Heat transfer equipment for installation within geological environments, such as the Earth, lakes and ponds, and including coaxial-flow heat transfer systems and structures comprised of a thermally-conductive outer tube structure co-axially arranged about an inner tube structure and provided with a cap structure, so as to seal off the inner and outer tube structures and provide an inner flow channel that extends down the inner tube structure and transitions into an outer flow channel between the inner and outer tube structures, and along which aqueous-based heat transfer fluid flows, and heat energy is transferred between the aqueous-based heat transfer fluid and the geological environment along the length of the outer flow channel