Solar powered thermal ballast - is it feasible?
Any dark colored mass would become a passive solar heater for when the sun goes away and the room begins to cool. The mass could be as simple as concrete blocks, tiles or as costly as eutectic tube solutions. Years ago,I was interested in solar systems  (I was years ahead in my imagination and thinking), and these were common ideas that were studied and written about. Before you give your brain an overload go to the local book store or library and read and study books on passive solar ideas. You will be amazed at the ideas people had to gather solar energy and use it at a later time. "Hope this helps".