This is a pretty cool idea that uses old soda cans, 2x4s, and black paint.
I think adding a couple of solar panels and some used DC fans to move the air through the cans would be a cool idea.
Maybe use it the winter to heat air for an area and then in the summer place it on the roof to remove heat from that area to try and make the house a little cooler.
Wow, someone had commericalized this:
Someone in Hungary has built a large one of these:
Another post left this website for determing what angle to place the panel:
A beer can solar heater:
Another solar heater design:
Garage solar heater