Design and optimization of heat extraction section in energy tunnel using simulated annealing algorithm

Energy tunnel has attracted increasing interest in preventing tunnel frost damage and providing energy for space conditioning through utilizing geothermal energy from ground source heat pump (GSHP) system. The energy lining system, composed of heat pump units, lining buried with geothermal heat exchangers (GHEs), and supply pipes, is one of the most popular energy tunnel systems. Despite their good potential for geothermal energy exploitation, studies on their design approach are still insufficient, particularly for cold region energy tunnels. In this work, we propose an innovative design framework for optimization of the heat extraction section in the energy tunnel by combing numerical simulation and Simulated Annealing (SA). The numerical model reproduces a horseshoe-shaped tunnel segment equipped with an energy lining system, generating heat power activated by circulating heat carrier fluid. And the simulation provides key input parameters, involving heat extraction power, the ground temperature field, and heating demand, for the algorithm, hence the heat extraction section range is optimized using SA. Influences of four design parameters (i.e., ground temperature, inlet fluid temperature, circulating fluid flowrate, and pipe spacing) and three axial temperature distribution characteristics (i.e., overall temperature along the tunnel axis, airflow characteristics, and temperature difference between the tunnel entrance and exit) dominating temperature profiles along the tunnel axis on the optimal range design are investigated. Major findings include: (1) the optimal range is sensitive to the temperature field along the tunnel axis, and the normalized optimal length decreases by 91.3% with the normalized feasible range increases 1099.6% as the average temperature inside the tunnel rises from −2.5 °C to 5 °C; (2) increasing flowrate and lowering the temperature of the inlet fluid can yield a reduction in the optimal length of up to 29.2% and 64.3% separately; (3) a relatively sparse pipe arrangement can save cost without comprising the optimal length. This study presents a smart and efficient approach for designing the heat extraction section of the energy tunnel, which is promising in more complex conditions and capable of guiding practical applications.