Abstract: A geothermal heat exchanger, comprising an inner tube bounding a first passage for a flowing heat exchanger fluid. The inner tube has one or more ribs on an outer side thereof. The inner tube and the one or more ribs are integrally formed by extrusion and entirely made of a thermally-insulating synthetic foamed material. The geothermal heat exchanger further comprises an outer tube made of thermally-conductive material concentrically positioned around the inner tube. The one or more ribs abut a first surface of the outer tube, and the inner tube and the outer tube cooperate to define an annular space which forms a second passage for the flowing heat exchanger fluid.