INTERNATIONAL JOURNAL OF LATEST TECHNOLOGY IN ENGINEERING,  
MANAGEMENT & APPLIED SCIENCE (IJLTEMAS)  
ISSN 2278-2540 | DOI: 10.51583/IJLTEMAS | Volume XIV, Issue XI, November 2025  
Validation of Single Well Geothermal Heat Pump System by Scaled  
Model Experiments  
Tok Gi Choe*, Jong Su Kim, Dong Guk Kim, Il Jin Ko  
Kim Chaek University of Technology, Pyongyang, Democratic People’s Republic of Korea  
DOI: https://doi.org/10.51583/IJLTEMAS.2025.1411000014  
Received: 10 November 2025; Accepted: 20 November 2025; Published: 02 December 2025  
ABSTRACT  
For shallow geothermal applications, the U-tube ground source heat pump (GSHP) system, the standing column  
well(SCW) geothermal heat pump system, and the forced external circulation single well(FECSW) geothermal  
heat pump system are recognized as the most advanced methods of geothermal cooling and heating systems, either  
by using no groundwater or by recycling groundwater. In order to improve the heat extraction efficiency of these  
single-well modes, it is vital to experimentally analyze the heat extraction efficiency in different experimental  
environments that can be encountered under realistic conditions. Based on the 1:20scaled model we design 10  
experimental scenarios considering different experimental conditions, such as soil moisture, groundwater  
convection, heat conduction, abstraction and injection interval selection, and compared the heat extraction  
efficiency according to the scheme. The experimental results shows that the order of increasing heat extraction  
capacity is U-tube, SCW, FECSW. Also, in three methods, the heat extraction capability is changed by the water  
flow into the ground, ground moisture, operating approaches, etc. The results of the scaled model experiments are  
used as a comparative validation data for numerical simulation of a single well geothermal heat pump system and  
contribute to the development of modeling method of geothermal single well in real conditions.  
Keywords: U-tube ground source heat pump, standing column well, forced external circulation single well, scaled  
model, heat extraction  
INTRODUCTION  
Nowadays, due to the worldwide depletion of energy and environmental pollution problems, the development of renewable  
energy for mankind to live in a clean environment is widely studied.  
With the rapid development of science and technology, a large production of high efficiency heat pumps has made active use  
of shallow geothermal energy with less development cost compared to deep geothermal heat in many countries.  
The U-tube GSHE is a shallow geothermal energy development method that embeds the heat exchanger into the ground and  
extracts the geothermal energy through heat exchange between the ground and the water flowing through the pipe. The U-  
tube GSHE used in practice includes single U-tube tube, multi-U tube and coaxial tube heat exchangers [1, 2, 3].  
There are two major heat transfer models for mathematically modeling U-tube GSHE: linear heat source model and  
cylindrical heat source model. The linear heat source model includes the Kelvin model, Ingersoll model, and IGSHPA model  
[4,5], which suppose several hypotheses to avoid and simplify the complexity of the actual heat transfer process and ignore  
several factors affecting the heat transfer process, such as interference between tubes and heterogeneity of temperature  
distribution. The cylindrical heat source model was proposed by Carslaw and Jaeger to develop a linear heat source theory  
model in some cases. Kavanaugh [6] developed a heat transfer model of a vertical U-tube GSHE with some modifications to  
the original cylindrical heat source model in 1991. In this model, the heat transfer process was considered only due to heat  
conduction, and the thermal interference between wells and seepage of groundwater in the soil was neglected. The cylindrical  
heat source model can be reasonably applied to the case operating for short periods of time with U-tube GSHE installed in  
large diameter wells.  
Several experimental and numerical simulation methods have also been proposed to study U-tube GSHE. Egidi et al. [7]  
proposed a mathematical model of U-tube GSHEs based on unsteady Navier-Stokes problems. They divided the heat  
exchanger into two computational domains: a straight tube in which the temperature field is calculated analytically, and a U-  
www.ijltemas.in  
Page 145  
INTERNATIONAL JOURNAL OF LATEST TECHNOLOGY IN ENGINEERING,  
MANAGEMENT & APPLIED SCIENCE (IJLTEMAS)  
ISSN 2278-2540 | DOI: 10.51583/IJLTEMAS | Volume XIV, Issue XI, November 2025  
curve tube in which the flow and heat exchange are calculated numerically based on Galerkin finite element method, and  
evaluated the generated effective energy. These results were analyzed for experimental measurements and validation obtained  
from the actual geothermal heat exchanger. Al-Khoury et al. [8] calculated the temperature distributions in all well heat  
exchanger elements and surrounding soil mass using fast Fourier transform in an axisymmetric shallow layer geothermal  
system consisting of a double U-tube GSHE embedded in the soil. Using 16,384 FFT samples and 100 Fourier-Bessell series  
samples in the time domain, the temperature distributions in the tube inlet, filler and soil were calculated. Eslammejad et al.  
[4] proposed an analytical model for predicting steady state heat transfer in a double U-tube GSHE with two independent  
circuits operating with different mass fluxes and inlet temperatures. The model can be used to describe the tube thermal  
resistance and thermal interaction of fluid in U-tube circuits and to investigate the new dual U-tube well structure with one  
circuit connected to a ground source heat pump and a solar thermal collector operating in heating mode. Gharibi et al. [5]  
carried out a three-dimensional numerical model with U-tube GSHEs in abandoned oil wells to analyze the effects of mass  
flux, fluid inlet temperature, length of insulation and tube diameter. Cimminino [9] proposed a numerical model and  
analytical model to calculate fluid and well wall temperatures in geothermal wells with multiple U-tubes. Tilley et al. [10]  
considered a simple Cartesian model consisting of two finite length parallel channels carrying heat transfer fluid embedded  
in the soil. One channel carries fluid from the surface to the bottom of the well, and the other carries fluid from the bottom  
of the well to the surface. Jalili et al. [11] studied the convective conductive heat transfer process between all components  
such as tube interior, tube exterior, and grout of a geothermal borehole heat exchanger (BHE) using an Modified Homotopy  
Perturbation Method(MHPM). Lyu et al. [12] established a three dimensional steady state numerical model including U-  
tubes, wells and geothermal layers based on geological data and studied the effect of important geological parameters such  
as bed depth, porosity, permeability and heterogeneity on heat exchanger performance. Maestre et al. [13] proposed a two-  
dimensional mixing model (RC2) for U-tube GSHEs and experimentally verified the mixing model under stop-start operating  
conditions. Santa et al. [14] conducted an experiment to investigate the cyclic thermal stress on the surrounding soil in the  
operation of BHE in closed loop geothermal systems. The results show that the irreversible compression effect induced by  
soil increases the salinity and lowers the soil freezing point. Lyne et al. [15] examined high conductivity phase change  
material (PCM) in the experimental model using annular region packing. The experimental results showed that PCM can  
reduce temperature fluctuations in annular and soil regions and increase the rate of heat consumption from heater to soil  
environment. Wang et al. [16] analyzed the heat injection temperature and initial water content during heat storage on the  
basis of a laboratory experimental setup using BHE. Gordon et al. [2] obtained an experimental study on a vertical coaxial  
BHE that increasing the inner tube diameter reduces the required length of the heat exchanger and increases the overall  
performance factor realized by the heat pump. Zhang et al. [17] evaluated the thermal conductivity and specific heat capacity  
of gravel, sand, sandstone and granite mainly affected by particle size, porosity, density and water content through  
experiments.  
A SCW is a ground heat exchanger that installs a circulating pipe in the well and extracts geothermal energy while circulating  
fluid. Several experimental and numerical simulation methods have been proposed to study the heat transfer characteristics  
in SCW. Bu et al. [3] developed a mathematical model describing heat transfer through the underground well as the  
surrounding rock, based on experimental studies on a single well, and simulated and evaluated the seasonal heat output. The  
heat output decreases with time, but the imbalance can be adjusted by changing the pouring temperature and speed. Beaudry  
et al. [18] developed a finite element model that combines heat transfer and groundwater flow with input data collected in a  
large-scale geothermal laboratory. Simulation results representing conditions of 6-day pumping test, 24-day thermal response  
test and 25-day dynamic winter operation were consistent with experimental data with mean absolute error of 7.3 cm, 0.15°C  
and 0.32°C, respectively.  
The FECSW is another type of combining the closed loop and open loop. Wu et al. [1] evaluated the influence of different  
parameters on the output temperature and efficiency of FECSW system through finite element numerical simulation. Thermal  
conductivity, volumetric heat capacity and porosity have a minor effect on the system sustainability and efficiency, and it  
has been confirmed that winter-summer (W-S) mode with active cooling has a greater effect than winter (W-S mode). Song  
et al. [19] designed a sandbox experimental setup for SCW and FECSW experiments. In the experiments, the effect of the  
outlet water temperature (OWT) and heat absorption (HAQ) of the hot well, especially the OWT improvement of SCW, on  
the flux of 0-30% was analyzed. Song et al. [20] performed model experiments and numerical simulations on a FECSW. A  
numerical model of FECSW was developed to analyze the motion of temperature and velocity profiles under flow and thermal  
breakthrough. When the porosity of the backfill material decreased from 0.35 to 0, the COP of the heat pump unit increased  
by 1.6% on average, and the thermal breakthrough strength decreased by 45.3% on average within 25 min. Through this  
quantitative analysis, theoretical suggestions for the heat transfer mechanism near the hot well wall are provided.  
Analyzing the results of previous studies, they use laboratory and field experimental data for comparative analysis, focusing  
on simulation calculations to enhance the performance and heat exchange efficiency of single well modes such as U-tube  
heat pump system, SCW heat pump system and FECSW heat pump system. In addition, the simulation calculations and  
experimental results are compared, limited to individual modes, and no three modes are studied in relation to each other. In  
www.ijltemas.in  
Page 146  
INTERNATIONAL JOURNAL OF LATEST TECHNOLOGY IN ENGINEERING,  
MANAGEMENT & APPLIED SCIENCE (IJLTEMAS)  
ISSN 2278-2540 | DOI: 10.51583/IJLTEMAS | Volume XIV, Issue XI, November 2025  
order to study and apply these modes to realistic conditions, the heat extraction efficiency of each mode should be compared  
with the study of the heat exchange characteristics of individual modes under different environmental conditions. In this  
paper, we designed a 1:20 scaled model and conducted 10 experimental scenarios considering different experimental  
conditions such as soil moisture, groundwater convection, heat conduction, abstraction and injection interval selection, and  
heat extraction experiments according to the scenarios, and compared the effectiveness.  
Design and Construction of Single Well Scaled Model  
A 1:20 scaled model was designed to perform geothermal extraction experiments. A single well scaled model consists of a  
well system, a water supply system, a cooling (heating) supply system, a temperature measurement system, and a flow  
measurement system, and a schematic of each scaled model is shown in Fig. 1.  
(a)  
(b)  
(c)  
Fig. 1. A single well scaled model system configuration  
(a) U-tube well, (b) SCW, (c) FECSW  
Single well system  
U-tube well  
In the U-tube well, the heat of the ground is generally extracted in the form of heat transfer, but in the experiments a system  
was also prepared to study and analyze the convective heat transfer due to groundwater flow. For U- tube well systems,  
Ф250mmUPVC pipe, Ф20mmUPVC pipe, Ф3.5mmPE tube and insulation materials were used. The Ф250mmUPVC pipe  
forms the outer wall of the scaled model (Fig. 2). The inner diameter of the pipe is 236 mm and the tube height is 1.5 m. The  
bottom of the tube is double fixed so that it is not lost by the load of the soil.  
The Ф20mmUPVC pipe is the well in the scaled model. In practice, the well wall generally consists of a casing (steel) and a  
drilled rock. However, there are also cases where poor diagenetic formations, such as Tertiary strata, are made by steel or  
plastic. In our experiments, we chose to install plastic tubes to replace the role of casings throughout the sample, since we  
use soil samples. At that time, the porous plastic tube allows the simultaneous convection and heat transfer with the soil and  
the non-porous plastic tube to examine only the thermal conductivity. The porous plastic tube made about 2,994 holes with  
Ф2mm to make 10% porosity. The non-porous plastic tube cuts off the water flow between in-hole and out-hole. This  
provides similar conditions to the practice of filling several impermeable fillers in the well to prevent groundwater  
contamination during anti-freezing agent leakage accidents.  
The Ф3.5mmPE tube is used as the U-tube. A plastic U-ring and Ф3.5mmPE tube were connected to make a U-tube. The  
tubes were separated by adhesive tape at intervals of 10 cm so that they did not adhere to each other. In practice, a Ф32 mm  
HDPE tube is used as a U-tube. HDPE (high density polyethylene) pipes are very effective in terms of corrosion resistance,  
strength and thermal conductivity. UPVC and PE pipes are shown in Fig. 2.  
www.ijltemas.in  
Page 147  
INTERNATIONAL JOURNAL OF LATEST TECHNOLOGY IN ENGINEERING,  
MANAGEMENT & APPLIED SCIENCE (IJLTEMAS)  
ISSN 2278-2540 | DOI: 10.51583/IJLTEMAS | Volume XIV, Issue XI, November 2025  
(a)  
(b)  
Fig. 2. UPVC and PE pipes  
(a) Ф250mmUPVC pipe and Ф3.5mmPE tube, (b) Ф20mmUPVC pipe  
Synthetic rubber insulation is used as insulation material. The Ф250mmUPVC tube, which is the outer wall of the scaled  
model, is subjected to external temperature during the experiments and may cause errors in the results of heat extraction in  
the soil, thereby covering the entire pipe (except the upper surface) to the floor with this insulation.  
SCW  
On the other hand, in the case of non-cracked impermeable rock, the water in the well cannot penetrate and migrate to each  
other with the surrounding strata (rock), so only heat conduction will affect the heat extraction. Non-porous plastic tube to  
be prepared for experiments in this case.  
The Ф3.5mmPE tube is used as the injection and pumping tube in a SCW model. Two Ф3.5mmPE tubes are inserted into  
the well, which is located in the upper part and in the lower part. The plastic plug is intended to prevent water from entering  
the well in case of non-porous plastic pipes.  
FECSW  
Ф2mm holes are drilled to Ф20mmUPVC pipe section corresponding to inlet and outlet section (10 cm each) with 10%  
porosity.  
In geothermal extraction with a FECSW, the selection of the seal section position and length (interval between the injection  
section and the pumping section) is very important. In general, it is reasonable that the injection and pumping intervals are  
in the best position for permeability. That is, it is necessary to select the site for the pumping and reinjection in the ground.  
If the length of the seal section is reduced, which adversely affects the heat pump operation by rapidly reaching the pumping  
section and resulting in severe temperature changes. On the contrary, if too large, the arrival time of the injected water is  
much slow, so that the amount of water in the water-poor zone cannot be provided.  
On the other hand, the seal method is also an important technical issue. The seal within the well can be divided into one- and  
two-barrier. If seal is not good, the injected water may leak through the screen or between the well and the ground soil,  
causing a loss of heat extraction efficiency.  
The problem to consider in seal technology is also how this is technically feasible since the pumping pipe is installed through  
seal section. In the experiment, a single seal structure was selected, the middle of the 1.5 m long Ф20mmUPVC tube was  
cut, covered with a seal plug (circular tree) and the seal was completed by passing through the Ф3.5mmPE tube into the tree  
In order to see if the experimental seal structure actually has a seal function, the Ф20mmUPVC tube was set vertically, filled  
with water in the upper part (water had a hydraulic pressure of 0.7 mH20) and kept for 48 h, there was no water leakage from  
the bottom of the tube.  
Water supply system  
Water supply is ideal to use a mini-pump, but considering the allowable flux 2.5~4 cm/s (0.5 m/s in practice), flux 0.6~1 L/h  
and cooling (heating) supply load in the scaled model, a very small amount flows into the Ф3.5mmPE tube and is therefore  
implemented using the static water head without using a mini-pump.  
www.ijltemas.in  
Page 148  
INTERNATIONAL JOURNAL OF LATEST TECHNOLOGY IN ENGINEERING,  
MANAGEMENT & APPLIED SCIENCE (IJLTEMAS)  
ISSN 2278-2540 | DOI: 10.51583/IJLTEMAS | Volume XIV, Issue XI, November 2025  
On the Ф250mmUPVC pipe, a cylindrical water tank of 270 mm in height and 236 mm in diameter (about 10 L in volume)  
was mounted, drilled at the bottom of the water tank, and a water droplet velocity monitor was installed to enable real-time  
water supply and flow measurements.  
Cooling (heating) system  
In practice, to determine the thermo-physical properties of the ground and evaluate the heat extraction capability, thermal  
response tests are performed to heat the water tank with an electric heater to circulate the heated constant temperature water  
into the ground. In the thermal response test, the hot water is supplied, but in the present experiment the cold water is supplied  
from the cold generator.  
For the experiments, the inlet pipe for the water supply tank and the outlet pipe for the cooling water extraction into the iron  
tank filled with water 2L volume are arranged at intervals such that there is no thermal interference and sealed to prevent  
water from flowing out of the iron tank. Iron box of this structure shall be installed in cooling chamber and covered with  
synthetic rubber insulation to ensure that cooling is maintained in it (Fig. 3).  
Fig. 3. Cooling chamber covered with synthetic rubber insulation  
According to the experiments, when the temperature in the cooling chamber was -14°C, the temperature in the cooling box  
(iron box) was -1.5°C, i.e., the cooling generated from the cooling system was fully transferred into the cooling box.  
On the other hand, for heat supply experiments, iron box may be put in heating chamber.  
Temperature measurement system  
In the experiments, the temperature measurement system is a key part with cold supply.  
For temperature measurement, we used the DS18B20 which is a digital temperature sensor that communicates in a single-  
line access mode. DS18B20 uses a bus communication protocol that implements bus communication using a single control  
signal bus. The DS18B20 uses a bus communication protocol that implements bus communication using a single control  
signal bus.  
A microcontroller computer (PIC18F452) is used to realize control over the measurement system, such as temperature  
measurement, display and data communication.  
The microcontroller (firmware) was developed in MPLAB IDE v8.5.  
The communication circuit uses the Max485 chip to transfer the processed temperature measurement data from the  
microcontroller to the upper computer in RS485 communication mode.  
In the experiments, six temperature sensors are placed in the corresponding position of the scaled model and the temperature  
in the cooling chamber, the inlet and outlet temperature of the model and the temperature at three locations in the model are  
measured simultaneously in real time.  
The temperature control unit automatically realizes the start and stop of the cooling system at a set time interval, while  
displaying the temperature at different locations on the LCD and transferring data to the upper computer by PIC control,  
ensuring the stability of the long-term operation of the cooling system. Using the set key, increment (decrease) key and left  
and right moving key, the experimental date, time, compressor start and stop time, etc. are set. The data transmitted in the  
temperature controller is displayed in real time and stored in a computer.  
www.ijltemas.in  
Page 149  
INTERNATIONAL JOURNAL OF LATEST TECHNOLOGY IN ENGINEERING,  
MANAGEMENT & APPLIED SCIENCE (IJLTEMAS)  
ISSN 2278-2540 | DOI: 10.51583/IJLTEMAS | Volume XIV, Issue XI, November 2025  
Real-time measured inlet and outlet temperatures and soil internal temperatures are recorded in the storage medium while  
graphically displayed on the computer. To accomplish this, the temperature data measured on a single-chip computer were  
acquired on a computer using USB-485 communications, and then the program for display and recording was written using  
Qt program.  
Flow Measurement System  
In order to accurately determine the heat extraction and to provide the inlet and outlet temperature difference to the scaled  
model, the flow measurement should be performed well.  
In the experiments, the flux is very small, with an average of 1 L/h, so a calibrated beaker is used considering the condition  
that the flux can be measured without the flow meter. Beaker volume is 100 ml.  
RESULTS AND DISCUSSION  
Soil property assessment test  
In the geothermal extraction experiments, fine sand and coarse sand soil were used as materials. Here, fine sandy soil is used  
as a filling material to fill in the well model.  
The characteristics of the sandy soil used in the experiments were determined by experimental methods.  
Hydraulic conductivity  
The hydraulic conductivity of soil was determined by the fixed head method.  
We filled the sample in the sample box, shaken it with constant shaking, and poured water to ensure sufficient humidity.  
Then, the graduated water supply box was filled with water and placed on the top surface of the sample upside down so that  
the hole went down. Since the water in the water supply tank is kept to match the water level above the sample, the hydraulic  
gradient I becomes 1 because of the same length L and water level H of the soil layer, and thus the hydraulic conductivity K  
can be determined using the flux Q and the cross-sectional area F flowing for time t.  
The experimental results for coarse sand, the soil sample filled in the model are shown in Table 1. The average time for three  
experiments was 38.6 s, and the hydraulic conductivity of coarse sand was determined to be about 81.3 m/d.  
Table 1. Experimental results of hydraulic conductivity measurement of coarse sand  
№ Time, s Flux, cm3 Section area, cm2  
1
2
3
41.6  
37.5  
36.9  
90.8  
90.8  
90.8  
25  
25  
25  
On the other hand, the experimental results for fine sand filled in well are shown in Table 2. The average time was 93.5 s,  
and the hydraulic conductivity of the fine sand was estimated to be about 33.6 m/d.  
Table 2. Results of the hydraulic conductivity Measurement of fine sand  
№ Time, s Flux, cm3 Section area, cm2  
1
2
3
76.6  
91.7  
90.8  
90.8  
90.8  
25  
25  
25  
112.1  
www.ijltemas.in  
Page 150  
INTERNATIONAL JOURNAL OF LATEST TECHNOLOGY IN ENGINEERING,  
MANAGEMENT & APPLIED SCIENCE (IJLTEMAS)  
ISSN 2278-2540 | DOI: 10.51583/IJLTEMAS | Volume XIV, Issue XI, November 2025  
Porosity  
Porosity determination experiments were performed by filling the sand well in the scaled model and supplying water to the  
bottom of the model to determine the amount of water needed to fill the soil pores. Sandy soil is compacted well at 1.5 m  
height and filled with water to another model with the same volume as the experimental model and connect the two models  
to the pipe. When the valve is opened, water flows into the sand filled model in the water filled model. When the water level  
is balanced, the drawdown value is measured and it is saturated to the top surface of the sandy soil by filling again. At that  
time, adding the measured drawdown values at each stage results in the water level in the air gap of the soil layer.  
Experimental results showed that when saturated up to the top surface of the soil, the height of the sandy soil was lowered to  
1.45 m and the height of water injected was 0.51 m. Since the cross-sectional areas of both models are the same, the porosity  
of the sandy soil filled in the model is determined to be n=0.51/1.45≈0.35.  
Experiments and results using U-tube GSHE scaled model  
In order to perform the geothermal extraction experiments in the U-tube GSHE scaled model, the Ф20mmUPVC tube was  
installed vertically at the bottom center of the Ф250mmUPVC tube and the Ф3.5mmU PE tube was inserted here (Fig. 4-a).  
In this situation, the coarse sand used as the formation material was filled with a certain thickness and filled with all by  
compaction (Fig. 4-b)  
(a)  
(b)  
Fig. 4. The U-tube well internal structure  
The well internal structure, (b) Internal structure of sand-filled experimental setup  
The sandy soil filled with Ф250mmUPVC pipe can be saturated by using the valve at the bottom of the pipe, else drained  
through it.  
Based on the experimental scenarios shown in Table 3, the U-tube GSHE scaled model experiments are conducted and the  
experimental results are analyzed.  
Table 3. U-tube GSHE scaled model experimental scenarios  
Condition  
Experimental purpose  
Scenario 1  
Scenario 2  
Scenario 3  
Scenario 4  
Sand in saturated state, use water as filler inside  
well, porous well  
Evaluation of heat extraction efficiency by heat  
conduction in the absence of filler  
Sand in saturated sand, use of fine sand as filler  
inside well, porous well  
Evaluation of heat extraction efficiency by heat  
conduction in case of filler  
Dry sand, use water as filler inside well, non-  
porous well  
Evaluation of heat extraction efficiency by heat  
conduction in dry impervious ground  
Partially saturated sand, water as filling material  
Evaluation of heat extraction efficiency by heat  
www.ijltemas.in  
Page 151  
INTERNATIONAL JOURNAL OF LATEST TECHNOLOGY IN ENGINEERING,  
MANAGEMENT & APPLIED SCIENCE (IJLTEMAS)  
ISSN 2278-2540 | DOI: 10.51583/IJLTEMAS | Volume XIV, Issue XI, November 2025  
inside well, non-porous well  
conduction in weakly permeable strata  
Scenario 1  
The sandy soil was completely saturated and tested in only water-filled condition without filler inside the well.  
The experimental time is 45,360s (about 0.525d), the flux is 0.0228 m3/d, and the average I/O temperature difference is  
5.37°C.  
Scenario 2  
The sandy soil was completely saturated and the fine sand of size 0.5~1mm inside the well was filled with filler and the  
experiments were carried out.  
The experimental time is 34,560s (about 0.4d), the flux is 0.021m3/d, and the average I/O temperature difference is 6.67°C.  
Scenario 3  
The soil is dry sand.  
The PVC pipe forming the well is a non-porous pipe and it is not possible to leak the filled water into the surrounding soil  
instead of the filler between the well and the U-tube.  
The experimental time is 30,780s (0.35625d), the flux is 0.0216m3/d and the average I/O temperature difference is 4.57°C.  
Scenario 4  
The soil used in Scenario 4 is water-filled and completely removed through the drain valve and partially saturated.  
The PVC pipe forming the well is a non-porous pipe, which acts as a barrier to prevent the water filled between the well and  
the U-tube from escaping into the surrounding soil.  
The experimental time is 61,300s (0.71d), the flux is 0.0234m3/d, and the average inlet and outlet temperature difference is  
4.56°C.  
The variations of the I/O temperature of the U-tube GSHE scaled model obtained during the experiments are shown in Fig.  
5.  
Fig. 5. I/O temperature variation in U- tube GSHE scaled model.  
(a) scenario 1, (b) scenario 2, (c) scenario 3, (d) scenario 4  
The results of the experiments of the scenarios for the U- tube GSHE scaled model were combined to analyze the heat  
extraction capacity (Table 4).  
www.ijltemas.in  
Page 152  
INTERNATIONAL JOURNAL OF LATEST TECHNOLOGY IN ENGINEERING,  
MANAGEMENT & APPLIED SCIENCE (IJLTEMAS)  
ISSN 2278-2540 | DOI: 10.51583/IJLTEMAS | Volume XIV, Issue XI, November 2025  
Table 4. Analysis of heat extraction capacity for U-tube GSHE test scenarios  
Average temperature  
Average flux, m3/d Thermal Extraction Capability, W  
difference, ℃  
Scenario 1  
Scenario 2  
Scenario 3  
Scenario 4  
5.37  
6.67  
4.57  
4.56  
0.0228  
0.021  
6.29  
6.79  
4.79  
5.38  
0.0216  
0.0234  
Experiments and results using SCW scaled model  
In order to perform the geothermal extraction experiments in the SCW scaled model, the Ф20mmUPVC tube was vertically  
installed at the bottom center of the Ф250mmUPVC tube and two Ф3.5mmPE tubes were installed at the inlet and outlet  
positions. The inlet pipe is located in the upper part of the well and the pumping pipe in the bottom.  
In this situation, the sandy soil used as a formation material was filled with a constant thickness between the well and the  
external wall and filled with all by compaction.  
Based on the experimental scenarios shown in Table 5, SCW scaled model experiments were conducted and the experimental  
results were analyzed.  
Table 5. SCW scaled model experimental scenarios  
Condition  
Experimental purpose  
Water filled instead of soil,  
non-porous well  
Evaluation of heat extraction efficiency  
by heat conduction in water  
Scenario 1  
Scenario 2  
Scenario 3  
Scenario 4  
Scenario 1  
Evaluation of heat extraction efficiency  
by heat conduction in ground with  
constant humidity  
Wet sand, non-porous well  
Dry sand, non-porous well  
Saturated sand, porous well  
Evaluation of heat extraction efficiency  
by heat conduction in impermeable rocks  
Evaluation of heat extraction efficiency  
by heat conduction and convection in  
saturated sand  
In scenario 1, the model was filled with water instead of soil and the experiment was performed.  
Since the well model uses non-porous pipes, only heat conduction occurs between the circulating water flowing inside the  
well and the water filled in the model around the well.  
The experimental time is 22,900s (about 0.265d), the flux is 0.0228 m3/d, and the average I/O temperature difference is  
5.87°C.  
Scenario 2  
In scenario 2, heat exchange experiments were carried out on partially saturated wet sandy soil.  
Also, the well is sealed and only heat exchange by the surrounding soil and heat transfer.  
www.ijltemas.in  
Page 153  
INTERNATIONAL JOURNAL OF LATEST TECHNOLOGY IN ENGINEERING,  
MANAGEMENT & APPLIED SCIENCE (IJLTEMAS)  
ISSN 2278-2540 | DOI: 10.51583/IJLTEMAS | Volume XIV, Issue XI, November 2025  
The experimental time is 41,040s (0.475 d), the flux is 0.0214 m3/d, and the average I/O temperature difference is 7.07°C.  
Scenario 3  
In scenario 3, heat exchange experiments were carried out on dry sandy soil.  
Since a well model is built with non-porous pipes, there is only heat conduction between the well and the surrounding soil  
as in Case 2.  
The experimental time is 34,560s (about 0.4d), the flux is 0.0214m3/d and the average I/O temperature difference is 4.69°C.  
Scenario 4  
In scenario 4, heat exchange experiments were carried out on saturated sandy soil.  
The use of porous pipes results in convective phenomena, not only heat conduction between the well and the surrounding  
soil, which increases the heat exchange efficiency.  
The experimental time is 40,180s (about 0.465d), the flux is 0.0228 m3/d, and the average I/O temperature difference is  
7.8°C.  
The I/O temperature variations in the SCW scaled model during the experiment are shown in Fig. 6.  
Fig. 6. I/O temperature variations in the SCW scaled model.  
(a) scenario 1, (b) scenario 2, (c) scenario 3, (d) scenario 4  
The experimental results are summarized in Table 6.  
Table 6. Analysis of heat extraction capacity for SCW scenarios  
Average temperature  
Average flux, m3/d Thermal Extraction Capability, W  
difference, ℃  
Scenario 1  
Scenario 2  
Scenario 3  
Scenario 4  
5.87  
7.07  
4.69  
7.8  
0.0228  
0.0214  
0.0214  
0.0228  
6.5  
7.89  
4.71  
8.46  
www.ijltemas.in  
Page 154  
INTERNATIONAL JOURNAL OF LATEST TECHNOLOGY IN ENGINEERING,  
MANAGEMENT & APPLIED SCIENCE (IJLTEMAS)  
ISSN 2278-2540 | DOI: 10.51583/IJLTEMAS | Volume XIV, Issue XI, November 2025  
Experiments and results with FECSW scaled model  
In order to perform the geothermal extraction experiments in the FECSW scaled model, the Ф20mmUPVC pipe was  
vertically installed at the bottom center of the Ф250mmUPVC pipe and two Ф3.5mmPE pipes were installed at the inlet and  
outlet positions.  
The injection pipe is located in the upper part and the pumping pipe passes through the seal section from the bottom of the  
well and rises to the upper part.  
In this situation, the sandy soil used as a formation material was filled with a certain thickness between the well and the outer  
wall and filled with all by compaction.  
We conducted a FECSW scaled model experiment based on the experimental scenarios shown in Table 7 and analyzed the  
experimental results.  
Table 7. Scenarios for FECSW scaled model experiments  
Condition  
Experimental purpose  
Injection in upper part, abstraction in lower  
part, porous well  
Evaluation of heat extraction efficiency by heat  
conduction and convection in the case of  
pumping under well  
Scenario 1  
Injection in the bottom, abstraction in the  
top, and porous wells  
Evaluation of heat extraction efficiency by heat  
conduction and convection in case of pumping in  
upper part of well  
Scenario 2  
Scenario 1  
As the middle part of the well is sealed, the water injected into the upper part penetrates into the surrounding soil through the  
injection section and enters the well again through the pumping section and is discharged out. The soil material is sand and  
fully saturated.  
The experimental time is 42,120s (0.4875d), the flux is 0.0384 m3/d, and the average I/O temperature difference is 6.73°C.  
Scenario 2  
In scenario 2, only the injection and pumping positions of the circulating water are changed, and the other conditions are the  
same as in scenario 1. Thus, the circulating water is injected into the bottom of the well and permeates into the surrounding  
soil and moves back to the top and then leaves out.  
The experimental time is 82,080s (0.95d), the flux is 0.0323 m3/d and the average I/O temperature difference is 4.64°C.  
The variation of the I/O temperature of the FECSW scaled model obtained during the experiments is shown in Fig. 7.  
Fig. 7. I/O temperature variation in the FECSW scaled model.  
(a) scenario 1, (b) scenario 2  
www.ijltemas.in  
Page 155  
INTERNATIONAL JOURNAL OF LATEST TECHNOLOGY IN ENGINEERING,  
MANAGEMENT & APPLIED SCIENCE (IJLTEMAS)  
ISSN 2278-2540 | DOI: 10.51583/IJLTEMAS | Volume XIV, Issue XI, November 2025  
The results of the experiments on FECSWs were combined to analyze the heat extraction efficiency (Table 8).  
Table 8. Analysis results of heat extraction efficiency for FECSW scenarios  
Average flux, m3/d  
Thermal Extraction Capability, W  
Average temperature difference, ℃  
Scenario 1  
Scenario 2  
6.73  
4.64  
0.0384  
0.0323  
12.57  
7.33  
CONCLUSIONS  
In the U-tube mode, the heat extraction capacity is high in scenarios 2 and 1, and relatively low in scenarios 4 and 3. In  
scenarios 2 and 1, the use of saturated sand as the formation material and the wells filled with water or fine sand in the  
borehole were made of porous tubes, so the heat transfer inside the borehole was extended to the ground area, resulting in  
increased heat extraction capacity. However, in scenarios 4 and 3, non-porous wells were used, and the heat extraction  
capacity was not high as a result of insufficient heat exchange with surrounding soil. Therefore, it is reasonable to use porous  
casing or gravel-filled materials because there is no possibility of groundwater contamination in the case of water without  
antifreeze in the U-tube mode.  
The order of magnitude of heat extraction capacity in SCW is scenario 4> scenario 2> scenario 1> scenario 3. Scenario 4 has  
a high heat extraction capacity compared to scenarios 1, 2 and 3 using non-porous pipes because of the inclusion of saturated  
sand and porous pipe. This shows that the effect of convection is much greater than heat conduction from the viewpoint of  
heat extraction. The heat extraction of scenario 4 was about 1.27 times higher than that of scenario 2 in U-tube mode using  
closed U-tube under similar conditions. This indicates that should increase the convective heat exchange with the circulating  
water and surrounding ground as much as possible to increase the geothermal extract.  
In FECSW, scenario 1 has about 1.7 times higher heat extraction capacity compared to scenario 2. The reason is that the low  
temperature water is injected in the upper part of scenario 1, so convection is caused by temperature difference in the soil,  
but convection does not occur in scenario 2 because the low temperature water is injected into the lower part. Therefore, it is  
considered effective to generate convection by injecting the top of the borehole during heating and injecting it into the bottom  
of the borehole during cooling.  
The results of the scaled model experiments can be applied in practice to select the well structure and operating mode to  
achieve high heat extraction efficiency.  
AUTHOR CONTRIBUTIONS  
Tok Gi Choe: Writing-original draft, Conceptualization, Methodology; Jong Su Kim: Investigation, Validation; Dong Guk  
Kim: Software, Il Jin Ko: Visualization, Review & Edition  
ACKNOWLEDGEMENTS  
We would like to thank the officers of Kim Chaek University of Technology who encouraged the geothermal heating and  
cooling project and our study.  
Data Availability  
The data that has been used is confidential.  
Conflict of Interest  
The authors declare that they have no conflict of interest.  
REFERENCES  
1. Wu, Q., Tu, K., Sun, H.Z., Chen, C.F., 2019. Investigation on the sustainability and efficiency of single-  
www.ijltemas.in  
Page 156  
INTERNATIONAL JOURNAL OF LATEST TECHNOLOGY IN ENGINEERING,  
MANAGEMENT & APPLIED SCIENCE (IJLTEMAS)  
ISSN 2278-2540 | DOI: 10.51583/IJLTEMAS | Volume XIV, Issue XI, November 2025  
well circulation (SWC) groundwater heat pump systems. Renewable Energy. 130, 656-666.  
http://doi.org/10.1016/j.renene.2018.06.107  
2. Gordon. D., Bolisetti. T., Ting. D., Reitsma. S., 2017. Experimental and analytical investigation on pipe  
sizes  
for  
a
coaxial  
well  
heat  
exchanger.  
Renewable  
Energy.  
1–23.  
http://doi.org/10.1016/j.renene.2017.08.088  
3. Bu. XB., Ran. YM., Zhang. D.D., 2019. Experimental and simulation studies of geothermal single well for  
building heating. Renewable Energy. 143, 1902-1909. http://doi.org/10.1016/j.renene.2019.06.005  
4. Eslaminejad. P., Bernier. M., 2011. Coupling of geothermal heat pumps with thermal solar collectors using  
double U-tube wells with two independent circuits. Applied Thermal Engineering. 31, 3066-3077.  
http://doi.org/10.1016/j.applthermaleng.2011.05.040  
5. Gharibi. S., Mortezazadeh. M., Jalaledin. S., Badi, H.A., Vatani. A., 2018. Feasibility study of geothermal  
heat extraction from abandoned oil wells using a U-tube heat exchanger, Energy. 1-26.  
http://doi.org/10.1016/j.energy.2018.04.003  
6. Kavanaugh. M., 2008. A 12-step method for closed-loop ground-source heat pump design. ASHRAE  
Transactions. 114, 328-337.  
7. Egidi. N., Giacomini. J., Maponi. P., 2018. A mathematical model for the analysis of the flow and heat  
transfer problem in U-shaped geothermal Exchangers, Applied Mathematical Modelling. 1-40.  
8. Al-Khoury. R., Focaccia. S., 2016. A spectral model for transient heat flow in a double U-tube geothermal  
heat pump system. Renewable Energy. 85, 195-205. http://doi.org/10.1016/j.renene.2015.06.031  
9. Cimmino. M., 2016. Fluid and borehole wall temperature profiles in vertical geothermal wells with multiple  
U-tubes. Renewable Energy. 96, 137-147. http://doi.org/10.1016/j.renene.2016.04.067  
10. Tilley. B.S., Yang. V.C., Baiense. J.C., Evans. S., 2017. Frequency-dependent thermal resistance of vertical  
U-tube geothermal heat exchangers, Journal of Engineering Mathematic. 102, 131–150.  
http://doi.org/10.1017/s10665-016-9881-7  
11. Jalili. P., Ganji. D.D, Nourazar. S.S, 2018. Hybrid semi analytical method for geothermal U shaped heat  
exchanger. Case Studies in Thermal Engineering. 1-19. http://doi.org/10.1016/j.csite.2018.07.010  
12. Lyu. Z.H., Song. X.Z., Li. G.S., Shi. Y., Mu. Z.H., 2018. Numerical analysis of characteristics of a single  
U-tube downhole heat exchanger in  
a
geothermal well. Geothermics. 72, 15–23.  
http://doi.org/10.1016/j.geothermics.2017.10.012  
13. Maestre. I.R., Gallero. F.J., Gómez. P.A., Blázquez.J.L., 2019. Experimental validation of a hybrid model  
for vertical ground heat exchangers under on-off operation conditions. Energy & Buildings. 204, 109517.  
http://doi.org/10.1016/j.enbuild.2019.109517  
14. Santa. G.D., Galgaro. A., Tateo. F., Colac. S., 2016. Induced thermal compaction in cohesive sediments  
around a well heat exchanger: laboratory tests on the effect of pore water salinity. Environmental Earth  
Science. 75, 181. http://doi.org/10.1007/s12665-015-4952-z  
15. Lyne. Y., Paksoy. H., Farid. M., 2019. Laboratory investigation on the use of thermally enhanced phase  
change material to improve the performance of well heat exchangers for ground source heat pumps.  
International Journal Energy Resources. 1–9. http://doi.org/10.1002/er.45222  
16. Wang. H.J., Qi. C.Y., 2011. A laboratory experimental study of high-temperature thermal storage in the  
unsaturated soil using a vertical well heat exchanger, International Journal of Low-Carbon Technologies.  
6, 187– 192.  
17. Zhang. Q., Zhang. Y.J., Gao. P., Yu. Z.W., 2014. Effect of natural cold source on groundwater source heat  
pump according to laboratory and field geotechnical thermal physical tests, Energy and Buildings. 84, 557–  
566. http://doi.org/10.1016/j.enbuild.2014.09.007  
18. Beaudry. G., Pasquier. P., Marcottev. D., 2019. The impact of rock fracturing and pump pumping location  
on the thermal recovery of a standing column well: model development, experimental validation and  
numerical  
analysis.  
Science  
and Technology for  
the  
Built  
Environment.  
1–33.  
http://doi.org/10.1080/237447731.2019.1648133  
19. Song. W., Ni. L., Yao. Y., 2017. Experimental research on bleeding performance of single well groundwater  
heat pump. Procedia Engineering. 205, 3162–3169.  
20. Song. W., Zheng. C.G., Zhang. B., Jin. Y., Liu. Y.Z., 2021. Simulation of the flow and thermal breakthrough  
of a forced external circulation standing column well, Energy and Built Environment. 32, 1-19.  
http://doi.org/10.1016/j.enbenv.2021.01.006  
www.ijltemas.in  
Page 157