# Long-Term Thermo-Hydraulic Numerical Assessment of Thermo-Active Piles--A Case of Tropical Soils
## 1 Introduction
(2020) performed a series of laboratory and field tests to evaluate the seasonal thermal performance of a thermo-active pile in a typical Brazilian tropical soil profile [18]. The studied unsaturated tropical soil thermal conductivity increases by 32% after the rainy season, leading to a heat transfer efficiency variation. It is also critical to evaluate the climatic temperature information, particularly in regions with inherent seasonal variability. [PERSON] et al. (2020) investigated  
Figure 1: The application of thermal-active piles based on energy demand of building: (**a**) Heating mode, and (**b**) cooling mode. the applications of thermo-active structures under tropical and subtropical climates in a commercial scope [19]. Compared to the subtropical regions, the thermo-active systems in tropical regions are less economically viable due to their inefficient performance and high cooling demand. The more balance between heating and cooling, the more economically feasible is the GSHP system. [PERSON] et al. (2021) carried out thermal response tests on a thermo-active micropile in a typical tropical sedimentary stratified soil profile [20]. The long-term assessment of the thermal balance of the GSHP system provides implications for its thermal sustainability. However, most of the existing literature ignores this point. [PERSON] and [PERSON] (2010) experimentally and numerically demonstrated that single borehole heat exchanger-coupled geothermal heat pumps can operate sustainably for decades with combined heating and cooling demands [21]. For the long-term performance, [PERSON] et al. (2012) used the finite-element method to investigate the role of groundwater flow on the 10-year performance of large borehole heat exchangers, it was shown that the groundwater flow does not affect the hourly peak thermal loads but improves the long-term system efficiency [22]. [PERSON] et al. (2016) studied numerically the effect of surface air temperature fluctuations on the GSHP heat exchange rate over 25 years, which would reduce the length of 30m-deep thermo-active piles by about 11% to achieve economic savings [23]. This effect was also emphasized by [PERSON] et al. (2018) [24]. [PERSON] et al. carried out a numerical analysis to assess 60 years of thermal perturbation from a low enthalpy heating plant. They showed that sustainability is related to the characteristics of the groundwater flow (direction and pressure) and the position of the geothermal system [25]. The lack of knowledge of the long-term performance of thermo-active piles in tropical regions motivates this study. Differing from other studies conducted in temperate regions, tropical climatic temperatures and geological conditions are taken into account here, and energy demands in three cities in Brazil are chosen. This work also performed parametric analysis through coupling thermo-hydraulic numerical simulations. The thermal behaviors of a thermo-active pile for 10 years are investigated. Additionally, a numerical approach compiled in the FISH language is used to quantify the heat exchange in terms of conduction and advection, which distinguishes the contribution of groundwater flow and the ground The distinction between conduction and advection divergence for the energy exchange with the presence of groundwater flow is therefore investigated. Some recommendations on the proper implantation conditions of thermo-active piles at the design stage are addressed at the end.