Evaluation of thermal performance of energy textile installed in Tunnel

A new geothermal energy source obtained from a tunnel structure has been studied in this paper. The geothermal energy is extracted through a textile-type ground heat exchanger named "Energy Textile" that is fabricated between a shotcrete layer and guided drainage geotextile. To evaluate the thermal performance of the energy textile, a test bed was constructed in an abandoned railway tunnel located in Seocheon, South Korea. The thermal conductivity of shotcrete and lining samples which were prepared in accordance with a common mixture design was measured in laboratory. An overall performance of the energy textile installed in the test bed was evaluated by carrying out a series of in-situ thermal response tests. In addition, a 3-D finite volume analysis (FLUENT) was adopted to simulate the operation of the ground heat exchanger being encased in the energy textile with the consideration of the shotcrete and lining thermal conductivity, fluid circulation rate, the existence of groundwater and the arrangement of circulation pipes. The results of numerical analyses show that the energy textile exhibits better thermal performance under the following conditions: the thermal conductivity of the tunnel wall is relatively high, and the velocity of the circulated fluid in the energy textile is low. Based on the in-situ thermal response tests, a numerical analysis is performed to simulate the performance of the system under the field condition. In addition, to observe the long-term thermal behavior, the difference in the temperatures of the inlet and outlet fluid of the energy textile is monitored using a constant-temperature water bath and a real simulation system for cooling operating. From the long-term monitoring, results show that the average heat exchange amount of the energy textile (2 unit) was 592–713 W, that is 0.08–0.09 RT for 1 unit.