Thermal performance of geothermal energy tunnel in unsaturated soils

Energy tunnel provides an innovative and efficient approach to harvesting geothermal energy. Understanding its performance and identifying the key influential factors are important to the application of this sustainable technology. In practice, soils surrounding an energy tunnel are often unsaturated, while previous studies either focus on fully saturated conditions (FS analysis) or simplify the soils above the water table as completely dry (CD analysis). These analyses may lead to inaccurate evaluation of thermal performance in unsaturated conditions because soil's thermal properties are moisture-dependent. Moreover, the effects of soil moisture conditions on thermal performance are expected to depend on various factors like soil and operational conditions. To address these problems, this study developed a coupled thermo-hydraulic finite element code to investigate the thermal performance of energy tunnel in unsaturated soils. The effects of soil property, groundwater, tunnel's internal airflow environment (TIAE), heat carrier fluid, and pipe conditions were investigated. Comprehensive results show that TIAE is one of the most important factors affecting the thermal performance in unsaturated soils. Another dominant factor is the groundwater flow velocity and fluid inlet temperature when the water table is relatively high (above tunnel crown) and low (below tunnel invert), respectively. Moreover, in the cases with a high water table and high TIAE (e.g., heat transfer coefficient ≥ 5.3 W/(m²∙K)), the conventional CD and FS analysis can give an acceptable performance evaluation for the sand and clay, respectively. In other cases, however, significant underestimation and overestimation, with a maximum value of 70 % and 130 %, are induced by the CD and FS analysis, respectively. These findings suggest that soil unsaturation should be properly considered in the design of energy tunnels.