Heat exchange mechanisms in energy tunnel systems

Ground Source Heat Pump (GSHP) systems are known for their efficiency in space heating and cooling, using the ground as a source of energy. The ground energy can be accessed by the implementation of GSHP systems into subsurface structures such as transport tunnels. This technology comprises the embedment of High-Density Polyethylene (HDPE) pipes into the tunnel lining, converting it into tunnel ground heat exchangers (tunnel GHEs). There are studies available on the thermal interaction between the ground and the tunnel GHEs in the current literature. However, to properly evaluate the thermal performance of tunnel GHEs, the two-way thermal interaction between the tunnel GHEs and both the ground and the tunnel air must be considered. This requires the consideration of the ground conditions and the tunnel air temperature variations. In this work, the tunnel GHEs, the air inside the tunnel and the surrounding ground are modelled using finite element techniques. The models account for groundwater flow in the ground and induced airflow in the tunnel area. Numerical results highlight the significant effect of groundwater flow velocity on the temperature distribution pattern in the ground, the air temperature variations within the tunnel area and the average fluid temperature within the tunnel GHEs. To accurately capture the temperature variations in the tunnel air, it is crucial to account for both conduction and (induced) convection inside the tunnel as numerical results show significantly different temperature distribution inside the tunnel area in comparison to when only pure conduction is considered. In practice, the very common under/over-estimation of GSHP systems performance (including tunnel GHEs) is related - to a certain extent - to the lack of understanding regarding the operational environment of these systems (i.e., groundwater flow and tunnel air interactions with the activated lining). It may also lead to an inadequate air ventilation system design.