Feasibility analysis of geothermal energy utilization in ultra-low temperature district heating networks: Effect of heat distribution in underground tunnels

The integration of ground-source heat pumps into ultra-low-temperature district heating networks represents a promising pathway for decarbonizing urban heat supply and mitigating the shortcomings of conventional high-temperature district heating systems. However, the feasibility of such systems depends on several technical factors related to operating conditions, demand characteristics and heat distribution. This study evaluates the technical feasibility and annual performance of a geothermal heat pump–based ultra-low temperature district heating system operating at supply and return temperatures of 35/20 °C under realistic boundary conditions. The investigated system consists of a ground-source heat pump connected to a borehole field comprising 135 boreholes, each 200 m deep, serving a district heating network with a peak demand of 5.2 MW. A system-level modelling framework implemented in MATLAB/Simulink is used, with district-heating boundary conditions derived from Leanheat® network simulations, to assess a partial peak-load coverage strategy, geothermal borefield behaviour, and the influence of network temperature level and distribution environment, including tunnel based pipe routing, on system performance. The results show that a heat pump sized to cover a fraction of the peak demand can supply approximately 69% of the annual heat demand while achieving a seasonal performance factor of 3.82. Borefield operation remains thermally feasible throughout the annual cycle, with no freezing observed at the borehole outlet. A comparative analysis shows that increasing the network temperature level to 60/40 °C results in similar demand coverage but significantly higher electricity consumption due to reduced heat pump efficiency. Furthermore, routing district heating pipes through an underground tunnel environment reduces annual distribution heat losses by approximately 183 MWh without significantly affecting heat pump performance or hydraulic behaviour. A sensitivity check shows that the tunnel environment partially offsets the increase in distribution heat losses associated with reduced pipe insulation.