# 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
The maximum temperature recorded at the core of the individual pile after 24, 48, 72, and 96 h is observed to be less than that of the group pile. At 96 h, the highest temperature at the center of the single pile is 0.66 \({}^{\circ}\)C lower than that of pile 5\(\#\), and on average, it is 0.57 \({}^{\circ}\)C lower than the temperature of the peripheral piles. This suggests that the adverse effects of continuous operational mode on the heat exchange efficiency of the central piles within the clustered pile system surpass those observed in the peripheral piles. Figure 11: The temperature difference of entry and exportation water between deep-buried tubular energy pile and pile 5\(\#\) varies with time. Figure 10: Temperature change of 20 m deep-pile group. 3.2 Comparison and Analysis of Rate of Thermal Exchange in Continuous and Intermittent Operational Mode  
To facilitate intermittent operation among the clustered piles of DBP-EP, enhancements were made to the preceding model by implementing a piecewise function to regulate the rate of thermal energy transfer medium within the heat exchange conduit. During periods of heat exchange within the pile, the fluid velocity in the heat transfer process within the conduit is maintained at 1 m\({}^{3}\)/h, while during inactive phases of the pile, the velocity of the fluid involved in the heat transfer process within the conduit is reduced to 0 m\({}^{3}\)/h. Combining with the actual operation state of the piles, four intermittent operational modes were set as follows: 1 8 h on cycle, 16 h off; 2 12 h on cycle, 12 h off; 3 16 h on cycle, 8 h off; 4 20 h on cycle, 4 h off. The intermittent ratios (n, the ratio of the duration of pile operation to the duration of rest) for the four operational modes are n = 1/2, n = 1, n = 2, n = 5, and the intermittent ratio for the continuous operational mode is set as n = \(\infty\). The duration of operation for these conditions is 30 days. Figure 13 illustrates the fluctuation in thermal exchange rate pertaining to the group DBP-EP piles, running in continuous operational mode for 720 h. The depicted figure reveals a consistent downward trend in the thermal exchange rate curves across various positions of the piles. Before 50 h of heat exchange operation, there were no substantial variations observed in the thermal characteristics among the piles. However, as the heat exchange operation progressed to 500 h, the differences in heat exchange rates among the piles gradually increased, with pile 5# having a lower heat exchange rate than the other piles. When comparing piles 6# and 5#, notable differences emerge: at the 600 h mark, the heat exchange rate of pile 6# surpasses that of pile 5# by 52.2 W, while at 700 h, this margin expands to 66.90 W. This observation underscores an amplification in the disparity of heat exchange rates between the central pile and its counterparts with prolonged operational duration. Figure 14 illustrates the temporal variations in heat transfer rate of pile 5# across distinct intermittent operational scenarios. As depicted in the graphical representation, the reduction in the heat exchange efficiency of pile 5# escalates as the intermittent ratio n increases, reaching its minimum when the intermittent ratio n = 5. This is because in the intermittent operational mode, the intermittent ratio n = 5 is lower than the temperature recovery degree of the circumambient stone with n = 2, 1, and 1/2, resulting in a decrease in the heat exchange rate of the pile in the subsequent heat exchange process. Figure 12: Changes of radial temperature of pile 4#, 5#, and 6# and the maximum temperature of single pile core at 20 m depth. Table 2 illustrates the mean heat exchange rates of the DBP-EP group across varied operational conditions over a 30-day period. Comparative analysis reveals that the average heat exchange rates of the DBP-EP exhibit an upward trend with increasing intermittent ratio (n), when contrasted with the continuous operational mode.