Here is a type of solar air heater that is easy to make and costs very little. This unit may be built with or without fans.
Preheaters can also be used.
Vacuum cleaner hose can be used as collectors. It can be found in trash.
Check out this article about DIY Solar. https://www.ecohome.net/guides/1075/all-about-solar-air-heaters-diy-or-purchased/
Solar Sponge Mechanical Drawing. This is a generalized drawing, showing the principle. The polycarbonate glass front is not shown well in the illustrations but is essential. Vacuum cleaner hoses are run between the inlet and output. The number of chamber and length/diameter of hose can vary.
Basic airflow for passive heating system.
Rooftop mounted Solar Sponge.
These are done with soda cans.
