Abstract: An elongated solar heat collector is formed by means of a heat exchanger having two thin-walled tubes arranged coaxially of one another, a cylindrical, transparent heat shield positioned coaxially around the tubes and an elongated parabolic reflector mounted to locate the focal axis of the reflector on the axis of the tubes and shield. The outer tube of the heat exchanger has helical corrugations extending along a greater portion of its length and the valleys of the corrugations are made to contact the outer surface of the inner tube so that a helical passageway is defined between the inner and outer tubes. The straight elongated passageway of the inner tube and the helical passageway between the coaxial tubes communicate with one another at one end of the elongated collector so that two fluid flow paths defined by the passageways are serially connected and permit a fluid heat exchange medium such as water to flow in and out of the exchanger.