Abstract: Thermal energy is extracted from geological formations using a heat harvester. In some embodiments, the heat harvester is a once-through, closed loop, underground heat harvester created by directionally drilling through hot rock. The extracted thermal energy can be converted or transformed to other forms of energy.

Abstract: The present invention is a heat recovery fluid to be used in a closed loop, hot dry rock system, where the fluid remains in a liquid state flowing down from the surface in an annular, downflow conduit to a terminal end heat exchanger. At the terminal end heat exchanger, the heat recover fluid partially vaporizes as a result of indirect heat transfer locally from the geological formation, resulting in a dual-phase fluid or vapor stream flowing out from the terminal end heat exchanger to an axial, return upflow conduit. In one embodiment, the upflow conduit is separated by a single wall from the downflow conduit. When the upflow conduit bears a dual-phase fluid, it has been found that the degree of vaporization of the heat transfer fluid does not result in substantial separation of the vapor and remaining liquid, i.e., which could result in plugging or slug flow in a sloping upward direction, but instead produces a stable flow regime for return of the heated heat recovery fluid to the surface.