# Heat Transfer Performance and Operation Scheme of the Deeply Buried Pipe Energy Pile Group
## 1 Introduction  
Shallow geothermal is a renewable and clean energy source [1; 2; 3], and shallow geothermal storage is one of the technologies used to store and extract stored geothermal energy when needed [4]. At present, there are two kinds of underground structures for collecting and developing shallow geothermal energy: (1) Ground source heat pumps (GSHP); (2) Energy pile. Shallow geo-thermal energy demonstrates a high energy conversion rate, which is facilitated by the heat exchange between the heat carrier fluid in the underground heat exchange tube and the adjacent soil [5]. In previous studies, there was considerable research on the heat transfer efficiency of GSHP [6; 7; 8; 9; 10; 11; 12; 13; 14] and inside-buried pipe energy piles [15], mainly including structural heat transfer characteristics, heat transfer simulation analysis methods, and geotechnical thermal properties [16; 17]. However, there has been less analysis on the optimization of the heat exchange system during the operational phase [18; 19; 20; 21]. [PERSON] [22] and others taking into account both the cooling and heating demands of the structure along with the intermittent operational mode of the GSHP, assessed the system performance of the energy pile through numerical simulation, and used the service life assessment model for comparison of the environmental performance differences between the two heat exchange systems. By studying the thermal effects of intermittent operational mode and continuousoperational mode on energy piles, [PERSON] et al. [23] observed that in the operational mode of 16 h operation and 8 h stop, the axial strain and stress of piles were relatively stable, and the displacement of pile end and pile bottom did not change significantly. [PERSON] et al. [24] studied the effect of phase-change backfill materials on the thermal performance of precast high-strength concrete energy piles under intermittent operational mode, and found that the heat transfer efficiency of phase-change backfill material energy piles under intermittent operational mode was more affected than that of ordinary grout energy piles. [PERSON] [25] and others developed a three-dimensional numerical simulation model to analyze heat transfer within a configuration involving multiple piles. The investigation focused on assessing the heating performance of the system under both continuous and intermittent operating conditions. The outcomes indicate that the energy pile exhibits superior long-term heat transfer performance during intermittent operation compared to continuous operation. [PERSON] [26] and colleagues investigated how intermittent operation affects the thermal storage capabilities of GSHP. Their findings suggest a progressive decline in the system's thermal storage capacity with the escalation of operating cycles. [PERSON] [27] and others conducted a numerical simulation of the heat transfer and flow characteristics of a single helical coil buried tube heat exchanger over a week, considering the coupled heat and moisture migration of backfill and soil, and found that intermittent operation can improve the thermal performance of the buried tube heat exchanger. [PERSON] [28] and others proposed an intermittent operation strategy for a composite GSHP system, designed the system based on actual engineering, formulated four operating conditions, and studied them through simulation. The findings demonstrated that implementing the intermittent operation strategy notably decreased soil thermal accumulation and effectively attained the optimal intermittent operating state. The aforementioned studies primarily concentrated on the analysis of heat exchange in single pile configurations. Enhancing the operational strategies of heat exchange structures can significantly enhance heat transfer efficiency and decrease operational energy consumption. GSHP and energy piles are often deployed in groups, making the study of heat exchanger arrays particularly important.