Solar Air Heating isn't sexy, but it can be effective if controlled properly. 'Solar Air Heating' refers to active systems which heat air directly, with collectors, fans, and ducts---not passive solar heating. My book The Solar House documents and discusses the history of this technology, beginning in the 1940s. George Löf was the most important figure in this technology, and his work is featured extensively in the book.
An advantage of Solar Air Heating is that the heat is collected outside the building, so it can be rejected when it isn't wanted, thereby reducing the risk of overheating the indoor spaces. The main problem with Solar Air Heating is storage. Solar heat is plentiful in the afternoon. You need it in the middle of the night. Storing heat in gravel beds and tubes became popular in the 1970s. I know of some 1970s-80s buildings where the rooftop collectors have been removed, but the basement remains full of gravel.
Today, it's my impression that flat-plate collectors (with glass), and gravel beds, are so uncommon as to be practically obsolete. The most common form of Solar Air Heating, I think, is the transpired solar wall. (SolarWall seems to be the biggest manufacturer.)
So I was surprised to learn about a new project in New Zealand (pictured above) using flat-plate collectors to heat air directly. The collectors, manufactured in Denmark, include PV cells to drive the fans, so no electricity is drawn from the grid. Pretty clever! More information about this project is available here.
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