Temperature response and thermal performance analysis of a super-long flexible thermosyphon for shallow geothermal utilization: Field test and numerical simulation

Abstract

• Super-long flexible thermosyphon (SFTS) fabricated by metal corrugated pipe, 32m in length. • Field tests of SFTS for shallow geothermal utilization. • Start-up and steady-state thermal performance of SFTS. • CFD simulation of long-term underground temperature variation near SFTS. • A method to avoid the heat transfer problem due to super-high liquid column of working fluid in super-long thermosyphon. A super-long flexible thermosyphon (SFTS) fabricated by the corrugated pipe, with a total length up to 32m, was field-tested in a drilling, in order to evaluate its potential for use in shallow geothermal utilization. The seasonable dependence of local underground temperature and air temperature, the corresponding temperature response of SFTS, and the its heat transfer characterists were investigated. The underground temperature recovery characteristics under the operation of uni-directional heat extraction were also conducted by a combination analysis of field tests and CFD simulation. The temperature response characteristics of SFTS in start-up of field tests show that the SFTS could operate effectively as the vaporization of working fluid occurs all over the whole evaporator. This indicates that employing a corrugated pipe prevents the formation of a super-high liquid pool at evaporator, which would inhibit the boiling of working fluid at the bottom. The maximum heat transfer power of SFTS obtained in field tests of this work is approximately 305W under the conditions with the inlet temperature of cooling water controlled at 5.5°C and the flow rate at 1500mL/min. The analysis of steady-state heat transfer results reveal that the underground temperature plays the most important role in the overall thermal performance of SFTS, followed by the flow rate and the temperature of cooling water. The long-term uni-directional heat extraction of SFTS from underground for heating is feasible and however the heat extraction rate is recommended to be maintained below 30W/m at the drilling site aiming to make sure a good sustainability. This work reports field tests of a super-long flexible thermosyphon in geothermal utilization for the first time.