Thermal power extraction from a deep, closed-loop, multi-level, multi-branch, U-shaped borehole heat exchanger geothermal system

Abstract

A novel closed-loop, multi-level, multi-branch, U-shaped borehole heat exchanger geothermal system is proposed to achieve a more efficient method to exploit deep geothermal resources. Compared with single U-shaped closed-loop geothermal systems, the new system can reduce the drilling cost and increase the heating length. In this study, first, a three-dimensional unsteady-state flow and heat transfer model was established. Then, the temperature distribution of the working fluid in the tube and a hot dry rock reservoir was analyzed. Third, the key factors that influence heat production were analyzed. The results show that increasing the injection rate blindly does not improve the heat production power; the maximum heat production power can only be obtained by controlling the injection rate to a certain extent. The heat production power has a good linear relationship with reservoir temperature and horizontal well length. Under different reservoir temperatures, the unit power generation price of the proposed geothermal system reaches the lowest value when the horizontal well length is 3000 m and the horizontal well branches number is 4. Overall, this novel design can improve the heat exchange efficiency, reduce the power generation cost and can serve as a reference for the study of exploiting deep geothermal resources.