Would you like to build and install a very inexpensive solar powered furnace that costs $60 in materials?
How long would it take to recover that $60 by lowering your heating bills? Think about that, you can heat an entire room comfortably with one of these units that fits into a window frame. Would you build several and really add up the savings?
This particular solar collector design is actually portable. You can try on one window and later move it to another. This is not a permanent install so that is really a bonus.
Check out the details courtesy of Instructables.
Window Mounted Solar Hot Air Furnace