Heat extraction performance of horizontal-well deep borehole heat exchanger and comprehensive comparison with the vertical well

•A horizontal-well borehole closed-loop heat exchanger is proposed.•Comprehensive comparison of two type wellbore exchanger is conducted.•Horizontal length and flow rate have opposite impact on system efficiency.•The horizontal well can achieve better efficiency and economic benefit. A horizontal-well deep borehole closed-loop heat exchanger (DBHE) is presented in this paper, enlarging the heat transfer area by the horizontal section compared with the vertical well. Previous studies about DBHEs focused on vertical wells and few of them concerned horizontal wells and system efficiency. Then, a new 3D transient flow and heat transfer model is established. Temperature characteristics of the working fluid and formation are analyzed. System efficiency is introduced, and finally a comparison with vertical wells is carried out. The results show that under conditions in this paper, the temperature rising degree of the working fluid in the horizontal section is 2.67 times of that in the vertical. Geothermal water flow velocity can enhance heat convection obviously when it beyond a critical value. Horizontal length and flow rate are the vital factors of heat extraction. The outlet temperature rises about 2 °C by every 500 m horizontal length longer. Increasing horizontal length and flow rate have significant promotion in system efficiency and economy compared with the vertical well in a certain extent. The key results can provide guiding significance for design and retrofit of DBHEs.