Experimental investigation on extraction of shallow geothermal energy by flexible two-phase closed thermosyphons: Effect of geometric parameters

•Field tests of super-long flexible thermosyphons (SFTSs) for shallow geothermal systems.•Effects of evaporator dimension and corrugated-pipe shape.•A better thermal performance of SFTS with helical corrugated-pipe.•The maximum heat extraction rate achieved by SFTSs is approximately 25 W/m.•Insights into the heat-transfer mechanism of geothermal thermosyphons. This study investigates the thermal performance of three super-long flexible thermosyphons (SFTSs) fabricated using metal corrugated-pipe under shallow geothermal conditions, aiming to uncover enhanced heat-transfer methods applicable to this innovative design. Field tests are conducted for SFTSs, each with a length of 52.5 m, under various cooling conditions in a drilling environment. The effects of evaporator dimensions and corrugated-pipe shape on the start-up and steady-state thermal performance of SFTSs are also discussed. The results indicate that the whole liquid pool of all SFTSs boils during start-up, even with an initial liquid column height of 15 m. The SFTS with the lower-diameter evaporator demonstrates better thermal performance during both start-up and steady-state operations, with an increase of approximately 31.8% in the heat transfer power under optimal conditions within the test parameters. Moreover, the helicon-shaped corrugated-pipe evaporator provides better thermal performance for SFTSs than the U-shaped corrugated-pipe evaporator, thus resulting in an increase in the heat transfer power by approximately 5.5% under optimal conditions. The maximum heat transfer power achieved in this study is approximately 1227 W when the inlet temperature of cooling water is 3 °C and the flow rate of cooling water is 4500 mL/min, demonstrating the potential of super-long flexible thermosyphon in extracting shallow geothermal energy. These results provide valuable insights into the heat-transfer and enhancement mechanisms of geothermal thermosyphons.