Influence of spatial variability of permeability on thermal performance of energy tunnels

Energy tunnels represent an innovative solution for meeting heating and cooling needs through heat exchange with the ground. Among the key factors influencing their thermal performance, there is the groundwater flow which drives continuous thermal recharge of the surrounding ground. The extent to which this recharge occurs depends significantly on the flow velocity, which is mainly governed by hydraulic gradient and permeability. While previous studies typically assumed homogeneous hydraulic properties, the effect of spatial variability on the thermal exchange remains unexplored. Therefore, this study investigates how spatial variability of intrinsic permeability impacts the thermal performance of an energy tunnel under seepage. A numerical model was developed and validated against field tests data available in the literature. Then, lognormally distributed, autocorrelated random fields of intrinsic permeability were generated and incorporated into the model as a series of Monte Carlo simulations (MCS). The results indicate that assuming homogeneity is acceptable when the coefficient of variation of permeability ( C O V κ ) is below 1.0, in both heating (winter) and cooling (summer) modes. However, for higher variability ( C O V κ > 1.0), this effect becomes significant and assuming homogeneous conditions may lead to a substantial underestimation or overestimation of the heat exchange. Only in cases with a large difference between internal air temperature and fluid temperature, the homogeneous assumption may remain reasonably valid even at high C O V κ up to 4.0. The effect of spatial variability increases with increasing groundwater flow velocity up to 0.5 m/d, beyond which the variability effect remains similar. This study highlights the critical importance of field measurements of hydraulic properties in order to accurately estimate the heat exchange rates of energy tunnels.