thermosiphon air panel it would still have vents top and bottom of the metal clading and the chamber would be on both sides so the air is heated from both surfaces of the clading. This would give the best efficiency of all as the thermosiphoned air would be exposed to a larger suface area of the absorber (metal wall clading).

Thanks for any input on this you have.

You are correct in your assumptions- a "backpass" thermosiphoning thermal air panel with no penetrations of the metal will work. It also has the advantage that the interior side of the glazing stays cleaner much longer, since it has no air flow dragging in dust or oily bits. I commissioned a building integrated backpass thermal air panel just 2 months ago- a bit late in the heating season here. I'll be measuring it's performance in November/December.

A few of notes about thermosiphing panels:

Taller is better (more air flow due to higher stack effect).

Bigger vent cross sectional area is better (again, more air flow).

Applying vanes/ridges etc in the air channel to induce turbulence across the metal breaks up the laminar flow, improving the heat transfer efficiency from the metal to the air. (60C metal in contact with a 55C laminar flow layer doesn't transfer much heat to the air as compared to 50C metal in contact with turbulent 20-40C air.)

Higher efficiency is achieved with high flow low delta-T operation as opposed to low-flow high delta-T. When it's 0C outside, if your exit air is over 45C your losses out the glazing side are substantially higher than if the exit air temp is under 35C. Since the glazing has very low insulation value, keeping the metal as cool as possible is important for efficiency. Many tinkerers seem to prefer to warmer exit air temps to reassure themselves that it's working- stick a hand in the flow and it's noticeably warm, but that's a hallmark of a lossy less-efficient collector. With active thermal air-collectors that can blow directly onto humans it's usually nicer to run it 40C+, but with the low exit air velocity of thermosiphoning versions there's no need to run 'em that warm. (There is scant wind-chill effect.)

Somewhat related question: how well could a dark panel hung inside my south-facing window approximate a dedicated thermal solar panel?

I have about 75 sq. ft in three big south-facing windows that can get full sunlight (no trees etc. in the way), plus some west-facing windows. The crude calc I've done shows that I might be able to get more than 1 therm's worth of solar heating on a given, sunny winter day from these (75sqft x 300btu per hr per sqft x 5hr per day = 112500 btu) , which is almost 1/3 of my peak wintertime daily usage.

There's a narrow space between the glass and the venetian blinds already up in these windows. I'm having trouble thinking of exactly how to take advantage of this: I could put a black plastic back up there, but it would be pretty ugly and probably spew plastic fumes. Cloth might touch the glass in places, disrupting air flow. Dark carboard would probably bleach pretty fast.

Anyone have any better ideas, or even know of any products out there for this purpose?

Posted By dcmeserve on 16 Jul 2010 03:00 PM
Somewhat related question: how well could a dark panel hung inside my south-facing window approximate a dedicated thermal solar panel?

I have about 75 sq. ft in three big south-facing windows that can get full sunlight (no trees etc. in the way), plus some west-facing windows. The crude calc I've done shows that I might be able to get more than 1 therm's worth of solar heating on a given, sunny winter day from these (75sqft x 300btu per hr per sqft x 5hr per day = 112500 btu) , which is almost 1/3 of my peak wintertime daily usage.

There's a narrow space between the glass and the venetian blinds already up in these windows. I'm having trouble thinking of exactly how to take advantage of this: I could put a black plastic back up there, but it would be pretty ugly and probably spew plastic fumes. Cloth might touch the glass in places, disrupting air flow. Dark carboard would probably bleach pretty fast.

Anyone have any better ideas, or even know of any products out there for this purpose?

It would be a net-zero increase in solar gain & efficiency, since there is no increase in collection air as compared to your windows without the solar absorber. The only difference is that the solar absorber takes the heat instead of your furniture, floors walls, dog, cat etc. 

A thermal air panel adds glazed collector area, but if properly designed has lower nighttime losses that windows.

My window blinds are white, to minimize heat gain in the summer, and they stay mostly closed in the winter so that the room interior doesn't get sun-bleached.

But I can see what you're saying, in that they're giving me some of that possible 112500 btu's of heating already, so adding a dark color will just get me a bigger chunk of it. Though I guess if I arranged it to give a proper thermosiphon effect it might do more (right now the blinds mostly trap the air between them and the window, adding some insulation).

Thanks for the response!