Ground Heat Exchangers from Artificial Ground-Freezing Probes for Tunnel Excavations

Ground-source heat pumps (GSHPs), despite their high efficiencies, are still not as cost-effective as air-source heat pumps, especially in urban environments, due to the necessity of drilling/excavation. Integrating GSHPs into existing geo-structures, such as underground tunnels, can play a vital role in reducing the overall costs of GSHP systems and promoting their use in cities. Tunnels can be realized through artificial ground freezing (AGF) by using probes for circulating the freezing fluid, which are left in the ground once the tunnel is completed. The novelty of the present work lies in the proposal of a sustainable reuse of AGF probes as ground heat exchangers (GHEs). The idea of converting AGF probes is both sustainable and cost-effective for GSHPs, as it can reduce installation costs by eliminating the drilling/excavation process. A test was performed for the first time in the Piazza Municipio metro station in Naples, Southern Italy, where several AGF probes, initially used for the construction of two tunnels, have then been converted into GHEs. The probes have been connected to a testing device called the energy box. The experiments included testing the heat transfer in the recovered AGF probes through cooling and heating operations. This work presents a numerical simulation of a test that has been validated against experimental results.