Multi-objective optimization of geothermal heating systems based on thermal economy and environmental impact evaluation

Abstract

High-temperature geothermal resources are increasingly being explored as an alternative to coal and natural gas for space heating. In light of the growing demand for energy conservation and emission reduction, it is crucial to conduct a comprehensive evaluation of geothermal energy according to environmental impact and energy cost. Based on geothermal heating systems in Xi'an, China, this study develops a model involving the life cycle assessment (LCA) method to assess the borehole heat exchanger (BHE) in heating systems, encompassing carbon intensity (C_intensity). The response surface method was employed to optimize the drilling depth, operating flow, and pipe diameter ratio, which influences carbon intensity. The findings revealed that the minimum C_intensity is 24.11 g(CO₂)·kWh⁻¹, corresponding to a 4000 m burial depth, 20 m³ h⁻¹ flow rate, and 0.54 diameter ratio. However, these results diverge from the minimum levelized cost of energy (LCOE) of $7.84/GJ, with 3912.8 m, 34.32 m³ h⁻¹, and 0.64. A dual-objective optimization indicates the allocation of weights to the objective functions influences the optimal drilling depth (the optimal value is between 3920.51 m and 3921.62 m) and operating flow rate (ranging from 28.8 to 31.4 m³ h⁻¹). The optimal outcomes for LCOE and C_intensity are contingent upon the decision-makers' weight allocation.