Recently, there has been a growing interest in renewable energies such as geothermal and wind, utilized to reduce CO2 emissions and mitigate global warming. In particular, “closed-loop” geothermal in which water is continuously circulated through the pipe, is gaining significant attention due to its ability to extract heat from underground while having a minimal environmental impact.
To accurately evaluate the economic feasibility of the closed-loop geothermal, a pipe-flow simulator capable of handling a two-phase flow of water-vapor through a pipe, and a formation simulator predicting the decrease in the formation temperature are required. However, most conventional Closed-loop geothermal simulators suffer from the issue of not addressing all these aspects.
Therefore, this research attempts to create a simulator that can deal with both two phases in a pipe and the temperature drop in the surrounding formation. This research was conducted in the following three steps. First, we created a one-dimensional, two-phase, U-loop-type pipe flow simulator. It can calculate fluid pressure, saturation, and temperature, and compute those values in a pipe in single-phase and two-phase conditions as accurately as a commercial pipe flow simulator. Second, we constructed a two-dimensional formation simulator. It can predict the temperature drop near the pipe and the calculated value was verified with the analytical solution of the two-dimensional heat equation. Lastly, we combined these two simulators to develop a “closed-loop” geothermal simulator and conducted some case studies to investigate how the injection flow rate, the formation properties, and the difference in the total thermal conductivity of a formation affect the results of the calculations.
As a result of these studies, it has been revealed that with increasing injection flow rate, the rate of heat gain increases while the fluid pressure and temperature simultaneously decrease. Furthermore, it was also clarified that selecting the appropriate total thermal conductivity of a formation between the parallel and distributed model is crucial due to the great impact on the heat transfer to a pipe.
