Optimal filling ratios and thermal performance of a two-phase closed thermosyphon pile in frozen soil under warming conditions

Two-phase closed thermosyphon (TPCT) piles have been employed since the 1960s to stabilize ground temperatures in permafrost regions, preserving the structural integrity of foundations. TPCT piles utilize phase change processes of working fluids, such as CO2 or ammonia, to extract heat from the permafrost and lower its temperature during winter. One important factor influencing the performance of TPCT piles is the filling ratio of the working fluid, which remains underexplored in existing literature. This study aims to fill this gap by investigating the thermal performance of TPCT piles in frozen sandy soil through experiments conducted in a cold room, using a soil box measuring 1.38 m by 0.9 m by 1.22 m. Two model piles, each with a diameter of 63.5 mm and a length of 810 mm, were installed in the soil box. One pile was a CO2-charged TPCT, leveraging the thermosyphon principle, while the other was a conventional uncharged pile for baseline comparison. The filling ratio of the TPCT pile varied from 10 % to 60 % to assess its impact on thermal performance. Five temperature cycles were applied, with the final two cycles incorporating a gradual warming rate of 2 degrees C per cycle to simulate climate change. Results indicated that a filling ratio between 30 % and 50 % optimized thermal performance. Furthermore, the cooling capacity of TPCT piles was reduced by 11.9 % with every 1 degrees C increase in air temperature during the cooling period, providing valuable insights for designing TPCT piles in permafrost regions under warming conditions.