Feasibility Investigation of Geothermal Energy Heating System in Mining Area: Application of Mine Cooling and Aquifer Thermal Energy Exploitation Technique

Abstract

Mine heat hazards have resulted in large amounts of high-quality coal resources in deep that cannot be mined. The mining industry is paying more and more attention to the extraction and utilization of geothermal energy in mines, while at the same time reducing the underground temperature to realize co-extraction of coal and heat. In addition, coal mines tend to burn large amounts of coal to heat mine buildings and provide hot water for workers’ daily baths, creating operating costs and increasing greenhouse gas emissions. Therefore, it is of great significance to investigate the feasibility of extracting geothermal energy to provide the daily heat load for mines. Currently, there is little research on the feasibility of geothermal energy extraction and utilization in productive mines instead of abandoned mines. In this study, according to the actual situation of Xinhu mine in eastern China, a combined geothermal water system and heat-pump heating system is proposed, aiming to effectively realize mine cooling and geothermal exploitation and utilization. The geothermal storage capacity in the area is analyzed, and an economic analysis is developed. The economic analysis indicates that the main factors affecting the feasibility of the system are the number of mine users, the distance from the geothermal production well to the mine buildings, and the coal price. The research shows that the economic efficiency of the system is better when the heating scale is larger and the distance is smaller. As coal prices rise, the combined geothermal water and heat-pump heating system will be more economical than traditional coal heating. If a mine has 2000 workers, the application of this system can prevent 334.584 t of CO₂ emissions per year.

1. Introduction

Shallow coal resources will be mined out gradually and deep coal-seam mining will become the main source of coal supply [1]. However, deep mines are facing serious heat hazards due to the heat transfer of deep high-temperature rocks and geothermal water [2]. Underground high temperature working environments not only endanger workers’ physical and mental health, but also cause coal seams to be unrecoverable, wasting a lot of high-quality coal resources [3,4]. In addition, mining buildings, such as workers’ dormitories and meeting rooms in industrial complexes, require thermal energy for their daily operations, including heating in winter and heating of bathwater. At present, coal mines generally rely on the burning of coal to provide thermal energy. This process releases a significant number of pollutants into the air [5], such as PM_2.5, which can cause smog and damage public health in mining areas [6]. It also produces harmful gases and greenhouse gases, such as CO₂, NO_x, and SO₂, thus degrading the air quality in the mining area [7]. Moreover, the descent of coal-burning flue gas into adjacent soil leads to pollution from heavy metals or sulfur [8,9], harming the environment of agriculture and the safety of food. At the same time, the coal price is showing an upward trend, so a coal-burning boiler heating program would lead to increasing heating costs, generating an urgent need to find a cleaner and more cost-effective alternative [10]. The clean and renewable energy sources used for heating in mining areas are mainly solar, wind, and geothermal. However, solar and wind energy are highly dependent on weather conditions and are not suitable for places that require stable temperatures or seasonal use, and they often require cooperation with storage facilities for other energy systems. This will inevitably lead to large initial investment and large ground-space occupation, which are not suitable for mining areas with short service periods [11].

Therefore, the mining industry is paying more and more attention to the exploitation and utilization of geothermal energy in mining areas [12,13]. The extraction of geothermal energy can facilitate mine cooling, and geothermal water is a stable and low-enthalpy resource [14] that has great potential for providing thermal energy for the daily heat load of the mine [15].

Numerous academics have conducted studies on geothermal utilization in mines [16]. E. Solik-Heliasz et al. [17] demonstrated that the water pumped out from hard coal mines in the Upper Silesian Coal Basin is characterized by a considerable thermal potential. They proposed an effective method of energy-acquisition in a low-temperature geothermal power plant. To realize the transformation of mine geothermal energy and waste heat from “hazard/waste” into “treasure”, Liu et al. [18] proposed the “PCB” model for the synergetic utilization of waste heat and geothermal and coal resources. Loredo, C. et al. [19] studied the geothermal use of the water stored in closed and flooded mine workings, emphasizing that the temperature of the water flooding from the mine voids is suitable for space heating and cooling by use of heat pumps. Alvarado, E. J. et al. [20] designed a geothermal heat-pump system to evaluate the quantity of geothermal energy retrievable from a mine’s dewatering mechanism. The development of thermal technologies is important for the environment; however, such technologies must be economically viable [21]. Jundika C. Kurnia et al. [22] investigated the potential of converting an abandoned oil well into a geothermal energy power plant, but the Levelized Cost of Electricity (LCOE) of the system was found to be almost double that of conventional geothermal technologies. Durjoy Baidya et al. [23] investigated the technical and financial feasibility of integrating a validated seasonal thermal energy storage with an exhaust heat-recovery system. They developed an economic analysis that looked at vulnerability of fossil fuel prices, heat exchanger effectiveness, and different set temperatures. Leyla Amiri et al. [24] performed a techno-economic assessment of waste heat recovery from data centers using an absorption chiller system, and investigated the feasibility of the system. Rajab, M. M. et al. [25] created a simulation–optimization method based on uncertainty to evaluate the technological and economic viability of CO₂-based geothermal generation in an enclosed reservoir system. Their results proved that economic feasibility is of great significance to mine investment [26]. Previous studies have suggested that heat in mines has great application potential, and using geothermal energy in abandoned mines can reduce drilling costs. However, there are few studies on extracting geothermal energy near coal seams to prevent thermal damage in mines, and few analyses on the economic feasibility of active-mine heat-pump heating schemes. In addition, with respect to the actual conditions of some mines, the application of geothermal energy heating could be more expensive than coal heating during their service lives, but with the rise in coal prices, geothermal energy heating schemes will become feasible, and this change constitutes a research gap.

In such a context, this study proposes a combined mine geothermal water and heat-pump heating system to meet the heat load of the mine by using the thermal energy in the aquifer. The scope of the study includes evaluating the technical and economic feasibility of applying the geothermal reservoir exploitation system to a typical mine in China, discussing the impact on the economic analysis of increasing coal prices, different mine personnel numbers, and distances from the geothermal production well to the mine buildings, so as to provide a reference for the formulation of greener and more economical mine heating schemes.

2. Engineering Background

2.1. Study Area

In order to make the research more realistic, this paper takes the 81 mining area of Xinhu Coal Mine as the study area. Xinhu Coal Mine is located in the eastern part of China and has abundant deep coal resources. As a result of exploration, the main coal seam in mining area 81 is approximately 8 m thick and 930 m deep. Below the coal seam, there is an aquifer with a thickness of 150 m about 130 m from the coal seam. Through investigation and analysis, it was ascertained that the ground temperature of Xinhu Coal Mine increases with the depth of the coal seam, the maximum rock temperature at −600 m level reaches 40.87 °C, and the geothermal water temperature in Xinhu Coal Mine area is abnormal. The volume of water is large, and the maximum water temperature can reach 52 °C when the well is 1000 m deep. The thermal release of the surrounding rock and mine geothermal water leads to a heat hazard in the mine that is directly caused by the current geothermal field of the mining area. The area and depth of the primary (31~37 °C) and secondary heat hazards (>37 °C) [27] in Xinhu Coal Mine are divided in Figure 1 according to the temperature measurement data of the boreholes in the mine. At a depth of 500 m, some areas have primary heat hazards, while at 700 m, all study areas turn into primary heat hazard areas, and secondary heat hazard areas begin to appear. It can be seen that coal mining encounters challenges within this geothermal environment.

Figure 2 shows the mine cooling and aquifer thermal energy exploitation technique. Before the extraction of coal seams, injection and production wells are positioned between the surface and the rock formations below the coal seam, with several injection and production channels set up within the aquifer. To effectively cool the coal seam, cold water is injected from the injection well, and geothermal water is extracted from the production well to the surface for geothermal utilization. After cooling the coal seam to a recoverable temperature (<26 °C), the working surface can be arranged for coal seam mining. For building a water-source heat-pump room on the ground, its placement should minimize the pipeline’s distance in order to reduce the cost of pipeline laying.

2.2. Geothermal Fluid Thermal Reserves

According to the storage capacity method, the total geothermal water storage can be divided into volumetric storage and elastic storage, which can be written as (Equation (1)) [28].

$$Q_n=A\cdot (k\cdot \phi +H\cdot S)/B$$

(1)

where Q_n is natural storage capacity of geothermal water; A is aquifer area, φ is porosity of aquifer rock; k is aquifer thickness; H is the height of pressure head calculated from the roof of aquifer; B is the volume coefficient of geothermal water, generally water density/geothermal fluid density, 1.01 m³/m³; S is elastic storage coefficient, which can be calculated by (Equation (2))

$$S=c\cdot \phi \cdot {\rho }_w\cdot g\cdot k$$

(2)

where c is comprehensive compression coefficient of aquifer and g is gravity acceleration; ρ_w is density of water. For the Paleozoic geothermal reservoir, the comprehensive compression coefficient c is taken as 4 × 10⁻⁴/MPa. Substituting into Equation (2), the S is approximately 7.484 × 10⁻⁵. Thus, the amount of heat stored in water can be found by (Equation (3))

$$Q_t=Q_n\cdot C_w\cdot {\rho }_w\cdot (t-t_m)$$

(3)

where Q_t is the thermal reserves of geothermal water; C_w is specific heat capacity of water; t is temperature of aquifer; t_m is annual mean temperature [29].

Table 1. Calculation parameters of geothermal fluid storage capacity in Xinhu Coal Mine.

Qn (108 m3)	A (108 m2)	k (m)	φ	H (m)	S	B
21.57	1.137	95.46	0.2	959.41	7.484 × 10−5	1.01

shows the calculation parameters of geothermal fluid storage capacity in Xinhu Coal Mine.

Table 2. Calculation result of geothermal fluid thermal reserves in Xinhu coal mine.

ρw (kg/m3)	Cw (kcal/kg·°C)	t (°C)	tm (°C)	Qt (kcal)	Standard Coal (106 t)
1000	1	59.01	17.1	9.04 × 1013	12.9

shows the calculation result. According to Equations (1)–(3), the recoverable thermal reserves of geothermal water in Xinhu Coal Mine are 9.04 × 10¹³ kcal, equivalent to 12.9 × 10⁶ tons of standard coal. As a result, the amount of geothermal water resources in coal field is relatively abundant and the supply of thermal energy is guaranteed.

2.3. Combined Mine Geothermal Water and Heat-Pump Heating System

The common method of recovering heat from geothermal water is to combine a water-source heat pump with an open or closed geothermal cycle development system. Water-source heat pumps can obtain low-grade thermal energy from groundwater, and can transform low-grade thermal energy to high-temperature thermal energy, thereby generating accessible high-temperature thermal energy [30,31]. The geothermal water produced by the geothermal reservoir exploitation system is rich in low-grade energy, and the perennial temperature and displacement of geothermal water are relatively stable, making it an ideal water-source heat pump [32]. Consequently, this section outlines the design of the combined mine geothermal water and heat-pump heating system (Figure 3), primarily consisting of five components: a geothermal reservoir exploitation system, a plate heat exchanger system, a water-source heat-pump system, a heating system for buildings, and a bath water-heating system. The system’s medium for transferring heat is water.

Figure 3 shows the relationship between the systems and the inlet and outlet temperatures of the heat transfer medium. According to the aquifer temperature in the study area, the extracted water temperature is about 40 °C. Due to the high mineral content and unidentified contaminants in geothermal water, fine pipes can easily become scaled or clogged. Therefore, after the geothermal water is pumped out of the aquifer, it flows into the plate heat exchanger through the desander and the well water pressure pump. The inlet and outlet temperatures of the plate heat exchanger system and the water-source heat-pump system are set, which also determines the operating power of the systems. Once the temperature of the water decreases, the geothermal water is filtered through an automatic self-cleaning filter and piped into the injection well. Then the hot water from the heat exchanger flows into the water-source heat-pump system through the recirculation pump, before the thermal energy extracted by heat pump is used for daily hot bathwater and building heating in Xinhu Mine’s cold-weather season. The heating temperature and bathwater temperature are both 45 degrees Celsius [33,34].

3. Financial Analysis

3.1. Mine Heat Load

The time for building heating in the province where the mine is located is typically from November to February of the following year, with heating required over a 3-month period, and the hot bathwater is provided throughout the year. For the calculation of equipment parameters based on the heat load in the building-heating season, the heat load Q can be found using (Equation (4))

$$Q=Q_{\mathrm{rs}}+Q_{\mathrm{gn}}$$

(4)

where Q is heat load in cold-weather season; Q_rs is heat load of heating bathwater; Q_gn is heat load of buildings heating. The thermal energy for heating system for 1 year E can be calculated by (Equation (5))

$$E=Q_{\mathrm{rs}}{T}_{\mathrm{rs}}+Q_{\mathrm{gn}}{T}_{\mathrm{gn}}$$

(5)

where T_rs is duration of heating bathwater; T_gn is duration of buildings heating. Under the Xinhu Coal Mine’s original scheme, the yearly expense for coal heating would be calculated by (Equation (6))

$$C=\frac{E}{q_m}\cdot P_m$$

(6)

where C is coal cost of original scheme; q_m is standard coal calorific value; P_m is coal price. The combined geothermal water and heat-pump heating system avoids this part of the coal cost, meaning C represents the yearly cost reductions.

Assuming that the mining area has approximately n employees and a total water consumption of about 40n kg per day, if the initial and final temperatures of the bathwater are 15 °C and 45 °C, respectively, and the duration for supplying hot water is three hours, the heat load of the hot bathwater can be calculated by (Equation (7))

$$Q_{rs}=\frac{C_w\cdot M_{rs}\cdot \Delta T_{rs}}{3\times 3600}$$

(7)

where M_rs is water consumption; ΔT_rs is bath water temperature difference.

Under the guidelines of the “Code for Design of Coal Industrial Buildings”, considering indoor public spaces, such as canteens and meeting rooms [35], the building heating area in the mining area A_b is 30n m² and the heating thermal index e is 70 W/m², so the building-heating heat load can be written as (Equation (8))

$$Q_{gn}=A_b\cdot e$$

(8)

3.2. Economic Analysis

The initial investment in the combined mine geothermal water and heat-pump heating system consisted mainly of the cost of geothermal wells and ground heat exchanger stations, calculated as (Equation (9))

$$C_{\mathrm{II}}=C_{\mathrm{well}}+C_{\mathrm{P}}+C_{\mathrm{HP}}+C_{\mathrm{B}}$$

(9)

where C_II is initial investment; C_well is cost of geothermal wells; C_P is cost of plate heat exchanger; C_HP is cost of water-source heat pump; C_B is additional cost.

The investment in geothermal wells is mainly divided into the drilling cost, the cost of the water-injection pump, and the production pump, which can be calculated by (Equation (10))

$$C_{well}=C_d+C_{inj}+C_{pro}$$

(10)

$$C_d=H\cdot P_{well}+L\cdot P_c$$

(11)

where C_d is drilling cost; C_inj is cost of water injection pump; C_pro is cost of production pump; H is drilling depth; P_well is cost per meter for drilling geothermal wells; P_c is cost per meter for injection and production channels.

For the heat pump, its full-load heating coefficient of performance, evaporator and condenser heat loads, and compressor power consumption can be calculated by (Equations (12)–(15)) [36]

$$COP=0.00605{\left(T_{\mathrm{con},\mathrm{w} }-T_{\mathrm{eva},\mathrm{w} }\right)}^2-0.511\left(T_{\mathrm{con},\mathrm{w} }-T_{\mathrm{eva},\mathrm{w} }\right)+15.48$$

(12)

$$Q_{\mathrm{eva}}=Q_{\mathrm{P}}$$

(13)

$$Q_{\mathrm{HP}}=\frac{COP}{COP-1}{Q}_{\mathrm{eva}}$$

(14)

$$W=Q_{\mathrm{HP}}-Q_{\mathrm{eva}}$$

(15)

where COP is performance coefficient of water-source heat pump; T_con,w is condenser outlet water temperature; T_eva,w is evaporator outlet water temperature; Q_eva is evaporator heat load; Q_P is plate heat exchanger heat load; W is condenser power.

Studies show that the COP of water-source heat pumps with full load is expressed as a polynomial with only water temperature as the independent variable [37]; that is, its uncertainty is related to T_con,w and T_eva,w, and the sensitivity coefficient is equal to 1. This suggests that changes in the temperature of condenser and evaporator outlet water temperature have the same effect on COP, with a 5% change in temperature causing a 5% change in COP, an effect that is temporarily ignored in this study.

For the plate heat-exchanger heat load, heat transfer area is calculated as follows (Equation (16)) [38]:

$$S_{\mathrm{P}}=\frac{Q_{\mathrm{P}}}{U_{\mathrm{P}}\Delta T_{\mathrm{P}}}$$

(16)

where U_P is heat transfer coefficient of plate heat exchanger; ΔT_P is logarithmic mean temperature difference, which represents the mean integral value of the temperature difference during the heat transfer between the two fluids in the heat exchanger.

$$\begin{array}{l}\Delta T_{\mathrm{P}}=\frac{\Delta T_1-\Delta T_2}{\ln \frac{\Delta T_1}{\Delta T_2}}\\ \Delta T_1=T_1-t_2\\ \Delta T_2=T_2-t_1\end{array}$$

(17)

where T₁ is heat-flow inlet temperature; T₂ is heat-flow outlet temperature; t₁ is cold-flow inlet temperature; t₂ is cold-flow outlet temperature.

The larger the heat exchanger’s heat transfer area, the higher the cost. The cost of the plate heat exchanger can be found by (Equation (18))

$$C_{\mathrm{P}}=S_{\mathrm{P}}\cdot P_{\mathrm{P}}$$

(18)

where C_P is cost of plate heat exchanger; P_P is plate heat-exchanger unit area price.

Additional costs include the cost of pipes and the cost of recirculation pumps. The longer the distance from the geothermal production well to the mine buildings, the longer the pipe to be laid and the higher the cost. Additional cost can be calculated by (Equations (19)–(20))

$$C_B=C_{pipe}+4C_{rpump}$$

(19)

$$C_{pipe}=P_{pipe}\cdot d$$

(20)

where C_rpump is cost of recirculation pump; P_pipe is unit price of pipe per meter; C_pipe is cost of pipes; d is distance from geothermal production well to mine buildings.

The annual operation and maintenance cost of the system C_OM mainly include equipment maintenance cost C_fix, labor cost C_per, and electricity cost C_ele, which are calculated as follows (Equations (21)–(24))

$$C_{\mathrm{OM}}=C_{fix}+C_{per}+C_{\mathrm{ele}}$$

(21)

$$C_{fix}=\gamma C_{\mathrm{TCI}}$$

(22)

$$C_{per}=N_{\mathrm{per} }{S}_{\mathrm{per} }$$

(23)

$$C_{\mathrm{ele}}=\left(E_{\mathrm{ele},\mathrm{HP} }+E_{\mathrm{ele},\mathrm{pump} }\right)P_{\mathrm{ele} }$$

(24)

where γ is equipment maintenance cost coefficient; N_per is number of operating workers; S_per is salary of workers; E_ele,HP is heat pump power consumption; E_ele,pump recirculation pump power consumption; P_ele is electricity prices.

The electricity cost mainly includes the operating electricity cost of the water-source heat pump and each recirculation pump, which is related to the heat demand of the system and distance. The higher the thermal energy demand of the user, the higher the heating power of the system, which thus consumes more electric energy. At the same time, as the distance increases, so does the electricity consumption in providing water to the mine buildings. The electricity cost of the water-source heat pump can be calculated by (Equation (25))

$$E_{ele,HP}=\frac{E_{HP}}{COP}=\frac{\left(1+\rho \right)E}{COP}$$

(25)

where ρ is thermal energy loss coefficient.

The longer the transport distance of the heat transfer medium, the greater the power of the circulating pump and the more energy it consumes. The electricity consumption of recirculation pumps is assumed to be about 0.5d kW h

4. Results and Discussion

4.1. Influencing Factors

The values of the parameters of (Equations (4)–(25)) are determined according to the actual situation of the mine, as shown in

Table 3. The values of the economic parameters [29,38].

Economic Parameters	Value
cp,w, J/(kg·°C)	4180
qm, kWh/kg	8.14
H, m	2350
L, m	3600
Pwell, $/m	114.87
Pc, $/m	138.90
Cinj, pro, $	13,890
CHP, $	138,900
UP, W/(m2·°C)	3000
Tcon,w, °C	45
Teva,w, °C	8
Crpump, $	13,890
Ppipe, $/m	41.67
γ	0.02
Nper, person	5
Sper, $/month	555.60
Pele, $/kWh	0.11
ρ	0.05

. The economic parameter results are shown in

Table 4. The calculation results of the economic parameters.

Economic Parameters	Results
Qrs (kW)	0.46n
Qgn (kW)	2.1n
Q (kW)	2.56n
E (kWh)	5044.6n
C ($)	86.06nPm
Cd ($)	769,985.21
Cwell ($)	797,765.21
COP	4.85
Qeva, QP (kW)	2.0224n
ΔTP (°C)	4
CP ($)	0.0351n
CB ($)	41.67d + 55,560
CII ($)	35.11n + 41.67d + 992,225.21
Cfix ($)	0.70n + 0.83d + 19,844.50
Cper ($)	33,336
Eele, HP (kW h)	1092.13n
Cele, HP ($)	121.36n
Eele, pump (kW h)	0.5d
Cele, pump ($)	0.05556d
COM ($)	122.06n + 0.89d + 53,180.50

.

From the above research, it can be seen that the initial investment and annual operation and maintenance cost in the economic analysis are related to factors such as number of mine users n, distance from geothermal production well to mine buildings d, and coal price P_m. Therefore, this paper takes these three factors as the influencing factors in the economic analysis, and selects levelized cost of heat, net present value [39], and internal rate of return [40] as the evaluation indicators.

Levelized cost of heat (LCOH) is calculated using (Equation (26)) [41,42]

$$\begin{array}{l}{C}_{\mathrm{LCOH}}=\frac{C_{\mathrm{II}}+{\displaystyle \sum _{t=2}^{10}\frac{C_{\mathrm{OM}}}{{(1+i)}^t}}}{{\displaystyle \sum _{t=2}^{10}\frac{E}{{(1+i)}^t}}}\\ \end{array}$$

(26)

where t is the year; i is rate of return, greater than 8% is considered to be an acceptable, which is set at 10%.

The first year is the construction period. The equipment and infrastructure for the heating system are acquired and installed in the first year and put into operation the following year to replace the original coal-burning heating scheme for the mining area. Thus, the net present value (NPV) and internal rate of return (IRR) are as follows (Equations (27) and (28)) [43]:

$$\mathrm{NPV}=-C_{\mathrm{II}}+{\displaystyle \sum _{\mathrm{t}=2}^{10}(C-C_{\mathrm{OM}})\cdot {(1+\mathrm{i})}^{\wedge }{}^{-\mathrm{t}}}$$

(27)

$$-C_{\mathrm{II}}+{\displaystyle \sum _{\mathrm{t}=1}^{10}(C-C_{\mathrm{OM}})\cdot {(1+\mathrm{IRR})}^{\wedge }{}^{-\mathrm{t}}}=0$$

(28)

4.2. Levelized Cost of Heat

LCOH is a function of number of mine users n and distance d (Figure 4). In order to visualize the impact of changes in n and d on the LCOH, the LCOH curve is developed in the ranges of 1000, 2000, 3000, 4000, and 5000 m, respectively (Figure 5).

Figure 5 illustrates that as the number of users n increases from 0 to 4000, there is a consistent reduction in the LCOH, which drops from 0.60 to 0.04 dollars/kWh. The reason is that thermal demand increases when number of users n increases. Thermal energy can better distribute the cost of investment, maintenance, and operation of the heating system, which improves its economic advantages. At the same time, the LCOH curve transitions from a significant decrease to a slow one, meaning the rate of this decline diminishes, and it stabilizes when the number of users n exceeds 2500. Under the condition of distance d, the decline rate of the standardized heating cost curve is different. As the distance d increases, the rate of decrease declines more gradually. The reason is that extending the distance for heating results in more investment in the system, consequently raising the cost per unit of heat. For instance, when the mining area has 500 workers and the distance d increases from 1000 m to 5000 m, the LCOH values are 0.125, 0.129, 0.132, 0.136, and 0.139 dollars/kWh, respectively.

Furthermore, Figure 6 shows the LCOH curves for different numbers of users: 100, 250, 500, 1000, 2000, 3000, 4000, and 5000. As the distance grows, there is an upward trend in the LCOH curves. However, a downward trend is seen in the value of the LCOH when the number of users increases, which is consistent with the previous analysis.

4.3. Net Present Value

Figure 7 displays the contour lines distribution diagram of NPV at coal prices of 0.208, 0.278, 0.292, 0.306, 0.347, and 0.417 dollars/kg, aiming to analyze how coal prices, distance, and number of users impact the system’s NPV over a decade. The number of NPVs exceeding 0 indicates that the system is profitable.

Figure 7 shows that when the coal price is less than 0.278 dollars/kg, the NPV remains negative during the period, regardless of the number of users and distance, indicating that the system is in a loss-making state in this situation and that the original coal-fired heating scheme is more economical. When the coal price exceeds 0.278 dollars per kilogram, a key curve (NPV = 0) begins to emerge, and the range of NPV greater than 0 widens as the price of coal rises. When the coal price is 0.292 dollars/kg, the system is profitable only if the number of users is greater than 4750 and the distance less than 1763 m. When the coal price is 0.306 dollars/kg, the system can realize profit when the number of users is greater than 4643. When the coal price is 0.347 dollars/kg, the system is profitable when the number of users is greater than 3250. When the coal price is 0.417 dollars/kg, the system can achieve profit when the number of users is greater than 2200.

Therefore, it can be seen that with the increase in coal price, the economic competitiveness of the original coal-fired heating scheme is becoming weaker, and the combined mine geothermal water and heat-pump heating system is can more easily realize economic benefits. Above all, the higher the coal price, the more users, and the shorter the distance, the better the economic efficiency of the system.

4.4. Internal Rate of Return

The IRR of the system changes when coal prices are 0.347, 0.417, 0.486, and 0.556 dollars/kg, respectively, as shown in Figure 8.

The variables n and d on line IRR = 0 ensure that the application of the heating system does not cause economic damage without economic risk, while minor economic hazards, such as currency devaluation and increase in loan interest rates, result in losses, making this line a critical value. Areas showing IRR < 0 indicate that the system is not risk-resistant and has been in a loss state, at which point the heating system is not feasible. An IRR > 0 indicates that the system is profitable. The larger the IRR, the higher the system’s ability to resist economic risks.

When the coal price is 0.347, 0.417,0.496, and 0.556 dollars/kg, the critical values of variable n at which the heating system can make profits are 3284, 2256, 1660, 1328, respectively. As the number of users increases and the distance decreases, IRR gradually increases, indicating that the system’s resistance to economic risks increases during the operation period. In conclusion, consistent with previous analysis, the higher the coal price, the more users, and the smaller the distance, the larger the profit margin of the heating system and the greater its economic risk resistance.

4.5. Environmental Performance

The application of the combined mine geothermal water and heat-pump heating system can effectively prevent a large number of harmful gas emissions from coal burning, which can be calculated by (Equations (29)–(31)) [44,45]

$$A_{C{O}_2}=\frac{E}{q_m}\cdot A_m$$

(29)

$$A_{N{O}_{\mathrm{x}}}=\frac{E}{q_m}\cdot A_{mNO}$$

(30)

$$A_{S{O}_2}=\frac{E}{q_m}\cdot A_{mSO2}$$

(31)

where A_CO2 is reduced carbon dioxide emissions; A_m is the amount of CO₂ produced by burning 1 kg of coal (about 2.7 kg/kg). A_NOx is reduced nitrogen oxide emissions; A_mNO is the amount of NO_x produced by burning 1 kg of coal (about 0.024 kg/kg). A_SO2 is reduced sulfur dioxide emissions; A_mSO2 is the amount of SO₂ produced by burning 1 kg of coal (about 0.007 kg/kg). Therefore, the application of the combined mine geothermal water and heat-pump heating system instead of the coal-fired boiler heating scheme can prevent the emission of 1669.36n kg carbon dioxide, 14.84n kg nitrogen oxide, and 4.33n kg sulfur dioxide per year.

4.6. Economic Performance

Based on the economic analysis of the combined mine geothermal water and heat-pump heating system, when the heating demand, heating distance, and coal price are known, the feasibility of deploying the heating system in the mine can be assessed based on LCOH, NPV, and IRR.

If the mine has 2000 workers, the distance from production well to mine buildings is 1500 m, and the coal price is 0.417 dollars/kg, the combined mine geothermal water and heat-pump heating system is more cost-effective. The economic performance of the system is shown in

Table 5. The economic performance of case for the combined mine geothermal water and heat-pump heating system.

Property	Value	Unit
Heat Demand	10,089,200	kWh/year
LCOH	0.051	$/kWh
Net Present Value	12,402.709	$
Internal Rate of Return	10.225	%
CO2 Emissions Reduction	333.872	tons/year
NOx Emissions Reduction	2.968	tons/year
SO2 Emissions Reduction	0.866	tons/year

. In this case, mine heating used about 10,089,200 kWh/year, the LCOH was 0.051 dollars/kWh, and the NPV was positive, suggesting that the system’s application would save mine electricity consumption compared to coal heating, for a total of 12,402.709 dollars over 10 years. The IRR of 10.225% indicates that the system has a certain ability to resist economic risks and has scope for profitability.

In addition, if NPV and IRR are negative in some cases, this means the heating system is temporarily unsuitable for the mines and the coal-fired boiler scheme is more economical for heating. However, the system will become feasible in the future with the rise in coal prices, as demonstrated by this study.

5. Conclusions

The research presented in this paper investigated the techno-economic feasibility of the combined mine geothermal water and heat-pump heating system, which uses geothermal water extracted from aquifers to exploit thermal energy through a water-source heat pump for heating buildings and bathwater in mining areas. This study established a geothermal well and heat-pump system before mining, then calculated the economic index of the system according to the service life of mine, and made an innovative analysis of the feasibility changes in the system caused by coal prices. In this paper, geothermal fluid thermal reserves in the study area were calculated, an economic analysis for the application of the system was established, and influencing factors were studied. By those means, the recoverable thermal reserves of geothermal water in Xinhu Coal Mine were found to be 9.04 × 10¹³ kcal, equivalent to 12.9 × 10⁶ tons of standard coal, showing sufficient heat supply capacity. Moreover, the economic analysis showed that when coal price increases gradually, the combined mine geothermal water and heat-pump heating system will be a more cost-effective alternative instead of the original coal burning heating scheme. The higher the coal price, the more users, and the shorter the distance, the better economic efficiency of the system and the greater the economic risk resistance. In addition, with the application of the combined mine geothermal water and heat-pump heating system, a significant quantity of CO₂, NO_x, SO₂ emissions can be reduced. In a mine with 2000 employees, annual CO₂, NO_x, and SO₂ emissions can be reduced by 333.872, 2.968, and 0.866 tons, respectively. Therefore, it was suggested that the use of the combined mine geothermal water and heat-pump heating system has important economic and environmental benefits. Furthermore, in the economic analysis of this study, the influence of geothermal well depth and seasonal variation in pumping volume on the geothermal well pump’s power has was studied in depth, suggesting future research could enhance this aspect. Regarding the geothermal reservoir exploitation system, there is a need for more research into how the design of geothermal boreholes and the pressure of water injection impact the underground environment and the working surface.