The vast majority of solar thermal installations are based around solar collectors which use a fluid for thermal transfer. However, there are some designs which use air as the transfer medium.

Air collectors are just like their liquid counterpart flat plate collectors: they come in a frame that's insulated on three sides and glazed on the fourth, they have a flat absorber plate, they're of similar size to liquid-based collectors and they work in a similar fashion.

The absorber plate is usually a single sheet of dimpled aluminum that is painted with a selective coating (or black paint) to increase efficiency, but there are no fins connecting the plate to risers. Instead, air is blown across the back of the absorber plate by a blower: the air heats and passes on through the building via a duct system.

Air collectors can produce as many Btus per square foot (or meter) as any other collector, even though they're much more basic. They tend to be cheaper as they use aluminum instead of copper and are a lot simpler in design.

As you might expect, the return on investment for air collectors can be very attractive if they fit your needs.

Transpired collectors

Another variation on the air-based theme, transpired collectors don't have a glazed side to their frame. Instead, the fourth side is made up of the absorber plate itself. Fresh air is drawn through tiny holes punched in the plate and is heated as it passes through.

Transpired collectors are only used to produce heated "makeup" air in buildings that change their air frequently throughout the day. Because they're simpler than even the basic air collector, they're comparatively very cheap and make for a sound investment where they can be used.