Two-phase closed thermosyphons (TPCT) for geothermal energy extraction: A computationally efficient framework

Two-phase closed thermosyphons are efficient passive heat transfer devices that have received substantial attention in ground heat extraction. However, modeling thermal energy recovery using thermosyphons is often associated with a high computational cost due to prolonged operation. This study proposes an efficient computational framework for geothermal utilization using two-phase closed thermosyphons. A 1+1D reduced order transient heat conduction equation is numerically solved in the ground and is coupled with the thermosyphon and a coiled heat exchanger through a thermal network equation. Newton–Raphson method is used to solve the resulting nonlinear thermal resistance equation to determine the total heat transfer rate. The 1+1D algorithm is verified and validated against numerical and experimental studies on geothermal energy extraction and energy storage. The proposed framework facilitates the optimization of design and operational parameters for thermosyphons, providing a runtime range of 4-14 h for 20-year simulations of heat extraction. Parametric studies reveal the pivotal role of ground temperature in influencing the thermal performance of the thermosyphon, wherein doubling the evaporator length results in an increase in thermal power output by up to 45%. Various combinations of cooling flow rates and inlet water temperatures at the helical coil are tested to evaluate the system’s potential for electricity generation. Under different conditions, the thermosyphon exhibits the capability to generate thermal power, with a maximum observed production rate of 109.55 W/m for the base case in the analysis. Raising the inlet water temperature to 80 °C, in conjunction with maintaining a circulating coolant fluid flow rate in the helical coil below 10 lpm, qualifies thermosyphons installed at depths ranging from 150 to 250 meters in the ground for clean power generation. Overall, this study highlights the potential and importance of utilizing natural circulating systems with low environmental impact and high efficiency to mitigate the effects of climate change.