A heat engine apparatus for use in association with a borehole and a method of operating a heat engine in association with a borehole. The apparatus includes a first heat exchanger assembly in fluid communication with a proximal segment of the borehole, a second heat exchanger assembly in fluid communication with a distal segment of the borehole, a circulation barrier for providing a seal between the apparatus and the borehole in order to isolate the proximal segment of the borehole and the distal segment of the borehole from each other, and a heat engine associated with the first heat exchanger assembly and the second heat exchanger assembly, wherein the heat engine is a gas phase closed cycle thermodynamic heat engine. The first heat exchanger assembly, the second heat exchanger assembly, the circulation barrier and the heat engine are all adapted to be inserted in the borehole.