Solar thermal door build

I finished this project this fall with the help of my wife and father.  Our barn has two 8' wide door openings on the South side that I wanted to put doors on.  I also wanted to try a passive solar thermal collector in the hopes of putting one on the upper floor of the same barn (storage in basement, wood shop on first floor).  So why not make doors that double as solar collectors?

For those who don't know, a solar thermal collector is basically a chamber you build on the side of your building with glazing on it and air ports back into the building.  Air enters through the lower port and is heated by the sun.  It rises and reenters the building through the upper port.  Here's a more nicely described example of a Thermosyphon Solar Thermal Collector.

First I beefed up the center post.  Post is an overstatement, I'm surprised it held up the barn as is.  I wrapped it with 2x12's and plywood and filled the deliberate space between the post and the wood with a loose cement mix.  Then I could feel ok with attaching hinges to the post.

I built doors from 2x4s and the thinnest/cheapest plywood I could get.  We cut out air passages at the top and bottom, leaving a bit of plywood in the middle to give the door more resistance to racking (ie 2 upper ports instead of one wide one).  Then we painted them dark brown (cheap Restore paint).  I got some metal window screen and stapled it into each door chamber so that the sun can heat the screen and the air passing through the screen would absorb that heat.  The screen isn't vertical, it starts against the glazing at the bottom and is attached to the plywood at the top.  That forces all the air to pass through it.  Sorry, no pictures for this part but the link above explains it nicely.

Then I put twin wall poly sheeting on the surface of the door.  8 hinges later they were installed.  On the upper port I put more window screen on the interior of the door and a piece of thin garbage bag material to act as a check valve.  It lets air rise in the chamber and enter the room when warm but at night when the air is cooling and trying to sink, the flaps prevent air movement.  You can see the flaps open as heat pours through in the interior pictures.

Sorry I didn't take more photos, hopefully the Build It Solar link above helps make it all make more sense.

I wish the twinwall poly panels were clearer than they ended up.  I couldn't tell how frosty they were in the store since they have protective film on them.  

I was pleasantly surprised how the dark bowels of the barn seem to be lit up better with the doors closed than with them open.  Something about the diffuse light through the relatively small air passageways sends light to the back of the barn nicely.

I use these doors about once a month since there's a man door on the West side of the barn.  If these were the only way to get into the barn I'd've build them much stronger.

Excellent, excellent, excellent! I'm in the design stage of our new shop on the new land and have this planned for the entire South wall. The shop is positioned with the length South- North just for that reason. How is the performance so far? How much was the build cost?

Thanks!  The performance is hard to judge.  The flaps are open as soon as the door has 25% sunlight on it.  The heat is clearly pouring into the room.  But I have gaps at the bottom that allow for easy door operation but let air in.  I plan to block those gaps off or put a threshold in.

Since there weren't any doors last winter, I don't have anything to compare to.  If I'm ambitious, I could block the air flow in the doors in January for a week and see how cold it gets in there.  Then unblock them for another week and see.  I'll put that on my to-do list.

For better analysis of how these work, that Build It Solar site has extensive testing data on these types of collectors.  I'd trust their info long before I'd use any data I can provide

The build cost (not counting post reinforcement or trim) was:
Twin wall sheets       $45 x4ea
2x4s                       $3 x 12ea
3/8" plywood           $13 x 4ea
Twin wall end cap    $20?
Hinges (restore)      $20
Paint (restore)         $5
Window screen        $40?
Total was about       $353


ps. I'm looking up prices online at Menards and now I see that they have both diffuse and clear twinwall polycarbonate.  So that's why mine are frosty.  The clear would have been cheaper too

I like the builditsolar site a lot. Many of my project ideas come from there.

As far as measuring performance, my goals are simple. If it's 10 degrees outside and gets to 35 inside, that's as precise as I need to be 😊.  I track the temps in my chicken greenhouse day to day, and I can easily get a 40 degree gain in there. Your post doesn't say how big the barn is or if it is insulated at all so it will be interesting to see what temp gains you get on a sunny day.

Yes, if we ever get a sunny day it will be nice to see how warm it is in there.  A guy I know with solar panels said we had 5 sunny days in November  I do have some buckets of apples in there and it's stayed above freezing in the back of the basement of the barn.  The basement is just uninsulated, partially buried cinderblock.  The first floor is mostly insulated but I'm not sure how much heat will rise from the basement to the first floor (no stairs or airflow passageways).  If I'm happy with these doors, I'll build another set of panels for the first floor.  The barn is about 20 by 28...

Build It Solar rocks!

You may want to test the temp of the air coming out on a nice sunny day. The temp should be 120-125 or so. If it's warmer then that your vents may need to be a little bigger. 120 or so seems to be the sweet spot where you have good temp gains and still enough air flow. The first prototype I made had temps around 145. I opened the vents til the temps dropped a bit and had better output.  I want to try one with a solar powered exhaust fan but just as an experiment. On the shop I want them to build purely passive so I don't have failure points.

Ok, when it's sunny (January?) I'll do some measuring.  I know the 3.5" chamber is a bit undersized for the 6' height of the panel.  So it's not optimal already but anything is better than a wide open hole in the barn...

Mike Jay wrote:Ok, when it's sunny (January?) I'll do some measuring.  I know the 3.5" chamber is a bit undersized for the 6' height of the panel.  So it's not optimal already but anything is better than a wide open hole in the barn...

I hope my post didn't sound patronizing. You probably knew the stuff about the temps already. I'm betting you will be pleasantly surprised at how well your panels work.  I know I was with my experimental panel. Reading about it is one thing. Standing there with the warm air from your panel blowing on your face is altogether different 😊

Don't worry, it didn't sound patronizing to me.  I remember seeing temperature test results on BIS but didn't remember there being an ideal temperature range.  So I'm glad you mentioned it.  Maybe my glibness about the lack of sun this winter is to blame...

If you want another heating experiment for your barn, I did this one in my girlfriend's living room 😊  I cut a hole in the floor on each side of the room and put vents in. Under one vent in the basement I made a duct that runs almost to the floor. The other I left alone. The pellet stove is in the basement. The vent that has the duct allows the colder living room air drop to the basement floor and the heat goes up thru the vent that doesn't have a duct. It worked pretty well and raised the living room temp a few degrees. Something like that may work in your barn. Not nearly as well as solar panels on the main floor but a cheap easy thing to try in the meantime.

Good point, not sure if this is the right name but I call those "cold chimneys".  If the basement of the barn gets really warm I was thinking I could do that.  

I haven't put one in yet but I'm scheming on one to get more heat into my office in the house.  Heat from the basement wood stove pushes up the staircase, I just need to allow it to drop from my office back into the basement.  

Nice projects!
Ive been thinking of a solar air heater going from my first floor window to my second floor  on the south facing wall of my home.
The drawback I see is having a heat collector strapped to my brick house in the middle of summer.

As long as you live far enough North (or South in the Southern Hemisphere) the summer shouldn't be that much of a problem.  Winter sun is low in the sky which pummels the solar collector with energy.  Summer sun is high so theoretically most of the energy that does hit the collector glances off.  If excessive heat does become a problem, even in the winter, you should be able to block off the air ports to keep air from circulating.

Connecting two windows sounds like a neat thing to try.

Mike Jay wrote:As long as you live far enough North (or South in the Southern Hemisphere) the summer shouldn't be that much of a problem.  Winter sun is low in the sky which pummels the solar collector with energy.  Summer sun is high so theoretically most of the energy that does hit the collector glances off.  If excessive heat does become a problem, even in the winter, you should be able to block off the air ports to keep air from circulating.

Connecting two windows sounds like a neat thing to try.

Nice collector build and installation.

William,
A shutoff is a great control to have on these heaters. They can convert light, even ambient, into massive amounts of heat. Vertical collectors with a shutoff or a shutoff and bypass should not cause off season issues, especially with insulation (isolation) and maybe awnings or overhangs.

Most diy and manufactured air heaters will eventually wear out from stagnation, especially depending on materials. 20 plus years though!

Insulation breakdown is the most common failure that i have seen while servicing these systems, followed by plastic glazing degradation, this can be avoided with proper arrangement.