Thermo-hydraulic assessment of energy tunnels under various thermal, hydrogeological and operational conditions

Abstract

Energy tunnels, promising structures for shallow geothermal energy extraction, require deeper insights into their thermo-hydraulic behavior. This study investigates the output power from the lining of energy tunnels numerically through a finite element model. After calibrating the model with both numerical and experimental data, the effects of various parameters and groundwater flow (GWF) on output heat were examined. Results demonstrate that pipe diameter and flow velocity dominate output power variation (74 %), exceeding soil thermal properties (8 %). Non-isolated conditions enhance heat exchange compared to isolated setups. GWF significantly boosts output power, with exponential increases up to 436 % at 10⁴-fold velocity rises in isolated, perpendicular flows. GWF amplifies the impact of pipe layout by 0.8 %–10.8 %, and reduces power discrepancies between isolated/non-isolated conditions over time by restoring soil temperatures. Short-term output power surpasses long-term due to soil heat capacity depletion during prolonged operation. Inclined seepage, modeled as multidirectional flow, enhances thermal compensation by 10 %, with two-dimensional flows improving soil temperature recovery beyond one-dimensional flows. This study emphasizes GWF's crucial role in optimizing energy tunnels through GWF velocity and directionality, noting minimal soil influence. It offers quantitative guidelines on pipe configuration and hydrogeological conditions to maximize geothermal system efficiency.