Vertically installed borehole heat exchangers based on the gravitational heat pipe principle seem to be an attractive alternative to conventional brine borehole heat exchangers. The heat pipe has to be filled with a suitable working medium and has to be pressurized. The coupling of cooling, for example by the use of a heat pump at the head of the pipe and heating of the heat pipe in the underground zone leads to a closed self-circulation system with no additional energy input. In this study the thermodynamic design for a deep gravitational heat pipe filled with liquid and gaseous carbon dioxide is presented. The thermodynamic analyses have led to an installation of a 250 meter deep steel heat pipe with an inner diameter of 65 mm. The heat pipe was placed in a relatively homogenous part of the Triberg-Granite formation at the black forest, Germany.
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