Geothermal potential of the NE extension Warsaw (Poland) metro tunnels

Thermoactive geostructures are considered an economically suitable and environment-friendly solution for heating and cooling buildings. Energy tunnels have gained growing interest in recent years because of the large ground volume involved in heat exchange in comparison to building foundations. Heat exchange is obtained by embedding a circuit of pipes into the precast concrete lining, resulting in a reduction of the initial costs of installation with respect to standard low-enthalpy geothermal solutions. In this paper, thermal activation of the twin tunnels of the II line of Warsaw metro, Poland, is considered with the aim of evaluating the geothermal potential of the line. Thermohydraulic finite-element analyses were performed on a number of representative cross-sections, considering site-specific hydrogeological conditions based on the Engineering Geological Database of the Capital City of Warsaw. Results demonstrate that the thermal activation of the two 1·6 km-long tunnels could exchange up to 5·3 and 5·8 GWh in the heating and cooling seasons, respectively. Furthermore, the influence of the local geological conditions on the exchangeable heat rate was investigated. The results form a basis for a preliminary assessment of the shallow geothermal energy utilisation potential for the design of further extensions of the metro line in Warsaw.