# Heat Transfer Performance and Operation Scheme of the Deeply Buried Pipe Energy Pile Group
## 4 Results and Discussions
### Heat Transfer of DBP-EP Group in Different Operation Schemes
#### 4.3.1 Continuous Heat Transfer Characteristics of Pile Group  
Figure 10 shows the transverse temperature gradient cloud map at a depth of 20 m of the deep-buried pipeline energy pile group within 30 days of operation (Figure 10a-c represent the tenth, twentieth, and thirtieth days of heat exchange, respectively). As shown in the diagram, the internal temperature of the piles gradually increases with the heat exchange operation, and the thermal influence area of each individual pile gradually expands. In the thermal exchange procedure of the group piles until the tenth day, the thermal diffusion area of the individual piles is small, and there is no thermal interaction phenomenon between the piles. When the piles have been in operation for 30 days, the thermal diffusion area of the individual piles increases, and there is thermal interaction between the piles, indicating a more obvious thermal accumulation phenomenon in the group piles. The rationale behind this phenomenon lies in the high thermal conductivity of the concrete utilized in the pile foundation, facilitating continuous heat absorption from the thermal carrier fluid. Nevertheless, the heat conductivity coefficient of the adjacent rock and soil is significantly inferior to that of the concrete composing the pile foundation, so the heat inside the pile foundation cannot dissipate in time. Simultaneously, there is a common heat transfer zone between the group piles, which exacerbates the thermal accumulation inside the pile foundation and the circumambient soil and stone. To illustrate visually the adverse influence of the group pile effect regarding the thermal interchange process of DBP-EPs, Figure 10 displays the temporal evolution of the temperature differential between the entry and exportation water of the central pile within the grouped piles compared to that of a single pile. Figure 11 depicts the temperature gradient between the entry and exportation heat carrier fluid of the central pile (i.e., pile 5#) and the single pile after 720 h of heat exchange. From the figure, it is evident that during the initial phase of thermal exchange, the most significant reduction in temperature differential between the entry and exportation of the two piles occurs. At this time, the differential in thermal transfer between the two piles is not significant. As the heat exchange operation progresses, the temperature contrast between the two stacks gradually stabilizes over time. After 300 h of heat exchange, the temperature contrast between the entry and exportation of pile 5# intensifies in comparison to the standalone pile. From 0 h to 168 h, the temperature difference between the entry and exportation of pile 5# is basically consistent with that of the single pile. At 720 h, the temperature differential between the entry and exportation of the single pile is 0.20 \({}^{\circ}\)C higher than that of pile 5#. Combined with Figure 10, it can be seen that the negative impact of the group pile effect formed under continuous operation gradually becomes significant regarding the thermal performance of the energy piles. Figure 9: Aerial view of the 2D domain with the nine points representing the foundation piles. Figure 12 depicts the radial temperature profile at a depth of \(-20\) m for pile configuration 4\(\#\), 5\(\#\), and 6\(\#\), as well as the highest temperature at the pile center for the single pile at 24, 48, 72, and 96 h. The temperature at the core of piles 4\(\#\), 5\(\#\), and 6\(\#\) exhibits a gradual rise, accompanied by an expansion in the region of heat diffusion. The temperature at the center of pile 5\(\#\) and the temperature of the circumambient soil and stone are higher than those of the peripheral piles. This is because the heat diffused by the peripheral piles is transferred to the central part of the soil and stone, thereby reducing the temperature difference between the circumambient soil and stone and the central pile, which negatively affects the subsequent heat exchange of pile 5\(\#\).