The rising global energy demand and the pressing need to reduce carbon emissions have prompted the exploration of sustainable energy solutions. Energy tunnels, which incorporate heat exchange pipes within tunnel structures to harness geothermal energy, offer a promising technology for achieving structural and thermal functions. This study focused on the energy shield tunnel associated with the underground relocation of the Beijing East Sixth Ring Road, a key initiative aimed at transforming urban infrastructure while supporting China's "dual carbon" goals (peak carbon emissions by 2030 and carbon neutrality by 2060). The study addressed critical gaps in the design and thermal performance evaluation of energy tunnels, particularly under varying geological and climatic conditions. By conducting in situ experiments and developing analytical models, the study aimed to provide reliable methods for calculating heat exchange power, assessing structural responses, and optimizing system design for large-scale applications.

A comprehensive experimental and analytical approach was employed to evaluate the thermal and structural performance of the energy shield tunnel. Two primary in situ testing methods were utilized: thermal performance tests and thermal response tests. These tests simulated extreme summer and winter conditions (35 ℃ and 5 ℃ inlet temperatures, respectively) to measure the tunnel's heat exchange power. Distributed fiber optic sensors were embedded in the tunnel's secondary lining to monitor temperature and strain variations during thermal cycles, ensuring accurate assessment of structural responses. Critical parameters, including heat exchange power, thermal influence radius, and temperature distribution, were derived using analytical solutions. For example, heat exchange power was calculated based on the temperature difference between inlet and outlet fluids, mass flow rate, and the specific heat capacity of water. The thermal influence radius was determined using a derived formula under constant heat flux conditions, validated against measured data. In addition, real-world engineering data were used to design a geothermal energy supply system for adjacent buildings, comparing its economic and environmental advantages with traditional air-source heat pump systems.

The experiments yielded significant results. Under extreme summer conditions (35 ℃ inlet temperature), the heat exchange power reached 45 W/m², whereas under extreme winter conditions (5 ℃ inlet temperature), it was -39 W/m², indicating effective energy extraction and storage. A linear relationship was established between the temperature difference (between inlet fluid temperature and ground temperature) and heat exchange power, expressed as Q_d=2.635ΔT_d. This formula proved adaptable to particular geological and airflow conditions. The thermal influence radius of the tunnel was calculated to be approximately 8 m after 120 days of operation, suggesting a minimum spacing of 16 m between adjacent energy tunnels to avoid thermal interference. Structural monitoring revealed negligible additional thermal stresses in the tunnel lining, with maximum strains of 155 με (tensile in summer) and 80 με (compressive in winter), confirming the safety of the integrated heat exchange system. The designed geothermal system demonstrated substantial economic and environmental benefits. For the South Management Zone (1226.3 m²), the energy tunnel system reduced initial costs by 49% compared to traditional borehole systems and realized 31.6% annual energy savings over air-source heat pumps. Carbon emissions were reduced by 43 tons annually, with potential savings of 1792 tons if implemented throughout the entire 10 km tunnel.

This study provides a comprehensive framework for designing and evaluating energy shield tunnels, addressing thermal performance and structural integrity. The derived formulas for heat exchange power and thermal influence radius serve as practical tools for engineers, promoting efficient system design across diverse conditions. The negligible structural effects confirmed the feasibility of retrofitting existing tunnels into energy tunnels without compromising safety. The energy tunnel system demonstrated significant economic and environmental advantages, aligning with global sustainability targets. To refine design methodologies and explore broader applications, future work should expand the dataset with long-term operational monitoring. By leveraging underground infrastructure for renewable energy, this research contributes to the advancement of smart, low-carbon urban development.