# Long-Term Thermo-Hydraulic Numerical Assessment of Thermo-Active Piles--A Case of Tropical Soils
## 4 Conclusions  
This work focuses on the long-term numerical assessment of a thermo-active pile in the tropical region by conducting 3D thermo-hydraulic coupled simulations using the finite difference method (FDM). The model parameters are based on in-situ results which therefore ensure its reliability. The numerical simulation permits to analyze the behavior of thermo-active piles in terms of temperature, and energy exchange of the selected control volumes. Parametric analysis of thermal solicitation and pile diameter as well as groundwater flow was performed.  
Figure 15: The thermal balance of the control volumes in the 1 st year and 10 th year for different groundwater flow velocities.  
In tropical regions, the thermal solicitation T3 (continuous cooling mode) would be ideal, but also the most dangerous demand. Compared to T1 and T2, it leads to a higher accumulated soil temperature. However, the maximum soil temperature around the pile in the studied cases does not exceed the threshold (35 \({}^{\circ}\)C). For T1 and T2, the maximum accumulated temperature occurs at the water table position, while T3 is around the pile center.  
The energy exchange per unit of soil control volumes around the pile can be predicted by distance functions. After 10 years, the average soil heat injection around the pile decreased by 26% for the T1 thermal solicitation, but the heat extraction increased by 30%. While the heat injection decreased by 20% for T2, with no significant change in the heat extraction. The heat injection after 10 years is only 0.2 % of the 1 st year for T3.  
Variation of the pile diameter and pipe arrangement does not affect the thermal disturbance range under the T1 thermal solicitation but affects the pile average temperature.  
A groundwater flow allows improving the thermo-active pile thermal sustainability. High groundwater flow velocity can nonlinearly reduce the average temperature of the pile-soil interface and pile body. Compared to \(v_{D}=0\) m/day, the average heat transfer per unit of soil control volumes increases about 6 times for \(v_{D}=1\) m/day. The thermal equilibrium in the 1 st and 10 th year remains quasi-constant for \(v_{D}=1\) m/day.  
Extensive studies should be conducted to better understand the thermo-active piles' performance when applied in tropical regions. More particularly, the interaction between the thermo-active pile groups cannot be ignored, considering the conversion efficiency of the entire system scale rather than one single functioning pile will allow to quantify its performance more precisely.  
Conceptualization, J.Z. and D.D.; methodology, D.D.; software, J.Z.; validation, D.D., Q.P., C.M. and C.d.H.C.T.; formal analysis, J.Z. and D.D.; investigation, J.Z., Q.P., C.M. and D.D.; resources, D.D. and C.d.H.C.T.; data curation, J.Z., Q.P., C.M. and D.D.; writing--original draft preparation, J.Z.; writing--review and editing, D.D., Q.P., C.M.; visualization, J.Z., D.D., Q.P., C.M. and C.d.H.C.T.; supervision, D.D. and C.d.H.C.T.; project administration, D.D. and C.d.H.C.T.; funding acquisition, J.Z. and D.D. All authors have read and agreed to the published version of the manuscript.  
This research was funded by China Scholarship Council, grant number 201908070075 and the laboratory 3 SR is part of the LabEx Tec 21 (Investissement d'avenir--grant agreement no. ANR-11-LABX-0030).  
Not applicable.  
The authors declare no conflict of interest.