Heat Extraction Performance of Horizontal-Well Deep Borehole Heat Exchanger and Comprehensive Comparison with the Vertical Well

The sustainable development of deep geothermal is restricted by resource quality and insufficient reinjection. A horizontal-well deep borehole closed-loop heat exchanger (DBHE) is presented in this paper, enlarging the heat exchange area between the working fluid and hydrothermal reservoir. Previous studies on heat transfer of DBHEs focused on vertical wells and hot dry rock development, however few of them concerned horizontal wells for deep hydrothermal reservoirs. Then, a new 3D transient flow and heat transfer model is established. Temperature characteristics of the working fluid and formation are analyzed. To confirm the efficacy of horizontal-well DBHE, system energy and economic efficiency are introduced, then comparison with vertical-well system is launched. The results show that the temperature rising degree of the working fluid in horizontal section is 2.67 times of that in the vertical section. The faster hydrothermal flow velocity can enhance heat convection obviously once it exceeds a critical value. Horizontal length and flow rate are the vital factors of heat extraction. The outlet temperature rises about 2 °C after 120 days for every 500m horizontal length longer. DBHE with horizontal-well type has significant promotion in system efficiency, linear thermal power and economy. The best profit can be achieved under the flow rate of 800m3/d. On the premise of controlling investment, horizontal length can be appropriately increased. The key results can provide guiding significance for design and retrofit of DBHEs