Visible light from the sun turns into heat when it strikes the dark interior wall of the greenhouse. This heated air tends to rise and concentrate near the top of the greenhouse.
It then pushes open the top plastic flap and enters the living quarters. The raised pressure of air in the house causes cold air near the floor to push the bottom flap open into the greenhouse.
The solar greenhouse will continue will continue to bring heat into the house like this until sunlight is blocked by clouds or evening approaches. After a short period of time without sunlight, the convection currents inside the greenhouse reverse. Air cooled by the glazing will tend to drop and the hot air from the living space will rise and try to enter the opening at the top of the greenhouse. Without these one way flaps all the heat gained during the day would soon be lost by this cooling process, but with these flaps daytime heat is trapped inside the house. The flaps turn this ordinary greenhouse into a solar greenhouse.
OK! Now you understand how a solar greenhouse works. Do you think you can build one. The larger you make it the better it works, but the more it will cost. My solar greenhouse is 10 feet high and 20 feet wide. It reaches a temperature of 160 F. in mid winter when the outside temperature of 0 F. You might not want to build one like this large, but you should be able to construct one about 2 feet high as an experimental greenhouse.
It would be nice if you could frame it with wood and install real glass for glazing. If wood and glass are out of the question you could always make it out of cardboard and plastic. Be sure to insulate the back wall and side walls with solid insulation. Where will most of the heat be lost? I'll leave the construction details and the methods of collecting data and summarizing data up to you. Please post the results of your findings on the Bulletin Board under SOLAR GREENHOUSE.