Presence Of Groundwater Flow Research Articles

Understanding the impact of flow mechanisms in unsaturated layer on heat transfer near a partially submerged geothermal heat exchanger

Heat transfer near geothermal heat exchangers in the presence of groundwater flow has been extensively studied over the past years. However, previous studies often overlooked the presence of an unsaturated layer in the shallow subsurface soil, or disregarded phase change and fluid flow within this layer. In this study, a comprehensive hydro-thermal model is developed to evaluate heat transfer near a partially submerged geothermal borehole in groundwater flow while incorporating phase change and liquid and vapor flow in the unsaturated layer. The effect of unsaturated soil hydraulic analysis on heat transfer and energy efficiency of the geothermal piles is explored under varying soil permeabilities, air-entry suctions, solid thermal conductivities, and soil types. Consideration of water content variation and unsaturated fluid flow in the model enhances heat injection rate by up to 11.23 % in highly permeable soils. However, in scenarios with higher solid thermal conductivity, or lower air-entry suction values, the heat injection rate reduces by as much as 16.13 % due to unsaturated soil hydraulic analysis. Furthermore, it is found that consideration of unsaturated soil hydraulic analysis is of greater importance in Bonny silt than sandy soil, lowering the heat injection rate by 8.53 % and 2.53 %, respectively.

A new repository of electrical resistivity tomography and ground-penetrating radar data from summer 2022 near Ny-Ålesund, Svalbard

Abstract. We present the geophysical data set acquired in summer 2022 close to Ny-Ålesund (western Svalbard, Brøggerhalvøya Peninsula, Norway) as part of the project ICEtoFLUX. The aim of the investigation is to characterize the role of groundwater flow through the active layer as well as through and/or below the permafrost. The data set is composed of electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) surveys, which are well-known geophysical techniques for the characterization of glacial and hydrological processes and features. Overall, 18 ERT profiles and 10 GPR lines were acquired, for a total surveyed length of 9.3 km. The data have been organized in a consistent repository that includes both raw and processed (filtered) data. Some representative examples of 2D models of the subsurface are provided, that is, 2D sections of electrical resistivity (from ERT) and 2D radargrams (from GPR). The resistivity models revealed deep resistive structures, probably related to the heterogeneous permafrost, which are often interrupted by electrically conductive regions that may relate to aquifers and/or faults. The interpretation of these data can support the identification of the active layer, the occurrence of spatial variation in soil conditions at depth, and the presence of groundwater flow through the permafrost. To a large extent, the data set can provide new insight into the hydrological dynamics and polar and climate change studies of the Ny-Ålesund area. The data set is of major relevance because there are few geophysical data published about the Ny-Ålesund area. Moreover, these geophysical data can foster multidisciplinary scientific collaborations in the fields of hydrology, glaciology, climate, geology, and geomorphology, etc. The geophysical data are provided in a free repository and can be accessed at https://doi.org/10.5281/zenodo.10260056 (Pace et al., 2023).

Modelling artificial ground freezing subjected to high velocity seepage

Artificial ground freezing (AGF) is a ground improvement technique for ensuring the safety of underground construction works in water-bearing soils. High velocity water transport in coarse-grain soils can prevent the ice wall formation in brine-based ground freezing methods. This can be overcome by using expendable refrigerants such as solid carbon dioxide and liquid nitrogen, which provide much lower temperatures. Presented here is a model for analysis and design of ground freezing by expendable refrigerants under high velocity seepage conditions. Non-local mathematical formulations of heat transfer and water flow in soils are developed within the framework of the Peridynamics theory. Their computational implementation uses an adaptive multi-grid peridynamic approach to analyse large domains efficiently. The simulations are tested successfully against several benchmarks. The developed model enables reliable analyses of the effects of main geological and technological parameters on the ice-wall delivery in the presence of groundwater flow.

Natural convection effect on heat transfer in saturated soils under the influence of confined and unconfined subsurface flow

Recently, it has been proven that natural convection significantly impacts heat transfer close to geothermal systems in saturated soils even in the presence of groundwater flow. However, this phenomenon has only been investigated in the presence of confined groundwater flow. Therefore, in this study, a fully coupled hydro-thermal model was developed to quantitatively determine the natural convection influence on heat transfer in the presence of an unconfined groundwater flow and to compare the hydro-thermal response of soil under these two different scenarios. Results showed that natural convection influence depends on the coupled effects of soil permeability, groundwater velocity, and heater temperature. According to the results, in the case of a groundwater flow velocity less than 10−7 m/s, natural convection role is non-negligible when Rayleigh number is greater than 50, while for other groundwater velocities, that matters only when Buoyancy ratio is greater than 20. It was also shown that choosing a confined groundwater flow over an unconfined one led to an error of 47 % to 173 % in the explored scenarios. Furthermore, it was revealed that when the relative distance of the heater from the surface alters from 0 to 15 and 30 m, the error in the soil temperature due to neglecting natural convection can be as high as 43 %, 48 %, and 60 %, respectively.

Energy tunnels: A review of the state of the art and knowledge gaps to harness renewable energy from underground infrastructure

The thermal activation of underground tunnels, also known as energy tunnels, has shown significant potential to harness geothermal and aerothermal energy as a low-carbon and economical solution for space heating and cooling, water heating and road de-icing. The heat and load transfer mechanisms in energy tunnels are, however, far less understood compared to other energy geostructures, due to their complex interaction with the ground and tunnel environment. This paper aims to provide a comprehensive overview of the current state of knowledge on the thermal and thermo-mechanical performance of energy tunnels based on recent analytical models, field tests, model-scale experiments, and numerical studies. Results suggest that a thermal yield ranging from 5 to 6 W/m2 is attainable for each degree Celsius difference between the fluid inlet temperature and the ground temperature in the presence of groundwater flow and tunnel airflow. In the reviewed studies, the thermally induced tunnel structural responses appear to be insignificant compared to those caused by tunnel excavation. However, the long-term thermally induced ground deformation in clay deposits can reach up to 15 mm, which may become a crucial issue and therefore needs to be examined in geotechnical design. As a new area of research, there is a lack of long-term experimental investigations to solidify the current knowledge of the thermal, structural and geotechnical performance of the system. Further research is also required to develop an integrated analysis and design method considering the techno-economic aspects and explore innovative business partnership models. This work can serve as a useful resource for researchers, engineers, and policymakers seeking to further develop and promote the adoption of this technology in infrastructure projects.

Numerical investigation on the thermal performance of energy tunnels under groundwater flow and tunnel airflow

The integration of underground tunnels with ground heat exchangers (GHEs), also known as energy tunnels, is a promising technology that has gained attention in energy geotechnics research. This study investigates the coupled effects of groundwater and tunnel-air flows on the energy tunnel system via 3D thermo-hydraulic numerical modelling. It is found that the combination of groundwater flow parallel to the tunnel and limited airflow velocity results in reduced operational efficiency of ground source heat pump (GSHP) due to the strong thermal interference along the tunnel. The study also highlights the importance of real-scale modelling to evaluate the thermal yield of long energy tunnels, particularly when dealing with parallel groundwater flow in geothermal tunnel design.
 The use of shallow geothermal energy is a renewable solution towards net-zero carbon emissions. In recent years, a variety of conventional geotechnical structures, such as piles, retaining walls and tunnels, have been transformed into energy geostructures to also serve as GHEs to harness renewable energy for space heating and cooling purposes [7]. The thermal performance of energy tunnels is significantly affected by the presence of groundwater flow [1, 6] and the airflow in the tunnel [2, 4]. The groundwater flow and tunnel airflow can both be present in the field, however, their coupled effects on the energy tunnel system are hardly explored in the literature. This study aims to bridge this gap via real-scale numerical investigations.
 This study uses 3D thermo-hydraulic numerical modelling developed with the finite element package COMSOL Multiphysics [3]. The numerical model was successfully validated against an energy tunnel model-scale laboratory experiment [5]. Figure 1 shows the overall geometry and boundary conditions of the numerical model. A 48 m long tunnel section is thermally activated with the embedment of HDPE pipes (outer diameter of 25 mm and 2.3 mm thickness) placed in the lining segments at a spacing of 200 mm. A single tunnel ring consists of six segments with absorber pipes joined between them. Every four adjacent rings are connected in series to form a complete loop (with a single inlet and outlet), which is then connected in parallel to a flow and return header pipe(s). The entire thermally activated section comprises six loops, with identical fluid inlet temperature and flow rates, as they are fed by the same flow header pipe. The outlet fluid from each loop is mixed along the return header pipe to the GSHP. A constant 36 kW cooling thermal load is applied, and the simulation is run for ten years to allow the system to reach a steady state condition considering the minimum groundwater flow and airflow. The thermal properties of the ground are prescribed as 2 W/(m·K) and 1100 J/(kg∙K) for thermal conductivity and specific heat capacity, respectively.
 Figure 2 shows the entering water temperature (EWT, header outlet) and the coefficient of performance (COP) of the GSHP after 10 years of operation for different groundwater flow velocities and directions for tunnel air velocities = 0.05 m/s and 2 m/s.
 Results show that an increase in and leads to a decrease in EWT and an increase in COP. However, the effect of groundwater direction and velocity is more significant when airflow is limited ( = 0.05 m/s). The parallel groundwater flow leads to a higher EWT and lower COP than when the flow is inclined and perpendicular to the longitudinal tunnel axis. Consequently, the thermal performance of the system under parallel flow with a higher can be even worse than the perpendicular and inclined flow with a lower . When = 0.05 m/s, the EWT of parallel flow increases by 15% to 20% compared to the perpendicular flow, resulting in a maximum decrease of 15% in the COP. The effect of groundwater flow is much less significant when airflow is fast ( = 2 m/s): for increases from 0.05 to 2 m/d, EWT and COP change by no more than 13% and 5%, respectively, and the impact of groundwater flow direction is minor.

Improving data accessibility and application for underground climate re-search at the British Geological Survey

Improving data accessibility and application for underground climate re-search at the British Geological Survey

Field hydrothermal identification with groundwater flow by conducting Distributed Thermal Response Test (DTRT) through Moving Line Source (MLS) theory

Thermal Response Test (TRT) is conventionally used to get the ground thermal properties by using Infinite Line Source (ILS) method. However, ILS method gets abnormally ground thermal conductivity when there is an apparent groundwater flow. Moreover, traditional TRT is not able to identify the ground thermal properties along the vertical profile, misleading the hydrogeological engineering applications. In this investigation, ILS and Moving Line Source (MLS) models considering the variation of time were adopted to estimate the ground hydrothermal properties along a heat exchanger with different heating loads by Distributed Thermal Response Test (DTRT). The results showed that carrying fluid temperature gradient varies in different layers, attributed to the variation of ground hydrothermal properties. Thermal conductivity is overestimated by ILS for layers with groundwater flow, while MLS estimates more realistic values. Specifically, thermal conductivity, ground thermal resistance obtained by ILS method can be respectively 2240 %, 66.7 % higher than the values obtained by MLS method, for the layer with apparent groundwater flow. The groundwater velocity was generally underestimated by MLS, and a parameter was adopted to get the real Darcy velocity. In addition, the heat load influences the TRT results, especially when there are boundary effects or the presence of groundwater flow. A higher heat load leads to more reliable ground hydrothermal properties, and it is recommended that the heat injection rate remains between 30 and 70 W.m−1. Test time is also important in TRT, and a test duration longer than 30 h is recommended to reach stable ground hydrothermal properties illustrated by ILS and MLS methods. The investigation is helpful to give guidance for the design of Ground Source Heat Pump (GSHP) systems, especially for the stratified ground with apparent groundwater flow.

Numerical modeling of mixed convection near a vertical heat source in saturated granular soils

Accurate design of energy geostructures such as energy piles requires an exact prediction of soil thermal responses close to the embedded heat sources in the ground. In saturated soil, both heat conduction and heat convection facilitate heat transfer in the soil medium. Conventional analyses of heat transfer in saturated porous medium classified heat convection into two categories: (i) natural convection that could happen in the hydrostatic condition and (ii) forced convection that occurs due to the groundwater flow in the soil medium. This paper demonstrates that, in the presence of groundwater flow, the natural convection in the form of buoyancy-driven flow plays a prominent role in temperature distribution through saturated soil as well as the forced convection. A finite element analysis is performed to study the effects of groundwater velocity and soil permeability on the behavior of a fluid-saturated porous medium (soil). The results confirm that neglecting the natural convection can lead to inaccurate results. Furthermore, a novel expression is defined to estimate the importance of natural convection in heat transfer mechanisms in the soil medium.

Enhanced electrokinetic remediation of heterogeneous aquifer co-contaminated with Cr(VI) and nitrate by rhamnolipids

Electrokinetic remediation is an in situ remediation technique that is gaining increasing attention in the field of groundwater contaminant removal. Aquifers co-contaminated with hexavalent chromium (Cr(VI)) and nitrate are becoming more common and research on the remediation of this pollutant is attracting worldwide attention owing to the considerable human health hazards of Cr(VI) and nitrate, including carcinogenicity, genotoxicity, and cytotoxicity. Electrokinetic remediation technology has significant advantages for the treatment of heterogeneous aquifers. This is the first study on the electrokinetic remediation of an aquifer co-contaminated with toxic heavy metals and inorganic salts. This study investigated the feasibility of electrokinetic remediation for layered heterogeneous aquifers co-contaminated with Cr(VI) and nitrate in the presence of groundwater flow, and it also assessed the ability of the anionic biosurfactant rhamnolipid to enhance the remediation effect. Changes in the pH, electrical conductivity, Cr(VI), nitrate, electric current, and the distribution of energy consumption and the voltage contour were analyzed through seven sets of experiments. The highest removal rates of Cr(VI) and nitrate were 97.30% and 87.08%, respectively. Rhamnolipids greatly enhanced the electrokinetic remediation process, especially in medium with moderately low permeability: 1) rhamnolipids improved Cr(VI) removal(from 1.62% of EK3 to 85.57% of EK6 and from 48.42% of EK4 to 94.18% of EK7) through migration and reduction; and 2) rhamnolipids improved nitrate removal (from 13.03% of EK3 to 71.72% of EK6 and from 23.69% of EK4 to 78.43% of EK7) through enhanced migration.

Convective transport from geothermal borehole heat exchangers embedded in a fluid-saturated porous medium

Multi-dimensional natural convection and mixed convection have been studied numerically for the vertical borehole, a heated cylinder characterized by length Lb and diameter D, of a ground source heat pump in a fluid-saturated porous medium. The effect of natural convection over a range of ground properties characterized by Rayleigh number (RaL) is found to be significant and deviates strongly from boundary layer solutions for Rayleigh number less than 106, due to the flow turning required by the ground surface. For instance, the natural convection heat flux deviates from boundary layer prediction by over 20% for RaL = 104. Only at Rayleigh numbers of less than 2 × 103 can the heat transfer be treated as simple conduction, with the deviation less than approximately 5%, for fluid-saturated soil properties. The presence of groundwater flow, due to a hydraulic gradient, produces mixed convection leading to a convective heat transfer depending also upon Peclet number (PeD). A sufficient range of properties are examined such that the multi-dimensional mixed convection is shown to be captured between asymptotes to natural and forced convection. The dimensionless group (Lb/D)PeD/RaL is identified and shown to parameterize the variation between these two limits, with values less than 1.4 asymptoting to natural convection and values greater than 2.9 asymptoting to forced convection. Finally, a correlation encompassing all three regimes, natural, forced and mixed, is proposed.

Analytical model for arbitrarily configured neighboring shallow geothermal installations in the presence of groundwater flow

This paper introduces an analytical model analyzing the effect of groundwater flow on heat transfer in an infinite conductive-convective porous domain representing shallow geothermal systems with arbitrarily configured cylindrical heat sources. The model is formulated based on the moving source concept and solved based on the spectral analysis method and the superposition principle. Compared to models based on the Green’s function and the Laplace transform, the proposed spectral model has a simpler formulation, computationally efficient and easy to implement in computer codes. It can handle random time-dependent thermal loads and any arbitrarily configured grid distribution. The verification and numerical examples demonstrate the computational capabilities of the model, and show how the groundwater flow can play an important role in the thermal interaction between heat sources. They also feature how to make use of the direction of groundwater flow to avoid undesirable thermal interaction between neighboring installations, rapid depletion of energy sources and unfair mining of geothermal energy.

Thermal and hydrogeological aquifers characterization by coupling depth-resolved thermal response test with moving line source analysis

In this paper for the first time the Moving Line Source (MLS) model is combined with a depth-resolved Thermal Response Test (TRT). The latter was performed in a heterogeneous and groundwater rich subsoil, composed by a layering of silty sand, medium-fine sand and coarse-medium sand, with a layer of clayey silt separating a shallow aquifer from a deep one.The temperature evolution in the ground along the vertical axis was analysed with both the standard Infinite Line Source (ILS) and the MLS. The two models lead to similar estimates of the thermal conductivity in those regions of the subsoil where conduction prevails, while the MLS performs better where a significant groundwater velocity is expected. In these layers, the MLS analysis allows to derive both the thermal conductivity and the Darcy velocity. The MLS results were validated by comparison with a numerical simulation on a multiple-layers ground model developed in MODFLOW/MT3DMS using a constant energy boundary condition. The combined depth-resolved TRT/MLS approach represents an important method for an accurate design of the Ground Heat Exchangers under the presence of groundwater flow.

Influence of geology and hydrogeology on heat rejection from residential basements in urban areas

Urbanization and limited land availability have resulted in the increased utilization of underground structures including residential basements in largely populated cities such as London, with an average addition of 200 basements per year in some boroughs. Residential basements kept at a comfortable temperature level throughout the year significantly contribute to heat fluxes in the subsurface as well as an increase in ground temperature. Understanding the ground thermal status is crucial in managing the significant geothermal energy potential in urban areas as well as the sustainable development of the urban underground, and in maintaining the energy efficiency of underground structures. In this proof-of-concept study, a 3D finite element approach accounting for coupled heat transfer and groundwater flow in the ground was used to investigate the influence of ground conditions on the heat rejection rate from basements. A detailed analysis was made of ground, above ground and underground built environment characteristics. This study demonstrates that the amount of heat from basements rejected to the ground constitutes a significant percentage of the total heat loss from buildings, particularly in the presence of groundwater flow. The extent of thermal disturbance in the ground varies depending on the ground characteristics. The volume of thermally disturbance ground inversely correlates with the groundwater flow rate in ground mainly consisting of highly permeable material. However, a direct correlation exists when the thickness of permeable soil layer decreases. A larger horizontal to vertical ratio of ground thermal disturbance is observed when the thickness of permeable soil layer increases.

A hybrid numerical-semi-analytical method for computer simulations of groundwater flow and heat transfer in geothermal borehole fields

The formulation of a hybrid numerical-semi-analytical method for cost-effective simulations of heat transfer in fields of vertical geothermal boreholes, in the presence of groundwater flow, is presented. An amalgamation of a co-located control-volume finite element method and a finite volume method is used to solve 1) a volume-averaged continuity and the Darcy-Brinkman-Frochheimer equations to obtain the distribution of the groundwater flow; and 2) an unsteady three-dimensional volume-averaged advection-conduction equation to calculate the related ground temperature distribution, assuming local thermodynamic equilibrium between the groundwater and the soil particles. The bulk temperature distribution of the working fluid (flowing inside the legs of a U-tube pipe inserted inside each borehole and kept in place by grout) and the related heat extraction (or addition) rate are obtained using a semi-analytical method to solve a quasi-steady quasi-one-dimensional model. The conditions of no-slip, impermeability, equality of temperature, and continuity of heat flux are used at the interface between each borehole and the groundwater-saturated soil in the borehole field. The proposed method is applied to test and demonstration problems to demonstrate its capabilities.

Modeling of vertical ground heat exchangers in the presence of groundwater flow and underground temperature gradient

The paper focuses on the methodology for modeling the thermal response of vertical Ground Heat Exchangers (GHEs), when imposing underground water flow and a temperature gradient on the numerical model to represent the temperature of the depth profile. Four underground layers with different properties were studied, in order to identify their contribution in the total response of the GHE.In more detail, vertical GHEs in water saturated and dry soils, with or without water flow, are modeled with the equations that govern the heat transfer being presented. The numerical solution of the equations is based on the Finite Element Method (FEM) with boundary values. The model is validated with actual data of a Thermal Response Test (TRT) carried out in Lakatameia, Cyprus. Using the validated model, the heat injection rate of a GHE is investigated by determining the effect of the (a) summer and winter mode of operation, (b) underground temperature variation in depths smaller than 7 m due to daily and seasonal changes, (c) borehole radius, (d) borehole grout properties, (e) U-tube diameter, (f) U-tube leg and borehole centers distance, and (g) groundwater flow velocity.

A laboratory apparatus to study Thermal Response Test in the presence of groundwater flow

The standard approach to Thermal Response Test, based on conduction heat transfer in the ground, turns out to be unsuccessful under significant groundwater flow. The applicability of the Moving Infinite Line Source model to interpret the TRT in this case still needs to be proved. In order to study the TRT in the presence of a groundwater flow, an original laboratory apparatus has been developed. The Sand Box design is based on a heat transfer similitude between the real scale TRT problem and the laboratory scale one. The Sand Box sizes (1,2 m x 0.6 m x 1.0 m) are then set in order to keep the boundaries unaffected by the heat source during the TRT. The U-pipe heat exchanger is reproduced through a twocables electrical resistance 1 m long. A hydraulic loop with a peristaltic pump allows to obtain a Darcy velocity across the sandy soil up to 6,7510-5 m/s. The measurement system consists in several thermocouples in the porous medium and in a flow meter. The TRT results at null groundwater velocity allow to derive a reference thermal conductivity. The first tests with groundwater flow show the suitability of the apparatus and allow to derive some preliminary considerations.

The use of open-source electronic platforms to monitor the efficiency of Borehole Thermal Energy Storage Systems (BTES) in porous materials by means of lab scale modeling

In the framework of heating and cooling of buildings, the underground can be used as thermal energy storage to smooth the difference between production and demand. The heat transfer performances vary significantly depending on the type of soil where the plant is set up, but also with the water content and the presenceof groundwater flow. A lab scale modelling aimed at the simulation of thermal energy storage behaviour in porous materials was begun by Giordano et al. (2013) in order to better design the field scale living lab set up in Torino (Giordano et al., 2016). In this work, an implementation of the laboratory device is proposed with a monitoring system made of a low-cost open-source electronic platform based on Arduino®. With respect to the previous version, the heat sources have been eplaced with small scale borehole heat exchangers fed by a heating bath circulator equipped with in/out temperature sensors and flow rate control. Temperature and moisture monitoring within the medium has been improved with new and more accurate sensors. In addition, load sensors have been set up below the box to monitor the moisture migration induced during the heat injection. All the sensorsare read by Arduino® boards and shields thanks to on purpose scripts and the data are monitored by a lab PC.

Groundwater Controls on Postfire Permafrost Thaw: Water and Energy Balance Effects

AbstractFire frequency and severity are increasing in high‐latitude regions, but the degree to which groundwater flow impacts the response of permafrost to fire remains poorly understood. Here we use the Anaktuvuk River Fire (Alaska, USA) as an example for simulating groundwater‐permafrost interactions following fire. We identify key thermal and hydrologic parameters controlling permafrost response to fire both with and without groundwater flow, and separate the relative influence of changes to the water and energy balances on active layer thickness. Our results show that mineral soil porosity, which influences the bulk subsurface thermal conductivity, is a key parameter controlling active layer response to fire in both the absence and presence of groundwater flow. However, including groundwater flow in models increases the perceived importance of subsurface hydrologic properties, such as the soil permeability, and decreases the perceived importance of subsurface thermal properties, such as the thermal conductivity of soil solids. Furthermore, we demonstrate that changes to the energy balance (increased soil temperature) drive increased active layer thickness following fire, while changes to the water balance (decreased groundwater recharge) lead to reduced landscape‐scale variability in active layer thickness and groundwater discharge to surface water features such as streams. These results indicate that explicit consideration of groundwater flow is critical to understanding how permafrost environments respond to fire.

Analysis of Geo-Temperature Restoration Performance under Intermittent Operation of Borehole Heat Exchanger Fields

Intermittent operation can improve the coefficient of performance (COP) of a ground source heat pump (GSHP) system. In this paper, an analytical solution to analyze the geo-temperature restoration performance under intermittent operation of borehole heat exchanger (BHE) fields is established. For this purpose, the moving finite line source model is combined with the g-function and the superposition principle. The model takes into account the heat transfer along the borehole, thermal interference between BHEs, and the influence of groundwater flow. The accuracy of the model is validated through comparison with an experiment carried out under intermittent operation. The model makes it possible to analyze the geo-temperature restoration performance and its influencing factors, such as BHE spacing, heat flow rate, operation mode, and groundwater flow. The main conclusions of this work are as follows. The heat transfer along the borehole should be considered when analyzing the geo-temperature restoration performance. When the BHE spacing increases, the soil temperature change decreases and the heat recovery improves. Therefore, adequate borehole separation distance is essential in the case of a multiple BHE system with unbalanced load. The presence of groundwater flow is associated with interference between the BHEs, which should not be ignored. In the case of long-term operation, the groundwater flow is beneficial to the geo-temperature recovery process, even for downstream BHEs. Finally, a greater groundwater flux leads to a better geo-temperature recovery.