A novel Ground Source heat exchanger in an underground metro tunnel

Using Ground Coupled Heat Pump (GCHP) systems in urban areas can be particularly difficult due to space or legislative constraints, other than excessive costs due to drilling. To overcome these problems, the authors proposed to use Artificial Ground Freezing (AGF) probes, used for tunnel excavation, as Ground Heat Exchangers (GHE). It is a widespread practice to seal the probes in the tunnel after the completion. The conversion of existing AGF probes into GHE for GCHP allows us to avoid additional drilling costs, and other space or legislative constraints associated with the use of GCHP systems in urban areas. Such systems could be developed to use underground urban transportation tunnels for heating and cooling in smart cities. These systems were tested after the construction of two tunnels, as part of the GeoGRID project, in Piazza Municipio, Naples, Italy. The data obtained from the experimental setup have been analysed and used for validation of a finite element model developed by the authors to simulate heat transfer between the probes and the surrounding ground. A simplified pipe flow model was introduced, in combination with a 3D model of the ground, to reduce the complexity and computational effort to solve the discretized equations. The simulation results have been compared with experimental data, showing a good agreement, and observing differences in the range of 2 – 5 %. The model can therefore be used as a predictive tool for the development of this type of innovative heat exchanger.