Factors affecting the efficiency of closed-loop geothermal wells

Geothermal energy has a promising prospect of becoming a feasible source of clean energy in the future. However, conventional open-loop geothermal systems need continuous water circulation, which limits their application in dry areas. On the other hand, long-term fluid injection into the reservoir may cause issues such as seismicity. To overcome these shortcomings, closed-loop geothermal systems are not requiring water withdrawal or water injection. However, the mechanism controlling the efficiency of closed-loop systems has not been well analyzed. To resolve the issue, a systematic study was conducted on factors affecting production efficiency of a closed-loop geothermal well. First, a coupled three-dimensional model was established, which was then validated against experimental results available in the literature. Using this model, the effects of several critical design factors were investigated quantitatively. These factors include cement thermal conductivity, cement thickness, fluid circulation rate, injected fluid temperature, heat capacity of circulating fluid, heat exchanger height, and tubing outer diameter. Our simulation results indicate that increasing the cement thermal conductivity enhances both produced fluid temperature and thermal power. The effect of cement thickness is correlated with the value of its thermal conductivity. As the circulation rate increases, the temperature of produced fluid decreases while thermal power increases asymptotically. Using a circulating fluid with high heat capacity would result in higher thermal power. A large heat exchanger height would boost both the production temperature and thermal power significantly. Finally, a dimensionless analysis was conducted, through which two dimensionless numbers were derived to integrate the effects of different factors. By comparison analysis, the accuracy of the dimensionless numbers was validated. Also, the impact of the dimensionless numbers on dimensionless produced fluid temperature was determined. With the dimensionless numbers, the effects of different individual factors can be evaluated comprehensively, which brings convenience to parameter design. The potential applications of the derived dimensionless numbers in parameter design of closed-loop systems were discussed. Results and conclusions obtained in this paper may provide a reference for optimally designing closed-loop systems.