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Article

Study on the Effect of Heat Transfer Characteristics of Energy Piles

by
Xiaoyang Wang
1,2,
Tongyu Xu
2,
Kaiming Zhao
3,
Yueqiu Xia
2,
Yuechen Duan
2,
Weijun Gao
1,2,* and
Gangqiang Kong
4,*
1
Innovation Institute for Sustainable Maritime Architecture Research and Technology, Qingdao University of Technology, Qingdao 266033, China
2
Faculty of Environmental Engineering, The University of Kitakyushu, Hibikino 1-1, Wakamatsuku, Kitakyushu 808-0135, Japan
3
School of mechanical engineering, Southwest Jiaotong University, Chengdu 610031, China
4
Key Laboratory of Ministry of Education for Geomechanics and Embankment Engineering, Hohai University, Nanjing 210024, China
*
Authors to whom correspondence should be addressed.
Buildings 2024, 14(11), 3593; https://doi.org/10.3390/buildings14113593
Submission received: 17 October 2024 / Revised: 5 November 2024 / Accepted: 8 November 2024 / Published: 12 November 2024
(This article belongs to the Topic Resilient Civil Infrastructure, 2nd Edition)
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Abstract

:
The thermal performance of energy piles equipped with new metal fins to improve heat transmission is examined in this research. The solid heat transfer module of COMSOL Multiphysics was used to create a 2D numerical model of the energy pile, utilizing the energy pile at a field test site in Nanjing as an example. By contrasting the experimental data, the COMSOL Multiphysics model’s correctness was confirmed. After that, a new kind of energy pile fin was created to improve the heat transfer of the pile. The impact of the new fin type on the energy pile’s heat transfer efficiency was assessed, and the temperature change within the soil surrounding the pile before and after the fin was set was examined by contrasting the parameters of pipe configuration, buried pipe depth, and concrete thermal conductivity. The results indicate that after setting the fins to run for 336 h, the temperature of the concrete area increases by 10.8% to 12.3%, and the temperature of the region surrounding the pile increases by 5.3% to 8.7% when the tube diameter is chosen to be between 20 and 40 mm; The fins maximize the heat transfer temperature between the surrounding soil and the concrete, and as the tube diameter increases, the temperature drops. For 336 h of pile operation, the temperature of the concrete may be raised by 10.8% to 12.3% after the fins are set, and the temperature around the pile can be raised by 5.3% to 8.7%. The heat transmission efficiency of the energy pile can be improved by raising the temperature of the soil surrounding the pile through an increase in the concrete’s thermal conductivity; however, the degree of improvement diminishes as the conductivity rises. It is intended that this study will offer insightful information on the best way to design energy pile heat transfer efficiency.

1. Introduction

One of the biggest environmental, social, and economic problems the world is currently confronting is climate change. With its extensive distribution, substantial reserves, and rapid rate of usage, geothermal energy is a sustainable energy source that may help address the growing energy scarcity. Energy pile technology, which combines ground source heat pump (GSHP) technology with conventional building pile foundation, was created [1] to maximize the use of shallow geothermal energy. Under the same heating and cooling settings, energy piles have a greater heat transfer efficiency of 42.2% and 48.7%, respectively, than GSHP [2]. In addition to support the structural loads, this novel construction enhances the system’s and the soil’s energy transfer efficiency due to concrete’s high thermal conductivity. Utilizing concrete’s thermal qualities to their fullest [3,4] not only increases the system’s heat transfer efficiency but also lowers drilling costs, maximizes the use of subterranean space resources, and lowers carbon emissions—all of which have clear technical and financial benefits.
Concrete, heat exchange tubes, and heat exchange fluids facilitate the heat transfer process between energy piles and the nearby soil. The design of the pile and embedded heat exchange tubes, as well as the thermal characteristics of the fluid, tubes, and backfilled concrete, all affect how well heat is transferred [5]. Additionally, site layouts, thermal load applications, foundation geometry, and the characteristics of the soil foundation material can all have a significant impact. By improving the thermal characteristics of the heat-carrying fluid, pipe material, and concrete mixture, pile sizes and pipe shapes may be optimized to improve the heat transmission process [6].
Low thermal conductivity in some heat exchanger [7,8,9] tubes causes heat loss and has a detrimental effect on energy production. Metal oxides, silver-coated polyamide particles, single-walled carbon nanotubes, multi-walled carbon nanotubes, carbon nanofibers, and graphite are examples of thermally conductive fillers that can improve the thermal characteristics of HDPE materials [10]. In cold climates, circulating fluids often include a specific quantity of antifreeze to the water to guarantee even heat transmission from the pile. Although liquid phase (water) uses are also possible, backfill materials are typically based on a solid phase (concrete). The various filler materials are the primary distinction between them [11]. The thermal performance of energy piles is somewhat improved by adding steel fibers, graphite [12], aluminum, and thermally conductive fillers (such as silica fume and sand silanes) to the solid-phase mixes. Cement fly ash gravel (CFG) piles have been increasingly popular in recent years [13,14] because of its benefits, which include easier construction, lower engineering costs, and increased bearing capacity. However, the mechanical and thermal properties differ from those of conventional energy piles, which are appropriate for the foundation of some low-rise buildings with simpler structures and lower bearing capacities, because the pile body is typically unreinforced, with a low cement content and a high fly ash content. Furthermore, one of the novel energy piles with a lot of research activity in recent years is pre-stressed high-strength concrete (PHC) energy piles [15,16,17,18,19]. The construction is more cost-effective and convenient when ground source heat pump technology is combined with PHC piles and the internal space of PHC piles is used to arrange the heat exchange pipes. By altering the internal concrete of the piles, the heat exchange efficiency of energy piles can be maximized. Changing the filling material of PHC energy piles has a greater impact on heat transmission than altering the piles’ thermal characteristics, according to Guo’s novel analytical model of heat transfer of PHC energy piles [20]. PHC energy piles also have greater energy efficiency and heat exchange efficiency indices than other energy piles [21]. When comparing the heat transfer between W-type and 3U-type PHC pile heat exchangers under simulations of continuous and intermittent operation, the longer pipe length of the 3U-type with the same specification results in a heat exchange rate that is approximately 15% higher in the intermittent operation mode [22]. Dai [11] examined the heat transmission of U-type, W-type, and helical PHC energy piles under two concrete working conditions where the backfill material was considered as solid phase as opposed to liquid phase. When considering merely the role of a single energy pile, it was determined that if the fluid filler has a low convective heat transfer coefficient, it will aid in expanding the radius of thermal effect of the PHC energy piles to efficiently use the geothermal energy.
The key parameters of energy piles, such as piping configurations, soil geography, concrete thermal conductivity, and complex stress changes in piles over time, have been optimized by researchers in recent years to maximize their heat transfer performance and accuracy. Energy piles have been the subject of numerous studies. A simpler method to approximate temperature changes for energy pile design was presented by Abdelaziz [23]. The average temperature variation in the pile cross-section is used to streamline the processes of thermal strain analysis for energy heaps with varying dimensions and numbers of heat exchange circuits, leading to considerable computational time and cost savings. A thermal response test (TRT) data model gathered from a series of full-size on-site geothermal energy piles in Colorado Springs, Colorado, was calibrated and parameterized by Caulk [24]. The findings show that, depending on where the measurements are taken within the pile cross-section, field temperature and strain readings might vary by up to 20% during the heating process. Faizal [25] investigated the thermal resistance of the surrounding soil and energy piles. The thermal resistance of the energy pile was evaluated using the temperature gradient between the heat exchanger pipe and the pile–soil interface. This highlights the significance of measuring the radial thermal gradient between the pipe and the pile–soil interface in order to evaluate the thermal resistance of the energy pile for design purposes. Abrupt increases or stabilization of the fluid temperature during testing of the subsurface’s effective thermal conductivity around energy piles, particularly when power variations exceed 10% of the total applied power, can result in the estimation of inaccurate ground thermal performance results. An improved infinite line source (ILS) model was put forward by Abdelaziz [26], which was effective in correcting the ground thermal conductivity inaccuracy and removing environmental disturbances.
Researchers typically use numerical modeling to examine these characteristics since it is more flexible and promising for complicated or real-world experimental circumstances. Cecinato [27] created a finite element numerical model that accurately investigated the processes of transient diffusion and convective heat exchange in geothermal structures. It has been demonstrated that optimizing the surface area utilized for heat transmission remains the most effective way to increase energy efficiency. It is more efficient to install more pipes within the pile cross-section rather than enlarging the pile. In his study of the thermal interaction between energy pile pipes’ inlet and outlet, Sani [28] discovered that the number of loops, the pipes’ location, and the thermal conductivity of the concrete and soil all significantly affect how much heat interacts between the inlet and outlet pipes. Higher intake temperatures, on the other hand, raise soil temperatures and cause water to migrate through soil pores, changing the thermal conductivity of the soil. This crucial element is typically overlooked by conventional models. The impact of soil thermal conductivity variation with temperature on the thermal behavior of energy piles was examined numerically by Xi [29], who also assessed the impact of these variations by group arrangement, pile position, and operation mode on the thermal behavior of energy pile groups. The subaqueous level influences the energy pile’s thermal behavior and temperature variation because it establishes the pile’s contact area with the seepage field. The energy pile’s thermal performance improves and the pile body’s temperature fluctuation decreases as the water table rises. Yang [30] conducted an experimental investigation on the effects of seepage velocity, groundwater level, dry sand, and saturated sand on energy pile heat transfer performance. It demonstrated that groundwater seepage significantly affects the thermodynamic behavior of energy piles and that raising the seepage rate can improve the energy piles’ capacity for heat transfer.
Due to the enormous potential of energy piles, current studies have expanded into other areas. For instance, the load-bearing and heat-transfer properties of energy piles in seasonal permafrost locations during freeze–thaw cycles [31,32]. Since backfilled wells receive concrete pile materials and greater pile widths enhance the thermal performance of energy piles, energy piles and solar energy can also be combined to create a coupled energy pile–solar collector system [33]. Another example is the creation of energy geo-structures out of foundational components such retaining walls, piles, and tunnel linings [34]. Furthermore, Liu [35] and Cao [32] showed that energy piles for melting snow on bridges are feasible.
Studies on the structural optimization of energy piles to enhance the heat transfer process are, nonetheless, somewhat few [36]. This study utilizes the solid heat transfer module of the COMSOL Multiphysics 6.2 to create a two-dimensional numerical model of an energy pile at a field test site in Nanjing. In the experiment, the heat transmission from the pile to the surrounding earth is simulated. For verification, the simulated and field temperature data were compared, assuming that the top and lower portions of the pile section are adiabatic boundaries and that heat is only transmitted horizontally. A new kind of energy pile fin was then created, and by comparing variables like pipe configuration, buried pipe depth, and concrete thermal conductivity, the temperature change in the soil surrounding the pile before and after the fin was set was examined. Additionally, the impact of the new fin type on the energy pile’s heat transfer efficiency was assessed. It is anticipated that it will be useful for assessing the changes in geothermal energy as well as for the optimum design and development of new energy piles.

2. Increase Heat Transfer Efficiency with Fins

Recent years have seen a surge in interest in studies on optimizing heat transmission from underground pipes due to the growing popularity of GSHP systems. Combining GSHPs and energy pile systems with air-source heat pumps [37] or solar energy systems [38,39] is a viable optimization strategy to address the load imbalance issue in hot-summer and cold-winter locations. Even though the load imbalance issue has been resolved, the combined system still has a number of dangerous issues, including low efficiency and excessive cost.
An efficient strategy to greatly enhance heat transmission is to increase the heat transfer area while keeping the traditional drilling depth in order to improve the thermal performance of GHEs [40]. Numerous studies on fins have been carried out by researchers, who have been constantly investigating the impacts of different factors and producing important optimizations, such as altering the fin arrangement and modifying the fins’ height and thickness. Considering that traditional horizontal coils need a significant amount of additional space for heat transfer between the soils. Alnaqi [41] installed a spiral fin at the buried pipe to boost the heat transfer efficiency of a horizontal GHE. By doubling the fins’ thickness from 2 mm to 4 mm, the heat transfer rate improved by 10%. In addition to optimizing the pipe, Saeidi [42] installed metal plates as fins or blades beneath the transversely buried pipe to cover a wider area of soil, which resulted in an equally good improvement in heat transmission. It was determined that by further modifying the fins, the output fluid’s temperature was lowered by 3.8% and the GHE’s overall efficiency was raised by 37% when only one aluminum plate was used in the cooling mode. The impact of fin type, shape, and inner tube eccentricity on heat transfer improvement performance was investigated by Choure [43] using three different fin types on a horizontal three-tube heat exchanger using phase change material (PCM). Conduction was shown to be more prevalent than convection in the fin-based heat transfer model; the use of longitudinal, triangular, and curved fins for arrangement can improve and speed up heat transmission.
For vertical GSHP systems, fins are highly effective in addition to optimizing heat transfer for horizontal GSHPs. A novel kind of helical fins was installed in a traditional U-tube by Roshani [44]. through the addition of phase change materials (PCM) to the backfill material, pitch adjustments, and single and double configurations. Following a number of tests, it was finally determined that the combination of 50% PCM at a 40 mm pitch and double helical fins enhanced the heat exchanger’s efficiency and the heat pump’s coefficient of performance by 7.5%. A novel design for aluminum rod fins in a helical vertical ground heat exchanger was created by Saeidi [45,46]. It was possible to raise the soil temperature differential by 0.84 °C and boost heat transmission by 31% by varying the fins’ form, pitch, and thermal conductivity. To improve heat transmission, Bouhacina [47] inserted longitudinal fins on the smooth surface inside the (GHE) pipe in addition to the fins on the exterior of the buried pipe. The U-tube with additional fins can enhance soil heat extraction by around 7% and enable faster temperature recovery as compared to the smooth U-tube variant.
A novel kind of buried pipe optimization has emerged in recent years: the use of the topology optimization approach in the design of buried pipe heat exchangers. In order to design the optimal structure of tree topology showing symmetry, Xu [48] further investigated the evolution law by using heat dissipation minimization as the topology optimization target, taking into account the volume percentage of the soil domain and various boundary conditions. The tree-structured buried pipe heat exchanger can efficiently increase the heat release rate and enhance the heat transfer performance by roughly 25.3% when comparing the thermal performance of the optimized GHE with that of the conventional serpentine GHE. This offers a new approach to buried pipe heat exchanger design that prevents heat accumulation. By perforating the fins, Dong [49] improved convective heat transmission in a novel fin structure that was built utilizing the topology optimization approach. The top portion of the fins and the heat exchanger’s core are where the majority of the holes are located. The results of the CFD simulation demonstrate that, in comparison to the prototype structure, the new structure not only decreases volume but also convective thermal resistance across a greater range of heat flow densities. Additionally, the new structure’s convective thermal resistance reduction rate increases as heat flow density increases.
Fin placement to maximize heat transmission has widely been the subject of research. Due to the increased area, its configuration not only reduces building costs but also greatly strengthens the effectiveness of heat transmission. It is without a doubt a superior heat transfer optimization technique, particularly in regions where the winter heat load demand exceeds the summer cold load. This research simulates a novel fin design for W-type energy stacks, as seen in Figure 1. Instead of weakening the energy pile’s effective structure as much as feasible, the fin is bent to balance the heat dissipation at the periphery. The study of this fin will be presented in a later section.

3. Establishment and Validation of FEM

3.1. Energy Pile Details

At a field test location in Nanjing, China, a single energy pile was built. C30 commercial concrete that had been drilled and grouted was used to construct the energy pile. The pile Lb measured 24 m in length and the pile Db was 0.6 m in diameter. In the pile, a steel cage measuring 0.5 m in diameter was already constructed. Figure 2 illustrates how the heat exchanger and vibrating wire sensor are arranged in the energy pile. Inside the pile, a W-shaped structure housing 92 m of the polyethylene pipe serves as the heat exchanger. The pipe has a 25 mm outer diameter and a 2 mm wall thickness. The pipe’s installation depth is 22.0 m. Water is the fluid that circulates in the pipe. The Chinese national industry standard (Code for Design of Concrete Structures GB/T50010-2010) [50] states that the linear coefficient of thermal expansion is 1 × 10−5 με/°C and the modulus of elasticity of reinforced concrete is 3.09 GPa. The reinforced concrete used had a thermal conductivity of 1.74 W/m/°C, whereas the heat pipe had a thermal conductivity of 0.46 W/m/°C. Temperature readings were taken using vibrating wire strain gauges (VWSGs), which were positioned vertically on various depth portions.

3.2. Ground Conditions

The energy piles were buried 35 m below the surface under a layer of clay. A number of laboratory tests were used to identify the clay layer’s primary properties, which are listed in Table 1. The measuring area’s soils were separated into seven strata based on their characteristics. The soils’ thermal conductivity ranged from 1.53 to 1.72 W/m/°C, and their unit weight was 19.6 to 21.0 kN/m3. The overconsolidation ratio (OCR) of the nearby soils ranged from 5 to 8, according to geotechnical studies conducted at the location. The coefficient of thermal expansion (CTE) of a severely overconsolidated clay (OCR = 16) is 6 × 10−6/°C, according to Donna [41]. Additionally, when OCR declines, the soil’s CTE falls. Therefore, it was assumed that all soil layers in this geographic region had a coefficient of thermal expansion of around 1 × 10−6/°C, which was obtained by interpolation.
Seasonal variations in the beginning temperatures were observed based on the monitoring of pile temperatures from July 2018 to August 2019 (See Figure 3). In August (summer) and December (winter), the average pile temperature was 22.2 °C and 18.5 °C, respectively. Since the outside temperature affected the ground temperature in the hetero-thermal zone (0~8 m), the ground temperature within 8 m from the surface varied significantly with the depth of the margin. Below 8 m below the surface, the earth temperature remains constant at 18.5 to 21.5 degrees Celsius. As a consequence, more precise findings may be obtained by investigating the soil temperature change surrounding the piles in the 8–20 m range based on the depth of the piles.

4. Numerical Simulation

The energy pile cross-section is geometrically modeled using AutoCAD, with a borehole diameter of 0.6 m and a heat transfer tube diameter of 25 mm. Meanwhile, the soil range surrounding the pile is chosen at 5 m × 5 m to prevent the border effect from affecting the simulation results. COMSOL Multiphysics 6.2 is imported in order to simulate and evaluate the experimental circumstances, as seen in Figure 4a. It is assumed that the upper and lower cross-sections of the pile section are adiabatic borders, the surrounding vertical boundaries are external natural convection boundaries, and that heat is transported purely horizontally, ignoring the impact of water flow in the tube. The heat transfer pipe’s enforced temperature boundary serves as the contact boundary between the pipe and the pile. The energy pile’s beginning temperature, the pipe’s intake and outlet temperatures, the model material (Table 2), and the limitations are all modified to match the measured data. A free triangular network with 4170 network cells was used for the meshing. To model and compute the transient temperature change throughout 336 h of operation, set the time step to 1 h. The measured position and the monitoring point’s coordinates match. As seen in Figure 4b, the sites with the distance ratios from the origin to R of 0.5, 1, 1.5, and 2 are likewise designated as crucial points.

4.1. Assumptions and Mathematical Model

The following assumptions guided the numerical simulations.
(1)
The energy piles’ materials were thought to be isotropic, elastic, and incompressible;
(2)
the soil layers and piles are modeled as linear thermoelastic materials;
(3)
numerical simulations depict quasi-static conditions with negligible inertial effects;
(4)
the impact of associated loads on changes in the soil water field is minimal;
(5)
the impact of water flow temperature changes on the heat exchanger tubes was disregarded, and the heat transfer in the pile soil was computed using a solid heat transfer model.
Under the above assumptions and neglecting the heat transfer tube effect, the heat transfer in the pile soil is modeled using a solid heat transfer model, which can be expressed as:
ρ C P ∂ T ∂ t + ∇   ( − k ∇ T ) = 0
where ρ is the density of the material; Cp is the heat capacity of the material; k is the thermal conductivity of the material; T is the temperature of the structure; and t is the heat transfer time.

4.2. Model Validation

The experiment’s water flow was at ambient temperature, and the heating apparatus progressively raised it to the desired inlet water temperature over time. Figure 5 displays the temperature variations at the entrance and output following 336 h of operation. The temperature increases quickly within the first 48 h, reaching 39.2 °C at the entrance and 36.9 °C at the outflow. The entrance temperature reaches 45.8 °C and the output temperature reaches 43.5 °C after 336 h, following which the temperature variations progressively level off after 48 h. An interpolation curve is created by simplifying the temperature change and then input into the model. Since the impact of water movement is disregarded, the inlet and outlet temperatures are fixed on the heat exchanger pipe’s wall. The output pipe is set to the outlet temperature in the pile cross-section model, while the other three pipes are all set to the input temperature.
Figure 6 illustrates how the temperature change throughout the 336 h of measurement matched the computed findings when the simulation results were compared with the observed temperature readings at the 2 m, 10 m, and 20 m sensor positions.
The test pile’s temperature rose from 28.8 °C to 39.4 °C at the 2 m monitoring point, from 21.1 °C to 37.1 °C at the 10 m monitoring point, and from 19.2 °C to 35.7 °C at the 20 m monitoring point while it was heated with continuous heating power. There was a maximum temperature difference of 1.06 °C with an error rate of 2.7% between the measured and simulated temperatures at 2 m, 1.78 °C with an error rate of 4.8% between the measured and simulated temperatures at 10 m, and 2.65 °C with an error rate of 7.4% between the measured and simulated temperatures at 20 m. The model has some discrepancies in the computation results because it disregards the impact of the water flow temperature change on the heat exchanger tube and sets the heat exchanger tube wall temperatures at different depths to be the temperature of the inlet and outlet water flow. This is because the heat loss of the water flowing in the heat exchanger tube increases with depth. The largest inaccuracy at the bottom of the pile is around 7.4%, even though the precision of the simulated data of this model diminishes as the geoid’s depth increases. When combined, these validation and verification results allow us to have confidence in the model.

5. Results and Discussion

Before and after structural optimization, the heat transmission process in the pile and soil around the pile is examined using a steady state heat transfer model for concrete energy pile segments. The material’s thermal characteristics and the radius range of each part are linked to the temperature change in the heat transfer space. The following pertinent characteristics are examined in order to increase the energy pile’s heat transmission efficiency.

5.1. Effect of Heat Exchanger Piping Configuration on Energy Pile Heat Transfer

The heat exchanger pipe’s diameter in the initial simulation and field test was set at 25 mm. data over the past ten years on the traditional pipe parameters used in energy pile heat transfer studies. In order to lessen the impact of pipe arrangement on the deterioration of the pile foundation’s structural stresses, U-type and W-type heat exchanger pipes predominate in energy piles, as shown in Table 3. High-density polyethylene is the predominant material used in pipes. The most common diameter for ordinary pipes is 25 mm, which falls between 20 and 40 mm.
In order to compare the temperature changes of the heat exchanger with different pipe diameters on the concrete (at point 0.5 and 1) and the underground soil (at point 1.5 and 2) before and after the structural optimization, the energy pile pipelines with pipe diameters of 20 mm, 30 mm, and 40 mm are chosen. The temperature monitoring points in this section are the points with the ratio of the distance from the center to the diameter of the energy pile at 0.5, 1, 1.5, and 2 on the X and Y axes, respectively (illustrated in Figure 3). The new fins’ impact on heat transport has been determined. Figure 7 shows the temperature graphs of important locations both before and after the energy pile cross-section was optimized for various pipe diameters. According to Figure 7, while the pipe is operating without fins, the heat transfer temperature in the concrete area along the pipe wall progressively rises as the pipe’s diameter grows. Around the pile, the soil temperature field range is essentially the same. After the fins are installed, the concrete area and the soil temperature field range surrounding the pile are essentially the same as the tube diameter increases.
The temperature fluctuation over a 72 h period at various points from the origin is seen in Figure 8. It is evident from the figure that in the concrete and soil range without fins, the temperature differential at each location between various pipe diameters is negligible. In contrast to the 20 mm diameter pipe, the 30 mm pipe’s concrete range has a maximum temperature difference of 0.47 °C and a maximum temperature difference of 0.32 °C in the surrounding soil; the 40 mm pipe’s concrete range has a maximum temperature difference of 1.96 °C and a maximum temperature difference of 1.1 °C in the surrounding soil. A greater pipe diameter results in a larger volume flow rate at the same fluid velocity, hence the temperature difference is negligible. The heat transmission between the concrete and the surrounding soil was considerably enhanced once fins were added. The temperature in the concrete range rises by 18.1 (i.e., 6 °C) for a 20 mm diameter pipe, 16.3 (i.e., 5.4 °C) for a 30 mm diameter pipe, and 11.9 (i.e., 4.2 °C) for a 40 mm diameter pipe as compared to the case without fins. For a 20 mm diameter pipe, the highest temperature rise in the surrounding soil area is 9.2 (i.e., 2.5 °C); for a 30 mm diameter pipe, it is 7.8 (i.e., 2.2 °C); and for a 40 mm diameter pipe, it is 5.3 (i.e., 1.4 °C).
The temperature fluctuation during 336 h at various positions from the origin is displayed in Figure 9. According to the figure, the maximum temperature difference between the concrete range of a 30 mm pipe and the surrounding soil is 0.62 °C and 0.48 °C, respectively, whereas the maximum temperature difference between the concrete range of a 40 mm pipe and the surrounding soil is 1.43 °C, and the maximum temperature difference between the two is 1 °C. This is in contrast to a pipe with a diameter of 20 mm. The heat transmission between the concrete and the surrounding soil range was enhanced with the addition of fins. 20 mm diameter pipes within the concrete range can have their temperature raised by up to 12.3 (i.e., 4.9 °C) compared to the case without fins; 30 mm diameter pipes can have their temperature raised by up to 10.8 (i.e., 4.4 °C); and 40 mm diameter pipes can have their temperature raised by up to 11.9 (i.e., 4.2 °C). For a 20 mm diameter pipe, the highest temperature rise in the surrounding soil area is 8.7 (i.e., 2.5 °C); for a 30 mm diameter pipe, it is 7.8 (i.e., 2.2 °C); and for a 40 mm diameter pipe, it is 5.3 (i.e., 3.6 °C).
Although adding fins to the model can raise the pipe’s heat transfer temperature and area, the fins’ ability to promote heat transfer diminishes with increasing pipe diameter. This type of fin provides the maximum extra heat transfer temperatures at a diameter of 20 mm for pipe diameters between 20 and 40 mm. The temperature of the surrounding soil progressively stabilizes as the working duration increases, and the fins’ extra heat transmission efficiency gradually declines.

5.2. Effect of Buried Pipe Depth on Heat Transfer in Energy Piles

The heat exchange pipe’s intended buried depth in both the original simulation and the field test is 22 m. According to Table 3, typical energy piles can be buried anywhere between 10 and 40 m, with the majority of applications occurring between 10 and 20 m. A heat exchanger tube with a diameter of 25 mm was chosen to compare the temperature changes in the heat exchanger at depths of 2 m, 10 m, and 20 m for the concrete (1D) and the subsurface soil (2D) before and after the structural optimization and to determine the impact of the new fins on the heat transfer. In this section, the temperature monitoring points are the points with the ratio of the distance from the center to the diameter of the energy pile at 1 and 2 on the X and Y axes, respectively. The temperature of the concrete region and the soil surrounding the piles significantly increases once the fins are set up for various ranges of buried depths, as shown in Figure 10.
The temperature variations from 0 to 336 h before and after the concrete structure optimization at various depths are displayed in Figure 11. After 336 h of operation, temperature variations at important concrete sites at various depths were noted in comparison to the operating state without fins. After structural optimization, the temperature of the X-axis key point rises by 21 (i.e., 21 (i.e., 2.5 °C) and the temperature of the concrete key point in the Y-axis direction increases by 24.9 (i.e., 2.3 °C) at a depth of 2 m; the temperature of the X-axis key point increases by 15.8 (i.e., 2.9 °C) and the temperature of the concrete key point in the Y-axis direction increases by 20.5 (i.e., 2.9 °C) at a depth of 10 m; the temperature of the concrete key point increases by 17.5 (i.e., 1.5 °C) and the temperature of the X-axis key point temperature increases by 17.7 (i.e., 3.5 °C); and the temperature of the Y-axis direction concrete key point temperature increases by 22.1 (i.e., 3.4 °C). For concrete sections with varying subsurface burial depths, the impact of setting fins is particularly noticeable. Areas in the X-axis direction are susceptible to rising temperatures from 15.8% to 21%, while areas along the Y-axis direction can obtain temperature increases from 20.5% to 24.9%.
The temperature change in the soil surrounding the pile at various depths from 0 to 336 h before and after the structure’s optimization is depicted in Figure 12. After structural optimization, the X-axis key point’s temperature increases by 23.7 (i.e., 1.5 °C) and the soil key point’s temperature around the Y-axis direction increases by 25.1 (i.e., 1.4 °C) at a depth of 2 m; at a depth of 10 m, the X-axis key point’s temperature increases by 20.5 (i.e., 1.9 °C) and the soil key point’s temperature around the Y-axis direction increases by 22.3 (i.e., 1.9 °C); at a depth of 20 m, the X-axis key point’s temperature increases by 21.5 (i.e., 2.2 °C) and the soil key point around the Y-axis direction increases by 23.2 (i.e., 2.1 °C).
For soil surrounding piles sections with varying subsurface burial depths, the impact of setting fins is particularly noticeable. Areas in the X-axis direction aren susceptible to rising temperatures from 20.5% to 23.7%, while areas along the Y-axis direction can obtain temperature increases from 22.3% to 25.1%.
After 336 h of operation under the same pipe diameter and flow rate conditions (15.8~24.9% for the concrete and 20.5~25.1% for the soil surrounding the pile), the temperature change in the concrete and the surrounding soil was essentially stable after the fins were added to the heat transfer device’s structure. The fins on the concrete and the surrounding soil at varying depths have almost the same optimal heat transmission efficiency for the 22 m deep energy pile.

5.3. Effect of Concrete Thermal Conductivity on Heat Transfer in Energy Piles

The most common type of concrete used in energy heaps is concrete. Numerous elements influence its thermal conductivity throughout the casting process: more humidity will result in higher thermal conductivity, and higher temperatures will also cause variations in thermal conductivity. The impact of concrete’s thermal conductivity on temperature changes prior to and during heat exchanger structural optimization is examined in this section. Ordinary concrete has a thermal conductivity of 0.2 to 1.25 W/(m·K), but heat-resistant concrete might have a thermal conductivity of 1.5 to 2.5 W/(m·K). In order to compare the effects of varying concrete thermal conductivity on the heat transfer of the heat exchanger pipe before and after the structure optimization, a 25 mm diameter heat exchanger pipe is chosen at a burial depth of 10 m (pile center). The temperature monitoring points are the points that have a ratio of 1 to 2 between the center of the X and Y axes from the origin and the diameter of the energy pile. Figure 13 displays the temperature field at key spots both before and after the energy pile cross-section structure was optimized under the impact of various concrete thermal conductivities. The concrete range temperature considerably rose when the fins were set. The temperature increase intensifies with the concrete’s increased thermal conductivity.
Working circumstances in concrete with thermal conductivity variations between 0.5 and 2.5 W/(m·k) are taken into consideration for a constant Cp. The temperature variations at crucial locations for concrete and surrounding soil under the impact of various thermal conductivities are displayed in Figure 14 before and after structural optimization for 72 and 336 h.
After 72 h of operation, the concrete’s thermal conductivity was adjusted between 0.5 and 2.5. In comparison to the operating situation without fins, the temperature in the concrete area was raised by 7.1% to 18 (i.e., 2.4 °C to 4.9 °C) at the X-axis key points and 8.6% to 11.9 (i.e., 2.6 °C to 3.1 °C) at the Y-axis key points. The temperature in the soil area surrounding the pile area was raised by 4.2% to 7.4 (i.e., 1 °C to 1.7 °C) at the X-axis key points and 4.2% to 5.2 (i.e., 1.5 °C to 2.8 °C) at the Y-axis key points. After 336 h of operation, as contrast to the no-fin condition. the temperature in the concrete area was raised by 5.3% to 18.1 (i.e., 2.1 °C to 5.8 °C) at the X-axis key points and 6.6% to 15.2 (i.e., 2.5 °C to 4.3 °C) at the Y-axis key points. The temperature in the soil area surrounding the pile area was raised by 4.9% to 12.6 (i.e., 1.5 °C to 3.3 °C) at the X-axis key points and 5.1% to 10.9 (i.e., 1.5 °C to 2.8 °C) at the Y-axis key points. As the concrete’s thermal conductivity rises, the temperature in the heat transfer zone rises as well, but the temperature increment falls as thermal conductivity rises.

6. Discussions

This article compared the thermal performance of individual energy heaps using a two-dimensional model and field test verification. Although the variations in pile–soil heat exchange temperature before and after energy pile fin optimization are examined, there are still certain aspects of this study that merit more investigation and improvement.
Without taking into account how the fluid affected the temperature change in the pipe wall, the heat transfer performance of the energy heaps was examined in this work utilizing a solid heat transfer module. The accuracy of the temperature readings decreases with increasing buried pipe depth when using the two-dimensional model of heat transmission through the pipe wall. At 20 m, the greatest temperature differential between the simulated and observed temperatures has already reached 2.65 °C (i.e., error rate of 7.4%). As a result, only energy pile heat transfer studies with underground pipe depths up to 25 m may use this model. To improve the accuracy of heat transfer simulation in the future, a solid–liquid coupled heat transfer model or a three-dimensional model should be taken into consideration.
Furthermore, it is thought that the energy pile’s functioning and heat injection times are significantly correlated with the variance in soil temperature surrounding the pile. Only the heat transfer impact of the pile operating for 336 h is taken into consideration in this study; the heat transfer effect of running the fins all year long is not considered. Therefore, it is still necessary to confirm the innovative fin’s application. To achieve a more thorough knowledge of how fins affect the heat transfer process, future research might focus on confirming the differences between energy piles operating year-round and those operating under fin optimization.

7. Conclusions

This study presents the calibration, validation, and parameterization of a W-type energy pile that models a field test site in Nanjing, China. The parametric findings support the model and suggest a novel energy pile fin configuration to enhance heat transfer efficiency. Before and after the energy pile structure is optimized, the impact of concrete thermal conductivity, burial depth, and pipe arrangement on heat transfer performance is carefully examined. Several implications regarding the distribution of soil temperature within and surrounding the pile may be drawn from these findings:
  • Under the optimum operation of fins, the heat transfer temperature within the concrete and surrounding soil range rises noticeably, and as the tube diameter grows, the temperature increase reduces. When choosing a tube diameter between 20 and 40 mm, the temperature of the concrete area rises by 10.8% to 12.3%, and the temperature of the region surrounding the pile rises by 5.3% to 8.7% after the fins have been running for 336 h.
  • The depth of the ground edge has a significant impact on the ground temperature within 8 m from the surface, while the ground temperature in the heterothermal zone (0–8 m) has little bearing on the enhancement of heat transfer through the installation of fins. After 336 h of operation with the fins set, the temperature of the concrete area grew by 15.8% to 24.9% within 20 m of the buried depth, and the temperature of the region surrounding the pile increased by 20.5% to 25.1%.
  • When concrete’s thermal conductivity is adjusted between 0.5 and 2.5, the temperature in the heat transfer zone rises as the concrete’s thermal conductivity rises, but the temperature increment falls as the thermal conductivity rises. After putting the fins to run for 336 h, the temperature in the concrete area rises by 5.3% to 18.1%, while the temperature in the area surrounding the pile rises by 4.9% to 12.6%.

Author Contributions

Conceptualization, W.G. and G.K.; methodology, Y.D.; software, T.X.; validation, X.W.; formal analysis, X.W., K.Z.; investigation, T.X., K.Z.; resources, G.K.; data curation, X.W.; writing—original draft preparation, X.W.; writing—review and editing, X.W.; visualization, Y.X.; supervision, W.G.; project administration, G.K.; funding acquisition, W.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflict of interest.

List of Acronyms and Symbols

CTECoefficient of thermal expansion
CFGCement fly ash gravel
GHEGround heat exchangers
GSHPGround source heat pump
HDPEHigh-density polyethylene
ILSInfinite line source
OCROverconsolidation ratio
PCMPhase change materials
PHCPre-stressed high-strength concrete
TRTThermal Response Test
VWSGsVibrating wire strain gauges
CHeat capacity of a material
DbDiameter of borehole [m]
DsDiameter of soil [m]
EElastic modulus [MPa]
LbDrilling length [m]
REnergy pile radius [mm]
ScShape factor
TStructural temperature [°C]
doExternal diameter of PE pipe [mm]
diInternal diameter of PE pipe [mm]
λThermal conductivity [W/m/°C]
cSpecific heat [J/kg K]
rDistance to the center origin [mm]
tHeat transfer time [h]
xPipe distance [m]
zSoil depth [m]
ρDensity [kg/m3]
γUnit weight [kN/m3]
vPoisson’s Ratio
αThermal expansion coefficient [/°C]

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Figure 1. Optimization effect of energy pile structure.
Figure 1. Optimization effect of energy pile structure.
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Figure 2. Schematic diagram of the energy pile heat exchanger.
Figure 2. Schematic diagram of the energy pile heat exchanger.
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Figure 3. Distribution of the initial ground temperature with depth.
Figure 3. Distribution of the initial ground temperature with depth.
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Figure 4. Cross-sectional network modeling information. (a) Cross-section network division; (b) schematic diagram of key testing points.
Figure 4. Cross-sectional network modeling information. (a) Cross-section network division; (b) schematic diagram of key testing points.
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Figure 5. Variation in measured water temperature at the entrance and exit of the energy pile for 336 h of operation.
Figure 5. Variation in measured water temperature at the entrance and exit of the energy pile for 336 h of operation.
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Figure 6. Comparison of measured and simulated temperatures at different depth monitoring points.
Figure 6. Comparison of measured and simulated temperatures at different depth monitoring points.
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Figure 7. Temperature field at critical points before and after optimization of energy pile cross-section with different pipe diameters.
Figure 7. Temperature field at critical points before and after optimization of energy pile cross-section with different pipe diameters.
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Figure 8. Temperature at key points before and after optimization of different pipe diameter structures in X and Y axes after 72 h of operation. (a) x-axis; (b) y-axis.
Figure 8. Temperature at key points before and after optimization of different pipe diameter structures in X and Y axes after 72 h of operation. (a) x-axis; (b) y-axis.
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Figure 9. Temperature at key points before and after optimization of different pipe diameter structures in X and Y axes after 336 h of operation. (a) x-axis; (b) y-axis.
Figure 9. Temperature at key points before and after optimization of different pipe diameter structures in X and Y axes after 336 h of operation. (a) x-axis; (b) y-axis.
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Figure 10. Temperature field at critical points of concrete before and after structural optimization of energy pile cross-section at different depths.
Figure 10. Temperature field at critical points of concrete before and after structural optimization of energy pile cross-section at different depths.
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Figure 11. Concrete critical point temperatures before and after structural optimization of energy pile cross-sections at different depths. (a) x-axis, (b) y-axis.
Figure 11. Concrete critical point temperatures before and after structural optimization of energy pile cross-sections at different depths. (a) x-axis, (b) y-axis.
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Figure 12. Temperature at key points of the soil around the pile before and after optimization of the cross-sectional structure of energy piles at different depths. (a) x-axis; (b) y-axis.
Figure 12. Temperature at key points of the soil around the pile before and after optimization of the cross-sectional structure of energy piles at different depths. (a) x-axis; (b) y-axis.
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Figure 13. Temperature field at critical points of concrete before and after structural optimization of energy pile cross-section at different concrete thermal conductivity.
Figure 13. Temperature field at critical points of concrete before and after structural optimization of energy pile cross-section at different concrete thermal conductivity.
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Figure 14. Temperature change at critical point before and after optimization with different thermal conductivity. (a) x-axis of concrete, (b) y-axis of concrete, (c) x-axis of soil around piles, (d) y-axis of soil around piles.
Figure 14. Temperature change at critical point before and after optimization with different thermal conductivity. (a) x-axis of concrete, (b) y-axis of concrete, (c) x-axis of soil around piles, (d) y-axis of soil around piles.
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Table 1. Critical material parameters for soils, piles and heat pipes.
Table 1. Critical material parameters for soils, piles and heat pipes.
MaterialsDepth
z (m)		Thermal Conductivity
λ (W/m/°C)		Specific Heat
Cu (MJ/m3/°C)		Unit Weight
γ (kN/m3)		Elastic Modulus
E (MPa)		Poisson’s
Ratio
v		Thermal Expansion Coefficient
α (/°C)
Clayey soil
-Layer 10–41.663.2021.02200.301 × 10−6
-Layer 24–81.573.2520.32000.30
-Layer 38–151.533.1620.02200.30
-Layer 415–181.613.0820.01800.32
-Layer 518–231.563.2119.62300.30
-Layer 623–261.483.1120.62300.30
-Layer 7>261.723.1920.32800.25
Concrete-1.742.5025.030,9000.201 × 10−5
Table 2. Key material parameters for finite element simulation.
Table 2. Key material parameters for finite element simulation.
MaterialDepth(m)Density
ρ (kg/m3)		Thermal Conductivity
λ (W/m/°C)		Specific Heat
c (J/kg·K)
Soil221001.661523.8
1020001.531580
2019601.561637.7
Backfill material25001.741000
Aluminum fins2700204896
Table 3. Conventional piping parameters for heat transfer studies in energy piles in the last decade.
Table 3. Conventional piping parameters for heat transfer studies in energy piles in the last decade.
Ref.TypeMaterialL (m)do (mm)di (mm)λ (W/(m·K))x (m)
[20]U-typehigh-density polyethylene152522.71.780.11
[51]4U-typehigh-density polyethylene1025-0.4-
[16]U-typehigh-density polyethylene232522.70.420.255
[24]W-typehigh-density polyethylene15.219160.480.448
[52]W-typepolyethylene pipe2332---
[53]U-typepolyethylene pipe40.242.234.50.35-
[28]U-typehigh-density polyethylene-32280.3850.436
[25]4U-typehigh-density polyethylene102520-0.2
[22]3U/W-typepolybutylene pipe10.2/9.82016-0.38
[54]3U-typehigh-density polyethylene14.225--0.175
[55]3U-typehigh-density polyethylene13.540--0.4
[26]U-typehigh-density polyethylene30.52520.41.41-
[6]U-typecross-linked polyethylene2032260.410.3
[14]W-typehigh-density polyethylene1825---
[56]U-typehigh-density polyethylene202621.50.390.05
[57]U-typepolybutylene pipe15.2/9.225.419--
[58]U-typehigh-density polyethylene1625.4 0.610.2
[59]U-typehigh-density polyethylene283226.20.40.5
[60]2U-typehigh-density polyethylene2025/40-0.42-
[13]W-typehigh-density polyethylene7225---
[2]2U-typepolybutylene pipe162521--
[61]U-typehigh-density polyethylene102520--
[62]2U-typehigh-density polyethylene2020160.350.402
[26]W-typehigh-density polyethylene303225.50.39-
[63]U-typehigh-density polyethylene1026220.42-
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MDPI and ACS Style

Wang, X.; Xu, T.; Zhao, K.; Xia, Y.; Duan, Y.; Gao, W.; Kong, G. Study on the Effect of Heat Transfer Characteristics of Energy Piles. Buildings 2024, 14, 3593. https://doi.org/10.3390/buildings14113593

AMA Style

Wang X, Xu T, Zhao K, Xia Y, Duan Y, Gao W, Kong G. Study on the Effect of Heat Transfer Characteristics of Energy Piles. Buildings. 2024; 14(11):3593. https://doi.org/10.3390/buildings14113593

Chicago/Turabian Style

Wang, Xiaoyang, Tongyu Xu, Kaiming Zhao, Yueqiu Xia, Yuechen Duan, Weijun Gao, and Gangqiang Kong. 2024. "Study on the Effect of Heat Transfer Characteristics of Energy Piles" Buildings 14, no. 11: 3593. https://doi.org/10.3390/buildings14113593

APA Style

Wang, X., Xu, T., Zhao, K., Xia, Y., Duan, Y., Gao, W., & Kong, G. (2024). Study on the Effect of Heat Transfer Characteristics of Energy Piles. Buildings, 14(11), 3593. https://doi.org/10.3390/buildings14113593

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