Efficiency enhancement Heat extraction 2 Collector efficiency 1 Convective flow 1 Convective zone 1 Cylindrical cells 1 Cylindrical solar collector 1 Cylindrical solar collectors 1 Dean number 1 Economic advantageous 1 Energy efficiency 1 Environmental technology 1 External heat exchangers 1 Extraction 1 Flow byes 1 Heat convection 1 Heat exchangers 1 Heat transfer 1 Heat transfer efficiency 1 Helical corrugation 1 Helical tube 1 Helical tubes 1 Lakes 1 Lower convective zones 1 Modulation 1 Modulation frequencies 1 Natural convection 1 Nonconvective zone 1 Overall efficiency 1 Performance factor 1