Performance Analyses of Deep Closed-loop U-shaped Heat Exchanger System with a Long Horizontal Extension

Abstract

Deep closed-loop U-shaped heat exchanger with a long horizontal extension (up to 5 km) is a new approach to harvest heat more sustainably. This system comprises two deep vertical boreholes which are connected using a long horizontal section. Such a closed-loop system avoids subsurface water contamination, greenhouse gas emission, seismic events, and scaling problems. Furthermore, it leads to mitigating pumping power, exploratory risk, and environmental footprint, which are associated with prolonged operation period and less uncertainty. However, the performance of this system is not yet characterized to observe its potential over conventional heat exchangers. In this study, the depth, horizontal extension, and diameter of boreholes, formation temperature gradient, flow rate, inlet temperature, thermal conductivity of formation, and length of insulation layer are considered as variable parameters to study the performance of the system. An in-house wellbore simulator, called MOSKITO, is used to calculate the outlet temperature and temperature and pressure distributions along the system, Then, the overall and sectional power output are optimized based on these parameters. According to the results, the temperature of the produced fluid is a nonlinear function of flowrate. This nonlinearity is due to the influence of flowrate on the displacement of the cooled region in the reservoir. To increase the power generation or output temperature while decreasing the flowrate it may be necessary to increase the insulation of the production well. It has resulted that for a particular range of flowrate, it is feasible to produce hot water with continuous temperature enhancement over 100 years of operation. Additionally, with simultaneous optimization of fluid velocity and geometrical factors, the continuous generation of 2.5 MW energy over one century is accessible. Finally, flowrate also has a nonlinear impact on the pressure drop and thermosiphon system since increasing flowrate magnifies the pressure loss due to friction in the horizontal section. However, it may increase the temperature difference between water columns in vertical wells. Further investigation of the results in both vertical and horizontal sections enables us to develop a conceptual multilateral wellbores system with a higher flow rate.