Numerical simulation of single well geothermal system based on the artificial geyser concept considering phase change and multiphysics coupling

A geothermal system based on the artificial geyser concept can be free of high-pressure circulation pump. The core component of this system is a flash steam chamber located at the high-temperature well bottom, where periodic spray and vaporization of fine droplets occur, thus generating high pressure to drive the vapor to rise through the well- insulated inner tube of a coaxial heat exchanger (CHE). Though there was a semi-analytical model to evaluate this process, it failed to consider the heat transfer between ascending hot vapor and descending cold water in the CHE. Therefore, a new multi-physics coupled numerical model has been developed to assess the system operation mechanism, including the phase change of water. This model employs full coupling throughout its operation: during the downward flow of water in the well, it accounts for heat sources from both the hot formation and the ascending steam; during upward flow of vapor, it assesses the degree of steam condensation due to heat loss. The thermal behavior of descending water in the annulus is validated against existing research on a closed-loop borehole heat exchanger. Through the sensitivity analysis, it can be concluded that changes in the radius of production well and variation in the flow rate of injection well have a significant impact on the system's thermal power. For an engineering case with the same initial and boundary conditions, the outlet temperature of our system is around 50 °C higher than that of the hydrothermal system, and the total heat extraction reaches 380 kW, indicating that the artificial geyser system offers superior performance in terms of thermal energy utilization. After comprehensive optimization of the injection rate of water, the production well's radius, and the bottom pressure, the total heat extraction can reach 1 MW. In the actual project, one should monitor the pressure between the production well and the flash chamber to ensure the outlet vapor remains dry and has a high flow rate.