Radiant energy collecting or emitting element and method and tool for manufacture thereof

A solar energy heat exchange element comprising the combination of a metal elongate outer member having a central bore of uniform radius, means forming a solar energy heat exchange surface extending lengthwise of and projecting radially outwardly from the outer member, a plurality of circumferentially spaced grooves formed in helical paths along the central bore, said bore having smooth surface portions extending along the spacing between adjacent grooves, a metal elongate cylindrical inner member having an outer surface and formed of a different metal than said outer member mounted within the central bore, a plurality of circumferentially spaced helical lands on the outer surface of said inner member, said lands projecting radially outwardly into interfitting engagement with the grooves for locking the inner and outer members against relative longitudinal and rotational displacement, said inner member having smooth surface portions between the lands in intimate surface contact with the surface portions of the outer member for maintaining said intimate surface contact under changing thermal and mechanical stresses, and passageway means formed along the length of the inner member for conducting a fluid heat exchange medium.