I am curious how the air flows through the air collector you have shown in your diagram. The hot air appears to come out the top but where does it enter? Is there a fresh air intake or does it draw indoor air somehow? Also is it a passive heater? You have not mentioned the use of a fan to drive the air flow. My research has shown that active fan driven solar air heaters can be more efficient because they keep the absorber at a lower temperature and radiant less heat back out through the glazing. I have built at solar air heater using a fan powered by a solar panel. The setup seems to be working quite well so far. You can have a look at all the details here: http://practicallyoffthegrid.com/category/solar-air-heating/