The coaxial-flow heat exchanging structures installed in the Earth, for facilitating the transfer of heat energy in the aqueous-based heat transfer fluid, between the aqueous-based heat transfer fluid and material beneath the surface of the Earth. Each coaxial-flow heat exchanging structure includes an inner tube section, a thermally conductive outer tube section, and outer flow channel between the inner tube section and the outer tube section. A turbulence generating structure is disposed along a portion of the length of the outer flow channel so as to introduce turbulence into the flow of the aqueous-based heat transfer fluid flowing along the outer flow channel, while its cross-sectional characteristics produce fluid flows therealong having optimal vortex characteristics that optimize heat transfer with the Earth.