Geothermal Energy Piles Research Articles

Heat transfer performance of energy pile and borehole heat exchanger: A comparative study

Geothermal structures are widely utilized to satisfy the heating and cooling demands of buildings, providing a promising alternative to the escalating scarcity of energy. As two types of geothermal heat exchangers, energy piles and borehole heat exchangers exhibit different efficiencies in extracting geothermal energy. This paper examines the thermal performance of an energy pile and a borehole heat exchanger under heating and cooling conditions through field tests, introducing the heat exchange rate per unit pipe length (q0) as a metric. Furthermore, a three-dimensional heat transfer numerical model was developed and validated. The parametric analysis was subsequently conducted to investigate the sensitivity of heat exchange efficiency to five parameters, including flow rate, inlet temperature, pipe diameter, soil thermal conductivity, and soil temperature. The results showed that the heat transfer efficiency of the borehole heat exchanger was 42.2 % and 48.7 % lower than that of the energy pile under the heating and cooling conditions, respectively. Moreover, the heat transfer efficiency of the energy pile and borehole heat exchanger is most sensitive to inlet temperature, with the latter showing greater sensitivity to pipe diameter. This study provides valuable insights into optimizing the design and operation of energy piles and borehole heat exchangers to meet the growing energy demands of buildings.

Model for dimensioning borehole heat exchanger applied to mixed-integer-linear-problem (MILP) energy system optimization

This paper introduces three novel approaches to size geothermal energy piles in a MILP, offering fresh perspectives and potential solutions. The research overlooks MILP models that incorporate the sizing of a geothermal borefield. Therefore, this paper presents a new model utilizing a g-function model to regulate the power limits. Geothermal energy is an essential renewable source, particularly for heating and cooling. Complex energy systems, with their diverse sources of heating and cooling and intricate interactions, are crucial for a climate-neutral energy system. This work significantly contributes to the integration of geothermal energy as a vital energy source into the modelling of such complex systems. Borehole heat exchangers help generate heat in low-temperature energy systems. However, optimizing these exchangers using mixed-integer-linear programming (MILP), which only allows for linear equations, is complex. The current research only uses R-C, reservoir, or g-function models for pre-sized borefields. As a result, borehole heat exchangers are often represented by linear factors such as 50 W/m for extraction or injection limits. A breakthrough in the accuracy of borehole heat exchanger sizing has been achieved with the development of a new model, which has been rigorously compared to two simpler models. The geothermal system was configured for three energy systems with varying ground and bore field parameters. The results were then compared with existing geothermal system tools. The new model provides more accurate depth sizing with an error of less than 5 % compared to simpler models with an error higher than 50 %, although it requires more calculation time. The new model can lead to more accurate borefield sizing in MILP applications to optimize energy systems. This new model is especially beneficial for large-scale projects that are highly dependent on borefield size.

Recent Advancements in Geothermal Energy Piles Performance and Design

Geothermal energy piles or ground heat exchange (GHE) systems embrace a sustainable source of energy that utilizes the geothermal energy naturally found inside the ground in order to heat and/or cool buildings. GHE is a highly innovative system that consists of energy loops within foundation elements (shallow foundations or piles) through which a heat carrier fluid circulates, enabling heat extraction or storage in the ground. Despite the innovation and potential of GHE systems, there are significant challenges in harmonizing their thermal and mechanical designs due to the complex interactions involved. This review critically examines state-of-the-art design methodologies developed to address these complexities, providing insights into the most recent advancements in GHE performance and design. Key findings include innovative techniques such as advanced numerical modeling to predict thermomechanical behavior, the use of different pipe configurations to optimize heat transfer, and strategies to minimize thermal stress on the foundation. Additionally, this review identifies research gaps, including the need for more comprehensive full-scale experimental validations, the impact of soil properties on system performance, and the long-term effects of thermal cycling on pile integrity. These insights aim to contribute to a better understanding of the thermomechanical behavior of energy piles, ultimately facilitating more accurate and effective design solutions.

Numerical Investigation on the Thermo-Mechanical Response of Geothermal Energy Pile in East Indian Climatic Condition

Numerical Investigation on the Thermo-Mechanical Response of Geothermal Energy Pile in East Indian Climatic Condition

Pore unit cell network modeling of the thermal conductivity dynamics in unsaturated sandy soils: Unveiling the role of spanning‐wetting phase cluster

AbstractAs the world struggles with climate change and energy crises, understanding the role of soil in the food–water–energy nexus becomes increasingly critical. Accurately estimating the soil thermal conductivity drying curve is essential for assessing the impacts of temperature on soil biota and crop growth, environmental changes due to forest fires and global warming, and for designing geo‐energy extraction techniques such as geothermal energy piles. Existing empirical models often fail to accurately estimate the soil thermal conductivity (TC), particularly in pendular soil moisture regimes where they do not capture sharp changes in TC. This study introduces a novel approach using a pore unit cell network model to more accurately describe the dynamics of TC in variably saturated soils. A quadratic parallel scheme within each soil pore unit cell links the TCs of solid, water, and air to the overall effective conductivity. By modeling air invasion in the pore network model and employing the proposed equation, we determined the unsaturated soil TC based on varying local conductivities. The model effectively captures the significant decrease in conductivity in the pendular saturation regime, associated with the shrinkage of the spanning‐wetting cluster. Quantitative analyses showed a substantial improvement in prediction accuracy compared to existing models, especially under varying moisture conditions. Our findings have significant implications for better characterizing soil thermal and hydraulic properties, which are crucial for resource management in a changing climate and advancing geo‐energy technologies.

A short recent review on geothermal energy piles

This manuscripts presents a short recent review of geothermal energy piles, emphasizing their problems, design elements, heat transfer fluids, and classification. Phase change materials (PCMs) are used as heat transfer fluids, and their beneficial effects on energy pile performance are highlighted. Design factors for the best energy pile performance are examined, including the usage of nanofluids and geometrical optimization. The analysis presented provides brief insightful information about the state of geothermal energy piles heaps now, laying the groundwork for future studies and advancements in this area.

Thermal Conductivity in Concrete Samples with Natural and Synthetic Fibers.

One crucial property of concrete, particularly in construction, is its thermal conductivity, which impacts heat transfer through conduction. For example, reducing the thermal conductivity of concrete can lead to energy savings in buildings. Various techniques exist for measuring the thermal conductivity of materials, but there is limited discussion in the literature about suitable methods for concrete. In this study, the transient line source method is employed to evaluate the thermal conductivity of concrete samples with natural and synthetic fibers after 7 and 28 days of curing. The results indicate that concrete with hemp fiber generally exhibits higher thermal conductivity values, increasing by 48% after 28 days of curing, while synthetic fibers have a minimal effect. In conclusion, this research opens the door to using natural alternatives like hemp fiber to improve concrete's thermal properties, providing alternatives for thermo-active foundations and geothermal energy piles which require high thermal conductivities.

The impact of soil layering and groundwater flow on energy pile thermal performance

Shallow geothermal energy pile systems have emerged as cost-effective and low-carbon alternatives for heating and cooling buildings, compared to traditional air-conditioning systems. Geothermal applications have been researched extensively in recent years under the assumption of ground homogeneity, and the effect of ground stratification remains mostly unexplored. To investigate this, a 3D finite element numerical model is developed and validated against laboratory-scale experimental data, to study the transient diffusion-convection heat transfer linked with Darcy groundwater flow around energy piles in multi-layered lithology. The model is used to undertake long-term assessments under balanced and unbalanced thermal loads to evaluate the thermal effects of soil layering and discrepancies against commonly assumed equivalent homogeneous stratum, for soil profiles with different thermal conductivity distributions. The groundwater flow effect at various depths and seepage velocities on the thermal performance of the energy pile is investigated as well. Results demonstrate the need to account for the spatial variability in thermal properties, particularly for unbalanced thermal loading scenarios. The thermal yield can be underestimated by up to 19.6 % due to an inaccuracy of 48.2 % in the accumulated temperature after 25-year of operation with respect to an equivalent homogeneous ground with depth-weighted average thermal conductivity. This discrepancy grows as the contrast between layers increases. An empirical derived formula is presented and tested, presenting a correction to the effective thermal conductivity of the layered systems in this study that considers the thermal contribution of the ground beneath the pile. Groundwater seepage is shown to have a positive impact on the heat exchanger efficiency, and in the layered geology the efficiency under-estimation becomes more critical at low to moderate Darcy velocities, if neglected or inaccurately measured. These findings contribute to a broader understanding of energy piles and can assist engineers and practitioners in optimising energy geo-structure design, boosting the technology’s viability.

A steel joint for driven precast concrete geothermal energy pile foundations

This invention describes a steel joint used for connecting two precast concrete driven energy pile (DEP) segments which is known as DEP joint. Precast concrete DEPs are cast in segments with a maximum length of 12 m at a concrete factory. DEPs are typically driven into the ground until bedrock; hence, in places where the bedrock is deeper than 12 m, two or more segments must be connected using a joint to produce longer energy piles. DEPs have not been frequently used because no suitable joint existed that could maintain structural integrity and provide leak-proof coupling between the pipes at the joint interface. The present invention addresses this problem by designing a steel joint that can meet the structural and hydraulic requirements of a suitable steel joint for DEPs. Each quadratic concrete energy pile segment is prefabricated in a concrete factory, where the heat transfer pipes are embedded inside the steel cage of each segment. The steel DEP joint is installed at one or both ends of each concrete segment, and has two or four sidewall channels, depending upon its size. Heat transfer pipes are coupled between every two segments, inside the sidewall channels, while the energy piles are installed at a construction site. The sidewall channels are protected using steel shielding plates that are riveted to the joint so that the pipes and the coupling inside the sidewall channels are protected against harsh frictions during the installation of the DEPs in the ground. The steel joint facilitates the installation of longer precast concrete energy piles up to the bedrock depth, especially in sites where the bedrock is deeper than a single segment length. The main advantages of precast concrete energy piles compared to cast-in-place piles are that they enable better quality control and quality assurance, as well as being easier, faster, and cheaper to install. The invented DEP joint has passed structural integrity tests as required according to the BS EN 12794 standard, and also passed hydraulic pressure tests according to the ASTM F2164 – 21; hence, it is certified to be used in construction projects. We are now looking for potential licensees to start manufacturing the joints and using them in the energy pile industry.

A novel joint for driven concrete geothermal energy pile foundations

Geothermal energy pile foundations are among the most common types of energy geostructures which provide both structural support and can be used as a source with ground source heat pumps for heating and cooling buildings. Energy piles can be categorized into cast-in-place and precast piles. Driven precast concrete energy piles can be installed up to the bedrock level, with higher quality, lower cost, and faster installation process, compared to cast-in-place energy piles. Precast concrete driven energy pile foundations have not been commonly utilised due to existing problems with a suitable type of joint that could connect precast segments and allow continuity of heat exchanging pipes. An innovative and novel steel driven energy pile (DEP) joint is presented in this paper which can provide structural integrity between the segments and leak-proof coupling between the heat exchanging pipes. Both sizes of DEP joints passed 1000 impact blows of 28 MPa, they remained undamaged during the bending tests with a flexural stiffness of 3500 kN.m2, and 7720 kN.m2 for the 267 mm and 350 mm joints, respectively. Additionally, the pipes used in the prototype joints and piles indicated no leakage or pressure drop in the hydraulic pressure tests subjected to 690 kPa pressure.

A simplified coupled method for geotechnical analysis of geothermal energy pile with single U-shaped tube heat exchanger

Geothermal energy pile (GEP) is an environmentally friendly energy source that utilizes heat energy present in the shallow earth surface to decrease building energy consumption. However, thermal stresses of pile shafts due to the temperature changes during the operation of GEP may cause distress to the supported structures e.g. buildings, and there is currently a lack of established calculation methods for the geotechnical design of GEP. This paper presents a simplified coupled method for geotechnical analysis of GEP with a single U-shaped tube heat exchanger. Primarily, leveraging the finite line heat source model and the superposition principle, we propose a segmentation superposition heat transfer model tailored for GEP featuring a singular U-shaped tube heat exchanger. This model adeptly computes the temperature variations along the pile shaft. Secondly, a hyperbolic curve is chosen as the load-transfer function, and the unloading and reloading characteristics of the relationship between the shaft friction/shaft displacement during temperature cycling are simulated by Massing’s criterion. Building upon the aforementioned, taking into account the uneven temperature rise of the pile and the influence of soil shear deformation, a coupled calculation method for the working characteristics of energy piles under thermal–mechanical conditions has been established. The method is validated on the basis of in-situ measurements of the loads and deformations experienced by heat exchanger test piles and a model test on the thermal–mechanical behavior of GEP with a single U-shaped tube heat exchanger.

Thermo-mechanical behavior of energy micropiles

Since the first application of piles as energy geostructure in Austria in the 1980s [1], the use of geostructures has steadily increased worldwide. Energy geostructures are complex systems that combine structural, geotechnical, and thermal performance. They are based on the exploitation of the soil thermal energy at shallow depths to provide heating and cooling to buildings [1], allowing to reduce both the consumption of non-renewable energy and the harmful carbon emissions. Pile foundations are currently among the most used geostructures. They are particularly suitable for exploiting the ground thermal energy, as they reach depths where soil temperature is quite constant and independent from daily or seasonal variations [2–4] .
 Micropiles are small, drilled, and grouted-in-place piles having a diameter between 90-300 mm and a length up to 20 m [5–7]. Generally, they are constructed by drilling a borehole, placing reinforcement, and grouting the hole [6]. Micropiles can be loaded directly to transfer structural loads, both axial (compression and tension) and lateral, to a deeper stable stratum like in new foundations or they can reinforce the soil to theoretically make a reinforced soil composite (reticulated piles) that resists applied loads like in underpinning of existing foundations [6]. Due to the relatively small cross-sectional area of micropiles, load carrying capacity resulting from end bearing is generally considered to be negligible in soil or weak rock, thus micropiles mainly rely on shaft resistance for load bearing [6,7]. Innovative and vigorous drilling and grouting permit high grout-bond values to be generated along the micropile’s periphery. High-capacity steel elements, occupying upto 50% of the hole, can be used as the principal load bearing element with the surrounding grout serving only to transfer by friction the applied load between the soil and the steel [6]. The axial capacities of micropiles are in the range of 50–500 kN and up to 1000 kN when installed using pressure-grouting techniques [7]. In the Titan system, the hollow bar simultaneously acts as the drilling rod, injection tube and reinforcement for the micropile [8]. This system is fast, simple and flexible, can be used in restricted sites when access is difficult, can be installed with low vibration and noise in any type of soils including unstable soils. In the context of building rehabilitation, retrofitting, and underpinning of existing structures, the energy micropiles stand frequently as the best solution over energy piles [6]. The Titan 73/53 Energy micropile is a dual used micropile: as a geothermal energy pile with an equal extraction capacity as double U-pipe and as a foundation pile with characteristic load capacity of RM,K = 900KN [9]. Although energy micropile (EMP) systems are very similar to energy pile (EP) systems, their behavior cannot be assumed to be similar both from the mechanical point of view (axial load supported mainly by lateral resistance with almost null end bearing capacity) and the thermal point of view (potentially lower energetic efficiency, due to shorter primary circuit and smaller contact surface with the soil) [10]. For example, pile diameter and pipe diameter have been found to play respectively an important role and an insignificant role in energy piles [11] while the contrary was observed for energy micropiles [12]. Nevertheless, the thermal performance of micropiles in terms of specific heat flux (about 50W/m in fine grained soils) has been found encouraging to predict their use in heating, ventilation and air-conditioning systems[10]. Thermal response tests conducted on TITAN 73/53 energy micropiles show that their specific extraction capacity can be 111W/m in wet sediments, and between 60-80W/m depending on the soil’s properties[9]. To date, several studies are dealing with the thermo-mechanical behaviour of energy piles [13] and also the thermal performance of energy micropiles has been looked at to an extent by various
 researchers [10,12,14]. However, the coupled thermo-mechanical behaviour of energy micropiles is not well understood. In fact, the short and long terms effects of combined thermal and mechanical cyclic loading, the “group effects” of energy micropiles, among others, still need to be investigated. This study investigates the thermo-mechanical behavior of a TITAN 73/53 energy micropile, with the aim of identifying suitable evaluation criteria and design parameter for optimizing the structural, geotechnical, and thermal performance of energy micropiles. To achieve this objective, thermal response tests on instrumented TITAN 73/53 energy micropiles, and 3D Finite element analysis will be carried out. The results of the in-situ tests will be used to calibrate the numerical model of our simulation. Then the short and long terms performances of the system in various thermal, geotechnical, and structural conditions will be evaluated.

Incorporating phase change materials in geothermal energy piles for thermal energy storage

Introduction
 Geothermal energy piles (GEPs) are foundation elements that are installed in the ground to support the weight of the building to a competent strata. Energy loops are installed into the piles to allow heat rejection or extraction via the circulation of fluid through the loops. In winter, low-grade heat is extracted from the ground (source) and transferred to the building (sink) to achieve space heating. Conversely, in summer, heat is removed from the building and rejected into the ground to achieve space cooling. The system relies on the temperature gradient between the ground and the ambient air temperature; where in winter, the ground temperature is higher than the air temperature, while in summer, the opposite is true. Thus, the system can sustainably and continuously supply the heating and/or cooling demand of a building once the temperature gradient between the ambient air and the ground is maintained. To improve the sustainability and performance of the system, the heat energy rejected into the ground can be stored and used at a later time when space heating is required [1]. Energy storage substances such as phase change materials (PCMs) can be incorporated into energy piles to store the heat that is rejected into the ground to improve the performance of the GEP system [2]. PCMs are materials that stores or releases heat energy during phase transition. Many researchers including [3,4] have reported using PCMs in buildings to improve thermal comfort and minimise the energy demand during heating and cooling periods, however, not much work has been done with regard to incorporating them into pile foundations to improve the performance of GEP systems.
 Thus, this study carried out a laboratory experiment to investigate the effect of the addition salt hydrate PCM on the energy performance of geothermal energy piles.
 Laboratory test and procedure
 In this study, lab-scaled models of energy piles were constructed, with and without PCM. The piles are characterised by a length and a diameter of 300 mm and 100 mm, respectively. In order to add PCM into the piles, the PCM was encased and sealed in a PVC tube with an inner and an outer diameter of 6mm and 8mm, respectively. The same size of PVC tube was used for the energy loops. The energy loop (U-shape), and the encased PCM incorporated tubes are attached together and placed in the mould prior to concreting, for the pile model with PCM (see Figure 1a). Whereas only the energy loop was installed in the control sample.
 In the test setup, a layer of sand (220mm thick) was placed and compacted in an insulated wooden box (520mm × 520mm × 520mm). Afterwards, the pile was then installed at the center of the box and then filled with sand, in layers and compacted. An insulation sheet was used to cover the top of the box to prevent heat loss and the ambient air temperature influencing the tests’ results. Water at a temperature of 32°C, was circulated through the pile continuously for a duration of 4 days (96 hours). Instrumentations were installed in the pile and soil to monitor and record the changes in temperature during the tests (see Figure 1b).
 Results and discussion
 Figure 1c shows the results comparing the average energy rejected, and stored, in the ground for the piles with and without PCM. The maximum heat energy stored at the beginning of the test was found to be about 460W and 360W for the control and PCM piles. However, at the end of the tests, the energy stored was 230W and 430W for the control and PCM pile, with an average energy stored as shown in Table 1. The addition of salt hydrate PCM absorbs the heat rejected into the pile, consequently improving its thermal performance by about 22% in comparison to the control pile. Similarly, the magnitude of temperature developed in the pile and surrounding soil were observed to be lower in the pile with PCM. This can ultimately reduce the induced stresses and strains induced in a pile due to heat energy extraction or rejection.
 
 Conclusion
 This paper investigated the feasibility of using salt hydrate PCM in energy piles for heat storage. It was found that the energy rejected in a pile with PCM increases by up to 22% compared to a non-PCM pile.

Casting and installation of segmental precast quadratic concrete driven geothermal energy piles

Geothermal energy pile foundations are used both for structural purposes and to provide sustainable, clean, and cost-effective ground energy for heating and cooling buildings [1]. The majority of energy piles are cast-in-place concrete piles, which require extensive and expensive drilling during construction. A better alternative is to use precast concrete segmental driven energy piles, which can be cast in large quantities with high-quality assurance at a concrete factory, after which they are transported and installed into the ground [2]. This study aims to present a novel method of casting and installing segmental energy piles that utilize an innovative steel joint [3] for connecting the precast concrete segments on-site. In addition to structural integrity, the steel joint provides the possibility of a leak-proof coupling and continuity for heat-exchanging pipes embedded in concrete segments. The precast-driven pile foundations are made of high-strength concrete with a 14-day compressive strength of almost 80 MPa and have a quadratic shape with a maximum length of 12 m and a square width of 270 mm or 350 mm. In the 270-mm piles, a single U-loop can be used, while in the 350-mm piles, two U-loops can be used as presented in Figure 1. The heat-exchanging pipes are coupled inside the sidewall channels of the steel joint which are shielded using a steel cover plate, riveted to the joint. At the tip of the bottom segment of the piles, there is a strong steel rock shoe with a hardened steel point that allows the piles to penetrate the bedrock [4]. At a construction site, the joint and pipes can be connected quickly without any welding or electric equipment that expedites the installation work and obsoletes the health and safety risk associated with electricity. Utilizing the precast energy piles presented in this study increases the quality, and speed of installation while decreasing the overall cost of the project compared to cast-in-place energy piles. To prove the proper performance of the piles under real construction conditions, structural integrity tests and hydraulic pressure tests were performed according to BS EN 12794:2005 [5] and ASTM F2164–21 [6], and the results are briefly presented and discussed in the present study. Structural integrity tests consisted of impact tests in which 1000 blows imposing a minimum stress level of 28 MPa were applied on the pile segments connected using the new driven energy pile joint. During the impact tests, the pile structure and the joints remained undamaged. After the impact tests, the pipes were pressurized up to 100 psi (690 kPa) and no leakage or pressure drop was observed. Then the pile segments were cut according to the standard into shorter segments and subsequent bending tests were performed to measure the bending capacity. The bending tests show that the driven energy pile joints have a similar bending capacity as conventional normal joints. The results of this study show that segmental precast concrete energy piles using the new steel driven energy pile joints, are a perfect alternative for cast-in-place energy piles, which can provide sufficient structural and bearing capacity and can be used both for heating and cooling buildings.

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.

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

Establishing zone of influence (ZOI) for cylindrical heat source in dry sand

ABSTRACT Safe, proper design and installation of several thermo-active structures necessitate establishing zone of extent of heat migration in soil mass. This zone is termed as ‘zone of influence’, ZOI, and its determination is imperative to avoid unwanted situations, which could be threat to the surrounding structures. This is where in-situ monitoring of the temperatures in the near- and far-field would be quite prudent, however would be an expensive exercise. Therefore, COMSOL Multiphysics® was employed to determine the ZOI. Nevertheless, the relative efficiency of these tools in yielding the ZOI remains a big question, and requires results from experiments to validate. Hence, an attempt was made in this study to establish the ZOI of cylindrical heat source, which replicates geothermal energy pile in dry sands, by adopting novel experimental methodology. ZOI has been established for cylindrical heat source experimentally and numerically. Furthermore, unique mathematical equation has been developed by using regression analysis.

In-field performance of continuous flight auger (CFA) energy piles with different configurations

In order to face the national energy crisis in Brazil, the use of geothermal energy piles (GEP) integrated with heat pumps is an attractive and cost-effective solution for the building cooling demand. On the other hand, there are very few field studies on the thermal performance of GEP systems in Brazilian soils and climate conditions, and for this reason, the implementation of this technology is still a challenge. Motivated by this need, continuous flight auger (CFA) energy piles were constructed to support a building in São Paulo city, and an experimental study, described in this paper, was carried out to provide useful data for the design of these GEPs. This work examined two key aspects of the design of GEP systems: (1) the influence of the heat exchange pipe configuration, and (2) the thermal response of GEPs and their effect on nearby piles installed in multi-layered ground. For this study, thermal response tests (TRTs) were conducted on four GEPs constructed with different pipe configurations, installed in a saturated sandy deposit intercalated by soft organic clay layers. The results obtained showed that: (i) the influence of pipe configuration on the GEP heat transfer is more significant during the first hours of pile heating process, and decreases with time due to the thermal interference between the pipe loops; (ii) the GEP temperature profile is impacted by the presence of soil layers of highly different thermal conductivities and groundwater flow velocities; and (iii) the temperature variation of nearby piles was also affected by the thermal characteristics of the soil layers, and therefore it should be considered for the structural design of GEPs.

Design of an Energy Pile Based on CPT Data Using Soft Computing Techniques

The present study focused on the design of geothermal energy piles based on cone penetration test (CPT) data, which was obtained from the Perniö test site in Finland. The geothermal piles are heat-capacity systems that provide both a supply of energy and structural support to civil engineering structures. In geotechnical engineering, it is necessary to provide an efficient, reliable, and precise method for calculating the group capacity of the energy piles. In this research, the first aim is to determine the most significant variables required to calculate the energy pile capacity, i.e., the pile length (L), pile diameter (D), average cone resistance (qc0), minimum cone resistance (qc1), average of minimum cone resistance (qc2), cone resistance (qc), Young’s modulus (E), coefficient of thermal expansion (αc), and temperature change (ΔT). The values of qc0, qc1, qc2, qc, and E are then employed as model inputs in soft computing algorithms, which includes random forest (RF), the support vector machine (SVM), the gradient boosting machine (GBM), and extreme gradient boosting (XGB) in order to predict the pile group capacity. The developed soft computing models were then evaluated by using several statistical criteria, and the lowest system error with the best performance was attained by the GBM technique. The performance parameters, such as the coefficient of determination (R2), root mean square error (RMSE), mean absolute error (MAE), mean biased error (MBE), median absolute deviation (MAD), weighted mean absolute percentage error (WMAPE), expanded uncertainty (U95), global performance indicator (GPI), Theil’s inequality index (TIC), and the index of agreement (IA) values of the testing data for the GBM models are 0.80, 0.10, 0.08, −0.01, 0.06, 0.21, 0.28, −0.00, 0.11, and 0.94, respectively, demonstrating the strength and capacity of this soft computing algorithm in evaluating the pile’s group capacity for the energy pile. Rank analysis, error matrix, Taylor’s diagram, and the reliability index have all been developed to compare the proposed model’s accuracy. The results of this research also show that the GBM model developed is better at estimating the group capacity of energy piles than the other soft computing models.

Thermo-Mechanical Behavior of Long-Bored Energy Pile: A Full-Scale Field Investigation

A geothermal energy pile is a revolutionary piling technique that combines a pile foundation with a ground source heat pump system that not only supports the structure but also provides heating and cooling for buildings and bridges. The thermo-mechanical behavior of long energy piles in soft clay has rarely been investigated, despite their increasing utilization. A long floating energy pile with a length-to-diameter ratio of 66.7 was evaluated on its own and monitored in service of the supported structure in the city of Kunshan, China. With vertical mechanical loads, the experiment involved alternate cooling and heating cycles, allowing for careful analysis and assessment of the pile's temperature, stress, and displacement. Temperature-induced stress, axial force, and friction resistance of the pile shaft, as well as the change in displacement of the energy pile throughout building, were all studied. The field observations revealed without any surprise that a longer energy pile outperformed a shorter one in terms of heating exchange capacity with a more homogenous temperature distribution along the pile. Following a quasi-linear relationship with the temperature variation, the thermo-induced additional axial force soared with the larger length-diameter ratio of the pile and may even reach four times that of the pile under pure mechanical loads. Important additional settlements were also observed especially in cooling conditions. The shaft frictions along the long bored energy pile were found to have a complicated distribution, which requires further investigations.