Decarbonization potential of district heating systems using deep coaxial borehole heat exchangers in salt dome geological settings

The decarbonization of district heating systems remains a major challenge in Central and Eastern Europe, where high-temperature networks are still largely supplied by coal-fired plants. This study evaluates the technical potential of integrating a deep coaxial borehole heat exchanger (DCBHE) into the HOCLOOP (HOrizontal Closed LOOP) concept, which considers a vertical depth of 3690 m and a horizontal section length of 2500 m for the borehole heat exchanger coupled with an existing 2nd generation district heating system. Based on numerical simulations, the share of renewable energy during system operation was quantified. A case study of the Goleniów city district heating network in SW Poland is analyzed using long-term numerical simulations of a DCBHE installed in a high-conductivity salt formation, coupled with time-resolved heat demand based on typical meteorological year data (TMY). Two integration pathways are assessed: direct low-temperature heat recovery and cooperation with large-scale heat pumps. Results over a 20-year period indicate stable thermal performance, with outlet temperatures exceeding 80–100°C depending on the circulation rate and operating mode. Integration with heat pumps enables multi-megawatt thermal output and substantial reductions in coal use and CO2 emissions. While geothermal energy alone cannot exceed a 50% share of renewables in the existing Goleniów system due to high power and supply-temperature requirements (∼20.5 MW, 135°C). If one considers a hypothetical scenario, analyses suitably matched users, as assumed in the paper, of approximately 8 MW for central heating and 530 kW for hot tap water, and 3rd-generation DH – requiring a supply temperature of 90°C – can achieve renewable energy shares above 50% using even a single unit of DCBHE combined with a heat pump. This indicates that matching the source's capabilities to the recipient's requirements is a key aspect of achieving good system efficiency. A 50% share of renewable energy sources is important for classifying the system as energy-efficient under the Energy Efficiency Directive (EED). The results demonstrate that long-term near-full energy self-sufficiency (∼0.92 share of RES) can also be achieved for a 3rd-generation DH system with a total power demand of ∼3.2 MW, assuming a single DCBHE module of design. It was estimated that the simple payback period for DBBHE expenditures is about 36-40 years, and the total energy cost is ∼32-38 €/GJ, similar to that for conventional energy carriers, assuming a 4.7%/year inflation in energy carrier prices.