Assessing the interaction of an energy tunnel with the underground thermal conditions in an urban area

In the very shallow depths of urban areas it is difficult to find natural undisturbed underground thermal conditions because of anthropic interventions. Moreover, these areas are being increasingly used for energy purposes, for example implementing the technology of shallow geothermal systems to provide clean thermal energy and supply the thermal demand of buildings in both winter and summer seasons. The heat exchanged by these types of renewable energy technologies, in particular ground source heat pump systems such as open loop and closed loop ones, in combination with further anthropogenic activities, results in altered thermal regimes in the subsurface. Energy tunnels, which are achieved by thermally activating the tunnel lining, have recently gained attention among closed loop geothermal systems. Therefore, when planning an urban energy tunnel, attention has to be devoted to the initial underground thermal conditions and to the interactions the pre-existing thermal regime will have with the energy tunnel itself. To this aim, the paper outlines a methodological approach which is then applied to a case study in Turin, Italy, where a new metro line is planned. Thermo-hydraulic numerical modelling is adopted to reproduce the thermally disturbed subsurface environment in the study area prior to the energy tunnel’s thermal activation, due to the presence of multiple heat sources (open loop and closed loop systems, underground buildings, car parks and infrastructures), as well as after its commissioning. Results are illustrated in terms of temperature maps and cross sections where the thermally affected zones due to open loop, mainly, and closed loop systems are depicted depending on the operational mode of the shallow geothermal systems. Those results highlight that, when the installation of a new energy tunnel is envisaged, it is necessary to consider the current geothermal exploitation of the area and the operation of neighbouring similar systems.