## 5 Conclusions  
This study introduced two reinforced energy piles designed to enhance latent heat and thermal conductivity by incorporating PCM and steel fibers. The pile temperature, phase change processes, soil temperature variations, heat transfer power, and thermo-mechanical responses were analyzed by a three-dimensional transient heat transfer finite element model. At the same time, targeted parameter analysis was carried out. The key findings are as follows.  
1. The FRPC pile effectively reduced the thermal influence radius surrounding the pile. However, incorporating PCM introduced uneven temperature distribution within the FRPC pile and a 9.4% reduction in heat transfer capacity compared to the FR pile. Compared with the heating condition, the FRPC pile is more suitable for cooling conditions.
2. The mechanical response of energy piles under thermal loading can be effectively reduced through PCM in pile concrete. Under long-term heat transfer conditions, the axial stress change at the top of the FRPC pile caused by thermal loading could be reduced by 85.9%. The pile-side friction generally shows a downward trend in heating conditions.
3. The phase change path of the phase change material in the pile is that the heat exchange tube gradually starts the phase change to the pile surface, and the phase change is completed quickly. Since the thermal conductivity of the phase change material in the liquid phase is generally lower than that in the solid phase, the FRPC pile is more suitable for cooling conditions compared with the heating condition.
4. The increase in flow rate had a greater impact on the initial peak heat power of the PCM pile. However, the relationship between the two was not proportionally linear. There is an optimal economic flow rate to balance the system's energy consumption and heat power in different conditions.
5. The improvement in the thermal conductivity of energy pile concrete enhanced the overall heat transfer capacity and reduced the mechanical response of the pile structures. Compared with the OC pile, the FRPC pile is more suitable for long-term heat exchange operations.  
In this study, due to the influence of the model tank size, the boundary effect was amplified. In the next study on PCM concrete piles, more emphasis should be placed on eliminating the influence of the boundary effect, and the heat transfer capacity of PCM concrete piles should be more accurately evaluated.  
Conceptualization, X.B. and H.C.; methodology, J.S.; validation, Y.L., J.S. and H.C.; formal analysis, G.C. and J.H.; investigation, J.H.; data curation, H.C. and G.C.; writing--original draft preparation, J.S.; writing--review and editing, Y.L. and X.B.; supervision, X.B.; project administration, H.C.; funding acquisition, X.B. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the State Key Program of the National Natural Science Foundation of China, grant number 51938008, and the National Natural Science Foundation of China, grant number 51925804.  
Not applicable.  
Author [PERSON] was employed by the company Sinohydro Bureau 14 Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.