Experimental analysis of a thermoactive underground railway station

Little is known about the real energy potential of thermoactive underground infrastructures, such as railway stations, that can act as a heating/cooling provider for the built environment. This study presents the results of thermomechanical full-scale in situ testing and numerical analysis of a thermoactive underground train station. The thermal performance and related geostructural impact of a portion of the new underground energy infrastructure (UEI) installed at the Lancy-Bachet train station in Geneva (Switzerland) are analyzed. Heating and cooling tests simulating real operative geothermal conditions are considered. Particular attention is given to ((i) the monitored wall-tunnel hydrothermal interactions, ((ii) the thermal response of the UEI to heating/cooling thermal inputs and ((iii) the thermomechanical behavior of the energy geostructure. Among the main results of this study, it is shown how the hydrothermal tunnel behavior considerably varies on a seasonal basis, while the train circulation completely drives the airflow in the tunnel. The UEI shows a strong heat storage potential due to the main conductive heat transfers between the geostructure and soil, while lower heat fluxes are detected at the wall-tunnel interface. The extraction potential is of lower magnitude with respect to storage because of the limited range of operative fluid temperatures and of the concurrent action of temperature variations at the tunnel boundaries affecting the materials within the UEI. Preliminary guidelines for the thermal response test execution on underground thermoactive infrastructures are also reported. The monitored thermomechanical behavior suggests different wall behaviors in the vertical and longitudinal directions. Low-magnitude strains are recorded, while the mechanical capacity of the existing geostructure can satisfactorily sustain concurrent thermomechanical actions.

Thermomechanical behavior of underground energy infrastructures

Jacopo Zannin

The employment of geostructures as structural supports and as heat exchangers represent an effective, renewable and sustainable way to satisfy thermal needs of the built environment. The structural support of conventional geotechnical structures is coupled with the heat exchanger role by inserting heat exchangers attached to the reinforcing cage of concrete geostructures. Hence, geostructures act as multi-functional elements, called energy geostructures. The concurrent dual role involves thermal and mechanical aspects to be considered during analysis and design, causing novel challenges for engineers. This doctoral thesis focuses on the thermomechanical behavior of underground energy infrastructures (e.g., vertical retaining walls, diaphragm walls, cut-and-cover tunnels, base slabs, multi-floored underground basements, etc...). Prior to this work, limited knowledge on this topic was available and the main challenges were: (i) to thoroughly understand the thermal and hydrothermal aspects linked to the heat transfer within and around the energy geostructure; (ii) to detect fundamental aspects on the thermomechanical behavior; (iii) to provide feedback from real installations. To address such challenges, this doctoral thesis employed experimental, numerical and analytical techniques. Firstly, fundamental aspects on thermomechanical behavior and on hydrothermal aspects linked to the thermal performance are presented. Secondly, aspects related to the early-stage thermal performance design are tackled, presenting a methodology for thermal performance design based on a flowchart. Thirdly, the only analytical model able of considering combinations of axial and flexural thermal and mechanical loads is presented and used to tackle several examples in the field of energy geostructures. Finally, an experimental, in-situ, campaign on an energy wall of an underground energy infrastructure in Geneva (CH) is presented focusing on: the execution of (i) a thermal response test (TRT) and (ii) heating/cooling tests, including aspects linked to test execution, thermomechanical monitoring, data interpretation, and (iii) the determination of the thermal potential of the entire underground energy infrastructure installation. The main results provided by this thesis are: (a) thermal behavior of underground energy infrastructures involves strong interactions with the surrounding environments (e.g., air interfaces); (b) the temperature variation distributions induced by thermal operations are nonuniform, inducing axial and flexural mechanical actions; (c) a modified TRT execution procedure is proposed, allowing for the consideration of non-negligible hydrothermal aspects occurring within the energy geostructure and its surroundings; (d) the in-situ testing campaign undertaken at the site in Geneva revealed a very strong thermal storage potential and a slightly lower extraction potential due to the influence of thermal boundary conditions.

EPFL2020

Thermomechanical behavior of underground energy infrastructures

Jacopo Zannin

EPFL2020