Help me build a thermosiphon solar heater for my garage

Inspired by a build thread of a solar collector garage heater and some dedicated googling, I think I'm going to build a thermosiphon solar heater for my garage.

Here's the idea:

https://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htm

Basically you build a greenhouse on the side of your building. A vent at the bottom allows cool air from your conditioned space to flow into the greenhouse, where it is heated by the sun. Then, the warm air rises. At the top, another vent allows the hot air to escape back into the conditioned space. A damper at the top prevents the system from working in reverse and pumping hot air out at night. 

I love it because it is 100% passive - once I install it it just works away with zero extra input needed (well, other than sun).

I took some photos of my southward facing walls at morning, mid day, and dusk to get an idea of where I can place the units.

It looks like I will have plenty of surface area to get a good size unit going. 

I'll probably copy the article for construction of the collectors, but I will have a slightly harder time since I have vinyl siding. I think the vinyl means I will not be able to fasten the collectors on top of the vinyl, but rather I would have to cut the vinyl out of the way entirely, and then finish the vinyl edges at the sides of the collectors.

Right now I'm planning to basically cover the entire wall that sees sun all day (the wall without the door). Even though it doesn't have the brightest sun in the morning, it sees the longest duration of sun, sits under a larger eave, has less stuff already in the way inside, and actually will be mostly shaded by vegetation in the summer when I don't want the heat. On that wall I can probably make a maximum 28 x 8 ft collector for 224 sqft. 

What are your thoughts?

I had once read about using empty tin cans painted black to draw and store the heat?

I've often thought about putting collectors on the south wall of my garage. I found a design that used black,metal bug screening inside sealed collectors to absorb the heat. The collectors were simple wood and glass/plexiglass affairs. The upper air outlet has a light fabric flap to prevent back  flow at night. I'll see if I can find the link again.

Edit:

https://www.builditsolar.com/Projects/SpaceHeating/ShopThermosyphon/ShopThermosyphon.htm

Ha!

I think I still have that issue of the Mother Earth News!

Very interesting idea.

Is there some kind of formula to figure out approximately how many square feet of collector will heat an area?

In reply to paranoid_android :

I'm sure there are formulae online. In the link that Robbie posted the builder 's tests showed that he could get as much as 3500 BTU/hr from each 16" space between studs. That would be nice to warm up my garage (600 sq.ft.) with about 15 spaces across the south wall.

I looked at doing that because I have a large flat south wall of my garage.  I sent a link to my old man, a real scientist, and he said "Don't waste your time."  Blah balh efficiency, blah blah no real gain, blah blah snake oil."  So I didnt.  

In reply to 93gsxturbo :

I deleted my "Negative Nancy" post.  But you had to go there!  Haha!!

Here's what I am noticing about the links and data...

-For some reason, all of the pictures of each setup seem to be in uninsulated walls.  I can't imagine the heat gain from the collectors exceeds the heat loss from an uninsulated structure.  It seems counterproductive.

- If the structure WAS insulated, the vents still need to be open both top and bottom.  The open square footage of the vents looks like it would be the equivalent of leaving 3 or 4 windows wide open.

- The claim is that it saves $300-400 per year in Bismark (In 2012 dollars).  That's not much.  

- I am glad to see a reasonable attempt at actual real life measurements of the heat output.  However, they are deceptive.  The issue is not how many BTUs the collector generates, it is how many BTUs the collector generates IN EXCESS of the BTUs that would be generated natural through normal conduction through the wall.  Every example shown is a large sun exposed wall.  These walls would have gain with NO collectors.  So how much MORE gain is there over standard construction?

So Robbie, if you build this, here's what I would love to see:

- Use an insulated wall

- Use insulated active vents (Yes, this would ruin the 100% passive nature of the collector)

- Do some measured comparisons of the actual BTUs generated, INCLUDING baselines without the collector (but WITH the insulation).  The goal being to see the DIFFERENCE, not the total BTUs coming off the collector.

It looks like a fun project, and reasonably inexpensive.  But I don't buy the data as presented.  It's a sales pitch, with junk science.

Having said that, thermosiphons are a real thing, and definitely work.  

Interesting article. Following this one with interest.  

In reply to SVreX :

Paul, in both structures there is insulation. Robbie's post has batts everywhere except where the collector is, and the one I posted has sprayed in foam. Both could have batts in the areas where the collector is mounted, don't know why they don't. The vents aren't open to the great outdoors either. They lead to a sealed "window". In the first post the builder says he would use the double wall glazing rather than that corrugated single stuff if he were to do it today. It would be a pretty large window area, but not any worse than a single glazed window. The back of both collectors is insulated with rigid foam sheet insulation next the building wall. I think a well sealed collector might be able to add quite a bit of heat to a work space. For a garage the biggest issue will be sealing up the doors to reduce the infiltration to a minimum.

As a point of reference I've been in a couple of homes here in Michigan that can be left without heat and won't drop below 57*. Granted, they were meticulously constructed with particular attention to preventing uncontrolled infiltration and oriented for passive solar gain through conventional windows. Not  a garage friendly style of construction. I too, would like to see Robbie go ahead and document the results. After all, who better than a bunch of GRMers to make something like this work.

I did it years ago, just a smaller scale. 

Build thread

Follow up post

in short, it works. No snake oil, just physics. I did it as a test-build so I also could build one on my south-facing garage wall. I ended up not building it for one reason. I discovered that the sun simply doesn't shine between December and march here in Michigan. When I got done with my panel it was 3 weeks before it produced heat, Michigan sucks for sunshine. 

But, it does indeed work!  That small panel I built was designed to be inefficient because my wife didn't want a solid door there. So there is a lot of heat energy passed through the door and not heating the air. 

Another thing, the best design is the one I built, it has three passes of black aluminum screen inside. There have been tests done and data collected. That's the best balance between air flow, heat exchange, cost, and simplicity. 

Mine is still up and working. Before you build one, do some digging online, you can figure out how much sun you can expect annually, and even look at your property to see the sun tracking across hour building. 

Will the trees when they have leafs block too much of the sunlight ?

Dr boost - thanks! I will review your threads!

I had read that 2 screens is better than 3 screens, but I'm sure that is probably a close match (airflow vs heat rise).

The thing is, I don't really care about BTUs. I could estimate my building btu loss, and then try to calculate a btu gain, but all I really care about is temperature inside the building (I do not mean temperature leaving the collector). 

I would be interested to do some tests and make some before and after temp graphs. Maybe show outside temp, and inside temp at a few different distances from the south wall. Problem is it is hard to get two days with similar temps and sunshine. But we can try. Anyone have a device that is super cheap and logs temperature every 5-10 minutes? I could try every 30 minutes manually maybe...

californiamilleghia said:

Will the trees when they have leafs block too much of the sunlight ?

I don't want heat in the summer ;)

We are pretty close on latitude, Robby, so I am interested to see if you can prove the naysayers wrong.  I have what amounts to a 10ft tall x 20ft wide space to build something similar to this, no trees or shade, genuine southern exposure, and my neighbor has a pool/concrete in front of it so I will get excellent reflectivity as well.  Basically a Best Case Ontario.  And my pops still told me I am nuts.     

The theory behind it is sound, and Dr Boost, you had it working, but while warm air does come out, its basically throwing pebbles into the ocean as far as actually heating a building as large as a 2 car garage with as much thermal mass as 2 cold cars, two big ol Matco roll-aways, etc etc etc.  Just the amount of energy required is obscene.  Not to mention here in the northern climes you will only get about 7 hrs of usable heating on the best days.  That leaves 17hrs for the temp to drop.  

Run some maths here.  A 3500lb car, to take from 20F to 60F, will take about 6000 watts.  Want to do that during your 7 hours of heating that you get?  That will take 3500 BTUs per hour.  Assuming you have the car in a vacuum, it might be doable.  Lose any heat thru the roof, doors, etc, and you are needing more energy.  That second car that is sitting in the corner of your cold garage, some tools, heck, even the 20 gallons of gas and chemicals will all need energy added to them to warm up.  Just doing some quick maths with a Viper, a Land Cruiser, and 2500lbs of misc in the garage, it will take 3KW per hour or 10,000 BTUs per hour 7 hours to warm up.  That is, again, in a vacuum.  

Not to mention air is a crappy conductor of heat.  Hence why insulation works so well.  

So yeah, you can build a gee-whiz unit and heat a small box with not much in it a few degrees.  To heat a huge garage with a bunch of cold stuff, it isn't happening.  

Now geothermal heat pumps - those are cool.  The ones that collect ground heat during winter and ground cooling during summer.  But on a suburban lot with no body of water, no real chance of making that happen.  

In reply to 93gsxturbo :

Funny thing is, there's lots of folks doing the impossible. 

I still walk by it and feel the heat coming out of it and am amazed something so simple works.   

If I were building one today, I'd be scouring goodwill and Craigslist for people getting rid of doorwalls cheap or free. Double pane, insulated frames, they would make a GREAT outside panel. 

Came by to make a joke about declination or the sun's angle of its dangle.  57 degrees inside? Nope. And I don't even use the heat that much if at all possible. 

In God we trust, all others bring data.

DrBoost said:

I discovered that the sun simply doesn't shine between December and march here in Michigan.

Honestly this is the main reason I haven’t attempted a garage solar project.

Interesting point re: sunshine days.

According to a quick Google, this site: https://www.currentresults.com/Weather/Illinois/average-sunshine-april.php

Shows January through April in Chicago averages about 50% of the time between sunrise and sunset the sun reaches the ground. December is a little less and November is a lot less (38%).

The good news is my building is well insulated - 2x6 walls and roof with fiberglass batting, good-sealing and insulated doors. The car doors are not roll up style, they swing out like French doors and are similarly built about 6 inches thick with insulation. I did that because it's more shop and less parking. I don't move cars in and out frequently.

DrBoost said:

In reply to 93gsxturbo :

Funny thing is, there's lots of folks doing the impossible. 

I still walk by it and feel the heat coming out of it and am amazed something so simple works.   

If I were building one today, I'd be scouring goodwill and Craigslist for people getting rid of doorwalls cheap or free. Double pane, insulated frames, they would make a GREAT outside panel. 

Hmmm, like this:

https://chicago.craigslist.org/nwi/mat/d/2-36x-80-anderson-window-wall/6742314541.html

In reply to 93gsxturbo :

In data we manipulate, some rely on personal experience. 

Robbie said:

DrBoost said:

In reply to 93gsxturbo :

Funny thing is, there's lots of folks doing the impossible. 

I still walk by it and feel the heat coming out of it and am amazed something so simple works.   

If I were building one today, I'd be scouring goodwill and Craigslist for people getting rid of doorwalls cheap or free. Double pane, insulated frames, they would make a GREAT outside panel. 

Hmmm, like this:

https://chicago.craigslist.org/nwi/mat/d/2-36x-80-anderson-window-wall/6742314541.html

Bingo!!

Or this:

https://swmi.craigslist.org/mat/d/large-tempered-glass-panels/6710586017.html