Energy Piles Research Articles

Analytical solution for heat exchange in energy pile ground considering smear effects

AbstractDue to the installation of energy piles, the surrounding soil will be inevitably disturbed, leading to a smaller void ratio and larger thermal conductivity. This study proposes a new closed‐form solution to calculate the heat exchange of energy pile foundations considering smear effects on the different soil thermal properties in the smear zone and undisturbed zone. The analytical model adopts the separation of variables to solve the governing equations. The eigenvalues are obtained according to a series of recurrence formulas and a transcendental equation. The solution is verified against existing analytical solutions in literature and numerical simulation, and an excellent agreement is obtained. The parametric study shows that a larger thermal conductivity of the soil in the smear zone results in a lower temperature at any given time. If the smear effects are not considered, the temperature will be overpredicted, especially for the pile and the soil in the smear zone. The proposed solution precisely predicts the soil temperature in the smear zone and undisturbed zone, which can contribute to the thermo‐mechanical behaviour of energy pile foundations in further work.

Behaviours of a belled energy pile under heating-cooling cycles

Behaviours of a belled energy pile under heating-cooling cycles

Development of geopolymer-based composites for geothermal energy applications

This paper intends to develop geopolymer-based composites for geothermal energy applications. Both silicon carbide (SiC) sand and SiC powder were applied to optimize the thermal and mechanical behaviours of geopolymer. The flexural and compressive strength of geopolymer composites with different mix designs were investigated. A novel test set-up to record the thermal conductivity were designed, and the thermal behaviours of geopolymer composites with different mix design were tested. It was found that the proper addition of SiC powder is conducive to the compressive and flexural strength of geopolymer composites, while the influence of SiC particles is insignificant. The relationship between flexural and compressive strength of geopolymer composites was also investigated, and it was found that the Portland cement association (PCA) code reached the best fitting precision. With the addition of SiC materials, the maximum thermal conductibility of geopolymeric composite can be as high as 5.35 Wm−1K−1, which is about five times higher than conventional cementitious composite. The combined application of SiC powder and SiC particle is practical to increase the thermal conductivity of geopolymer, making it a suitable material for geothermal energy applications. Based on the numerical model, it was concluded that the addition of silicon carbide is beneficial to the heat exchange rate of the energy pile and improves the heat utilization efficiency of the energy pile.

Thermomechanical performance of energy retaining pile influenced by surrounding utility tunnel via the regression tree model

The thermomechanical performance of energy retaining pile differs from that of axisymmetric energy pile due to utility tunnel excavation. This study presents four field tests of energy retaining pile conducted before and after utility tunnel construction. A regression tree (RT) model for predicting the thermally induced stress of energy retaining pile was established based on the measured water temperature, pile temperature and previous stage thermally induced stress to evaluate the effect of shallow buried utility tunnel on the thermomechanical performance of adjacent energy pile. The established thermally induced stress RT prediction model could acceptably predict the thermally induced stress of the future short-team. Surrounding utility tunnel excavation alters the distribution of the axial thermally induced stress. The axial thermomechanical response of the energy retaining pile after utility tunnel construction is approximately 50% that before utility tunnel construction. The asymmetrical boundary conditions cause a limited bending moment of the energy retaining pile at the top or bottom of the utility tunnel, which is not enough to threaten the pile safety. Moreover, the thermal performance of the energy retaining pile slightly increases due to the heat exchange between the pile and the air in the surrounding utility tunnel.

Machine-learning-based performance prediction of the energy pile heat pump system

The coefficient of performance (COP) is one of the important indicators for evaluating ground source heat pump systems, and a certain amount of machine learning algorithms have been applied for COP prediction. However, most of the current models use a large number of input variables, and it may be difficult to obtain all the required parameters for diagnosis of the sensors. Most of the influencing factors contain fluid temperature, which is not easy to measure, and there is a risk of leakage when sensors are deployed in the system. In this work, two-year field tests were conducted based on an energy pile heat pump system, where in situ results were used as sample points, and the more easily measured ambient temperature and humidity, room temperature and humidity, and hourly power consumption were used as input parameters. Spearman correlation analysis is used to rank the influencing factors, and the prediction of COP was developed by a hybrid artificial neural network (ANN), optimized via the particle swarm optimization algorithm (PSO), and finally compared with empirical models and their updates. Results showed that among the input variables, the most and least important factors affecting the COP are the ambient temperature and ambient humidity, respectively. The prediction results of PSO-ANN were in the best agreement with the measured values, and the accuracy was higher than that of the empirical regression models. The proposed method noted in this study can be used as a supplement to the current COP prediction.

Experimental and Numerical Investigations on the Heat Transfer Performance of a New Type of Energy Pile

The ground‐source heat pump heating and cooling system is currently the most important development and utilization method of shallow geothermal resources, but its key component is the ground heat exchanger system. Herein, a new type of ground heat exchanger, the soil‐cement energy pile, is proposed. The similarity model comparison experiments of the soil‐cement energy piles and the soil energy pile in summer and winter were conducted respectively, and the 3D numerical models of heat transfer of the two kinds of energy piles are established, their related thermal properties are verified. Then the heat transfer calculation methods of the soil‐cement energy pile are presented and validated. The research has shown that the soil‐cement energy pile has good thermodynamic performance, which effectively reduces the largest thermal resistance in the heat transfer system of the energy pile, the thermal resistance of pile body material, and the heat transfer per linear meter of the soil‐cement energy pile is 21.20%–55.60% higher than that of the soil energy pile. The soil‐cement energy pile is of great significance to improve the thermal efficiency of the ground‐source heat pump system and can reduces the overall project cost.

Centrifuge modelling of an energy pile group with groundwater flow

For several decades, energy piles have been developed to extract heat from the ground by using foundation piles as heat exchangers. However, the thermal loading of these piles directly affects their mechanical behaviour. Centrifuge modelling of a thermally loaded pile group within a groundwater flow makes it possible to improve the understanding of the hydro-thermo-mechanical behaviour of this type of foundation. In this study, groundwater flow was established in a centrifuge model to study an energy pile group. The focus of this study was to enhance research findings on energy geo-structures such as foundations, which are very often located in soil with seepage. The groundwater flow allows reduction of thermomechanical effects and therefore plays the role as a stress and deformation stabiliser in addition to favouring the thermal recharge of the soil.

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.

Study on heat-exchange efficiency and energy efficiency ratio of a deeply buried pipe energy pile group considering seepage and circulating-medium flow rate

Seepage and circulating-medium flow rate are both primary factors affecting the heat-exchange efficiency and energy efficiency ratio of underground energy structures. In this study, a thermal-seepage coupled model of a deeply buried pipe energy pile group is developed, and the effects of circulating-medium flow rate, seepage velocity, and thermal migration caused by seepage on heat-exchange efficiency and energy efficiency ratio are analyzed. The results show that increasing the circulating-medium flow rate can improve heat-exchange efficiency, but aggravate heat accumulation, resulting in energy efficiency ratio reduction. Seepage can eliminate the heat accumulation as well as improve the heat-exchange efficiency and energy efficiency ratio. The heat accumulated around upstream piles will migrate along the seepage direction, generating thermal disturbance to the downstream piles. An increase in the circulating-medium flow rate of upstream deeply buried pipe energy piles will aggravate thermal disturbance. This phenomenon is slightly retarded if seepage velocity is increased. Additionally, when the circulating-medium flow rate of the deeply buried pipe energy pile group under seepage is within the optimal range, slightly reducing the circulating-medium flow rate of the upstream deeply buried pipe energy piles can reduce thermal disturbance to the downstream piles without affecting the overall heat-exchange efficiency.

Numerical study on the influence of embedded PCM tubes on the energy storage properties of the geothermal energy pile

Geothermal energy pile is a remarkable alternative energy source that can provide heating and cooling energy to meet the energy demands in buildings. This study aims to quantify and expand the knowledge on the thermal storage performance of the geothermal pile system embedded with phase change material containers as compared to the one without, as a goal to expand the wide application of the system. In this study, numerical predictions were conducted on lab-scale energy pile systems, namely Design A (i.e., no PCM) and Design B (i.e., with PCM). The work aimed to investigate and quantify the energy storage properties of geothermal energy piles, and the influence of the incorporation of paraffin wax PCM tubes in the pile, at different flow rates. Incorporating PCM in a geothermal energy pile demonstrated considerable improvement in the amount of energy stored and extracted compared to the one with no PCM. For 735, 1470 and 2100 mL/min, Design B exhibited an increase in the energy stored by 42.21%, 44.71% and 46.23%, respectively compared to Design A. On the other hand, for the flow rate 735, 1470 and 2100 mL/min Design B illustrated an increase in the energy extracted by 45.87%, 53.09% and 59.55%, respectively compared to Design A. The use of PCM in the energy pile has a promising solution to enhance the storage capacity and improve the heat exchange capacity.

Prediction of thermal conduction of energy piles using various machine learning models

ABSTRACT The global energy crisis has led to a shift towards sustainable energy sources, such as geothermal energy, which can be efficiently used in energy piles, walls, and tunnels. This paper compares six machine learning algorithms for predicting the thermal performance of energy piles. A random 80:20 split of collected data points from the literature was used for training and testing. The input parameters were the number of tubes, inner and outer tube diameters, tube thermal conductivity, pile diameter, pile thermal conductivity, and soil thermal conductivity. The output was energy pile thermal conductance. Among all the ML models used for training and testing, the Gaussian Process Regression (GPR) model outperforms with the highest R2 value of 0.994 and RMSE of 1.886. After sensitivity analysis, the thermal conductivity of the pile was the key factor reducing energy pile thermal resistivity, followed by the number of U-tubes and pipe diameter.

Study on the mechanical and thermal properties of one-part geopolymer composite for high performance in energy piles

One-part geopolymer is a single-component, easy-to-use geopolymer material that offers simplified and cost-effective production processes for eco-friendly building materials. This study aimed to optimize the thermal and mechanical properties of one-part geopolymer for geothermal energy applications by incorporating both silicon carbide (SiC) sand and SiC powder. Various mix designs of one-part geopolymer composites were investigated, and their flexural and compressive strength were analysed. Thermal conductivity was measured using a novel home-made test set-up. Results showed that the compressive strength of one-part geopolymer composite can be as high as 83 MPa under ambient curing condition. The relationship between flexural and compressive strength was also studied, with the PCA code providing the best fitting precision. Furthermore, the thermal conductivity of one-part geopolymer composite was significantly increased with the combined application of SiC powder and SiC particles. These findings suggest that geopolymer composites with SiC materials are suitable for geothermal energy applications.

Full-scale in situ experimental study on the bearing capacity of energy piles under varying temperature and multiple mechanical load levels

Full-scale in situ experimental study on the bearing capacity of energy piles under varying temperature and multiple mechanical load levels

Investigation on thermo-mechanical response of energy piles in saturated clay

In recent years, energy piles have been widely used in the collection of shallow geothermal energy. However, the engineering characteristics of the soil and the mechanical behaviour of the pile are affected by temperature, particularly in clay. To explore the influence of heating–cooling cycles on the thermo-mechanical response of an energy pile in saturated clay, a laboratory model test was designed. The energy pile in saturated clay was subjected to ten heating–cooling cycles. A fibre Bragg grating (FBG) system was adopted to monitor the pile strain. Meanwhile, pile and soil temperature, pore water pressure and pile top displacement data were collected using a dynamic acquisition instrument. The results show that FBG sensors can achieve good results in measuring the axial strain of a pile. The distribution and transmission of thermally induced axial force and stress are affected by soil properties and pile constraints. Temperature cycling will lead to the thermal consolidation of saturated clay, improving the bearing performance of energy piles, and to an irreversible settlement of the energy pile.

Recent advances in geothermal energy reservoirs modeling: Challenges and potential of thermo-fluid integrated models for reservoir heat extraction and geothermal energy piles design

Recent advances in geothermal energy reservoirs modeling: Challenges and potential of thermo-fluid integrated models for reservoir heat extraction and geothermal energy piles design

Stress effects on thermal conductivity of soils and heat transfer efficiency of energy piles in the saturated and unsaturated soils

Soils surrounding energy piles are often subjected to high overburden pressure. So far, the stress effects on the thermal conductivity of soils and hence the thermal performance of energy piles have not been well investigated. This study proposed a new model for the stress-dependent thermal conductivity of saturated and unsaturated soils. It was validated using the data from a series of new tests and the literature and is able to capture stress-induced variations of thermal conductivity. Then, it was applied to finite element analysis of energy piles’ heat transfer efficiency. The numerical results reveal that ignoring stress effects on thermal conductivity, as in the conventional analysis, underestimates the heat exchange rate between energy piles and ground. The underestimation is more significant for piles with a larger aspect ratio and a faster pipe flow velocity, especially when the ground is drier and more compressible. For example, when the pile is 0.6 m in diameter and 50 in aspect ratio, the underestimation is up to 18%, 15% and 4% for the clay, silt and sand, respectively. These results suggest that stress effects on thermal conductivity should be considered for better assessing the heat transfer efficiency of energy piles and other thermal geostructures.

Synergetic performance improvement of a novel building integrated photovoltaic/thermal-energy pile system for co-utilization of solar and shallow-geothermal energy

Soil thermal imbalance due to excessive heat extraction and photovoltaic efficiency decline caused by temperature increase are two bottleneck problems that hinder the utilization of solar and shallow-geothermal energy. The existing research is still blank in terms of the coupling performance of photovoltaic/thermal collectors coupled with energy-pile ground source heat pumps, and cannot effectively solve the soil thermal imbalance of the energy piles. Therefore, a novel building-integrated photovoltaic/thermal-energy pile system is proposed to address soil thermal imbalance, improve electric efficiency, and reduce the building load simultaneously. In this coupled system, the waste heat is recovered from the photovoltaic/thermal collectors to charge the ground for thermal balance; the low-temperature fluid from energy piles is used to cool the photovoltaic/thermal collectors for electric efficiency enhancement, and the building-integrated photovoltaic/thermal reduces the heating load as well. The coupled system model is built and the comprehensive performance is optimized considering three important parameters (photovoltaic/thermal installation angle, roof absorptivity, and energy pile number). Results show that the electric efficiency of photovoltaic/thermal collectors increases to 16.04% with a growth of 22.72% compared with the photovoltaic collectors, the decline in soil temperature is only 1.5 °C, the heat pump unit maintains a ten-year coefficient of performance of 3.21 with an energy saving of 46.47 MWh, and the accumulative building heating load is reduced by 1.65%. The optimal design suggests an installation angle of 45°, a roof absorptivity higher than 0.36, and as many energy piles as the building foundation can hold. This work reveals the coupling and the synergetic performance improvement of the novel system, which could facilitate the efficient utilization of solar and shallow-geothermal energy toward carbon neutrality.

Thermo-mechanical analysis of pipe energy piles in layered cross-isotropic soils

This paper aims to propose a method for the thermo-mechanical analysis of pipe energy piles in layered cross-anisotropic soils. Firstly, the pipe pile is modelled as a bar and the stiffness matrix equation considering thermo-mechanical loads is established based on the finite element method (FEM). The soils are considered to be layered cross-anisotropic and the boundary element method (BEM) is employed to obtain the deformation-force relationship of the soil-pile nodes. Afterwards, the coupled BEM-FEM equation is developed according to soil-pile deformation coordination conditions. The proposed method is verified with responses of pipe piles, solid energy piles and pipe energy piles. Parametric analyses show that the performance of the pipe energy pile induced by heating needs considering in the design of the pipe pile thickness, and the induced tensile stress should be taken into account in the design in hard soils. Besides, the influence of cross-anisotropy on the performance of the pile under cooling is larger than that under heating, and a softer surface soil layer yields a smaller axial force of the pipe energy pile for layered soils.

A simple load transfer method for energy pile groups

Various analysis methods have been developed to simulate the performance of energy pile groups subjected to thermal and mechanical loads. Although they considered the interactions between piles in the group, the ‘sheltering-reinforcing’ effect was usually ignored during the analysis, or the methods were not experimentally validated. Therefore, to address this gap, we propose an experimentally validated load transfer model that considers the ‘sheltering-reinforcing’ effect for the thermomechanical analysis of energy pile groups. The proposed method modeled two well-documented full-scale field tests and two finite-element simulation cases. The comparison results showed that the proposed method could capture several essential aspects of the energy pile group, including thermally induced strain, stress, displacement, and group effects. Furthermore, a parametric analysis was performed to evaluate the effects of relevant parameters on the thermomechanical behavior of the energy pile group.