# Finite Element Analysis of Combined Energy Piles with Long and Short Heat Exchanger Tubes
## 5 Conclusions and Prospect  
A new energy pile technology with the combination of long and short heat exchanger tubes is proposed for implementations in coastal areas. Numerical simulation was performed for a Ningbo rail transit project. The following main conclusions are gained:  
1. The new energy pile technology considers the groundwater condition., which enhances the heat exchange between the tubes and the soil. It also helps to dissipate the heat from the soil, effectively addressing the issue of heat accumulation. 2. The temperature change of the cement soil in the pile is consistently higher than the temperature of the pile body by 1-2 \({}^{\circ}\)C. As the length of the heat exchanger tubes on the inside of the pile increases, the stable temperature change of both the pile body and the cement soil gradually decreases. 3. When there is groundwater flow, the temperature change of the pile gradually decreases with the acceleration of groundwater flow. The maximum temperature change can be reduced by 11.91% (1.69 \({}^{\circ}\)C) and 8.82% (0.83 \({}^{\circ}\)C) for cooling changes. Additionally, the pile body reaches a stable temperature become faster under groundwater flow condition; and the higher the flow rate is, the faster the stabilization will be reached. 4. Under the coupling effect of heat and force, the displacement is positively correlated with the temperature change. Although the arrangement of heat exchange tubes inside the pile increases the amount of heat exchange, the maximum changes in the uplift and settlement of the pile top only increase by 0.66 mm and 0.41 mm, respectively. 5. Under the action of thermal coupling, the expansion of the pile shaft results in a decrease in the lateral friction resistance of the upper section and an increase in the axial force of the lower section. The axial force change of the pile shaft is positively correlated with temperature change. The axial force change increases for the new energy pile, but the maximum change is only 61 kN and \(-\)19 kN for temperature heating or cooling condition, respectively. 6. Compared with \(L_{2}\) = 80 m, \(v\) = 2 \(\times\) 10\({}^{-4}\) m/s and \(L_{2}\) = 0 m, \(v\) = 2 \(\times\) 10\({}^{-4}\) m/s, the temperature difference of the average temperature of the pile body is within 1 \({}^{\circ}\)C; hence, after the heat exchanger tubes on the inside of the pile are laid, the axial force  
Figure 14: Impact curve of groundwater seepage rate on lateral friction resistance of pile shaft. of the pile body and the displacement of the top of the pile are not obvious. The average temperature of the heat exchanger tubes on the outside of the pile and the outlet of the heat exchanger tubes on the inside of the pile are 45.50 \({}^{\circ}\)C and 34.22 \({}^{\circ}\)C, respectively, and the total heat exchange increases by a factor of 1.75. * This paper focuses on the effect of different tube types on heat exchange efficiency and the mechanical properties of pile and soil layers. The design of heat exchanger needs to consider many factors, such as building environment, energy demand, geological conditions, structural design, etc., which is worthy of further study. In addition, comparative studies that include theoretical analyses, numerical simulations, and field measurements are needed to further promote the application of the new energy pile technology. Conceptualization, S.C. and Y.D. ; methodology, Y.D. ; software, S.C. and S.N. ; validation, Y.D. and R.Z. ; formal analysis, G.C. ; investigation, S.N. ; resources, R.Z. ; data curation, S.C.; writing--original draft preparation, S.C. and S.N. ; writing--review and editing, Y.D. ; visualization, S.C.; supervision, W.M. ; project administration, W.M. ; funding acquisition, Y.D. and R.Z. All authors have read and agreed to the published version of the manuscript.