# Group Pile Effect on Temperature Distributions inside Energy Storage Pile Foundations
## 5 Conclusions  
This paper studies the effect of group piles on temperature change and distribution inside the concrete pile and surrounding soil subjected to cyclic thermal loads. The thermal load is generated from the compressed air stored inside the pile. Two-dimensional symmetric models were developed for various pile spacing cases. The dynamic thermal transfer simulations were then carried out using these models. The following conclusions are drawn from the simulation results.  
1. The group pile effect tends to increase the temperature up to more than 100 \({}^{\circ}\)C depending on the location inside both the soil and the concrete section due to the additional heat transferred from the adjacent pile. The amount of the increase becomes more significant when moving away from the center of the pile at a stabilized state.
2. As the increase of the loading cycles, the group pile effect firstly starts influencing the soil at the center line between two piles and then gradually affects other locations in the soil and concrete section when moving closer to the inner surface of the pile.
3. Temperature increase from the group pile effect is faster for the cases with smaller pile spacing in both the concrete section and the soil. However, the temperature tends to converge to a similar magnitude after enough cycles regardless of the spacing of the piles. The final stabilized temperature can be as high as 120 \({}^{\circ}\)C in the concrete pile and 110 \({}^{\circ}\)C in the soil after numerous loading cycles.
4. The group pile effect increases uniformity of the temperature distribution in the middle of the cycle but causes a less uniform distribution near the end of the cycle along the concrete section.
5. The group pile effect increases uniformity of the temperature distribution in the soil. After a sufficient amount of cycles, the distribution tends to become constant along the radial direction.
6. For the group pile case, the nonlinear thermal model with temperature-dependent soil thermal conductivity shows similar temperature distributions in both the concrete and the soil as that in the linear thermal model. For the isolated pile case, the nonlinear model predicts higher temperature in the concrete section and nearby soil layers but shows a lower temperature in the soil far from the concrete pile.
7. The temperature profiles obtained in this paper for different pile spacings are important for future thermal induced mechanical response studies. The temperature magnitude is about 4 times higher than that in typical thermal-active energy piles. The group pile effect is significant on the temperature distribution of the pile foundation and needs to be considered for the design of the energy storage pile foundation.  
Figure 18: Comparison of temperature in the soil between the linear and nonlinear thermal models for the isolated pile cases: (**a**) maximum temperature at different locations and (**b**) temperature distribution along the radial direction at the end of 1000 cycle.  
**Author Contributions:** Conceptualization, D.Z.; data curation, D.S. and Z.M.; formal analysis, D.S. and Z.M.; funding acquisition, D.Z., J.R.K., and D.L.; investigation, D.Z., Z.M., C.-S.S., and D.L.; methodology, D.Z., C.-S.S., and D.L.; project administration, J.R.K.; resources, C.-S.S., D.L., and J.R.K.; software, C.-S.S.; supervision, D.Z., C.-S.S., D.L., and J.R.K.; validation, D.S. and Z.M.; visualization, D.S. and Z.M.; writing--original draft, D.Z., Z.M., and D.S.; writing--review and editing, D.Z., C.-S.S., D.L., and J.R.K. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was supported by the Nazarbayev University Research Fund under grant #SOE2017001 and by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2020R1F1A1048422).  
**Conflicts of Interest:** The authors declare no conflict of interest.