U-tube Borehole Heat Exchanger Research Articles

Investigation by monitoring and numerical simulation of the performance of a ground-source heat pump system with and without regeneration in different configurations and operating modes

This paper is focused on restoring the thermal imbalance of the ground caused by conventional GCHP systems through seasonal and continuous regeneration using a geothermal-solar facility with a vertical GCHP combined with photovoltaic (PV) and PV-thermal (PV/T) panels in a heating-dominated climate, located in Timisoara, Romania. Therefore, the performance of a GCHP system integrated into the heat supply system of an experimental office, operating in single and double U-tube borehole heat exchanger (BHE) configuration, with regeneration in the summer months by injecting heat into the ground, is experimentally evaluated. Exploratory research has indicated an increase in the average temperature of the working fluid by 2.4% and an improvement in system performance when using heat injection (seasonal performance factor (SPF) by 3.5% and 6.6% higher and CO2 emission by 1.2% and 3.8% lower in the single and double U-tube BHE configurations, respectively). Additionally, numerical simulations with Polysun software are also used to test the performances of the hybrid GCHP-PV/T system with continuous regeneration throughout the year by PV/T panels and of a conventional GCHP integrated into the heating and DHW production systems for a single-family building. The simulations were experimentally validated through measurements on the geothermal-solar facility for heating and DHW production in the experimental office. The simulation results show that the SPFHP-PV/T values of the hybrid GCHP-PV/T system in the single and double U-tube BHE configurations, equal to 6.00 and 5.79, respectively, are 32% and 34% higher than the SPFHP corresponding to the conventional GCHP system, equal to 4.56 and 4.32, respectively. In addition, the SPFHP-PV/T in the single U-tube BHE configuration is 3.6% higher than in the double U-tube BHE configuration for the GCHP combined with the PV/T array (31.8 m2).

Analysis of the Combined Effect of Major Influencing Parameters for Designing High-Performance Single (sBHE) and Double (dBHE) U-Tube Borehole Heat Exchangers

In this paper, a comprehensive analysis of the combined effect of major influencing parameters on heat transfer in a single U-tube BHE (sBHE) and a double U-tube BHE (dBHE) with two independent circuits was performed by using a validated numerical heat transfer model. Geometrical parameters, such as shank spacing (with maximum, average, and minimum values), borehole diameter (large, medium, and small borehole sizes), and borehole depth (shallow, average, and deep borehole depths) as well as the thermal conductivity of soil and grout, which ranges from minimum to high values, were considered. The combined impact of these parameters was included under the following four major cases: (1) the combined effect of borehole depth, borehole size, and shank spacing; (2) the combined effect of borehole depth and soil and grout thermal conductivity; (3) the combined effect of soil and grout thermal conductivity and borehole size; and (4) the combined effect of soil thermal conductivity, borehole size, and shank spacing. Each of these major cases has nine different design options for both sBHEs and dBHEs. A series of results of heat transfer per unit borehole depth were generated for all the considered various cases. With the given parameters, the BHE case that provides the highest heat transfer among the various cases of sBHEs and dBHEs were obtained.

A comprehensive transient heat transfer simulation of U-tube borehole heat exchanger considering porous media and subterranean water seepage

A comprehensive transient heat transfer simulation of U-tube borehole heat exchanger considering porous media and subterranean water seepage

Global sensitivity analysis and uncertainty quantification for design parameters of shallow geothermal systems

To improve the design process of geothermal systems, it is important to know which design parameters particularly affect the performance of the system. This article presents investigations on design parameters for borehole heat exchangers in the shallow subsurface. The study is based on numerical simulations with one double U-tube borehole heat exchanger and approximated models obtained using machine learning. As a result of the global sensitivity analysis, relevant parameters are identified and their respective influence on the performance of a borehole heat exchanger is compared. For example, according to this analysis, the three parameters with the highest sensitivity are the initial temperature, the heat demand and the share of the borehole heat exchanger that is surrounded by groundwater flow. Finally, the effects of uncertainties in the parameters identified as relevant for the design of a borehole heat exchanger are considered in an uncertainty quantification for a fictitious site. Uncertainties for regulatory compliance with respect to temperature limits as well as a large probability of oversizing the system were identified for the considered example. The results of the exemplary uncertainty quantification indicate that it has the potential to be a useful tool for planning practice.

Experimentally investigating and numerically simulating the performances of various low-temperature hydronic heating/cooling systems connected to a vertical closed-loop ground-coupled heat pump

The method used in this study consists of evaluating different terminal heating/cooling systems in an experimental office at the Polytechnic University of Timișoara, Romania, through experimentation and numerical simulation. This paper is focused on the energy efficiency exploratory research of various low- and very low-temperature hydronic radiant heating/cooling systems (floor, wall, ceiling, combined floor-ceiling) and of the medium-temperature radiator heating system, connected to a vertical ground-coupled heat pump (GCHP) in double U-tube borehole heat exchanger (BHE) configuration. The main performance parameters are obtained for five weeks of operation in heating mode and three weeks in cooling mode, and a comparative analysis of these parameters together with indoor thermal comfort is carried out. The four simple heating systems (radiator, floor, wall, ceiling) have relatively small differences (maximum 7.4 %) in their coefficient of performance (COP) values, but the number of starts/stops in the case of radiator heating is twice that of floor heating, which leads to greater wear and tear on the heat pump equipment. Under the same operating conditions, the CO2 emission of the radiator heating system are 0.7–16 % higher than those of the heating system with radiant panels (floor, wall, and ceiling). The floor-ceiling combined radiant heating system has the highest COPsyst of 5.45 as well as the lowest CO2 emission value of 2.15 kg. The predicted mean vote (PMV) index (pair 1.1 met-0.29 clo) has values approximately equal to 0 only in the case of the floor-ceiling heating, when the percent of likely dissatisfied with the thermal comfort is 5 %. The three cooling radiant systems show differences in their energy performance, namely the radiant ceiling cooling system has the best COPsyst, 39.8 % higher than that of the radiant floor and only 5.9 % higher than that of the radiant wall. The values of the PMV index (pair 1 met-0.9 clo) for the radiant wall cooling system are 31–41 % lower than those of the radiant floor and 10.4–14.2 % lower than those of the radiant ceiling. Finally, numerical simulations of the useful thermal energy and the system COP in heating and cooling modes are performed with TRNSYS software for a duration of 8760 h and then analysed and compared with the experimental tests to validate the simulation models.

Study on effects of multi-climatic parameters on performance of ground source heat pump through coaxial borehole heat exchanger

ABSTRACT The accurate calculation for heat flux of borehole heat exchanger (BHE) is of great importance for design of ground source heat pump (GSHP); the climatic conditions influence GSHP performance through GHE. In this study, a 2-D heat transfer model for coaxial BHE was built to investigate the effects of multi-climatic parameters on BHE heat transfer and GSHP performance. The outlet temperature, ground temperature profiles and COP of GSHP system for the cooling period were concerned. The effects of climatic conditions under different borehole depths, ground thermal conductivities and flow rates were studied. The studies showed that the outlet temperature from BHE increased about 10% and system COP declined about 5% in cooling mode with considering the effect of climatic conditions. The comparison indicates that the effects of climatic conditions on coaxial BHE are stronger than that on U-tube BHE. Generally, the climatic conditions influence BHE heat transfer mainly through the shallow ground and the influences strengthen with time. The effects can be more significant with a larger ground thermal conductivity, but it weaken with increase of borehole depth. Also, the effects of climatic conditions on BHE and GSHP seem to be independent of flow rate.

Estimation method of layered ground thermal conductivity for U-tube BHE based on the quasi-3D model

The distribution of ground thermal conductivity (λs) is a significant consideration for the design of ground source heat pump systems. This paper proposed a new method to estimate the layered λs for U-tube borehole heat exchanger (BHE) using particle swarm optimization (PSO) and distributed thermal response test (DTRT) data. Firstly, a quasi-3D heat transfer model considering geothermal gradient for U-tube BHE had been built to simulate fluid temperature profiles. And the experiment in DTRT was performed using a 45 m sandbox. Then, the layered λs were estimated through PSO and DTRT data based on the established model. The results showed that the convergent λs distribution could be reliably obtained by short-term DTRT. Furthermore, the estimation results are independent of the number of layers and data points. The mean difference in λs distribution between 9 layers and 45 layers is 0.007 W/(m∙K) and the layered λs also could be obtained when the number of layers is more than the number of temperature data points, and the root mean square error between 30 and 220 points is less than 0.3 W/(m∙K).

Experimental investigation of effectiveness variation of borehole heat exchangers for cooling mode operation

• Parallel configuration shows higher effectiveness of 0.88 than series configuration with 0.81. • Borehole limiting ratio for parallel configuration is higher than that of series one. • Experimental study of space cooling operation using ground dource heat pump system. • Maximum rise in the tube-interface temperature is observed at 73 m depth of borehole. In the present research work, a novel technique is proposed to estimate real time variation of borehole temperature based on measured interface temperature along the depth of a double U-tube borehole heat exchanger (BHE). Experimentally determined transient variation of effectiveness of BHE is discussed in detail for experimental simulation of line source model and actual operation of a GSHP system for cooling mode operation. The real time borehole temperature computed using the present technique during actual operation of a GSHP system makes it possible to compute the actual variation of effectiveness of BHE. The above set of experiments were performed for both single and double U-tube BHEs considering series and parallel mode operations during peak summer conditions at a location situated closer to the Himalayan region in northern part of India. Effectiveness of BHEs was computed using varying borehole temperature obtained from line source model and the real time varying borehole temperature proposed in the present work. For the actual cooling mode GSHP experiments, the average value of thermal effectiveness of double U-tube BHE computed with constant borehole temperature is observed to be 0.72 for both series and parallel mode operations. However, with the use of real time varying borehole temperature, the effectiveness is observed to increase by 12% and 22% for series and parallel flow operations respectively.

Performance evaluation of U-tube borehole heat exchangers employing nanofluids as the heat carrier fluid

• All nanofluids showed a higher rate of heat transfer enhancement than pressure drop. • Employing nanofluids for purpose of reducing the borehole length is not promising. • Electrical energy cost is negligible compared to the capital cost of nanoparticles. • The optimum decrement of BHE thermal resistance occurs by a lower volume flow rate. • A dimensionless correlation is proposed allowing evaluation of outlet temperature. The present study aims to investigate comprehensively the performance of various nanofluids in single U-tube borehole heat exchangers (BHEs). Seven common nanoparticles with the volume fraction ranging from 0.1% to 2.0% are selected to be evaluated as the heat carrier fluid. Firstly, a comparative techno-economic analysis is performed in order to highlight the merits and drawbacks of each nanofluid. Then, a sensitivity analysis is performed to optimise the decrement percentage of BHE thermal resistance. Finally, by means of the linear regression of numerical results obtained for different nanofluids, simple equations are proposed allowing evaluation of the outlet fluid temperature for nanofluids. The obtained results indicate that Ag- and Cu-based nanofluids are characterised by the highest heat transfer enhancement, although this improvement is at penalty of a higher pressure drop and up to 31% higher required pumping power. On the other hand, SiO 2 - and TiO 2 -water nanofluids are the worst cases in terms of thermal performance, but at the same time, they are characterised by the lowest pressure drop. The optimum decrement percentage of thermal resistance yielded in presence of Cu-water nanofluid is equal to 4.31%. Furtheremore, it is shown that employing nanofluids for the purpose of BHE length reduction is not a promising choice. Economic analysis revealed that the cost of electrical energy for nanofluids due to the higher energy consumption of pump is negligible in comparison with the capital cost of nanoparticles. The SiO 2 nanoparticles with a capital cost ranging from 5.8 to 17.5 €/m is the cheapest nanoparticle to employ, unlike the Ag nanoparticles.

Experimental Study of Heat Extraction and Soil Recovery During Space Heating Application Using Ground Source Heat Pump System

Abstract The present research paper reports experimental results on the coupled phenomena of heat extraction and heat recovery by a double U-tube borehole heat exchanger (BHE) and its thermal performance for space heating application using a ground source heat pump (GSHP) system. Experiments were performed on a 5-ton cooling capacity GSHP system installed at Roorkee located in the northern part of India, during peak and moderate winter seasons. The heat interaction by the GSHP system with the ground is achieved by five double U-tube BHEs and one single U-tube BHE of 120 m depth each. Experimental trials were conducted for parallel and series flow of heat transfer fluid (water) in the BHE. Temperature of water and power consumption by the GSHP was monitored continuously during heat extraction for 12 h and heat recovery of soil as well. For the peak winter season, during heat extraction, a 5 °C temperature rise was observed in BHE water for series connection, which is about 10% higher than that of the parallel case. Maximum heat extraction of 14.8 kW was obtained for the series connection, which is 20% higher than the parallel one, thus resulting in a 20% higher value of the overall coefficient of performance (COP) of the system for series mode compared to the parallel mode operation during the peak winter season. However, the ground near the BHE has taken lesser time for heat recovery in parallel mode compared to the series mode.

Analysis of the transient performance of coaxial and u-tube borehole heat exchangers

While coaxial borehole heat exchangers often have lower borehole thermal resistances than u-tubes, studies in the literature have mixed results as to whether the coaxial design always outperforms the u-tube. This study contributes a systematic comparison between the two designs, accomplished using a custom numerical model in OpenFOAM that provides detailed predictions of the heat transfer within the ground heat exchangers. The modelling and subsequent analysis of system thermal resistances demonstrated that the u-tube and coaxial heat exchanger performances differed the most during the early, transient phase of operation. Furthermore, modelling showed that the coaxial design does not necessarily exceed the u-tube performance long term; testing of coaxial heat exchangers with polyethylene piping showed small differences in outlet temperature compared to the u-tube after 72 h. Additional testing showed that using a steel coaxial outer tube could provide a 22% improvement to performance over the u-tube design. These findings are used to compare predictions of thermal resistance with available analytical models. The impact of the results on the potential for length reductions is also discussed.

A Novel Semi-Analytical Solution for Thermal Response Analysis of Single U-Tube Borehole Heat Exchangers (Bhes)

A Novel Semi-Analytical Solution for Thermal Response Analysis of Single U-Tube Borehole Heat Exchangers (Bhes)

Prediction and parametric analysis of 3D borehole and total internal thermal resistance of single U-tube borehole heat exchanger for ground source heat pumps

The borehole and total internal thermal resistance are both significant parameters in evaluating the thermal performance of the ground source heat pump. This study aimed to obtain the accurate correlation of the 3D borehole and total internal thermal resistance (Rb,3D and Ra,3D) and analyze the impacts of parameters on the Rb,3D and Ra,3D. Firstly, eight parameters affecting the Rb,3D and Ra,3D, including the borehole diameter, pipe diameter, pipe-pipe distance, borehole depth, soil thermal conductivity, grout thermal conductivity, pipe thermal conductivity, and fluid velocity inside the pipe, were considered and an L-54 design matrix was generated. Then, the 3D numerical model, coupling with the four-resistance model, was proposed to calculate Rb,3D and Ra,3D for each case. After that, the response surface methodology was employed to obtain and verify the correlation of Rb,3D and Ra,3D, which were compared with the existing resistance calculation methods. Lastly, analysis of variance was carried out to reveal parameters that have statistically significant impacts on the Rb,3D and Ra,3D. Results show that the rationality and accuracy of the correlation of Rb,3D and Ra,3D can be verified by the determination coefficient and P value of regression model, as well as the P value of lack-of-fit. The existing resistance calculation methods are more or less inaccurate and the discrepancies in some cases can be up to 86.74% and 111.35% for the borehole and total internal thermal resistance. The pipe and grout thermal conductivity, pipe and borehole diameter, and the pipe-pipe distance can be seen as the significant contributory factors to the variation of Rb,3D and Ra,3D.

Nanofluid suspensions as heat carrier fluids in single U-tube borehole heat exchangers

The borehole heat exchanger (BHE) is a critical component to improve energy efficiency and decreasing environmental impact of ground-source heat pump systems. The lower thermal resistance of the BHE results in the better thermal performance and/or in the lower required borehole length. In the present study, effects of employing a nanofluid suspension as a heat carrier fluid on the borehole thermal resistance are examined. A 3D transient finite element code is adopted to evaluate thermal comportment of nanofluids with various concentrations in single U-tube borehole heat exchangers and to compare their performance with the conventional circuit fluid. The results show, in presence of nanoparticles, the borehole thermal resistance is reduced to some extent and the BHE renders a better thermal performance. It is also revealed that employing nanoparticle fractions between 0.5% and 2 % are advantageous in order to have an optimal decrement percentage of the thermal resistance.

Heat transfer analysis of single and double U-tube borehole heat exchanger with two independent circuits

Borehole heat exchanger (BHE) is a very suitable and cost-effective technology for heat storage and extraction of heat from the ground. In this paper, comprehensive analysis of major influencing parameters on thermal performance of single U-tube BHE (sBHE) and double U-tube BHE (dBHE) was performed by using validated numerical heat transfer model. Combined effect of increasing borehole size and soil or grout thermal conductivity (for different cases of shank spacing) on performance of sBHE and dBHE was studied. Detailed comparative analysis of thermal performance was made among sBHE and dBHE with small and large borehole sizes. The impact of borehole depth and diameter, fluid inlet temperature, mass flow rate, U-tube pipe material type and diameter on total heat transfer per unit borehole depth, borehole thermal resistance and thermal effectiveness was also investigated. The simulation result revealed that when the shank spacing is kept at high or low value in sBHE and dBHE (both with small and large borehole size), dBHE with larger borehole size has the highest thermal performance while sBHE with smaller borehole size has the lowest performance; and the highest and the lowest thermal resistance is obtained for sBHE with smaller borehole size and dBHE with larger borehole size, respectively. Grout with high thermal conductivity is preferred in terms of improving thermal effectiveness and reducing thermal resistance of the BHE when the soil thermal conductivity is sufficiently high; while low grout thermal conductivity is preferred with large borehole size and high shank spacing. For high soil thermal conductivity (4W/m.K), the heat transfer rate per unit borehole depth increases with grout thermal conductivity to a maximum value; and then further increasing of the grout conductivity results in smooth declining of the heat transfer in the BHE. Thus, ground and grout thermal conductivity have an optimal range to achieve a lower value of the BHE thermal resistance, and consequently for better BHE sizing. With increasing fluid inlet temperature (15oC to 42oC), the total heat transfer is improved by 51.5 W/m and 36.2 W/m for dBHE and sBHE respectively; and more heat is injected into the borehole when dBHE is used than sBHE, particularly at higher inlet temperature showing that dBHE is preferable than sBHE for system that works with higher fluid inlet temperature. The result obtained can be used as a quick reference for the design, operation optimization and performance study of ground coupled heat pump system where BHE is used as aborehole thermal energy storage.

Comprehensive application of hydrogeological survey and in-situ thermal response test

Thermal properties of rock-soil are of great importance for the design of borehole heat exchangers (BHEs). In the process of testing thermal properties of rock-soil through an in-situ thermal response test (TRT), groundwater seepage directly affects the evaluation of thermal properties in TRTs and leads to a varied arrangement and engineering costs of BHEs. Therefore, comprehensive hydrogeological surveys of groundwater and TRTs to determine geothermal properties are necessary. In this study, we made a preliminary attempt to use hydrogeological surveys and correlated TRTs to investigate the heat exchange efficiency of a single U-tube BHE. The geological conditions were used to predict and analyze the groundwater form, groundwater depth, and flow trajectory in the test area based on the natural electric field frequency selection method. Using a comprehensive dataset, the groundwater velocity was determined with ANSYS. The results showed that the initial temperature of rock-soil was 21.8 °C and that the comprehensive thermal conductivity of rock-soil was 1.942 W/(m·°C). Confined and phreatic aquifers existed in the test area, and the depth of the confined aquifer was approximately 70 m. The groundwater velocity in the test area was considered to be approximately 2–2.5 m/d. The results of this study confirm that TRTs combined with groundwater surveys can improve the test accuracy and consequently provide guidance for engineering practices in ground-coupled heat pump systems.

Impact of Employing Hybrid Nanofluids as Heat Carrier Fluid on the Thermal Performance of a Borehole Heat Exchanger

In this numerical study, 4 types of hybrid nanofluid, including Ag-MgO/water, TiO2-Cu/water, Al2O3-CuO/water, and Fe3O4-multi-wall carbon nanotube/water, have been considered potential working fluid in a single U-tube borehole heat exchanger. The selected hybrid nanofluid is then analyzed by changing the volume fraction and the Reynolds number. Based on the numerical results, Ag-MgO/water hybrid nanofluid is chosen as the most favorable heat carrier fluid, among others, considering its superior effectiveness, minor pressure drop, and appropriate thermal resistance compared to the pure water. Moreover, it was indicated that all cases of Ag-MgO/water hybrid nanofluid at various volume fractions (from 0.05 to 0.20) and Reynolds numbers (from 3200 to 6200) could achieve better effectiveness and lower thermal resistances, but higher pressure drops compared to the corresponding cases of pure water. Nevertheless, all the evaluated hybrid nanofluids present lower coefficient of performance (COP)-improvement than unity which means that applying them as working fluid is not economically viable because of having higher pressure drop than the heat transfer enhancement.

Thermo-Hydraulic Performance of U-Tube Borehole Heat Exchanger with Different Cross-Sections

For reducing the initial GSHP investment, the heat transfer efficiency of the borehole heat exchange (BHE) system can be enhanced to reduce the number or depth of drilling. This paper proposes a novel and simple BHE design by changing the cross-sectional shape of the U-tube to increase the heat transfer efficiency of BHEs. Specifically, in this study, we (1) verified the reliability of the three-dimensional numerical model based on the thermal response test (TRT) and (2) compared the inlet and outlet temperatures of the different U-tubes at 48 h under the premise of constant leg distance and fluid area. Referent to the circular tube, the increases in the heat exchange efficiencies of the curved oval tube, flat oval tube, semicircle tube, and sector tube were 13.0%, 19.1%, 9.4%, and 14.8%, respectively. (3) The heat flux heterogeneity of the tubes on the inlet and outlet sides of the BHE, in decreasing order, is flat oval, semicircle, curved oval, sector, and circle shapes. (4) The temperature heterogeneity of the borehole wall in the BHE in decreasing order is circle, sector, curved oval, flat oval, and semicircle shapes. (5) Under the premise of maximum leg distance, referent to the heat resistance of the tube with a circle shape at 48 h, the heat exchange efficiency of the curved oval, flat oval, semicircle, and sector tubes increased 12.6%, 17.7%, 10.3%, and 7.8%, respectively. (6) We found that the adjustments of the leg distance and the tube shape affect the heat resistance by about 25% and 12%, respectively. (7) The flat-oval-shaped tube at the maximum leg distance was found to be the best tube design for BHEs.

Comprehensive simulation based thermal performance comparison between single and double U-tube borehole heat exchanger and sensitivity analysis

In the ground coupled heat pump system that delivers space heating and cooling to the buildings, often questions are raised as to which borehole heat exchanger configuration (single or double U-tube) provides higher thermal performance. To answer this question, in this paper, detailed comparative transient heat transfer analysis between single and double U-tube borehole heat exchanger (BHE) with two independent circuits (that can operate with the same or different mass flowrate and inlet fluid temperature) is conducted. Validated numerical heat transfer model was applied to perform the dynamic simulation and analysis. The two borehole heat exchangers are compared from the perspectives of their thermal performance in the heating and cooling mode of operation. Detailed sensitivity analysis of the BHE has also been performed by investigating the impact of the important borehole parameters, thermal properties and working fluids on the thermal performance of the double U-tube BHE. The simulation result revealed that the heat injected and extracted by the double U-tube BHE at steady state is 21.8 (77%) and 10.2 W/m (71.8%) more than that of the single U-tube BHE respectively. The simulation results also indicate that the thermal effectiveness of the single and double U-tube BHE at steady state are 0.24 and 0.31 respectively. The single U-tube BHE showed higher borehole thermal resistance than that of the double U-tube BHE with values of 0.47 and 0.31 m.K/W, respectively. The results of the study also revealed that among the studied working fluids, CaCl2 (25%) solution showed better thermal performance than other anti-freeze solutions with thermal effectiveness of 35.6%. The simulation result of this work is crucial as an input for the optimization studies made to obtain a single set of optimal borehole paramaters and thermal properties that results in the best performance of BHE coupled to ground source heat pump system designed to provide space heating and cooling of buildings.

Analysis of double U-tube ground heat exchanger for renewable energy applications with two-region simulation model by combining analytical and numerical techniques

This research is concerned on heat transfer analysis and modelling in a double U-tube borehole heat exchanger (BHE), via dividing the borehole into two separate regions. The inner part region of the borehole was analyzed analytically, while the outer region of the borehole was studied numerically. The two analyses were joined with the borehole wall temperature. The transient heat transfer of the borehole outer region was analyzed with thermal resistance model using the implicit numerical technique. In the contrary the thermal capacitance of the inner region of the borehole was ignored in this analysis as the inner portion of the borehole is insignificant compared to the outer borehole region and its modelling was only treated with analytical method. Variation of borehole temperature with time, the variation of rate of heat transfer between the borehole inner region to the outer region, radial temperature profile of the ground, and variation in mean temperature of ground with respect to time were investigated for three borehole operation scenarios (i.e. double charging, double discharging, simulations charging and discharging).