The invention relates to a field of open-flow solar collectors, and specifically to flat solar collectors with wetting the underneath sides of their solar radiation absorbing plates with liquid heat transfer medium. More specifically, the invention proposes the flat solar collector, which operates with relatively low flow rate of the heat transfer medium on the underneath side of the solar radiation absorbing plate, with flow in form of some rivulets. The invention describes some technical solutions, which restrict meandering rivulets' flow. The proposed flat solar collector can be applied for heating water or other liquids and for evaporation and concentration of aqueous solutions.