Analysis of thermal interaction mechanisms and thermal-economic-environmental optimization of deep borehole heat exchanger arrays

The use of deep borehole heat exchanger (DBHE) arrays to exploit deep geothermal energy for clean heating in large-scale buildings has attracted increasing attention. However, existing studies have mainly focused on the heat transfer performance of a single DBHE, while the thermal interaction mechanisms within DBHE arrays and the effects of key operating and design parameters on their thermal, economic, and environmental performance remain insufficiently understood. Moreover, system-level multi-objective optimization and the identification of optimal operating conditions for DBHE arrays are still lacking. This study develops a numerical model for DBHE arrays to analyze inter-borehole thermal interaction mechanisms and quantify the effects of inlet temperature (T _in), flow rate (G), and borehole depth (D) on the coefficient of system performance (CSP), return on investment ratio (E ₀ ), and carbon emission reduction (ΔCO₂). A multi-objective optimization framework is further established by coupling the NSGA-II algorithm with the TOPSIS-entropy decision-making method to determine the optimal parameter combination. The results show that inter-borehole thermal interference plays a critical role in the long-term performance of DBHE arrays, and a borehole spacing greater than 15 m is required to alleviate performance deterioration. In addition, T _in , G , and D exert significant but distinct effects on the three objectives: increasing T _in and G leads to diminishing performance gains, whereas increasing D improves CSP and ΔCO₂ but reduces E ₀ because of the higher drilling cost. The optimal operating conditions are identified as T _in = 5.3 °C, G = 6.5 kg/s, and D = 2760 m, corresponding to CSP = 4.75, ΔCO₂ = 2019.91 t, and E ₀ = 0.42. This study provides new insights into the thermal interaction mechanisms of DBHE arrays and offers a practical basis for their optimal design and operation in sustainable building heating applications.