The borehole heat exchanger (BHE) has been widely used for geothermal development, but its heat production is limited. To enhance the heat transfer, an open loop geothermal system (OLGS) is proposed to exploit hydrothermal resources. For OLGS, the injection and production intervals are perforated in upper and lower wellbore and an insulated inner tube is installed in the wellbore. The OLGS provides a hydraulic circulation in a single well. The working fluid is injected into the reservoir from the injection interval, then extracts heat from the reservoir, and finally returns to surface through the production interval and insulated tube. Few studies were conducted to comprehensively investigate the heat extraction performance of OLGS. Hence, a 3D unsteady-state numerical model considering fluid flow and heat transfer processes of borehole and reservoir is established. Subsequently, the influences of the key factors, including injection flow rate, inlet temperature, injection-production interval, geological conditions, on the OLGS performance are analyzed. The results indicate that a large flow rate is beneficial to obtain higher thermal power. The injection-production interval has a significant effect on OLGS performance, and a longer interval could effectively avoid early breakthrough of circulating fluid. Furthermore, the permeability plays a remarkable role in heat production, while the influence of porosity is negligible. And temperature field analysis depicts that the OLGS has a larger impact scope than conventional BHE, which acquires greater heat extraction performance. The main findings of this study can offer guidance for the optimal design of OLGS.
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