Ive seen these things done before at the local solar fair, and i i figured it cant be too hard to make a small one
Step 1: Materials,
-One 80mm 12V computer fan and grill and mounting screws (these can be taken from computer cases or power supplies)
-Two, 9 volt batteries (I hope to replace these with a 12V solar panel soon)
-Two 9volt battery clips (probably moot if replaced with solar, but I may keep them for use in partial sun)
-Some cardboard, or black foam board
-Some black Duct Tape
-One length of dryer hose (probably best if its less than 10 feet mine is about feet long
I decided to make mine length wise, so i decided on what would be a top and a bottom, (both ends have handle loops so either end is just fine. just pick one.
I then took my foam board (black in this case, but cardboard painted black would work as well, probably better) I 2 rectangles about 6"x17.5" and duct taped them inside the top and bottom of the bag. I plan to get more foam board and make some sides and internal baffles but so far it works ok as it is.
I then punctured the corner holes and using the same screws and my trusty screwdriver that held the fan into a power supply mounted up the fan blowing upwards through the bottom of the bag.
PC computer fans all most always blow towards the label but there is usually a little direction of rotation and direction of airflow arrow on the outer plastic of the fan somewhere.
At this point i tested it, by hooking up the batteries and battery clips in series. (red fan wire goes to one battery clip red wire, the black wire of that clip connects to the red wire of the 2nd battery clip, and the remaining black wire goes back to the fan via the fans's black wire the fan was plenty to plump up the bag like one of those wacky inflatable Christmas lawn decorations.
This is where the sun heated air is going to come out as a slightly flowing warm breeze
Zipped up the bag, and then added a fan grill, (probably not needed, the fan blades are kinda sharp and can whack you, but I mostly put the fan grill there to protect the fan, and to keep critters out.
I tested it the duct, the bag no longer plumps up totally full, because it can relieve some air pressure down the duct, but it does keep from collapsing.
Step 5: Conclusison and Improvements.
In addition to baffles, Im thinking ov building a more rigid sides, and perhaps line the bottom of the "bag" with matte black card board, to keep it from collapsing/flopping. and perehaps to keep it from loosing any heat out the back side.
I want to experiment with making this solar, so the fan only blows when the panel is in the sun. jsut replace the 2 9v batteries with a 12v or 3 5v solar panels.
I'm also interested in building a reversed thermostat, so it will turn off the fan if the temp in the box is below a threshold. (will outline this one as I figure it out)
I plan to test this one to bring a little winter warm into my workshop, but i may build more and bigger ones for my house and other places.