# Long-Term Thermo-Hydraulic Numerical Assessment of Thermo-Active Piles--A Case of Tropical Soils
## 1 Introduction  
Heating and cooling demands account for most of the energy consumption of domestic and commercial buildings. Traditional solutions for heating and cooling buildings consume large amounts of electricity, natural gas, or fuel. According to REN21 (2022) [1], fossil fuel sources compose about 78.5% of the worldwide total energy consumption, whereas only 12.6% is generated from renewable energy sources. To lower the use of fossil fuels and develop greenhouse gases emission, the Ground Source Heat Pump (GSHP) systems, also known as geothermal energy systems, have been developed within the last few decades representing both environmentally and economically friendly solutions to fulfill the increasing energy demand. Among various kinds of geostructures (piles, diaphragm walls, basement slabs or walls, barrettes, tunnel linings) in which heat exchanging pipes can be installed and act corporately in providing both structural and heat exchange purposes, thermo-active piles are the most widely applied and are the focus of this study [2; 3; 4]. As shown in Figure 1, piles inject heat into the soil at higher climate temperatures and extract heat from the ground at lower temperatures. The circulation of the heat exchange fluid is controlled by the heat pump. Up to now, most of the existing thermo-active piles have been characterized by a temperate climate, where the ground temperature stays relatively constant between 10 \({}^{\circ}\)C and 15 \({}^{\circ}\)C over depths from 10 m to 50 m [5]. Whereas in tropical and hot dry climate zones, the energy demand for cooling is very important (around 56% of electricity consumption),and heat is injected continuously into the ground due to the GSHP system operation. The use of thermo-active piles or geostructures systems may cause an increase in the ground temperature and then affect their long-term performance. Although the implementation of thermo-active piles/geostructures has been increasing rapidly, till now no common design codes and methodologies exist. Guidelines and recommendations for the design of thermo-active pile foundations can be found in Germany (VDI,2001) [6], Switzerland (SIA, 2005) [7], UK (NHBC, 2010) [8], and France (French Recommendations, 2016) [9]. The responses of thermo-active piles are highly affected by the applied thermal load, the ground conditions, the degrees of end restraints as well as pile characteristics (geometry and concrete elastic modulus) [10; 11; 12]. The main features caused by thermal solicitations of thermo-active piles are characterized by the axial deformation of the structure, the stress/load variations induced by the constrained thermal expansion and contraction, and the resistance changes resulting from heating and cooling [3; 13]. The design approaches for thermo-active geosystems affected by groundwater flow have not been well defined yet. Therefore, analytical solutions for the ground temperature response functions based on the infinite line heat source approach are adopted in this work to assess heat transfer with the presence of groundwater flow [14]. Thermo-hydraulic coupled simulations can provide a more thorough understanding of the system temperature influence on the surrounding environment and heat diversion. In addition, the assessment of thermo-active system performance in various aspects such as the long-term heat exchange behavior is still an open issue. The thermal operation of thermo-active structures strongly depends on the regional heating and cooling energy demands [15; 16]. When they are nonsymmetrical over seasons, the sustainability will be directly affected, especially in tropical climates or cold regions. [PERSON] et al. (2019) conducted oedometric tests regarding the tropical Bangkok sand. In this region, the ground temperature near the pile-soil interface can increase from 28-30 \({}^{\circ}\)C to 50 \({}^{\circ}\)C due to the thermo-active pile's operation [17]. [PERSON] et al.