When the sun’s rays fall on a solar water heater collector, made of either an Evacuated Vacuum Tube system (EVT) or Flat Plate Collector system (FPC), the solar radiation heats up the water present in glass tubes (in EVT) / metal tubes (in FPC) due to high transmittance of the covering glass medium.
The heated water, now less dense, rises in the various tubes eventually reaching the tank at the top. The relatively colder and denser water in the tank descends into the tubes, in turn getting heated — this cycle continues.
The Thermosiphon effect ensures the movement of hot water upwards into the tank because of difference in density, eliminating the need for an external pump. The insulated tank ensures that the hot water remains hot for a long time