Multi-factor optimization of thermo-economic performance of coaxial borehole heat exchanger geothermal system based on Levelized Cost of Energy analysis

The deep coaxial borehole heat exchangers (BHEs) are commonly employed for geothermal heating purpose. In this investigation, using the established thermo-economic model and fast algorithm for heat extraction prediction, the response surface approach was adopted for investigating the thermo-economic performance and optimizing the design parameters of the coaxial BHE system. The results show that the heat exchange rate is enhanced from 235 kW to 1,061 kW when the depth varies between 2,000 m and 4,000 m. A change in flow rate from 20 m3/h to 80 m3/h yields a 30% augmentation in heat exchange rate. However, increasing the depth will cause extra drilling costs. Extremely large or small flow rates and pipe diameter ratios result in additional equipment and operating costs. The system's levelized cost of energy (LCOE) is contingent upon the interaction among the depth, flow rate, and diameter ratio. The optimal design parameters to meet the minimum LCOE are 3912.8 m depth, 34.32 m3/h flow rate and 0.64 pipe diameter ratio. The optimized LCOE is $ 7.84/GJ with an improvement of 22.02-63.02%, demonstrating a competitive thermo-economic performance to traditional energy sources. This research will contribute to the parameter design of BHE and effective utilization of geothermal energy.