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
Drill motor
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.

Step 2: Cut and Attach Absorber Screen

Now, lay your roll of black window screen over the frame, you may need a helper or a scrap of wood to hold down one end of the screen to prevent it from rolling back up as you're trying to work with it. You are going to cover the entire side of the frame with two layers of screen. The first layer only needs a few staples to hold it in place. If your screen is larger than your frame, leave enough room between your staple and where you need to cut to allow you to get your scissors and hand in there to cut, then tack it down after cutting.

I found it easier to attach the screen and then cut slightly in from the edge so the screen is slightly smaller, about 1/4", than the frame in all directions. You don't want any frayed edges to stick out from your frame and catch or scratch your window frame.

Step 3: Attach Screen to Upper Vent.

Since we're working with the window screen, let's flip our frame over and attach one layer of screen to what will be the upper outlet vent. This screen is necessary to prevent the flap valve from being sucked into the frame at night. This screen provides a seal for the flap valve so that at night when it's cooling off, your warm air doesn't get drawn into the solar collector and dumped out the bottom vent as cold air.

Just as warming air in the solar panel creates a convection current where warm rises out the top and draws cooler air in the bottom, the reverse is also true. Without some means of preventing reverse air flow through your panel, your warm air would be drawn into the top vent, cooled down, and dumped into your living space, thereby negating any benefits from solar gain collected during the day.

Step 4: Attach Film to Frame.

Lay your plastic film/mylar over what will be the side of the panel facing the interior (opposite side from the screen side) of your room and stretch and staple the film to the frame. Be careful not to stretch so hard that you rip the film through the staples. It just needs to be taut enough to minimize any wrinkles in the film. Like the screen, we just tack the film to hold it in place. We will finish with thin molding to complete the project. Unlike the screen, excess film can overhang the frame and be trimmed off with a sharp knife after completing the frame.

Step 5: Cut and Attach Molding.

If you are using purchased molding, now is the time to measure and cut it. We are only going to frame the inner opening with this molding, 52" X 29" in my case. So, three pieces 29" long and two pieces 50-1/2". The third 29" piece will create the hinge for the upper flap valve, set this piece aside for now.

Starting at the bottom of the upper vent, place one short piece of molding over the film so it lines up with the bottom frame of the vent and attach with screws, nails or staples. Then we do the same with the bottom, aligning it with the top of the bottom vent. Next, we measure and trim the longer side pieces so that they just fit between the two end pieces, and fasten.

We're almost done.

Step 6: Attaching Upper Flap Valve.

Now, we need a piece of light weight plastic as wide as the frame and 1/2 - 1" higher than the vent opening for the valve. We will trim any excess valve material upon completion. I used mylar here again because I had it and it is extremely light. Plastic trash can liner will also work. You need to have one good straight edge to but up against the trim molding edge. Carefully align the edge of the plastic with the edge of the molding at the bottom of the upper vent. When you are satisfied that there is not too much resistance to interfere with the flap opening and closing freely, staple the valve along the top edge only. Test the valve action again to make sure nothing got out of alignment. Satisfied? Good, now we can complete the project by adding the third piece of trim molding that we set aside in the last step to the top of the outlet vent. As with the last step, I drilled and screwed this piece to the frame.

If you picture a pet door in your door that swings in and out as your pet comes and goes, you can get the idea of what we're doing here. The flap needs to swing to and fro easily, just like the pet door.