## 5 Conclusions  
This paper investigates the heat transfer response of the concrete pile foundation subjected to temperature changes from the compressed air for the renewable energy storage purpose. The non-steady state heat transfer analysis was conducted using the 2D plane strain model with different thermal properties for concrete and pile section geometries. The temperature changes and distributions in the concrete pile foundation have been identified from the numerical results. The temperature changes and distributions obtained in this paper can further be used for thermal mechanical analyses to investigate the structural and geotechnical safety and stability for the foundation system and surrounding soil. The following conclusions can be drawn for the general heat transfer response in the concrete pile foundation:  
1. The heat generated from the compressed air is gradually transferred from the inner to the outer surface of the pile section and causes temperature changes, which become smaller and slower when moving from the inner to the outer surface. 2. The temperature distribution along the radius direction in a loading cycle varies with time, especially when the temperature loading starts to drop. Residual temperatures in the pile section are observed at the end of the loading cycle. 3. The temperature in the pile foundation and nearby soil gradually increases with cycles because of the cumulative effect of the residual temperature. As moving from the inner surface to the outer, the increase of temperature with cycles and the temperature variation within each cycle become smaller. 4. The maximum temperature in each loading cycle increases with the loading cycles but starts to be converged at a stable value after a sufficient amount of cycles. As moving from the center of the pile foundation, more cycles are needed for the maximum temperature to be converged. Moreover, for the effects of the studied parameters (\(d_{i}\), \(k\), and \(c\)), the following conclusions can be drawn:  
1. As the \(d_{i}\) increases, the section thickness of the pile foundation reduces, which requires less time for the heat transfer from the inner surface to the outer. Therefore, the temperature change in the larger \(d_{i}\) cases is more significant, but the temperature distribution along the radius direction is more uniform. The convergence of the temperature variation between cycles in the pile foundation is faster for the larger \(d_{i}\) cases. 2. As the \(k\) increases and \(c\) reduces, the heat transfer speed increases, which results in larger temperature changes and higher temperatures in the concrete pile foundation. The convergence of the temperature variation between cycles in the pile foundation also becomes faster as \(k\) increases and \(c\) reduces. The effect of \(c\) is much smaller than that of \(k\). The research work presented in this paper initiates an investigation of the temperature distribution in the proposed energy storage pile foundation. The numerical model and simulation have several assumptions and limitations including: the cracking and nonlinearity of the concrete thermal properties were not considered; the heat transfer at the bottom surface of the pile foundation was ignored; the temperature in the heated compressed air was assumed to be uniform inside the pile foundation. Future experimental research is needed for verification purposes. Conceptualization, D.Z. ; data curation, Z.M. and D.S. ; formal analysis, Z.M. and D.S. ; funding acquisition, D.Z. and J.R.K. ; 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, Z.M. and D.S. ; visualization, Z.M. and D.S. ; 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.