A whole-process simulation for energy piles in unsaturated soils considering the coupled heat and moisture transfer

Abstract

Compared with traditional borehole heat exchangers, energy geostructures are more common to be embedded in unsaturated soils. The coupled heat and moisture transfer in unsaturated soils makes the thermal performance of energy geostructures more complicated. In this study, a three-dimensional heat-water–vapor mathematical model has been developed through theoretical derivation, considering the changes in thermodynamic properties. The spatially and temporally varying moisture and temperature profiles were generated with a finite element method. Then, a comprehensive simulation of the whole operation process for energy piles was carried out by coupling the validated mathematical model with the non-isothermal pipe flow and solid heat transfer physical fields. The results showed that after operation at constant power for one month, the liquid content of the unsaturated soil surrounding the pile decreased by 16.9 %, and the thermal conductivity dropped from 1.4 W∙m−1∙K−1 to 1.31 W∙m−1∙K−1. In addition, after three months of operation at a given inlet temperature, the heat exchange rate of the fully-coupled model was 9.6 % lower than that of the model considering heat transfer only. The intermittent operation mode could effectively reduce the heat accumulation and alleviate the decrease of liquid content, helping improve the long-term heat transfer performance. During a three-year operation period, the cooling power generally presented a downward trend while the heating power was the opposite. Therefore, it can be concluded that the application of energy piles in unsaturated soil areas requires more accurate evaluations and long-term monitoring.