What's with the cans? Solar furnace...

I see people building solar boxes and filling up the box w/ aluminum cans, dryer vent, etc. I don't get it. How is this more efficient than using a plain sheet of the foam board w/ the metal coating (or any kind of non-tubular flat surface)? Why would it be any better to run the air through a black tube (heating the air indirectly) than it would to heat the air directly under a glass panel? And it can't be a thermal mass for storage -- you would want any storage inside the home, not inside the furnace, right? I suppose corrugated metal increases your surface area, but even then, there is only so much energy shining into the window at any one time.

AND ANOTHER THING!
I've seen other people who build a furnace/can box, and then just tape it to an existing window. In the words of "Oh Brother..." "That don't make no sense. X amount of suns rays/energy/heat are entering that window whether there is a box or not. The advantage of a solar furnace is just that it, in essence, increases your home's window area -- more passive solar (albeit, without the heat loss of a regular window since it doesn't cut a massive hole in your house like a window does).

I building a solar furnace for my chicken coop. Warm birds are laying birds.

Light doesn't convert to heat unless it hits something. If the only thing it hits is the shiny insulation, most of the heat will reflect back out. Painting the insulation black will probably just make it degrade from the heat. You need some kind of absorber that can transfer the heat to the air, thus the cans. A simple piece of corrugated roofing painted black will work.

Adding a box over an existing window probably won't make much of a difference, unless you are improving the overall insulation of the window. Adding an extra piece of glass over any window will help, but, like you said, it's probably better to increase the area heated outside the window.

Let us know how the chickens like it!

Chris