Air-based space heating systems work very well in areas with high load requirements and, although they cannot be used to heat a domestic water supply, can generate as much heat as a liquid-based system. They are also very cost-efficient because the equipment is simpler and cheaper. Air-based systems come in two main types.
Air recirculation
The more common of the two options uses the air already inside the building. A wall-mounted collector takes cool air from the building, warms it and returns it. Some simple ducting, a controller and a blower are all that is needed to complete this setup, which commonly can provide for as much as 25% of a building's heating needs.
This setup is very popular in large workspaces such as warehouses, shops and other workplaces. It is less important to maintain an absolutely stable temperature in these locations and the simple air recirculation setup can maintain a reasonable baseline temperature, even with variations caused by constant external door opening. The large mass of air inside the building is heated during the day and slowly releases that heat at night.
Transpired collectors
The less common setup uses unglazed transpired collectors. External, cool air is drawn through the collectors and is heated as it goes. The air is then delivered into the building. This setup is extremely simple and is often used on very large buildings which need a lot of make-up air during the day.
The collectors for this setup are installed vertically on the walls, where they capture a lot of low winter sun but no summer sun. The heated air travels through ductwork in the same way as the recirculation option.
PV alternatives
It is possible to build an air-based solar thermal space heating system with a blower run by a PV panel. The advantages of this are that the system only operates when there is sunlight available to heat the air, so it is self-regulating, and that the blower doesn't use any grid-tied power.
On the down-side, DC blowers are very hard to find and the setup only works on small, wall-mounted installations. In other words, it's a very limited option that usually isn't sufficient for most buildings, though it could be used in single-room applications.