Efficient Development and Utilization of Geothermal Energy: An Analysis of the Operational Strategy for Deep-Buried Pipe-Type Energy Piles Considering Seepage Effects

Abstract

As a shallow geothermal energy development technology, energy pile contributes to sustainable development. The seepage effect has a positive effect on the heat transfer performance of the energy pile, and the heat transfer efficiency of the energy pile can also be improved by optimizing the operation strategy. Combined with the structural characteristics of the deep-buried energy pile, the heat transfer characteristics of the deep-buried energy pile are analyzed under continuous and intermittent operation conditions, and the effect of seepage on the heat transfer performance is further investigated under the intermittent operation mode. The results show that the long-term operation of the deep-buried energy pile will reduce its heat exchange performance and aggravate the heat accumulation phenomenon inside the pile body, and the intermittent operation can maintain a higher instantaneous heat exchange rate (HER) in the long-term operation compared with the continuous operation. Considering the energy demand, when the intermittent ratio is 5, the average HER of the pile body only decreases by 68.93 W, and the overall energy efficiency of the pile body is improved by 7.7%. Combined with the operating effects of different intermittent ratios, the optimal range of the circulating medium flow rate for deep buried pipe energy piles should be selected from 1.0 m³/h to 1.2 m³/h. Groundwater seepage can weaken the degree of heat accumulation inside the DBP-EP piles and improve the overall heat exchange efficiency of DBP-EP, and combined with the intermittent operation mode will be able to further alleviate the DBP-EP heat buildup. The two factors promote each other and have a positive impact on the piles, positively affecting the soil’s long-term heat exchange.

1. Introduction

There is a close relationship between the development of shallow geothermal energy and sustainable development. As a technology for the development and utilization of shallow geothermal energy, energy pile conforms to the direction of sustainable development in terms of environmental protection, resource conservation and economic benefits, effectively promoting energy transformation, promoting energy conservation and emission reduction, and improving residents’ quality of life [1,2]. Energy pile technique is used to harvest and employ subterranean thermal energy. With the benefits of stability, economics, and environmental protection, energy pile is a type of subsurface structure able to both carry and exchange heat [3,4,5]. Although constrained by the length of the pile body, its heat exchange space range is limited, and it typically needs to be aided by other heat supply methods to meet the building energy supply needs, the buried pipe type dominates energy piles nowadays [3,6]. Setting up a deep hole in the pile, which passes through the bottom of the pile to reach a heat exchange depth of more than one hundred meters, the deep buried tube energy pile will essentially increase the total amount of heat exchange by showing structural shape in Figure 1 [6]. Furthermore, advantageous for this kind of construction are high energy pile heat exchange efficiency and land area preservation via buried tube construction.

Continuous operation of ground source heat pump (GSHP) systems and energy piles in real engineering applications may induce thermal imbalance in the surrounding soil and rock mass, therefore impeding effective geothermal energy extraction [7,8,9,10,11]. According to current research, optimizing the running strategy of underground heat exchange pipes significantly lowers thermal accumulation around the system, enhancing the heat transfer performance. Developing a three-dimensional heat transfer model for multi-energy piles, Gui [12] et al. investigated the heating performance of the system under both continuous and intermittent running situations. The results revealed that intermittent operation improves performance. Accounting for heat-moisture coupled transmission in backfill and surrounding soil, Liang [13] et al. numerically simulated heat transfer and flow characteristics in a single helical pipe heat exchanger during one week. Their results showed that intermittent operation improves buried heat exchanger thermal performance. Melis [14] et al. built a lifetime assessment model in addition to a finite element model to evaluate the heating and cooling needs of a reference building and the intermittent operation of GSHPs. The average pile temperature of energy pile under intermittent operation condition is lowered by 59.3% and the average soil temperature by 72.9% compared with the continuous operation mode, according to He [15] et al., on the effect of intermittent ratio on heat transfer efficiency of energy pile under intermittent operation condition. The long-term continuous running of energy piles causes a thermal imbalance that intermittent operating mode can help to reduce. As the research object, Hu [16] et al. took the energy pile with phase change material and investigated the heat transfer performance of the energy pile under the running conditions of 12 h, 12 h halt, and continuous operation. Under the intermittent operation mode, the results revealed that the heat transfer rate per length-meter of the energy pile and the wall temperature recovery rate of the pile were much enhanced after thirty days of operation. Under intermittent operation, Shang [17] et al. investigated the heat exchange and coefficient of performance (COP) of GSHP systems. The findings showed that when the piles rest before starting activity, heat exchange and COP rise. Yang [18] et al. compared continuous and intermittent operation of GSHPs and found that intermittent operation significantly reduced thermal accumulation in the soil, as well as electricity consumption. Zhang [9] et al. developed a GSHP model and compared simulation results with those from commercial buildings. The results indicated that intermittent operation maintains a balanced ground temperature. In conclusion, optimizing the operating strategy of the heat exchange system effectively improves heat transfer efficiency and reduces operational energy consumption. The upper heat exchange tube of deep-buried tubular energy pile is wrapped by concrete with fast heat transfer, and the lower part is wrapped by rock and soil. Its heat transfer along the depth direction belongs to the condition of variable cross section and variable heat transfer medium, which is different from common ground source heat pump or energy pile. Therefore, it is of great significance to study and optimize the operation strategy of deep-buried tubular energy pile.

In addition, most of the heat exchanger tubes in the energy pile are situated in the saturated soil area below the groundwater level. Therefore, the heat transfer between the pile and the earth will be influenced by the lateral seepage of groundwater. Consequently, it is important to take seepage into account while calculating the heat transfer between the pile and the ground-buried pipe heat exchanger [19,20]. Zhang [21] et al. established a three-dimensional mathematical model of groundwater seepage heat transfer in energy piles by building a three-dimensional mathematical model of groundwater seepage and investigated the influence of three-dimensional groundwater seepage on the heat transfer between the pile and the soil. The results reveal that groundwater seepage can efficiently enhance the heat transfer efficiency of the energy pile and help to distribute heat around it. Lian [22] et al. investigated the seepage heat transfer model of DBP-EP cluster piles with various layouts as the research object, and the results suggest that seepage can weaken the degree of heat accumulation around the piles of the DBP-EP cluster piles, thus further reducing unnecessary heat loss. In seepage situations, You [23] et al. suggested an analysis model of the spiral coil pipe energy pile group and investigated the heat transfer performance of the system in groundwater seepage conditions. The results reveal that groundwater seepage efficiently lowers the soil temperature around the energy pile, thereby lowering the system energy consumption; the heat exchange between the pile body and the soil located upstream of the seepage is shown to be larger. Chen [24] and colleagues examined the flow rate of the circulating media within the heat exchange tubes in addition to groundwater seepage. They computed the effect of the flow rate of the circulating media and the seepage rate on the heat transfer efficiency and energy efficiency ratio (EER) of the group piles by means of a thermal-seepage coupling model of the group piles. The results showed that although improving the heat transfer efficiency of the energy piles, a rise in the flow rate of the circulating medium increases the degree of internal heat accumulation within the piles, therefore lowering the energy efficiency ratio. Groundwater seepage can improve the heat transfer performance of the group piles and remove the internal heat accumulation brought about by the rise in the circulation medium. Yang [25] and colleagues investigated the thermodynamic performance of energy piles under seepage conditions utilizing them as the test items. The results revealed that the heat transfer performance and degree of temperature recovery between the piles and the ground are more important when the seepage rate is quite high, therefore reducing the pile displacement and the soil pressure at the pile tip.

Ground-source heat pumps (GSHP) [24,25,26,27] and energy piles [28,29] with pipelines buried inside the piles dominate current studies on the operating methods of energy underground structures. However, limited analysis exists regarding the intermittent operation of DBP-EP systems. Compared to traditional underground pipe heat exchangers, DBP-EP systems include deep heat exchange wells that extend hundreds of meters underground. The impact of groundwater seepage on the heat transfer characteristics of DBP-EP systems must not be ignored. This study examines DBP-EPs under various intermittent operation conditions. In situ field experiments are conducted to analyze the heat transfer characteristics during operation. The variations in heat transfer performance between continuous and intermittent operations are investigated using a numerical model of DBP-EPs under several operating modes. Aiming to further maximize the operational strategy of DBP-EPs, the study further assesses the effects of two fundamental operational parameters—circulating fluid velocity and inlet temperature—on the heat transfer performance of the intermittent operating mode. Following the choice of the ideal intermittent operating strategy, the effect of groundwater seepage on the heat transfer properties of the DBP-EP system under this operational mode is investigated, therefore providing a reference for the engineering use of DBP-EPs.

2. Test Site Overview

2.1. The On-Site Overview

We conducted this experiment at the Energy Pile Demonstration Base [14]. Comprising a drilled, cast-in situ pile with a diameter of 800 mm and a length of 23 m, the energy pile boasts a C30 strength grade. Drilled vertically from the center of the pile foundation to a depth of 100 m, a heat exchange deep well standing at 150 mm in diameter and a depth of 100 m, the deep well is backfilled with fine sand. Embedded in the reinforcement cage are heat exchange pipes and sensors; attached optical fibers convey temperature sensing to the pipe walls. Control tubes link the heat exchange pipes to a central manifold. Arranged in a double U-parallel pattern, the heat exchange pipes have an inner diameter of 20 mm and an outer diameter of 25 mm.

2.2. Experimental Design

Summertime saw the experiment carried out, with an average ambient temperature of 28 °C. DBP-EP was the test subject. Using a thermal response device, the experiment linked the heat exchange pipes in the energy pile so heating the circulating fluid within the pipes. The fluid flow speed was controlled with a pump. Using a Distributed Temperature Sensing (DTS) system, the ground temperature was noted at first as 18.5 °C before the test. The test on temperature responsiveness took seven days. Recorded during the experiment were the entrance and outflow water temperatures as well as the circulating fluid flow rate. The thermal response device helped to keep the heat exchange pipe’s inlet water temperature at 30 °C while setting the inlet flow rate to 1 m³/h (velocity: 0.52 m/s).

3. Analysis of Test Results

Figure 2 shows the curves in temperature variation for the DBP-EP’s intake and exit. The figure shows that the heating of the flowing fluid inside the pipes by the thermal response apparatus causes the temperatures at both the inlet and the outflow of the pile to rise gradually and stabilize. As the thermal response equipment runs in the early phases of the experiment, the temperatures at the inlet and outlet of the heat exchange pipe rise gradually. Along with a quite high rate of temperature growth, the temperature difference between the intake and outlet of the heat exchange pipe quickly rises during this period. The temperature differences at the heat exchange pipe’s input and exit stabilize as the experiment goes on, therefore slowing down the pace of the temperature rise. The temperature difference between the heat exchange pipe’s input and output eventually finds equilibrium and settles at around 3.3 °C. The heat exchange between the pile and the surrounding ground progressively reaches a balance during this phase.

4. Verification of DBP-EP Model

4.1. Model Building

4.1.1. Governing Equation

The cross section of the DBP-EP heat exchange tube is set to a circle and the circulating medium in the tube is set to liquid water during the heat exchange procedure. Within the heat exchange tube, the water flow control equation is:

$$\rho {\displaystyle \frac{\partial u}{\partial t}}=-{\nabla }_{\mathrm{t}}\rho ·e_t-{\displaystyle \frac{1}{2}}{f}_D{\displaystyle \frac{\rho }{d_h}}\left|u\right|u+F·e_t$$

(1)

The Darcy friction coefficient can be expressed as:

$$f_D={\displaystyle \frac{64}{Re}}$$

(2)

In this study, there is groundwater seepage. The stratum is assumed to be a porous medium layer where seepage can exist. The heat transfer of the stratum can be expressed as:

$${\left(\rho C_p\right)}_{eff}{\displaystyle \frac{\partial T_0}{\partial t}}+{\rho }_w{C}_{p,w}{v}_w·\nabla T_0+\nabla ·q=\mathcal{Q}$$

(3)

$$q=-k_{eff}\nabla T_0$$

(4)

$v_w$ represents the velocity field of liquid water, and its expression is:

$$v_w=-{\displaystyle \frac{\kappa }{{\mu }_w}}\nabla p_w$$

(5)

The temperature variance between the entrance and exportation of the heat exchange pipes determines the complexly related heat exchange efficiency of DBP-EPs. Here is the mathematical expression:

$$\mathcal{Q}=v\rho C_p{∆}_t$$

(6)

The EER is chosen as the metric to evaluate the heat exchange efficiency (The meanings of each character in the formula are shown in

Table 1. Abbreviations used in this paper.

Nomenclature	Physical Significance	Nomenclature	Physical Significance	Nomenclature	Physical Significance
$\rho$/${\rho }_w$	Fluid density (m3/h)	$p_w$	Groundwater osmotic pressure (pa)	P	Power of the heat pump system (kg/h)
u	Average cross-sectional velocity	${\mu }_w$	Dynamic viscosity	h	Head of the water pump
$f_D$	Darcy friction factor	$\kappa$	Permeability	g	gravitational acceleration
F	Volumetric force density	$\mathcal{Q}$	HER (W)	n	Efficiency of the heat pump system (0.9)
$d_h$	Average hydraulic diameter	v	Volumetric flow rate of the heat carrier fluid (m3/h)	$C_{p,w}$	Heat capacity of water at constant pressure (J·kg−1·°C−1)
$v_w$	The velocity field of water	$C_p$	Specific heat capacity of the heat transfer medium (J/(kg °C))	${∆}_t$	Temperature differential between the entry and exportation water temperatures of the pile (°C)

):

$$EER={\displaystyle \frac{\mathcal{Q}}{P}}$$

(7)

$$P={\displaystyle \frac{vhg}{102n}}$$

(8)

4.1.2. Model Assumption

The DBP-EP is modeled with the following presumptions due to the complicated geological composition and to prevent interference from outside elements influencing the heat exchange behavior of the energy pile:

① Assumed to be homogeneous with a consistent starting temperature is the soil. Neglected impacts on heat transport in the DBP-EP are those of steam formation and thermal radiation within the soil.

② One assumes that the backfill material within the pile makes perfect contact with the wall of the heat exchange pipe. Assuming pure heat conduction, the thermal characteristics of various soil layers are regarded as constant; the contact thermal resistance is therefore omitted.

4.1.3. Mesh Generation

The mesh is polished at the junction between the heat exchange pipe and the pile to increase the accuracy of heat exchange calculations. A tetrahedral mesh is used to discretize the model with 236,228 boundary elements. The mesh remains unchanged throughout the calculations. The DBP-EP pile diameter is 800 mm, pile length is 23 m, heat exchange well diameter is 150 mm, well depth is 100 m. The soil volume is 15 m long, 15 m wide and 120 m high. The calculation area is based on the ground and extends downward, and the value of the ordinate ranges from 0 m to −120 m. The model grid division and soil layer distribution are shown in Figure 3. In order to achieve the purpose of comparison with the test, the initial constant flow rate in the heat exchange tube is set at 1 m³/h according to the test standard.

4.2. Model Feasibility Verification

The accuracy of the model was confirmed by simulating the heat exchange process in the operation of the DBP-EP using COMSOL program. The model was built with the previously provided data, with the same dimensions of the energy pile. Using a porous media heat transfer technique, heat transfer to the soil and pile body was replicated in the model configuration. Furthermore, described by a non-isothermal pipe flow model were the water flow conditions in the heat exchange pipe. The flow rate inside the pipe was fixed at 1 m³/h and the input temperature of the circulating fluid was set to correspond with the experimental settings.

Along with the recorded temperatures at various pile depths following seven days of operation, Figure 4 shows the fluctuation in the circulating fluid temperature within the heat exchange pipe. The temperature changes at various depths quite closely correspond with the recorded values. Strong consistency exists between the simulation findings and experimental observations shown by the computed root mean square deviation between the two data sets: 0.148.

4.3. Mesh-Dependent Verification

Finally, the grid dependence of the model is verified by comparing the running results of the model under different grid numbers. The DBP-EP group is set to run for 30 days. As can be seen from Figure 5, during the 30-day operation, the difference in the number of grids has a small impact on the outlet temperature, which can be ignored. The results show that the mesh number has little effect on the outlet temperature, and the mesh number is finally determined as 282,912 to balance the calculation efficiency and accuracy.

5. Comparison and Analysis of Intermittent Operation and Continuous Operation Simulation Results

Based on the actual operating schedules of energy piles, four intermittent operation modes were defined: ① 8 h of operation and 16 h of rest; ② 12 h of operation and 12 h of rest; ③ 16 h of operation and 8 h of rest; ④ 20 h of operation and 4 h of rest. The corresponding intermittent ratios (n) for these modes are: n = 1/2, n = 1, n = 2, and n = 5, representing the ratio of operating time to resting time for each pile. For the continuous operation mode, the intermittent ratio is defined as n = ∞.The operation duration for all conditions is fixed at 30 days.

5.1. Comparison Between Continuous and Intermittent Operation

Comparing intermittent and continuous operating modes during a 7-day period, Figure 6 shows the change in the HER over time for DBP-EPs. The HER of the pile lowers in both operation modes from 0 to 12 h, displaying swings surpassing 59%. The rate of heat exchange reduces and progressively stabilizes in continuous operation mode during 12 to 168 h. The HER falls monotonically within each cycle and then further falls as the cycle period rises in intermittent operation modes. In all kinds of intermittent operations, the HER falls with a rising intermittent ratio. Furthermore, in every intermittent operation mode the HER is higher than in the continuous mode.

Table 2. Average HER for 30 days in continuous and intermittent operation mode.

Intermittent Ratio/n	∞	5	2	1	1/2
Average HER (W)	4429.03	4360.10	3916.99	3118.89	2675.69

lists two operating modes’ average HER during a 30-day period for DBP-EPs. Extracted were the instantaneous HERs of the pile on days 7, 14, and 30; the fluctuations are illustrated in Figure 7. The instantaneous HER of the pile reduces with an increasing intermittent ratio (n) at these intervals. The instantaneous HER of the pile reduces with increasing operation times under the same running conditions. The instantaneous HERs of the pile on days 7, 14, and 30 are 845.86 W, 1040.03 W, and 1050.94 W greater, respectively, when (n = 1/2) than when (n = ∞). The intermittent operation mode thereby improves the pile’s immediate HER. Still, the instantaneous HER by itself is not enough to establish the superiority of a specific operational situation. Table 2 shows that although the pile’s instantaneous HER under the intermittent operation mode is higher than that under the continuous operation mode, the average HER stays lower in the intermittent mode. The pile’s average HER rises with the intermittent ratio. The average rate at n = 5 is almost exactly the same as under the continuous operating mode. The average HER of the two modes differs just by 68.93 W.

Figure 8 shows the energy efficiency ratio (EER) fluctuation of DBP-EP over 7 days in continuous and intermittent operation mode. It can be observed from the trend shown in the figure that EER of piles under these two operating conditions presents a trend of gradual decline, indicating that the energy efficiency of the system will be affected to some extent during long-term operation, no matter continuous operation or intermittent operation, showing a phenomenon of performance attenuation. The EER changes in DBP-EP in intermittent operation mode are more consistent and fluctuate less, which indicates that intermittent operation can maintain the stability and high efficiency of DBP-EP more effectively.

In addition, relevant studies have shown that EER changes have a certain relationship with the intermittent ratio. EER values increased as the intermittent ratio decreased, indicating that intermittent operation can effectively improve the overall energy efficiency of the system. In particular, when the interval ratio is 1/2, that is, when the system runs for a period of time equal to the pause period, the EER value reaches the maximum, reaching 6.83.

Table 3. EER mean for 30 days of operation of DBP-EP in continuous and intermittent operation mode.

Intermittent Ratio/n	∞	5	2	1	1/2
EER mean	4.39	4.73	4.80	5.16	5.60

shows the average EER values of DBP-EPs over a 30-day period, comparing two operating modes. The EER in the intermittent operating mode is constantly higher than in the continuous mode as the intermittent ratio rises, as the table shows. This happens as the heat pump system does not run consuming electrical energy during off periods in the intermittent mode. Longer off intervals brought upon by a decreased intermittent ratio increase instantaneous HER. Thus, in practical engineering applications, the intermittent ratio should be minimized, given that the heating and cooling needs of the building are satisfied, to guarantee the effective and energy-saving operation of the energy piling system. When the operating state is characterized by n = 5, the average HER of a single energy pile lowers by less than 70 W compared to the continuous mode, while the EER increases by over 7%, so this operating mode is more beneficial considering both the average HER and EER.

Figure 9 illustrates the temperature variation at the center of the energy pile, located at a depth of 20 m, under both intermittent and continuous operation modes. The figure shows that the temperature at the pile center increases quickly during the first phase of heat exchange in the continuous operating mode; the first 12 h cause the most notable increase, surpassing 44.4%.The temperature curve then keeps rising gently until settling. The pile center temperature stays less than that in the continuous mode in the intermittent operating mode. The intermittent ratio suggests higher heat accumulation in the pile as the temperature rises since the peak temperature rises. The intermittent operation mode also helps to reduce the internal heat generation in the pile.

Combined with the above, it is known that the HER of the DBP-EPs gradually decreases during operation under all conditions, while the pile center temperature rises as the HER decreases. As the pile center temperature increases, the HER decreases further. This indicates that heat accumulation resulting from the increase in pile temperature negatively impacts the performance of the DBP-EP. In comparison to the continuous operation mode, different intermittent operation modes lead to varying degrees of temperature reduction at the pile center. The smaller the intermittent ratio, the lower the peak temperature at the pile center, leading to improved instantaneous heat exchange performance in the intermittent operation mode.

5.2. Effect of Circulating Medium Flow Rate on Intermittent Operation

The medium’s circulation flow rate is adjusted in intermittent operating mode to examine how various flow rates impact the HER and EER of the pile. Figure 10 shows the average HER of the pile under several intermittent flow rates and operating circumstances. Under all intermittent ratio settings, the average HER of the DBP-EP rises as the figure shows with the circulation medium flow rate. The average HER of the pile shows more increase with increasing flow rates as the intermittent ratio rises. The average HER of the pile rises by 613.62 W at an intermittent ratio of n = 1/2 and by 926.66 W when the flow rate rises from 0.8 m³/h to 1.4 m³/h. This is explained by the longer heat exchange loop of the DBP-EP, which permits more thorough heat exchange when the flow rate rises by the same amount, therefore producing a more complete heat exchange over the longer operation duration. When the flow rate increases, the frequency of contact between the heat medium and the heat exchange surface increases per unit time, which helps to transfer heat from the medium to the system more effectively, improving the overall heat exchange efficiency. Therefore, in the case of a longer operation duration, a higher flow rate can make the heat exchange process more complete, thus achieving a higher heat release rate.

The average HER of the pile decreases with increasing circulation medium flow rate under the same operating conditions. For example, the average HER of the pile rises by 399.66 W at an intermittent ratio of n = 5 when the flow rate rises from 0.8 m³/h to 1.0 m³/h. The increase is just 210.3 W, though, when the flow rate increases from 1.2 m³/h to 1.4 m³/h. Therefore, although raising the average HER by increasing the flow rate helps the pile, very high flow rates have little influence on additional improvement. Ultimately, the most efficient flow rate range should be found by considering the energy efficiency ratio; so, the ideal circulation flow rate is not always the greatest.

Figure 11 shows the trend of average energy efficiency ratio (EER) of DBP-EP in intermittent operation mode under different circulating fluid flow conditions. Analysis of the graphical data shows that the EER of DBP-EP system shows a downward trend with the increase in circulating fluid flow rate, and the EER decrease gradually intensifies with the increase in flow rate. This shows that the higher flow rate does not continuously improve the energy efficiency of the system but rather leads to a significant decrease in efficiency.

The EER reduction was particularly significant at the smaller interval ratio condition, where the EER of DBP-EP decreased by 0.99 at the interval ratio n = 1/2 when the flow rate increased from 0.8 m³/h to 1.4 m³/h, while the EER decreased by only 0.92 at the interval ratio n = 5. This phenomenon suggests that the negative impact of increased flow on EER at smaller intermittent ratio settings is more significant, possibly because the increased flow during shorter intermittent periods does not effectively match the heat exchange process, resulting in a decrease in system operating efficiency. It can be seen that with the increase in circulating fluid flow, the increase in the average heat transfer capacity of the system gradually decreases. This shows that the increase in flow rate tends to be saturated after a certain critical point. Although the increase in flow rate helps to improve the heat exchange per unit time, beyond a certain flow range, the improvement of heat exchange efficiency will be significantly slowed down, which may lead to insufficient use of heat medium in the heat exchange process, affecting the improvement of thermal efficiency. The increase in flow rate will also lead to the increase in power consumption of the heat pump unit. Although the increase in flow rate may promote the improvement of heat transfer capacity in the initial stage, too high flow rate will cause the increase in power consumption and power demand of the pumping system, thus negatively affecting the energy efficiency of the overall system. Therefore, simply increasing the circulating fluid flow rate is not necessarily able to effectively improve the heat exchange efficiency of the system but may lead to unnecessary energy waste.

As an example, the influence of different circulating fluid flow rates on the temperature variation at the pile center is analyzed under the condition of (n = 2). Figure 12 illustrates the temperature variation at the center of the pile, located at a depth of 20 m, as the circulating fluid flow rate changes under the (n = 2) condition. It is observed that the temperature at the center of the DBP-EP increases with the circulating fluid flow rate, though the rate of increase gradually diminishes. A higher temperature at the pile center indicates greater heat accumulation within the pile structure. Therefore, increasing the circulating fluid flow rate in DBP-EPs leads to greater heat accumulation during operation. As shown in Figure 9, as the circulating fluid flow rate increases, the rate of increase in the average heat exchange coefficient of the pile decreases, while the heat accumulation becomes more pronounced, which is detrimental to heat exchange. Therefore, considering the EER of DBP-EPs at different flow rates, and maintaining a constant average heat exchange coefficient, the optimal circulating fluid flow rate range should be between 1.0 m³/h and 1.2 m³/h.

5.3. Influence of Inlet Temperature Variation on Intermittent Operation

The impact of varying the inlet temperature of the circulating medium on the HER and coefficient of performance (COP) of DBP-EPs under intermittent operation conditions has been analyzed. Figure 13 shows the influence of inlet temperature of different circulating media on average HER of energy pile under intermittent operation mode. It can be clearly seen from the data in the figure that with the increase in circulation medium inlet temperature, the average HER of DBP-EP presents a trend of gradual increase, which is verified under different intermittent ratio conditions. Specifically, the increase in inlet temperature significantly promoted the heat release efficiency of DBP-EP, and the increase in HER was more significant under the condition of higher intermittent ratio.

For example, when the inlet temperature of the circulating medium rises from 28 °C to 32 °C, the average HER of DBP-EP increases by 920.87 W at the intermittent ratio n = 1/2, while the increase in HER is more significant at the intermittent ratio n = 5, reaching 1489.32 W. The results show that increasing the inlet temperature can not only improve the overall heat exchange efficiency of the system, but also the improvement effect is more prominent at a higher intermittent ratio, which may be related to a longer heat exchange cycle and a more adequate heat exchange process.

Under the same operating conditions, the influence of the increase in inlet temperature on the average HER shows some nonlinear characteristics. For example, when the intermittent ratio n = 5, when the inlet temperature increased from 28 °C to 30 °C, HER increased by 695.09 W, and when the temperature further increased from 30 °C to 32 °C, HER increased by 794.24 W. This change indicates that the influence of temperature rise on HER presents an accelerated effect in different temperature ranges, especially in the higher temperature range, and the influence of temperature rise on HER is more significant. This trend may be due to the increased temperature difference in the heat exchange process under high temperature conditions, which promotes more heat transfer to the heat exchange medium, resulting in a further increase in the heat release rate of the system.

Figure 14 illustrates the variation in the average EER values of the pile body at different inlet temperatures under intermittent operation conditions. As shown in the figure, the EER values of the pile body increase with the inlet temperature under different intermittent operation modes. For a given circulating medium inlet temperature, a smaller intermittent ratio results in a higher average EER of the pile body. The inlet temperature of the pile does not influence the electrical energy consumption of the heat pump system. However, longer operating times lead to higher energy consumption, which results in a lower EER. Therefore, when the operating state is characterized by n = 1/2 and the circulating medium inlet temperature is 32 °C, the EER of the pile body reaches its maximum value of 6.6. Conversely, when the intermittent ratio is n = 5 and the circulating medium inlet temperature is 28 °C, the EER value is minimized at 3.8. Based on the above observations, under different intermittent operation conditions, an increase in the circulating medium inlet temperature improves both the EER and the average HER of the DBP-EP. The most significant improvements occur when the temperature is between 30 °C and 32 °C.

Figure 15 displays the fluctuation in the pile core temperature depending on the intermittent ratio of n = 2 with the inlet temperature of the circulating media. The figure shows that the rate of increase becomes more noticeable when the intake temperature rises and the pile core temperature of the deep-buried pipe energy pile increases. Simultaneously at the same inlet temperature, the peak core temperature rises with increasing heat exchange cycles, showing more heat accumulation inside the pile. The highest core temperature difference between the first and seventh heat exchanges cycles is 0.81 °C when the inlet temperature of the circulating medium is 28 °C. This difference reaches 0.99 °C at an inlet temperature of 32 °C. Consequently, a rising inlet temperature of the circulating medium speeds up heat accumulation in the pile. A well-crafted intermittent ratio can help to reduce this effect.

6. Analysis of DBP-EP Heat Transfer Performance Under Seepage

6.1. Change in HER

The operating state is characterized by n = 5, when the DBP-EP exhibits best heat transmission performance. The DBP-EP at n = 5 was investigated under several groundwater seepage velocities. Under different groundwater seepage velocities, Figure 15 shows the fluctuation in the HER of the DBP-EP over a period of 24 to 168 h. The HER of the DBP-EP, with and without groundwater seepage, shows in the figure a comparable pattern over time. The HER of the DBP-EP steadily lowers with the no-seepage condition over time. Under seepage, though, the HER is more than under no-seepage conditions. By 543.13 W, 943.35 W, and 1246.41 W, respectively, the instantaneous HER of the DBP-EP with seepage velocities of 30 m/a, 60 m/a, and 90 m/a exceeds that of the no-seepage condition at hour 44. At hour 164, the variations are 1393.65 W, 1045.57 W, and 580.77 W, respectively. The findings show that instantaneous HER of the DBP-EP increases with increasing groundwater seepage velocity. Groundwater seepage helps to more effectively perform heat exchange within the pile and the adjacent soil by lowering heat accumulation inside the pile during operation.

Table 4. Average DBP-EP HER in 30 days under intermittent and seepage conditions.

Intermittent Ratio/n	∞	5	2	1	1/2
Average HER (W)	4429.03	4360.10	3916.99	3118.89	2675.69
Seepage velocity (m/a)	No seepage	30 m/a	60 m/a	90 m/a	-
Average heat exchange rate (W)	4360.10	4883.54	5248.63	5515.50	-

shows the changes in HER under seepage condition and intermittent condition of DBP-EP for 30 days.

Table 5. The mean HER of the DBP-EP over a 30-day period under seepage conditions.

Seepage Velocity (m/a)	No Seepage	30 m/a	60 m/a	90 m/a
Average heat exchange rate (W)	4360.10	4883.54	5248.63	5515.50
Increase rate	-	12.01%	20.38%	26.50%

shows a comparison of seepage and non-seepage circumstances, with the average HER of the DBP-EP during a 30-day period. It also shows how, in comparison to the no-seepage situation, seepage increases the HER. Figure 16 and Table 5 show that the average HER and the enhancement ratio of the DBP-EP rise as seepage velocity increases. But the instantaneous HER under all seepage situations steadily lowers as the running duration increases. When the seepage velocity is 30 m/a, 60 m/a and 90 m/a, the average HER increases by 12.01%, 20.38% and 26.50%, respectively, compared with the condition without seepage.

6.2. Analysis of EER and Thermal Radius

Figure 17 illustrates the variation in the EER of the DBP-EP over a 7-day period, considering both seepage and no-seepage conditions. The enhancement of the DBP-EP’s heat transfer performance due to seepage increases with the seepage velocity. On day 1, the EER of the DBP-EP with seepage velocities of 30 m/a, 60 m/a, and 90 m/a is 0.93, 1.21, and 1.43 higher, respectively, compared to the no-seepage condition. On day 7, the EER is 0.81, 1.29, and 1.67 higher, respectively.

Table 6. Average EER of DBP-EP under seepage condition for 30 days.

Seepage Velocity (m/a)	No Seepage	30	60	90
Average EER	4.73	6.15	6.61	6.96
Increase rate	-	30.02%	39.75%	47.15%

presents the average EER of the DBP-EP over a 30-day period under both seepage and no-seepage conditions, along with the enhancement ratio of the EER with seepage relative to the no-seepage case. Under seepage velocities of 30 m/a, 60 m/a, and 90 m/a, the EER of the DBP-EP is 1.42, 1.88, and 2.23 higher, respectively, compared to the no-seepage condition. These results suggest that higher seepage velocities lead to greater heat transfer rates within the pile and more significant improvements in the DBP-EP’s heat exchange performance.

Figure 18 illustrates the temperature field distribution surrounding the DBP-EP under seepage conditions. During the initial heat exchange phase, the temperature field around the DBP-EP shows minimal variation. As the operating time progresses, the temperature field evolves into a fan-shaped pattern. A faster seepage velocity results in less pronounced longitudinal stretching of the temperature field. This suggests that intermittent operation partially alleviates thermal accumulation in the DBP-EP, while groundwater seepage reduces the thermal accumulation within the pile and the adjacent soil. These factors act synergistically, positively impacting the long-term heat exchange between the pile and the soil.

6.3. Potential Applications and Developments

With growing global concern about climate change, countries have introduced more stringent environmental protection regulations, especially in the construction and energy sectors. DBP-EP systems generally comply with these environmental regulations because they use renewable energy sources and significantly reduce dependence on fossil fuels. In many countries and regions, building energy efficiency standards and carbon emission regulations require the adoption of green building technologies, and the DBP-EP system meets these green standards and can obtain relevant certifications (such as LEED, BREEAM, etc.). In this paper, the differences and similarities of the DBP-EP heat transfer performance under different operating strategies are analyzed by the method of field test and numerical simulation. Some suggestions for the subsequent construction scheme of DBP-EP are put forward. However, there are still shortcomings in the discussion process, and many problems need to be further improved:

① The selection of intermittent operation scheme and operation factor change is limited, and the selection range of operation time should be further expanded to facilitate more extensive research and analysis.

② The application of intermittent operation mode is worth promoting, and more practical engineering data will be conducive to further research.

③ The influence of different geographical locations and groundwater seepage environment on the heat transfer performance of DBP-EP system needs further study.

7. Limitations

When environmental factors are considered, in addition to ambient temperature, the heat transfer performance of DBP-EP is also affected by wind, precipitation (rain, snow or ice) and solar radiation. In this study, DBP-EPs was investigated under different intermittent operating conditions. The heat transfer characteristics during operation were analyzed by field experiments. The DBP-EPs numerical model was used to study the change in heat transfer performance under different operating conditions. In order to further optimize the operation strategy of DBP-EPs, the effects of two basic operating parameters, circulating fluid velocity and inlet temperature, on the heat transfer performance in intermittent operation mode were further evaluated. On the basis of selecting the ideal intermittent operation strategy, the influence of groundwater seepage on the heat transfer performance of DBP-EPs system is studied, which provides a reference for the engineering application of DBP-EPs. The influence of different geographical location and environment on the heat transfer performance of DBP-EPs system needs further study. In the follow-up work, the study rule of this paper can be verified or modified by long-term monitoring of the DBP-EP heat transfer performance under intermittent operating conditions in multi-season conditions, so as to enhance the purpose of the expansion of this study.

8. Conclusions

This study combines in situ experiments and numerical simulations to examine the HER (HER) and EER of deep-buried pipe-type energy piles (DBP-EP) under both intermittent and continuous operation during the summer. Additionally, it investigates the impact of different circulating fluid flow rates and inlet temperatures on the intermittent operation of these piles. The gain effect of groundwater seepage on the intermittent operation model is further studied. The conclusions drawn from this study are as follows:

(1)

Intermittent operation benefits long-term performance: In the heat exchange process of DBP-EPs, an intermittent operation strategy contributes to improved long-term performance. The operating state is characterized by n = 5, the operation strategy for the DBP-EP is optimal. Compared to continuous operation, the EER increases by over 7%, while the average HER decreases by less than 70 W, which helps alleviate the internal thermal accumulation in the pile.

(2)

Effect of circulating fluid flow rate: The average HER of the pile rises with increasing circulating fluid flow rate. This does, however, also cause a drop in the EER and aggravates internal thermal accumulation. The ideal range of circulating fluid flow rate for DBP-EPs is between 1.0 m³/h and 1.2 m³/h based on the observed effects at certain intermittent ratios.

(3)

Greater average HER of the pile results from a greater circulating fluid inlet temperature. The average HER as well as the EER improves as the inlet temperature rises. Moreover, a suitable intermittent ratio can allow the pile to avoid internal thermal accumulation.

(4)

Groundwater seepage improves general heat exchange efficiency and lessens the degree of thermal buildup in the DBP-EP. Seepage reduces thermal accumulation even more when combined with intermittent operation, therefore enhancing the long-term heat exchange within the pile and the adjacent soil.

(5)

Since the study on the effect of groundwater seepage on heat transfer efficiency only considers the single variable of intermittent ratio of operating parameters, and the change in other operating parameters also affects heat transfer, minor adjustments can be made in accordance with the intermittent operating conditions recommended in this study in the actual operation period to achieve the best heat transfer effect.