A thermo-siphon solar heater installed on the southern wall of a building can be used to provide heat on most days. This is recommended for workshops or other out buildings that are used mostly in the day time. On this project, we used old skylights that had been removed from a building because they leaked. (There are lots of rejected leaky skylights available on craigslist.org!) We put aluminum studs in between the glass and wall board to act as absorber plates and painted everything black. With cement board over this we added thermal mass and lateral bracing. Cold air from the interior moves into the panel from a low vent, heats up inside the panel and exits the panel into the room as warm air. On a 40 degree day, the air coming out of the heater is around 90 degrees. Free heat.
In the summertime the southern side of this building is in complete shade from a cotton wood tree.