99600c1d_Collector___Solar_Water_Heater__Evacuated_Tube_Collector
[S.No] 9 [Flat Plate Collector] Heavy scaling of the copper/aluminum tubes which cannot be cleaned manually as the bore dia. is 12.50 mm. giving rise to substantial loss in efficiency of the system. [Evacuated Tube Collector] 
Negligible scaling of tubes which can be cleaned manually (inner tube dia. Is 37mm). Loss of efficiency consequently is minimal
Comparison between Evacuated Tube Collector & Flat Plate Collector - Solar Water Heater
Evacuated Tube Collector