A coaxial-flow heat transfer system for installation in a geological environment and facilitating the transfer of heat energy between an external heat energy producing system and the geological environment. In the coaxial-flow heat transfer system, aqueous-based heat transfer fluid is pumped through the external heat energy exchanging system so as to transfer heat between the aqueous-based heat transfer fluid and the external heat energy exchanging system. The coaxial-flow heat transfer system comprises a coaxial-flow heat transfer structure for installation within the geological environment and having a proximal end and a distal end for exchanging heat between a source of fluid at a first temperature and a geological environment at a second temperature. The coaxial-flow heat transfer structure comprises a thermally conductive outer tube section, and an inner tube section having an inner flow channel and being coaxially arranged within the outer tube section. An outer flow channel is formed between the inner and outer tube sections, and a helically-arranged fin structure is disposed along the outer flow channel, so as to form at least one helically-extending outer flow channel, for constantly rotating the aqueous-based heat transfer fluid flowing between the inner and outer flow channels, and thereby improving the transfer of heat energy between the aqueous-based heat transfer fluid and the geological environment along the length of the outer flow channel.