Impact of thermosiphoning on long-term behavior of closed-loop deep geothermal systems for sustainable energy exploitation

Circulation of working fluid in closed geothermal loops is an alternative environmentally friendly approach to harvest subsurface energy compared to open hole geothermal doublet systems. However, the rapid decline of production temperature, low generated thermal power, and difficulties in deepening the system are major limitations. Herein, synthetic studies are presented to investigate the system's performance and improve its longevity for better use of this clean baseload power. The investigations are conducted by implementing appropriate equations of state to model state-of-the-art thermal and hydraulics processes in wellbores and considering various geometrical configurations to adopt proper design strategies. They provide insight for maximizing the generated thermal power, decreasing pumping energy, and avoiding production temperature drawdown. The results indicate that a stable thermal condition could be reached in which not only the temperature breakthrough is avoidable, but also the generated thermal power and production temperature continuously enhance over the project lifetime of one century. Analysis of the thermosiphon effect in the designed systems revealed that even with the pressure loss of 900 kPa at surface installations, the triggered natural flow rate is larger than 11 L/s. This thermosiphon flow rate yields the thermal power production of 2 MW and Cumulative extracted energy of 15 PJ over the project lifetime of 100 years. Restriction of this flow rate to 5 L/s leads to an average extraction temperature of 80 °C. It is also found that a change in the subsurface temperature gradient does not affect the optimal 2 km isolation length of the production well.