This is a solar air convection heater for my garage that is powered by the low angle winter sun. It's easy to build and it works very well. With enough building insulation, this can be a primary heat source with a secondary needed for cloudy days. The idea and design is from Mother Earth News December/January 2007
glass, plexiglass, or some kind of clear material.
black aluminum window screen
caulking, paint, screws, lag screws, staple gun + other tools
I have some results on temperature differences from the first weeks of operation. Just like when you put up your first wind generator, the wind won't blow for days/weeks, I had overcast and mostly cloudy weather the first few days operating this thing. The heater puts out 87 to 104 degrees F at the vents on partly cloudy days to sunny days. The uninsulated garage hits about 17 to 30 degrees F above outside temps, very comfortable for me. It seems the colder it is outside, the bigger the temperature difference between the garage and outside. If you want more heat in an uninsulated area I would recommend possibly 1/3 more collector area than the ratio I give in the instructions. Seal up any drafts, it will do a lot to hold the heat. This thing works great.
Step 1: Frame and Fit
Make a box to mount on the wall of building (A south wall, SE and SW also can work.) This should be mounted vertically on a wall; the higher in the sky summer sun won't hit it as directly as the low winter sun. The walls of the box are 2x6 lumber; the angled "roof" of the box is a 2x8. I found it good to prebuild the box on the ground and then mount it to the wall. Make the box based on stud spacing in the walls; I have 24" studs, so my box is 8' wide with vents cut into four sections.
I sized the solar collector based on the numbers from the Mother Earth article where the guy built 160sq ft. of collector for his 700 sq ft building; using that ratio I built a 48 sq ft collector for my 200 sq ft garage.
Step 2: Paint and Add Screen Mounting Boards
Paint the wood and then mount the boards that the aluminum window screen will mount to. Insert a 2x4 horizontally near the bottom leaving room for vent holes, insert 2x2 horizontally at the top; make it even with the 2x4 (look at the drawing to understand this better.)
I notched the middle board for a 2x2 that will later support the glazing.
Step 3: Mount to Wall and Seal
Use a level, floor jack and support wood to set it straight. Have a neighbor hold it while predrilling holes from the inside of the building for the lag screws. I used six 4" lag screws.
Seal the box from the inside with window and door foam. Also seal the bottom 2x4 that holds the screen.
Step 4: Cut Vents Into Wall
Cut the holes in the wall like in the diagram, a vent on the bottom and on top for each stud section. Watch out for electrical wiring, etc.
My vents are 4" x 16"
The cuts are not pretty, use foil tape to trim the inside edge of the vents
This was the toughest part because I didn't have a good saw.
Step 5: Install Metal Window Screen
Cut and staple black metal window screen (two layers) to the wood inserts in the box.
Note, this needs to be metal window screen for good heat exchanging; fiberglass screen won't work well.
I used 48" wide screen and had to trim the edge some with a utility knife.
Step 6: Install Glazing and Vent Valves
On the top vents in the building you'll need to put a flapper valve made from plastic sheeting or a trash bag, this keeps the warm air from leaving at night as the heater would work in reverse. Also put some screen on the vents to keep most of the critters out and to keep the plastic valves from sucking into the vent.
Using recycled wood and some on-hand hardware, I spent around $100 for this; now I'm enjoying free heat.
Ducting with a fan and a thermostat switch is what they use on commercial versions installed on homes. That works well for precise heat control. I like convection, no moving parts; and I will just close the vents when the weather gets warmer with some cardboard stapled to the wall.