If you get any sum on your flat roof some solar thermal or PV panels
As above, the idea of storing heat obtained cheaply, E7, solar or anyway else, is not a runner.
Water would be the obvious storage medium, but the volume needed to provide space heating would be absolutely vast. If water was in any way suitable, storage heaters would not be full of special bricks.
A lot of systems do indeed use big tanks of water for storage / buffering.