Experimental and numerical investigations on the energy and structural performance of a full-scale energy utility tunnel

Abstract

The present study evaluates the performance of a novel tunnel-type ground heat exchanger, an ‘energy utility tunnel (EUT)’. A field test and 3-D time-dependent thermo-mechanical numerical models were utilized to analyze the energy and structural performance of a full-scale EUT during geothermal extraction processes in both continuous and intermittent modes. By utilizing the experimental data to calibrate the numerical model, the impacts of the intermittent operation on the energy and structural performance of the EUT were examined. The results showed that the extracted thermal powers of the EUT were comparable to that of energy traffic tunnels. During geothermal extraction, the EUT experiences additional strains and stresses, which are mainly influenced by the degree of restraint and temperature change. In particular, the axial component of the thermally induced stresses in the EUT is found to be the largest and should be given due attention during the design phase. Furthermore, sensitivity analysis has revealed that as the intermittent ratio (IR) increases, an exponential augmentation (q/qIR=0 = 16.46 × eIR/0.57 + 85.4) is observed in the extracted thermal power of the EUT. Simultaneously, there is an exponential decay in the temperature change and thermally induced strain and stress of the EUT. Therefore, the operating hours of the system should be dynamically adjusted in real-time to the daily heat demand of the energy consumption side, thus achieving maximum efficiency and reduced operating costs while simultaneously minimizing the impact of EUT operation on its thermal–mechanical response.