The role of ground conditions on energy tunnels heat exchange

Geotechnical structures are being increasingly employed, in Europe as all around the world, to exchange heat with the ground and supply thermal energy for heating and cooling of buildings and de-icing of infrastructure. Most current practical applications are related to energy piles and retaining walls, but some examples of energy tunnels were recently proposed. With respect to building foundations, tunnels involve a larger volume of ground and surface for heat exchange. When mechanized tunnelling is used, tunnel lining segments can be equipped and optimized for heat exchange by including hydraulic circuits into the concrete. This paper deals with the use of a tunnel segmental lining to exploit heat. Thermo-Hydro Finite Element analyses able to capture the key aspects of the problem are performed to investigate the heat exchange of the system under different underground scenarios. In particular, the influence of ground properties (hydraulic and thermal conductivities), and ground conditions, (groundwater temperature and flow velocity) are investigated and discussed. Parametric analyses are employed to define design charts useful for the preliminary quantification of exploitable heat and the assessment of the applicability to different ground conditions.