Free heat with minimal investment. Having a serious interest in renewable energy, I recently got the urge to try and create a solar insert that would simply slide into my existing window casings to help with the heating of my 1996 mobile. I saw on the local news one night how a local man built a solar thermo siphon (TAP) air panel to do just that using black aluminum window screen and plywood. OK, that's fine if you don't mind living in a cave as it totally blocked the view and incoming light through the window. The concept is fine and it works. He made a comment about how much he saved on heating, but I don't remember what he said. Having seen many designs on the internet of solar space heaters using black window screen, I decided to test the idea last winter as I had a difficult time believing window screen could possibly generate much heat. So, I grabbed a couple of screens I just happen to have and attached them to the front door with magnets salvaged from a micro wave. Amazing!! I think I was getting over 120 deg. I didn't write it down so I am relying on my (poor) memory.
Anyway, I wasn't too keen on losing the view or light so I set my mind to wandering and came up with the idea of using poly film instead of plywood. The first prototype was made using poly film and the initial test gave me 90 - 94 deg. with the sunlight hitting about 60% of the window screen. Not bad. OK. Now, how can I improve this? If I can reflect the sunlight onto the backside of the screen, it should generate more heat. Aluminum foil? You would need a substrate to attach it to and it would also eliminate the view and light. Then it hit me! Aluminized Mylar (emergency blanket, rescue blanket etc.)!! I recalled that as I was playing with other solar projects that these blankets reflect light and heat yet are also semi transparent, so I tried it. Super! It is like having polarized windows and it boosted the output temperature about 4 deg. This winter will tell the tale.
Now, on with the instructions...
What you need:
3/4" square molding for basic framework
Black window screen - the window screen I see around here isn't really black but Charcoal. Still dark enough to serve as solar absorber.
1/4" X 3/4" molding for attaching film to frame, screen bead molding?
Saw - table saw or miter box
Screwdriver or power driver
wood glue - optional
a sharp utility knife to trim excess film
heavy duty scissors or tin snips for cutting window screen - you might also consider gloves when working with aluminum screen
#6 X 1-1/2" wood screws - I used 8 per frame
#6 X 3/4" wood screws for attaching trim molding
countersinking drill bit for the above screws
Staple gun and staples - 1/4" - 3/8"
POly drop cloth, emergency blanket or other transparent sheet to enclose air chamber and for anti siphon flap valve. Another option that may improve efficiency would be a thin polycarbonate sheet screwed to the frame.
Step 1: Build the frame
Measure your window opening and cut 3/4" square molding accordingly. My windows are 29" X 59" so, two pieces 28-15/16" for top and bottom frame, 2 pieces 57-7/16" for side frames and two pieces 27-716" for inlet/outlet frames. I cut the pieces for the main frame 1/16" short for a slip fit into the window opening. A tight seal isn't really necessary unless you have serious air infiltration around your windows. In that event, I would either properly seal the leaks or make another frame that you can seal tightly against the existing window and cover it with plexiglass/polycarbonate.
assemble the top and bottom rails to the ends of the upright rails using one #6 X 1-1/2" wood screw at each joint. Here is where we might add wood glue for a more durable joint. Make sure your frame is relatively square and fits well into your opening. Not too tight and not too loose. A competent woodworker might dado the frames for a more professional look.
Now, add the inner rails that will create your inlet/outlet openings. Considering my window size and emergency blanket size (52" X 84") if I make the inner opening 50-1/2" high, I can use the 52" width of the blanket with minimal waste. That makes my vent openings 2-3/4" high by the width of the frame. There is a formula for determining the vent size but me and formulas don't get along too well. Some percentage of total collector area.
Now you might paint or stain your frame if desired.