Single Energy Pile Research Articles

Thermal interference process between two energy piles in 2D model using transparent soil

Energy piles have the potential to contribute to the use of shallow geothermal energy. The heat exchange efficiency of energy piles in groups decreases compared with that of a single energy pile. Understanding the thermal interference mechanism between piles and its impact on the temperature response of the soil is crucial. In this study, the evolution processes of the temperature field of transparent soil around one and two energy piles in a 2D model were obtained by establishing the relationship between pixel intensity and soil temperature. This relationship was verified using a single energy pile test under heating conditions. Thermal images of transparent soil under different thermal loads (heating or cooling) were studied to investigate the phenomenon of thermal interference between two piles. The temperature superposition effect becomes more evident closer to the center of the two piles. Analyzing the temperature distribution in the soil and introducing a thermal interference coefficient revealed that the thermal accumulation effect between piles is more significant under lower thermal loads.

An approach for analysis of a single energy pile subjected to a horizontal load in sand

Many studies on horizontally loaded energy piles have tended to ignore the impact of axial frictional resistance in the analysis processes or lacked experimental validation. The effect of axial frictional resistance may be considered by conveniently calculating the frictional resistance and revealing the mechanism of axial frictional resistance. Our proposed displacement analysis approach takes into account pile-sand axial friction during heating. Validation was carried out through a centrifuge test case. Subsequent parametric analyses explored the influence of pile diameter and Young's modulus on the lateral response of the pile. Comparative results between the centrifuge test and proposed approach underscored its effectiveness in capturing the impact of friction resistance on energy piles during heating. The parametric analyses show that the increase in pile diameter from 0.5 m to 0.7 m and Young's modulus from 10 GPa to 20 GPa led to a decrease in normalized pile top displacement by 50.68% and 28.33%, a decrease in maximum soil pressure in front of the pile by 16.18% and 11.59%, and a decrease in maximum bending stress of the pile by 3.27% and an increase by 14.48%, respectively.

The mechanical response of energy pile groups in layered cross-anisotropic soils under vertical loadings

The mechanical response of energy pile groups in layered cross-anisotropic soils under vertical loadings is studied with the aid of the coupled finite element method - boundary element method (FEM-BEM). The single energy pile is simulated based on the finite element theory, which then is extended to energy pile groups. The global flexibility matrix for soils is obtained by considering the coupling effects of vertical and thermal loadings. The coupled FEM-BEM equation for the interaction between energy pile groups and soils is derived based on the displacement compatibility condition at the pile-soil interface. According to the displacement coordination condition and force balance in the rigid cap, the displacement of the cap and axial forces of pile groups can be solved. The presented theory is validated by comparing the calculated results with numerical simulations and field test results in existing literature. Finally, effects of the thermal loading, pile-soil stiffness ratio, pile spacing, cross-anisotropy of Young's modulus and the stratification are discussed.

3D FE modeling of a real-world energy piled foundation

3D FE modeling of a real-world energy piled foundation

Numerical investigations on effects of mechanical and geometrical characteristics on thermally induced group behavior of field-scale energy piles

Energy pile groups generally exhibit different thermomechanical behavior from single energy piles. Aiming to investigate the group behavior of energy piles, three-dimensional numerical models were developed and validated against field test data for a 2 × 2 energy pile-raft foundation. The validated models were subsequently adopted to comparatively analyze the thermomechanical behavior of pile groups operating either partially or fully. The results highlighted that the group action effects would induce lower thermal stress and greater head displacement imposed upon piles. Partially operating grouped piles exhibited differential displacement, thus inducing group tilt in the raft. Besides, parametric studies were made to evaluate the effects of mechanical and geometrical parameters on the group behavior of energy piles. The effects of soil thermal expansion were directly correlated with the heating extent of the soil. Under geological conditions with high soil thermal expansion coefficients, heating-induced tensile stresses and excessive head displacements could occur during the long-term operation of multiple piles. Higher soil elastic modulus resulted in higher restrictions on the thermal movement of piles, thus reducing the head displacement and group tilt, which was considered beneficial in practice. Special attention should be paid to the group action effects for closely-spaced energy pile-raft foundations.

Effects of pile configuration on the group behavior of semi-floating energy piles

To investigate the effects of pile configuration on the group effects of semi-floating energy piles, a series of laboratory tests were performed. The tests include three distinct pile configurations, namely, a single energy pile, a 2 × 2 energy pile group, and a 3 × 3 energy pile group. Different operation modes of energy pile groups, including all energy piles being cooled and partial energy piles being cooled, were considered. The monitoring and analysis of temperature changes in energy piles and surrounding soil, axial load variation, and displacement of energy piles and non-thermally activated piles were performed. The interactions among energy piles, non-thermally activated piles, raft, and surrounding soil within semi-floating pile groups with varying numbers of energy piles and operation modes were discussed. The findings reveal that the group effects of energy piles can be attributed to two aspects, namely pile-soil interactions and pile-raft-pile interactions. Both of these interactions contribute to reduced thermally induced axial loads and increased thermally induced displacements of energy pile groups. Under similar thermal loading, the 3 × 3 energy pile group exhibits a 19% increase in displacement and a 64% decrease in maximum thermally induced axial load compared to a single energy pile. Furthermore, the group effects of an energy pile group were affected by the pile configuration and are more pronounced with an increase in the number of energy piles within a pile group. The findings presented in this paper contribute to a more precise assessment of the thermo-mechanical response exhibited by such pile foundations.

Numerical evaluations on the effects of different factors on thermo- mechanical behaviour of an energy pile group

Energy pile is a kind of economical and efficient underground energy structure. Obtaining the effects of different factors on the performance of energy pile help to improve the working efficiency of the energy pile. However, the thermo-mechanical behaviour of energy pile groups are more complex than that in single energy pile due to the mutual interference of energy pile groups. In this paper, the energy pile group with the layout of 3 × 3 was simulated by a 3-D numerical model. The influences of soil type, pile spacing, pile material performance and pile arrangement form on the thermo-mechanical behaviour of the energy pile group were investigated. The results show that increasing pile spacing can not only improve the thermal performance of energy pile group but also reduce the pile displacement, but it will lead to the increase of pile axial force because of the weakening of pile group effect. The thermal performance of energy pile group is the highest under the rock-soil condition, followed by sand and clay. However, the pile displacement is the largest when the soil type is sand, followed by clay, and the rock-soil is the smallest, and the influence of soil type on pile axial force is just opposite to pile displacement. At the same time, the increase of thermal conductivity of pile material benefits to heat transfer performance of energy pile group, but it also aggravates the heat accumulation and results in a larger top displacement and axial force of pile. As for concrete compression modulus, increasing the concrete compression modulus can increase the axial force of pile, but the change of concrete compression modulus has no effect on the pile displacement. In terms of layout of energy pile group, the per unit soil volume heat exchange rate of energy pile group in the cross arrangement form is larger than that in sequential arrangement form. However, the variation range of soil temperature in the cross arrangement form is larger than that in the sequential arrangement form, thus increasing pile displacement and reducing pile axial force.

An Overview of Sandbox Experiment on Ground Heat Exchangers

As an energy-efficient and low-carbon technology, ground-source heat pumps are promising to contribute to carbon neutrality in the building sector. A crucial component of these systems is the ground heat exchanger, which has been extensively studied through sandbox experiments. These experiments play a vital role in understanding heat transfer characteristics and validating simulation results. In order to facilitate the improvement of ground heat exchangers and the development of ground-source heat-pump systems, this article provides a comprehensive summary of existing sandbox experiments. The borehole sandbox experiments are classified into the single borehole experiment, borehole group experiment, seepage experiment, and multi-layer soil experiment. It was observed that the heat transfer efficiency of a single spiral tube is only 80% compared to that of a double spiral tube. Moving on to energy-pile sandbox experiments, they are further divided into mechanical performance, thermal performance, and thermal-mechanical coupled performance tests. It was revealed that the heat transfer distance of a single U-shaped energy pile in the radial direction is three times greater than in the vertical direction. For the mentioned sandbox experiments, the sandbox design, experiment conduction, testing conditions, and result analyses are summarized. To improve the sandbox experiments, there are still some difficulties in building a similarity experiment, testing the temperatures in a small error, controlling the boundary conditions accurately, and testing the thermophysical properties of soil accurately. Furthermore, the perspectives of sandbox experiments of ground heat exchangers are also proposed. The sandbox experiments under complex environment conditions or with novel composite energy geo-structures or ground heat exchangers with new materials and new technologies would be further investigated. By addressing these aspects, this review aims to provide guidelines for the design, construction, operation, and optimization of sandbox experiments for different ground heat exchangers, ultimately promoting the wider adoption of ground-source heat pumps in achieving carbon neutrality.

Analytical interpretation and numerical analysis of multiple energy pile thermal response tests

Thermal response tests (TRT) are an essential in situ test for the sizing of ground source heat pump systems. Its application on energy piles is becoming an established practice, involving key differences in relation to testing boreholes. Recently, group TRTs emerged as an alternative to testing single short energy pile elements. This work assesses the execution and interpretation of those tests through different analytical heat transfer models implemented in a unit-response calculation methodology. To assess the suitability of the method, a comprehensive parametric analysis using a validated numerical model is undertaken, considering different energy pile sizes, thermal properties for both concrete and soil and group pile arrangements. The analytical methodology is validated against experimental and numerical results, and then tested to interpret the numerically simulated TRTs using a curve fitting algorithm. The broad parametric investigation and the evaluation of the capacity of different analytical methods on modelling each scenario provide directions to execute and interpret group TRTs. The soil effective thermal conductivity and energy pile thermal resistance are obtained with less than 5% precision error for most scenarios and less than 10% considering all scenarios. These results can be further improved with specific individual G-functions for the pile elements, that can be implemented on the proposed methodology.

A Simple Method for Predicting the Response of Single Energy Pile Considering Temperature Variation of Pile–Soil Interface

To capture the influence of temperature variation of pile–soil interface on the response of a single energy pile, numerical simulation and theoretical analysis were adopted in this paper. The temperature variation mechanism of a single energy pile was clarified using numerical simulation, and the temperature variation model of the pile–soil interface was established. Considering the influence of temperature variation on the conventional load transfer model of the pile–soil interface, the improved hyperbolic functions were proposed, and the parameters related to the improved load transfer model were determined. To predict the response of a single energy pile considering the temperature variation of the pile–soil interface, an iterative algorithm was developed using load transfer methods. Comparisons of the load-settlement response of three well-documented cases between the present computation results and the results derived from other methods were made to verify the reliability of the calculation method. Furthermore, a parameter analysis was carried out to assess the influence of soil properties and the parameters related to the load transfer models on the thermomechanical response of a single energy pile.

Study on the thermal response of spiral energy piles based on field test

Field tests of spiral energy piles under the combined effect of temperature and loading are relatively few. Based on the field test, the heat transfer efficiency, pile strain, axial force and shaft friction of two spiral energy piles were studied. The major findings of the experimental studies were: First, when the double spiral energy pile was heated, the temperature distribution was more uniform; the total heat transfer and the heat transfer rate were higher than those of the single spiral energy pile. Second, the pile strain distribution was such that smaller values were noticed at both pile ends while larger values were in the middle part of the pile. The additional tensile stresses of the two piles generated during cooling reached 4.06 MPa and 4.75 MPa, which exceeded the tensile strength of concrete. Finally, during heating, the shaft friction was negative in the middle and upper pile and positive in the middle and lower pile. The single spiral energy pile showed two neutral points. The downward load generated by the single spiral energy pile was about 885 kN higher than that generated by the double one. The aforementioned changes should be focused on in the actual project.

Laboratory investigations on the thermo-mechanical behaviour of an energy pile group operated in heat release mode

Compared to the single energy pile, the thermo-mechanical behavior of pile group become more complicated due to mutual influences of different piles. A model test device of pile group with 2 × 2 layout was built to study the thermo-mechanical characteristics of pile group with different number of energy piles, inlet temperatures and connection forms of buried pipes. The results show that compared to single energy pile, the energy pile group mode will lead to the decrease of heat exchange performance and the increase of pile tip soil pressure and pile head displacement. But, the side friction and axial force of pile will decrease as a result of pile group effect. Meanwhile, increasing the inlet temperature can enhance heat exchange characteristics of pile group, but it will also result in the increase of the soil temperature, pile head displacement, pile tip soil pressure, pile shaft axial force and pile side friction. Additionally, reducing the number of energy piles in pile group can improve the heat exchange rate per unit pile depth, but the total heat transfer amount of energy pile group will decrease. With the increase of the number of energy pile in pile group, the temperature change and pile head displacement of the pile increase, but the pile side friction and pile body axial force decrease. Under current experimental conditions, the pile head displacement and pile body axial force in parallel connection mode are larger than those in series connection modes, and the pile body excess temperature is the smallest in parallel mode of four energy piles and the largest in series mode of four energy piles. For the series connection mode, the heat exchange performance, pile body excess temperature, pile head displacement and pile body axial force gradually decrease along the series direction due to the decrease of temperature change.

Analytical Solutions for Thermomechanical Soil–Structure Interaction in Single Semifloating Energy Piles Embedded in a Layered Soil Profile

Analytical solutions for axial displacement, strain, and stress in a single semifloating energy pile embedded in a layered soil profile subjected to thermal and mechanical loads were derived. An analytical solution for the location of zero displacement, also known as a null point, was derived for a pile subjected to thermal load. It was shown that the location of the thermal null point corresponds to the location of the maximum magnitude of thermal axial stress. It also was shown that the location of the null point moves toward the pile tip as the stiffness of the bedrock below the pile tip increases. The analytical solutions were validated against in situ full-scale energy pile tests. The solutions along with the validation process delineated the load transfer mechanism in energy piles subjected to thermal, mechanical and combined thermomechanical loads embedded in a four-layer soil profile. Flowcharts delineating the procedures for obtaining the analytical solutions for a single energy pile embedded in an arbitrary number of layers, and subjected to thermal and mechanical loads are provided. Although the continuity of stresses and displacements at the interface of different soil layers is maintained, displacement, strain, and stress diagrams exhibit a lack of smoothness, the amount of which depends on the difference in the stiffness of these layers. In summary, the presented solutions provide a rational, mechanics-based framework for advancing the understanding of thermomechanical response of energy piles that not only is essential for analysis and design, but ultimately contributes to a wider use of energy piles and increased sustainability of civil engineering infrastructure.

Long-term performance investigation of a GSHP with actual size energy pile with PCM

• Long term Performance investigation of ground source heat pump system. • Energy pile coupled with phase change materials under an actual building load. • Optimization of Phase change materials’ location inside the energy pile concrete shell. • Effect of utilizing multiple PCM melting temperature inside a single energy pile. The use of geothermal energy has increased significantly (90 time) since 1995. Among these increases, Ground Source Heat Pumps (GSHP) has contributed by 40 times in an effort to reduce the burning of fossil fuels and contribute to the reduction of green house gas emissions. The space requirements and high initial cost of borehole fields hinder the widespread use of GSHPs. The use of foundation piles, used as a ground heat exchanger, has been proposed to overcome these limitations. Research has shown that foundation piles have a lower depth and smaller spacing compared to boreholes. Phase Change Materials (PCMs) have been proposed as a potential solution to increase the storage capacity and decrease the thermal radius of energy piles. In the current study, a 3-D finite element model was developed to study the effects of PCMs on the performance of energy piles against a real building load for a complete year, while integrating an actual heat pump (HP) performance curve into the numerical model. The effects of the PCM location and melting range were also investigated. The study revealed a 5.2% enhancement in the Coefficient of Performance (COP) during the melting of the PCM, and a negative effect of up to 1.8% during the completely solid-state. Locating the PCM cylinders inside of the concrete shell led to better performance compared to when the PCM cylinders were located outside of the concrete shell. The best locations were found to be between the center of he pile and the U-loop. The PCM melting range of (4–6)°C was better than the melting range of (1–3)°C for the current study load. Lastly, the use of multiple PCM melting temperatures for a given design was investigated. The results revealed that the use of multiple PCM melting temperatures led to a performance enhancement of up to 26%.

Determination of optimal designs for geothermal energy piles in the soil supporting a multi-storey building

The article presents the optimal design of geothermal energy piles supporting a multi-story building at different pile spacing. The optimization model OPTPILE, based on the construction cost of the single pile, was used for this purpose. It was provided with geotechnical and structural design constraints that satisfy the requirements of building codes. The optimal design of a geothermal energy pile was studied for a 10-storey building with different pile spacing. So far, only a single energy pile has been optimized and therefore the spacing between the piles has not yet been considered in the design. However, in this work, pile spacing is taken into account by considering the entire load distribution of the multi-story building. A 3D beam-slab frame was created to determine the pile loads. The recommendation for the optimal design of geothermal energy pile spacing for a 10-storey building was developed. The results show that the optimal pile spacing (square distribution) for a 10-storey building is about 7 m. The construction cost for all thermal pile foundations and concrete structural components for a 10-storey building is estimated to be 101.1 €/m2. The optimal architecturally reasonable spacing of piles with a square distribution for a 10-storey building is 8.5 m. In this case, the cost of the concrete structural elements of the building and the piles increases by 1% to 102.1 €/m2. The cost of installing the heating pipes in the pile is about 1 €/m2.

A simple method for analyzing thermomechanical response of an energy pile in a group

This study develops a simple method for rapid estimation of the thermomechanical response of an energy pile in a group. Based on the equilibrium condition of the force, the governing equations for a single energy pile are established using the finite difference method. An exponential model is used to describe the relationship between the unit skin friction and pile–soil relative displacement as well as the relationship between the unit end resistance and displacement at the pile end. Based on the Pyke model for the loading–unloading behavior of the pile–soil interface, the incremental relationship between the unit skin friction and pile shaft displacement, and the incremental relationship between the unit base resistance and pile end displacement, are presented in the unloading and reverse loading stages. Considering the mechanical and thermal loads and the reinforcing effect of neighboring piles, a simple method is proposed to analyze the thermomechanical response of an individual energy pile in a group. Comparisons between the measured and computed results of six well-documented cases are presented to verify the reliability of the proposed method.

Experiment and simulation research on heat exchange of different types of underground heat exchangers

Compared with traditional bored pipe heat exchanger geothermal collection technology, the continuously optimized energy pile technology is more efficient and economical in the application of building energy-saving projects. According to the structural characteristics of three different underground heat exchangers of deep buried pipe energy pile, inside buried pipe energy pile and traditional bored and buried pipe heat exchanger, we analyze the heat transfer characteristics through field test and numerical simulation. The results show that at the same depth, the heat exchange and heat exchange efficiency of single deep buried pipe energy pile are higher than those of traditional bored buried heat exchanger. Under the same pile length, the heat exchange rate of single deep buried pipe energy pile is higher than that of the inside buried pipe energy pile. The high thermal conductivity of the pile foundation can significantly improve the heat exchange effect of the heat exchanger. The well-pile section heat exchange ratio of the deep buried pipe energy pile reaches 1.95. The installation of the deep well can not only reduce the thermal interference caused by the pile foundation thermal accumulation, but also compensate for the adverse effect of the small heat exchange tube spacing on the heat exchange. By optimizing the construction process of the heat exchanger and reducing the thermal interference effect between the heat exchange tubes, the overall heat exchange effect of the heat exchanger will be effectively improved. <fig fig-type="abstract-image" id="F1" orientation="portrait" position="float"><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="SP.J.1249-2022-39-1-20/48BEB0D3-18F9-4a9d-93C3-E7253C6DB4C2-F001.jpg"/></fig>

A Numerical Study of Groundwater Flow's Influence on Energy Extracted from a Single Energy Pile

A Numerical Study of Groundwater Flow's Influence on Energy Extracted from a Single Energy Pile

Numerical simulation of thermo-mechanical behavior of floating energy pile-raft foundation

The 3D nonlinear finite element model is developed to investigate thermo-mechanical response of a piled raft equipped with floating energy piles on sand subjected to combined thermo-mechanical loadings. Attention is focused on thermally induced group effects， the variations in temperature of the surrounding soil and the effect of the number and the layout of the energy piles on the thermally induced additional axial stress in piles and the differential settlements among the pile heads. The results show that when all the piles are heated， the additional axial compressive stress is much smaller for a pile in the energy pile group than that for single energy pile in the same group. In a pile-raft foundation with partial energy pile， the additional axial compressive stress is distributed in parabolic shape along the energy pile， and it decreases with the increasing number of energy piles. While the additional axial stress in a non-energy pile is distributed in S shape， and the additional axial tensile stresses in the upper parts of non-energy corner piles and edge piles increase with the increase of the number of energy piles. The layout of energy piles has great influence on the additional axial stresses in the energy piles， but has insignificant effect on the additional axial stresses in the non-energy piles. The maximum differential settlement among the pile heads is larger when the layout of energy piles is asymmetrical. The temperature of the surrounding soil depends on the number of energy piles around it. <fig fig-type="abstract-image" id="F1" orientation="portrait" position="float"><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="SP.J.1249-2022-39-1-67/9F0446D4-9E61-4374-AD12-215148B7C310-F001.jpg"/></fig>

Performance of a full-scale energy pile for underground solar energy storage

This study presents a field test to investigate the thermal injection performance of a full-scale energy pile for underground solar energy storage (USES). The tested energy comprises a full-scale bridge pile foundation and a spiral-shaped pipe. Numerical modeling was carried out to provide complementary results. Sensitivity analyses of the pipe configuration and operation mode on the thermal injection performance of the energy pile were performed based on the numerical modeling. The results showed that 84% of the injected thermal energy could be transferred to the surrounding soil by the energy pile, and the total amount of the thermal energy stored by a single energy pile was −3.2 GJ over 29.5 days. The maximum average value of the thermal injection rate of the energy pile was equal to −129 W/m, which was 2–3 times larger than the thermal injection rate of the geothermal boreholes. Compared to the continuous USES operation of the energy pile, the intermittent mode of the energy pile was more efficient for USES. Increasing the length of the heat exchange pipe of the energy pile was more effective in enhancing the thermal injection performance of the energy pile for USES than changing the pipe shape.