Heat extraction performance of multi-level, multi-branch, closed loop coaxial horizontal borehole heat exchanger geothermal system

• A multi-level and multi-branch coaxial horizontal borehole geothermal system was proposed. • A 3D heat transfer model in HDR and a 1D convective heat transfer model in wellbore were established. • MLMB-CHBHE can obtain higher outlet temperature than that of similar heat exchangers. • Optimal injection and well layout parameters for the MCBHE geothermal system were obtained. • The lowest electricity cost is $0.070/kWh when 4 level, 5 branch, 3000 m length of horizontal well. To overcome the problem of limited heat exchange area and highly drilling costs, this study proposes a multi-level and multi-branch coaxial horizontal borehole heat exchanger geothermal system. Compared with multi-branch U shape heat exchanger system, the new system only requires drilling one vertical well, which can save drilling costs. Compared with single coaxial horizontal borehole system,the new system has multiple horizontal wells that can increase the heat transfer area. A 3D heat transfer model in hot dry rock (HDR) and a 1D convective heat transfer model in the wellbore were established. To studied the heat extraction performance and the key parameters affecting the heat extraction efficiency of the geothermal system. The results indicate that the maximum heat production power can only be achieved by controlling the injection rate to a certain extent. The heat production power increases linearly with the horizontal well length and branches. The heat production power increases linearly with horizontal well level 1–4, and then the growth rate decreases at 5 level. When the horizontal well levels are 4, branches are 4, and length is 3000, the unit electricity cost is the lowest. Through parameter optimization, we have found the optimal parameters for improving the heating power efficiency of the system, which can provide reference for the design of deep geothermal development processes.