Analysis of Potential Use of Freezing Boreholes Drilled for an Underground Mine Shaft as Borehole Heat Exchangers for Heat and/or Cooling Applications

Abstract

Borehole engineering encompasses the part of mining that involves the process of drilling boreholes and their utilization (e.g., for research, exploration, exploitation, and injection purposes). According to legal regulations, mining pits must be closed after their use, and this applies to pits in the form of boreholes as well. The Laboratory of Geoenergetics at AGH University of Krakow is involved in adapting old, exploited and already closed boreholes for energetic purposes. This includes geothermal applications, as well as energy storage in rock formations and boreholes. Geoenergetics is a relatively new concept that combines geothermal energy with energy storage in rock formations (including boreholes). One type of analysed borehole is a freezing borehole. They are used, for example, in drilling mining shafts that are in the vicinity of aquifers and are drilled using the rotary drilling method with a reverse circulation of drilling mud, or in peat bogs. For borehole heat exchangers based on freezing boreholes for long-term mathematical modelling, several heating scenarios were considered with several thermal loads. The maximum average power obtained after one year of usage of four boreholes with variable temperatures was 11 kW. With the usage of 10 boreholes the power reached over 27 kW. The heat-carrying temperature was assumed to be 22 °C during early summer (June and July) and 2 °C during the rest of the year. When considering stable exploitation during a 10-year period with four boreholes with the same temperatures, a heating power of over 12 kW was obtained, as well as a power of over 28 kW when considering using 10 boreholes. The maximum amount of heat obtained during the 10-year period using 10 boreholes was over 8.8 thousand GJ. Once they have fulfilled their function, these boreholes lose their technological significance. In the paper, the concept is outlined, and the results of the analysis are described using the numerical program BoHEx.

1. Introduction

Mining is continuously developing and will continue to develop globally, and soon also in the universe (likely starting on the Moon, and later on other planets in the Solar System), beginning with borehole mining [1]. Despite the advancement of technologies such as electricity generation through geothermal energy [2,3], traditional mining remains a cornerstone of the global economy. Prospects for increased demand for mineral resources continue to drive new technologies in mining and other economic sectors [4].

Freezing boreholes (FBs) have long been used in the construction of mining shafts, primarily to freeze water in aquifers [5]. This is necessary, for example, to penetrate layers without flooding the drilled shaft with groundwater [6,7,8] or when drilling tunnels [9]. In the case of this study, the analyzed FB was used to freeze groundwater during mineshaft construction. During a mineshaft construction, the drilling mud is pumped in a reversed order, with it going down the borehole in the annular space. Freezing groundwater prevents mud losses, thus allowing the drilling process to proceed. Another example is maintaining permafrost during drilling in Arctic and sub-Arctic areas [10]. These boreholes often reach significant depths.

From a thermodynamic perspective, freezing boreholes operate similarly to borehole heat exchangers (BHEs) due to the fact that both types of boreholes are drilled in order to force a transfer of energy between the borehole and surrounding rock formation, with FBs being used to freeze surrounding groundwater and BHEs to source geothermal heat [11,12,13]. However, they can have much greater depths than typical drilled BHEs, reaching up to 300 m below ground level [14,15] with constructions in the form of single, double, or triple U-tubes. Therefore, they can be classified as deep borehole heat exchangers although their definition is not yet precisely defined.

In BHEs and FBs, heat is “extracted” [16], most often at low temperatures. In this paper, it is considered low temperature when it prevents its direct use for heating [17]. It can only be useful as a result of using geothermal heat pumps [18,19]. In rock formations, the process of extracting heat from the Earth results in freezing groundwater (in FBs) or may have the same effect (in BHEs). However, it is not always a necessity [20]. Because of the previous existence of FBs and the fact that they are obsolete upon the construction of the main shaft, they should be considered for adaptation to BHEs in order to heat the surrounding infrastructure. The main goal of this paper is to prove that they can indeed work as BHEs. Currently, there are no FBs adapted to BHEs, further proving the feasibility of this paper.

For deep BHEs, coaxial design is preferred because of the depth [21]. A more conductive rock mass shows that the coaxial borehole heat exchanger provides better thermal performance [22]. The main reason for coaxial construction is lower pressure losses of heat carriers than in the corresponding U-tube [23]. U-tube construction is recommended by authors for BHES of depth up to 300 m. Many research studies show that coaxial construction for deep BHEs is favorable [24,25,26]. As an example, the unit heat extraction rates were calculated at the end of the numerical simulation [23] as 32.8 W/m (single U-tube), 36 W/m (double U-tube) and 39.1 W/m (coaxial). The necessary condition for a coaxial BHE is no leaks. When leaks exist, the U-pipe with grouting is a better solution. The internal pipe should be thermally insulated adequately to the depth of the borehole [27,28]. The distribution of temperature in the annulus and inside the internal pipe is shown in Figure 1.

Favourable circumstances present when steel is used as the material of external columns. Old deep oil or other boreholes have steel pipes used as casing. It causes the borehole resistance of used old boreholes to be beneficial [29,30]. The internal column (tube) should be centralized to increase the area of heat exchange between the heat carrier and rock mass.

The reuse of old boreholes, including closed ones, gives a good opportunity for deep BHE construction. The basic condition is the presence of potential heat consumers in close proximity to the borehole. If there are no heat consumers near the borehole, it can also be adapted for further use. This possibility is provided by storing electricity as part of geoenergetics. The boreholes can work as gravity or pressure borehole energy storage, e.g., to store surplus energy from photovoltaic panels or wind turbines [31,32]. Running electric lines is much cheaper than heating networks.

Figure 1. Temperature profiles in coaxial DBHE of 3000 m depth for different values of thermal conductivity coefficients of inner column (1—0.01 Wm⁻¹K⁻¹, 2—0.12 Wm⁻¹K⁻¹, 3—1.16 Wm⁻¹K⁻¹, 4—46.1 Wm⁻¹K⁻¹) [33].

Figure 1. Temperature profiles in coaxial DBHE of 3000 m depth for different values of thermal conductivity coefficients of inner column (1—0.01 Wm⁻¹K⁻¹, 2—0.12 Wm⁻¹K⁻¹, 3—1.16 Wm⁻¹K⁻¹, 4—46.1 Wm⁻¹K⁻¹) [33].

It is impossible to count the number of boreholes drilled in the world. Most of them were constructed as wells to access water. There are oil and gas boreholes, which also have greater depths. Boreholes drilled in the Carpathian flysch over 150 years ago were most often located away from buildings. Currently, as a result of urbanization, many of them may be adapted into BHEs. Freezing boreholes can be used for heating (and/or cooling) purposes for buildings connected with underground mines.

1.1. Studied Boreholes

The analysis of the possibility of converting freezing boreholes (FBs) into borehole heat exchangers (BHEs) was conducted on 10 freezing boreholes scheduled for closure. They were used in the construction of a shaft with a depth exceeding 1000 m below ground level. The boreholes were arranged in two groups of five each, spaced 1.25 m apart along the perimeter of the shaft. Freezing boreholes are drilled with control over their axis, using special drilling methods [34]. In the analysis, it was assumed that all freezing boreholes from their bottom to a depth of 300 m would be cemented. Boreholes from a depth of 3.5 m below ground level to 300 m below ground level would be available for conversion and use (approximately 296.5 m each).

For the purpose of freezing the analyzed shaft to a depth of 690 m, the following were designed and executed:

• 20 freezing boreholes with odd numbers (from X1 to X39) drilled from a freezing circle with a diameter of 16.0 m, to a depth of 700 m.
• 20 freezing boreholes with even numbers (from X2 to X40) drilled from a freezing circle with a diameter of 16.0 m to a depth of 440 m.

The freezing boreholes were alternately located in a pattern of deep borehole–shallow borehole. The designed arrangement of freezing boreholes allowed for the freezing of Cenozoic formations with forty boreholes and the middle-variegated sandstone with twenty boreholes. The nominal distance between freezing boreholes in Cenozoic formations and variegated sandstone was 1.25 m, while in the middle-variegated sandstone, it was 2.5 m. The schematic of the analyzed shaft with boreholes is presented in Figure 2.

1.2. Mathematical Model

The analysis was based on the use of the BoHEx simulator [35] to determine the number of boreholes utilized for geothermal (geoenergetic) purposes. BoHEx is an original mathematical model used for BHE forecast preparation, based on [20,35] adapted for this work. Boreholes functioning as BHEs can have a negative impact on each other, meaning two boreholes share one supply, resulting in a lower supply for both boreholes [36,37].

The obtainable low-temperature heating power was also determined, assuming in one scenario the collaboration of boreholes with a water-to-water heat pump, and in another scenario assuming that the working fluid would be a 25% solution of monopropylene glycol (i.e., ground-source heat pump GSHP).

Modeling the heat exchange process between borehole heat exchangers and the formation is a challenging task. The main difficulty arises from large dimensions (several hundred meters) and the time scale (several to several dozen years), as well as the complexity of phenomena (turbulent flow, groundwater flow through various rocks, phase changes in the rock formation, borehole construction, etc.). Even disregarding these aspects, several factors, whose determination and even estimation are very difficult, affect the ability to accurately model the entire system, such as the velocity of deep groundwater movement or the distribution of physical parameters of the rock formation.

To simplify the model’s construction and solution, the analyzed system presented in Figure 3 is considered as being composed of two separate subdomains (1—pipe heat exchanger, 2—rock formation), similar to [20]. The first domain is associated with processes occurring inside the borehole (including the borehole wall). In this subdomain, there is a flow of the heat carrier through the pipes, heat penetration into the walls, heat conduction through the pipe walls, sealing, and the borehole wall. The main task in this area is to determine the fluid temperature along the entire length of the channels depending on the borehole temperature. Often, due to the spatial scale, this area is treated as a linear or cylindrical heat source of a stationary nature. In this work, this area was analyzed as a non-stationary, non-linear heat source, allowing for the consideration of phenomena with non-uniform heat extraction [35]. This mathematical model provides a suitable solution for the problem at hand, which is why it was chosen. The aforementioned BoHEx simulator [35] uses this model, therefore providing an automated tool.

The second subdomain is associated with the rock formation and extends from the outer surface of the borehole (this surface connects two subdomains with a common boundary condition) to the outer edges of the modelled space. Heat transport dominates here through conduction, or when groundwater flow is present, it becomes mixed, convective-diffusive. Thermal processes in this subdomain, due to the scale and mass of the system, proceed very slowly. In the rock formation area, heat exchange is described using a three-dimensional, non-stationary heat transport equation, which in Cartesian coordinates takes the form:

$${\rho c}_p\frac{\partial T}{\partial t}=\frac{\partial }{\partial x}\left(k_x\frac{\partial T}{\partial x}\right)+\frac{\partial }{\partial y}\left(k_y\frac{\partial T}{\partial y}\right)+\frac{\partial }{\partial z}\left(k_z\frac{\partial T}{\partial z}\right)+s$$

(1)

where: ρ, c_p, k denote physical properties—density, specific heat, and thermal conductivity coefficient in each layer of the rock formation.

Most often, information about the lithology of layers is available from well logging, as well as from literature [38,39] regarding the variability of these parameters with depth, i.e., in the vertical direction. The source term s allows for the consideration of parameters such as natural heat sources, phase change heat, or formation water in individual layers.

Assuming no chemical reactions or phase changes in the considered heat carrier, modelling in the first domain boils down to solving the Fourier–Kirchhoff equation. In Cartesian coordinates, this equation takes the form:

$${\rho c}_p\frac{\partial T}{\partial t}+{\rho c}_f\mathrm{v}\cdot \nabla \mathrm{T}=\nabla \cdot \left(k_i\nabla T\right)+s$$

(2)

To determine the heat transfer coefficient, the Nusselt number relationship for flow through a tube proposed by Seider and Tate, as well as Hausen [40] was applied. This relationship takes the form of laminar flow (Re < 2100):

$$\frac{hD}{k}=0.116\left[{\left(\frac{DG}{\mu }\right)}^{\frac{2}{3}}-125\right]{\left(\frac{c\mu }{k}\right)}^{\frac{1}{3}}{\left(\frac{\mu }{{\mu }_w}\right)}^{0.14 }\left[1+{\left(\frac{D}{L}\right)}^{\frac{2}{3}}\right]$$

(3)

where D, k, c, G, μ, μ_w represent the inner diameter of the pipe, thermal conductivity coefficient, specific heat, mass flow rate, fluid viscosity, and fluid viscosity at the wall temperature, respectively. The thermal resistance resulting from heat conduction through the wall of the pipe and the filling was determined by the formula:

$$R_k=\frac{1}{2\pi k}\ln \frac{r_o}{r_i}$$

(4)

where r_o, r_i, k denote, for each successive element, the outer radius and inner radius of the pipe, and its thermal conductivity coefficient (material of the pipe). The volumetric flow rate of the circulating heat transfer fluid in the borehole heat exchanger was analyzed in the turbulent flow region.

To solve the model Equations (1) and (2), the control volume method on a Cartesian grid was applied. For the time derivative approximation, a Two-level method was used. The time step was variable. The subdomains were solved iteratively until the required convergence was achieved. The system of linear equations resulting from the discretization was solved using the Conjugate Gradient Squared (CGS) method.

Boundary conditions used during the analysis were as follows:

(a)

$$T\left(0,x,y,z\right)=T_g\left(z\right)$$

(5)

(b) For z = 0

$$\frac{dT\left(x,y,z\right)}{dx}=0$$

(6)

$$\frac{dT\left(x,y,z\right)}{dz}=q(t)$$

(7)

$$T\left(x,y,z\right)=f\left(t\right)$$

(8)

$$q\left(t\right)={\sum }_{i=1}^N{q}_i\left(t\right)$$

(9)

(c) For z = z_max

$$\frac{dT\left(x,y,z\right)}{dz}=qz$$

(10)

where qz denotes the natural earth heat stream at a depth of “z”.

(d) boundary conditions that are considered at the edge of the boundary:

$$\frac{dT\left(x,y,z\right)}{dx}=0$$

(11)

$$\frac{dT\left(x,y,z\right)}{dy}=0$$

(12)

$$T\left(x,y,z\right)=T_g\left(z\right)$$

(13)

$$T_{in}=const or Q_t=const$$

(14)

$$V_{in}=const$$

(15)

(e) at the border between the two simulated zones, it is assumed:

$$q=\alpha (T_f\left(z\right)-T_g\left(z\right))$$

(16)

2. Freezing Borehole Characteristics

Each of the freezing boreholes is equipped with two columns of pipes:

• an outer, steel pipe with a diameter of Φ139.7 × 8 mm, butt-welded (in a gas shield) and terminated with a bottom (the actual freezing pipe),
• an inner, PE pipe with a diameter of Φ85 × 5, with a weight adapted to the depth of the freezing borehole (the casing pipe).

All external columns of pipes were cemented into the rock formation. The scheme of the freezing borehole is presented in Figure 4.

At a depth of 300 m, a cement plug was placed in both the 139.7 mm and 85 mm pipes. The 85 mm pipes were perforated in the interval from 299 to 300 m. The flow of fresh atmospheric air in the shaft occurs through the full diameter (7.5 m) with a capacity of 45,000 m³/min. The barrier is formed by the casing with the construction shown in Figure 2. For design purposes, the average monthly temperatures of atmospheric air were adopted for the nearest meteorological station. Due to the availability of documented data, the temperature profile in the XSA borehole, located about 40 m from the analyzed shaft, was adopted. The profile is presented in

Table 1. Temperature profile adopted for the calculations.

Depth, m	Temperature, °C
84.0	15
106.0	16
156.7	17
187.9	18
204.8	19
237.5	20
247.8	21
293.4	22

. It does not cover Kasuda and Achenbach’s [41] relation because the first value of temperature is significantly below periodic heat transfer temperature.

It was assumed that there was no groundwater movement within the shaft. Depending on the thermal capacity of the rock formation, two types of air conditioning systems inside the facility can be considered: a “classic” system with input/output parameters of 6/12 °C for the working fluid, or a “high-temperature” system with parameters of 16/22 °C. In case it is not possible to provide sufficient cooling power at lower working fluid parameters, the possibility of using the second solution should be considered. In such a case, cooling beams or surface systems absorbing heat can be installed in the facility. Therefore, the option of a higher working fluid parameter on the return, i.e., 22 °C at the inlet to the borehole heat exchanger, was taken into account. Based on the project, it will be possible to select operational parameters and design the borehole heat exchanger system with a typical, reversible geothermal heat pump or a refrigeration unit. Two numerical grids were created:

• with all boreholes,
• excluding boreholes 17, 18, and 19, as well as 37, 38, and 39.

3. Existing Wells

Regardless of the number of heat exchangers adopted, they should be connected in parallel. The most advantageous construction will be a concentric design, due to the fact that it is characterized by low hydraulic losses due to friction and the best energetic efficiency when compared to typical U-pipe configuration. The design is shown in Figure 5.

The analyzed area had dimensions of 150 × 150 × 350 m³ in all directions. The geometry of the analyzed area is depicted in Figure 6. Real, time-varying air temperatures prevailing in the selected region were assumed on the ground surface.

The shaft shown in Figure 2 is present in the analyzed area. This shaft is located at the centre of the computational plane x-y, i.e., at coordinates (75.0, 75.0), and it intersects all layers down to a depth of 350 m. Fresh air flows down the shaft at a rate of 45,000 m³/min and at a temperature identical to the air temperature prevailing at the analyzed location. However, if the incoming air temperature drops below 1 °C, reheating is applied to ensure a minimum inlet temperature of 1 °C. The graph illustrating the changes in air temperature at the shaft inlet throughout the year is presented in the chart in Figure 7.

Thermophysical parameters of the rock were assumed to be constant and referenced to a temperature of 20 °C. The basic parameters for air are presented in

Table 2. Thermophysical properties of air at 20 °C.

Parameter	Symbol	Value
Density	ρ	1.205 kg/m3
Specific heat	Cp	1005 J/(kg·K)
Kinematic viscosity	ν	15.11 × 10−6 m2/s
Thermal conductivity	k	0.0257 W/(m·K)

.

In the analyzed rock mass, 10 or 4 freezing boreholes were considered, arranged in 2 groups of 2 or 5 boreholes. The distance between each pair of boreholes in each group is 1.25 m. The freezing boreholes are located at depths ranging from 3.5 m below ground level to 300 m below ground level. The freezing boreholes were treated as concentric (coaxial) borehole heat exchangers. A water–glycol solution with a concentration of 25% monopropylene glycol flows through these borehole heat exchangers [42] The thermophysical parameters of the solution used in the calculations are presented in

Table 3. Thermophysical properties of monopropylene glycol at 20 °C.

Parameter	Symbol	Value
Density	ρ	1024.85 kg/m3
Specific heat	Cp	3910.5 J/(kg·K)
Kinematic viscosity	ν	2.439 × 10−6 m2/s
Thermal conductivity	k	0.457 W/(m·K)

.

The complex analyses were conducted using the numerical simulator BoHEx. The analyzed area was initially discretized into 180 × 180 × 100 (3.24 million) control volumes, employing an irregular grid with local refinements of 10–100 times near the heat exchangers and the analyzed shaft in order for the model to give a more complex understanding of energy transfer in the borehole itself. The computational grid for the shaft and the borehole heat exchanger, including the filling and considering the thickness of cement sheaths, is illustrated in Figure 8 and Figure 9.

To establish the correct initial temperature of the rock formation, a yearly computational cycle was conducted for the entire analyzed geometry. This assumed the airflow through the shaft with a nominal value of 45,000 m³/min and excluded the flow of monopropylene glycol solution through the borehole heat exchangers. Additionally, for the initial calculations, the temperature profile of the rock formation was based on another nearby shaft, as presented in Table 1.

In all calculations, it was assumed that there was no groundwater movement. An adaptive time step (Δt_max = 3600 s) was used in the calculations, ensuring numerical residuals for velocity and temperature below a value of 10⁻⁶. The initial temperature field in the x-z cross-sectional plane is shown in Figure 10.

Due to all proposed boreholes sharing the same depth and geological conditions, the simulation described up to this point was applied to all boreholes.

Table 4. Input parameters.

Case Number	Characteristics
1	+22 °C at inlet of 4 BHEs
2	+22 °C in months VI-VII, +2 °C in the rest of the year, at the inlet of 4 BHEs
3	+22 °C at inlet of 10 BHEs
4	+22 °C in months VI-VII, +2 °C in the rest of the year, at the inlet of 10 BHEs
5	+22 °C in months VI-VII, −2 °C in the rest of the year, at the inlet of 4 BHEs
6	+22 °C in months VI-VII, −2 °C in the rest of the year, at the inlet of 10 BHEs

contains the input parameters used in the analysis.

4. Results

Table 5. Average powers.

Case Number	Characteristics	Average Power Obtained from Every BHE, W, W/m
		after 1 Year		after 10 Years
		Sum, W	Per Meter, W/m	Sum, W	Per Meter, W/m
1	+22 °C at inlet of 4 BHEs	4994	4.16	4937	4.11
2	+22 °C in months VI-VII, +2 °C in the rest of the year, at the inlet of 4 BHEs	11,249 *	9.37	-	-
3	+22 °C at inlet of 10 BHEs	10,656	3.55	7807	2.60
4	+22 °C in months VI-VII, +2 °C in the rest of the year, at the inlet of 10 BHEs	27,030 *	9.01
5	+22 °C in months VI-VII, −2 °C in the rest of the year, at the inlet of 4 BHEs	-	-	12,170 *	10.14
6	+22 °C in months VI-VII, −2 °C in the rest of the year, at the inlet of 10 BHEs	-	-	28,217 *	9.41

* Average heating power.

contains the average powers obtained from 4 to 10 borehole heat exchangers in the 1st and 10th year of the system’s operation.

The total energy flows are presented in

Table 6. Total energy flows.

Case Number	The Amount of Exchanged Energy with the Geological Formation during the First Year of Operation, GJ.	The Amount of Exchanged Energy with the Geological Formation during the Tenth Year of Operation, GJ.	The Minimum Amount of Exchanged Energy with the Geological Formation over the Course of 10 Years of Operation, GJ. **
1	181	156	1 560
2	354 *	-	-
3	335	98	980
4	851 *	-	-
5	-	383 *	3830 *
6	-	889 *	8890 *

* Sum of two-way heat flows; ** Value of the neighbouring column multiplied by 10.

.

On the basis of literature data, it was calculated that the thermal conductivity of the geological formation at the shaft location, in the depth range from the surface to the bottom of the boreholes, was 1.178 W/(m·K), and the volumetric heat capacity was 1.293 MJ/(m³·K). For calculations, extremely unfavourable (minimum) values of thermal conductivity were adopted. Using the formulas [43]:

$$q=13·\lambda +10=13·1.178+10=25.31\frac{w}{m}$$

(17)

and

$$q=20·\lambda =20·1.178=23.56\frac{w}{m}$$

(18)

it was calculated that the average unit power of a single heat exchanger will be 24.44 W/m. However, this is a value that can be considered for a single borehole heat exchanger [43]. With a greater number of them and additionally a very close distance between them, the unit power will be significantly lower or will only occur for short-term heating loads.

5. Conclusions

1. All freezing boreholes should be utilized. Heat should be exploited in a bidirectional mode (heating/cooling).

2. The applied well construction should have an additional pipe slid onto the existing inner PE column. It should be a larger-diameter pipe, with a wall thickness as large as possible, made of PP, as illustrated in Figure 2. It is more advantageous to use both inner column pipes made of PP, but this increases investment costs (removal of the existing PE pipe). The inter-pipe space of the inner column can be filled with a material with thermal insulation properties. Such a construction should also be the subject of numerical modelling.

3. There is a thermal interaction between the shaft and potential borehole heat exchangers. This stabilizes the power and annual heat quantities after approximately 3 years of system operation. Due to the high airflow in the shaft, the temperature change of its walls is minimal.

4. TRT tests should also be performed on the well up to 300 m with a single and double inner column pipe. This will enable the most accurate calculations under the conditions of a calibrated numerical model.

5. The thermal conductivity of surrounding rock formations plays a vital role in the design process, so whether it is of a sufficiently high value should be considered at the earliest stages of the planning process.

6. The theoretical load capacity of a single borehole heat exchanger is 24.44 W/m. It was determined based on extremely pessimistic values of the thermal conductivity of the drilled rock profile. Such a value cannot be used for design calculations due to the small distance between the wells and their thermal interference.

7. The value of the unit heating power exchanged with the borehole and the heat carrier (glycol solution) was determined to be 10.14 W/m. This corresponds to the situation of case 5 after 10 years of system operation with 4 borehole heat exchangers operating in a cyclical change of the direction of heat flow. This value was obtained based on equally pessimistically assumed thermal parameters of the rock mass. The value for 10 wells is only slightly lower, at 9.41 W/m.

8. The minimum amount of heat exchanged between the rock mass and the heat carrier over 10 years of system operation was determined under these conditions to be almost 9000 GJ.

9. The presented work can be used by owners of future, existing and closed shafts with freezing boreholes. For future boreholes, specific characteristics necessary for good installation quality can be predicted. For existing or abandoned boreholes, it can be taken into consideration to use BHEs.