Sidney Explains’ Youtube video on cheap solar heater piqued my interest on this DIY, solar-powered zero electricity project to heat up off the grid buildings.
Essentially it’s a box with aluminium cans like soda or bean cans, painted in black and attached to each other. There’s a hole in the bottom to intake cold air that flows through the cans that get heated by the sun and then exits from the top into the building.
Solar space heating — active and passive solar projects for space heating has more details and different types of projects like this.