Similar idea to the "Coke Can" solar heaters. If you've got a south facing wall or roof then a system of ducts could be laid out. It'll be heated by the sun and set up a convection current. Stick a pipe on the bottom leading to your roof space and it will draw hot air out - vent it to atmosphere. In winter if you can bring the vent back into the building you get a degree of free heating. Plus if it's on the roof then it's effectively a layer of insulation that will keep some of the sun off the roof in the first place.
Iain
Iain