Thermo-economic comparison of CO₂ and water as a heat carrier for long-distance heat transport from geothermal sources

Deep geothermal energy has tremendous potential for decarbonizing the heating sector. However, one common obstacle can be the mismatch between geologically attractive regions in the countryside and urban areas with a high heat demand density, which are therefore attractive for district heating systems. In the last years, an increasing number of regions consider the transport of geothermal heat into urban clusters. One example of such a region is the South German Molasse Basin in Upper Bavaria. However, such heat transport pipelines come along with massive upfront investment costs due to the required large pipe diameter and insulation thickness. While the classic concept foresees the use of water as a heat carrier in such long-distance heat transportation pipelines, CO₂ can be an attractive alternative. This study investigates the thermo-economic performance of CO₂ as a heat transport carrier for a potential long-distance heat transmission pipeline with a length of 20 km, which could connect a planned geothermal project in the South of Munich with the existing district heating network of Munich. The results of the base case scenario demonstrate that for both heat carrier options water and CO₂ rather low LCOH for the transport of the heat can be achieved. The resulting additional LCOH by the long-distance heat transport of around 0.6 cC/kWh are rather small compared to the typical overall LCOH of geothermal district heating systems. Comparing the thermo-economic performance of water and CO₂ reveals rather similar achievable LCOH, with a slight advantage for the classical concept of using water. However, this changes if the installation of a high temperature heat pump (HTHP) is considered in order to increase the thermal capacity of the heat transport system. In the case of using CO₂, the additional temperature increase takes place directly within the CO₂ stream by just installing a compressor, while in the case of the water system, a complete HTHP system needs to be installed. In combination with a higher achievable COP, the CO₂ HTHP configurations results in lower overall LCOH compared to the water system.