Research Progress of Heat Damage Prevention and Control Technology in Deep Mine

Abstract

As mine mining extends to greater depths, the challenge of heat damage in high-temperature and high-humidity deep mines has emerged as a significant obstacle to the safe mining of deep mines. This paper reviews the causes of mine heat damage, evaluates heat damage mechanisms, and explores deep mine cooling technologies. Traditional deep mine cooling technologies employ mechanical refrigeration to cool air. While these technologies can mitigate heat damage, they are associated with issues including high energy consumption, insufficient dehumidification, and significant cold loss. To address the high energy consumption and fully utilize geothermal resources, heat pump technology and combined cooling, heating, and power technology are employed to recover waste heat from deep mines, thereby achieving efficient mine cooling and energy utilization. To enhance the effectiveness of air dehumidification, the integration of deep dehumidification with mine cooling technology addresses the high humidity ratio in mine working faces. To enhance the refrigeration capacity of the system, liquid-phase-change refrigeration technology is employed to boost the refrigeration capacity. For the future development of deep mine cooling technology, this paper identifies four key directions: the integration of diverse technologies, collaboration cooling and geothermal mining, deep dehumidification and cooling, and intelligent control.

1. Introduction

Mineral resources are among the most abundant and widely distributed energy resources globally, playing pivotal roles in economic development, energy security, and industrial production [1,2]. With the gradual depletion of shallow energy and mineral resources, mine mining has progressively extended to greater depths [3]. Currently, the maximum mining depth in China is 1500 m, with over 130 mines exceeding 700 m in depth [4]. In China, shallow coal resources constitute less than 20% of the total reserves whereas coal resources below 1000 m account for 80%. Deep mineral resources are abundant, significantly enhancing the stability and sustainability of energy supply and representing the principal direction for future mine development. During deep mine mining, heat damage problems become particularly pronounced. As indicated by statistical data, when mining depths reach 1000 m, the surrounding rock temperatures can reach 35 °C to 45 °C, with geothermal gradients increasing by 1 °C to 3 °C per 100 m. The mining depths and surrounding rock temperatures of typical deep mines in China are shown in Figure 1a and Figure 1b, respectively. It can be observed that an increase in the mining depth in deep mines results in higher temperatures of the surrounding rock.

As the mining depth increases, the temperature and humidity ratio of the mine working face also increases. Studies have demonstrated that air temperatures in certain deep-mine working faces exceed 40 °C, with a relative humidity exceeding 90% [5,6]. The challenge of mine heat damage has significantly impeded the safe mining of deep mines, which has had a negative impact on the safety of miners and the efficient operation of mine equipment [7]. According to the ‘Coal Mine Safety Regulations’, the air temperature at production mine working faces must not exceed 26 °C. Operations must cease if the working face air temperature exceeds 30 °C [8]. The ‘Safety Regulations for Metal and Non-Metallic Mines’ stipulates that the wet bulb temperature in personnel’s continuous workplaces must not exceed 27 °C. Operations should be halted if the wet bulb temperature exceeds 30 °C [9]. Consequently, addressing the challenges of high-temperature and high-humidity environments in mines is imperative for advancing deep mining technology.

Deep-mine heat damage has emerged as a significant constraint to mine production safety and occupational health. Traditional mine cooling technologies depend on the vapor compression refrigeration cycle, which utilizes refrigeration units to cool the airflow [10,11]. To achieve energy conservation and carbon reduction, sustainable mining technologies are needed to mitigate environmental impacts. Consequently, there is a pressing need to develop innovative, cost-effective, and non-traditional deep mine cooling and geothermal technologies for the efficient, energy-saving, and environmentally friendly extraction of deep mineral resources.

The causes of the main heat sources in deep mines and the damage caused by high-temperature and high-humidity conditions are summarized. The evaluation index for mine cooling technology is given. The development of deep mine cooling systems is reviewed, the technical characteristics of traditional deep mine cooling methods are analyzed, and novel green-energy-saving mine cooling technologies are summarized, including deep-mine heat damage resource utilization systems; combined cooling, heating, and power (CCHP) technology; deep dehumidification and cooling technology; liquid-phase-change refrigeration technology; and heat pipe cooling technology. Building on this foundation and focusing on the trend toward high-efficiency and low-cost cooling technologies, this paper outlines four key future directions for deep-mine heat damage control: the integration of diverse technologies, collaboration cooling and geothermal mining, deep dehumidification and cooling, and intelligent control. These directions offer innovative strategies for mitigating deep-mine heat damage and provide a framework for achieving sustainable and safe deep mining.

2. Causes and Hazards of Deep-Mine Heat Damage

Deep-mine heat damage is a significant factor affecting both safety production and miners’ physical and mental well-being. A comprehensive analysis of the causes and hazards of mine heat damage has revealed its complexity and diversity.

2.1. Causes of Deep-Mine Heat Damage

The primary heat sources that cause deep-mine heat damage include surrounding-rock heat dissipation, production equipment heat dissipation, air compression heat dissipation, and others.

(1)

Surrounding-rock heat dissipation: Surrounding-rock heat dissipation is a significant contributor to mine heat damage. The geothermal gradient is a key factor that influences the surrounding rock temperature. As the mining depth increases, the surrounding rock temperature also increases, leading to an increase in thermal discharge into roadways [12]. The heat within the surrounding rock is transferred to the roadway wall via conduction. When the airflow enters the roadway, convective heat transfer occurs along the wall. Owing to the temperature difference between the airflow and the surrounding rock, the mine airflow temperature increases [13]. In deep mines, surrounding-rock heat dissipation is the primary cause of elevated temperatures.

(2)

Production equipment heat dissipation: Electromechanical equipment heat dissipation refers to the release of energy as thermal energy during the conversion of electrical energy to mechanical energy during mechanical operations, thereby increasing the temperature of the mine environment.

(3)

Air compression heat dissipation: Air compression heat dissipation is attributed to pressure changes as air flows through the mine owing to varying depths. Pressure causes air compression, converting potential energy into heat and increasing the enthalpy [14]. As the mining depth increases, the air compression intensifies, leading to a substantial increase in temperature. In a 1000 m deep mine, potential energy changes can increase the air temperature by 10 °C when the air is delivered to the working face.

(4)

Other factors: Other factors such as mineral oxidation and mine water heat dissipation also contribute to mine heat damage. Mineral oxidation involves exothermic chemical reactions between ores and oxygen during mining, producing oxides and releasing heat [15]. This process not only elevates the working face temperature but may also induce spontaneous ore combustion, posing significant safety hazards. Mine water heat dissipation occurs as mine water transfers heat to the working face air via heat exchange, thereby increasing the ambient temperature [16]. In deep mines, which are affected by geothermal gradients, the mine water temperatures are typically higher. Additional heat sources in mines include blasting, filling material heat release, and human body heat dissipation, all of which further increase air temperatures.

Figure 2 shows the proportion of various heat sources contributing to mine heat damage in typical mines. As depicted in Figure 2a, in the Sanshandao Gold Mine, Shandong Province, China, other heat sources, including mineral oxidation and mine water, accounted for 35.9% of the total mine heat, and the surrounding-rock heat dissipation constituted 30.5%. Figure 2b shows that the surrounding-rock heat dissipation contributed 52.6% of the total mine heat in the Xiadian Gold Mine, Shandong Province, China, making it the primary factor causing high temperatures. To address deep-mine heat damage, targeted technical designs should focus on predominant heat sources to ensure effective thermal management and safe mining.

2.2. Hazards of Deep-Mine Heat Damage

Deep-mine heat damage manifests as high-temperature and high-humidity air conditions on the working face, negatively impacting the production process. The hazards primarily affect four key areas: the surrounding rock, working environment, supporting structure, and other hazards. Figure 3 illustrates the relationship between the causes of mine heat damage, their impact targets, and the resulting heat damage.

2.2.1. Surrounding Rock

The impact of mine heat damage on the surrounding rock is evident in its physical and chemical properties as well as in its stability [19]. First, high temperatures induce thermal expansion in the surrounding rock, thereby generating internal thermal stress. When the strength limit of the surrounding rock is exceeded, fractures or deformations may occur. Second, the strength of the surrounding rock diminishes at high temperatures, including both compressive and shear strengths, thereby increasing the likelihood of roof falls, spalling, and other accidents. Third, in deep mining, the combined effects of high temperatures and elevated ground stresses can readily induce dynamic disasters, such as rock bursts, posing significant threats to mining safety. Wang et al. [20] investigated the impact of high-temperature environments on the thermophysical properties and mechanical parameters of surrounding rock. The simulation results indicated that as the temperature increased, the specific heat capacity of the rock increased whereas the thermal conductivity decreased. Temperature significantly affected the damage variables of the surrounding rock. Li et al. [21] examined the effects of temperature on the physical and mechanical properties of sandy mudstone. When the temperature increased from 25 °C to 400 °C, the peak stress and elastic modulus of the sandy mudstone gradually diminished. Elevated temperatures reduced sandstone cohesion and induced macroscopic crack formation.

2.2.2. Working Environment

The adverse effects of mine heat damage on the working environment are primarily manifested as high-temperature and high-humidity conditions at the working face, which negatively impact miners’ health, work efficiency, and the service life of electromechanical equipment. The impact of the ambient temperature on human physiological performance is shown in Figure 4. In high-temperature environments, critical physiological functions such as body temperature regulation are disrupted, leading to elevated body temperatures, tachycardia, and hypertension [22]. Prolonged exposure to high-temperature and high-humidity environments predisposes miners to heat fatigue, heat stroke, and heat exhaustion, with severe cases potentially resulting in fatalities [23,24]. Ismail et al. [25] investigated the physiological responses of workers to high-temperature and high-humidity environments, indicating that significant impacts on heart rate and maximal oxygen consumption occurred. Prolonged work under these conditions leads to miner fatigue and weakness, thereby reducing work efficiency and safety [26]. Additionally, such environments accelerate equipment wear and aging, thereby increasing the failure rates.

2.2.3. Supporting Structure

During mining, bolt, concrete, and steel frame supports are critical for ensuring safe operations. In high-temperature environments, the bonding force between the bolts and surrounding rock diminishes, and the surrounding rock deformation increases [28]. This reduces the bolt anchoring strength, thereby compromising roadway stability. Additionally, under high-temperature and high-humidity conditions, metal support structures, such as bolts and steel frames, are susceptible to corrosion, reducing their load-bearing capacity [29,30]. Mctyer [31] investigated the impact of temperature on the anchorage performance of resin bolts and found that under sustained high temperatures, the resin bolt stiffness decreased, and the peak load capacity was reduced by 20%.

2.2.4. Other Hazards

High-temperature mine heat damage significantly exacerbates coal spontaneous combustion and gas accidents. As the mining depth increases, the rising stratum temperatures cause coal body fracturing and enhance surface activity, accelerating coal oxidation and increasing the risk of spontaneous combustion, thereby posing significant safety hazards. Experimental analysis by Qin et al. [32] revealed that increasing stratum temperatures lower the characteristic temperature and ignition points of lignite, thereby accelerating spontaneous combustion processes. Under the coupled effects of temperature and pressure in high-temperature rock strata, the surrounding rock strength diminishes, inducing gas outbursts. Under subjected to high-temperature and high-pressure conditions, gas reacts with coal or surrounding rock chemicals, producing hazardous compounds. Additionally, high-temperature conditions serve as a potential ignition source for gas explosions, severely impacting mine safety. Zhang [33] investigated gas adsorption and desorption rates under varying temperature and pressure conditions. Higher experimental temperatures and pressures led to increased gas desorption rates.

3. Deep Mine Cooling Technology

Deep mine cooling technology is a highly effective measure for mitigating deep-mine heat damage. The primary objectives are to enhance miner safety and comfort, boost production efficiency, and ensure operational stability. Here, traditional refrigeration cooling technology is reviewed and the advantages and disadvantages of traditional mine cooling technology are analyzed. Given the limitations of traditional cooling technologies, green mine cooling technology is summarized, and cooling technology with development potential is introduced and analyzed.

3.1. Evaluation Index

To comprehensively evaluate the performance of mine cooling systems,

Table 1. Evaluation index of mine cooling system.

Performance	Index	Equation		Meaning	Reference
Cooling performance	System temperature difference	$∆T=(T_{\mathrm{i}\mathrm{n}}-T_{\mathrm{o}\mathrm{u}\mathrm{t}})$	(1)	Air temperature difference between inlet and outlet.	[34]
	Cooling capacity value	$K={\displaystyle \frac{m_{a}L∆T}{1000\times 1000\times {∆T}_{\mathrm{d}}}}$	(2)	Cooling capacity of the system.	[35,36]
	Unit cooling investment value	$K_1={\displaystyle \frac{E}{K}}$	(3)	The investment required to reduce the air temperature by 1 °C.	[35,36]
Dehumidification performance	System humidity ratio difference	$∆d=d_{\mathrm{i}\mathrm{n}}-d_{\mathrm{o}\mathrm{u}\mathrm{t}}$	(4)	Air humidity difference between inlet and outlet.	[34]
	Dehumidification capacity value	$M={\displaystyle \frac{m_a{q}_{\mathrm{w}}∆d}{1000\times 100\times {∆d}_{\mathrm{d}}}}$	(5)	Dehumidification capacity of the system.	[35,36]
	Unit dehumidification investment value	$M_1={\displaystyle \frac{E}{M}}$	(6)	The investment required to reduce the air humidity ratio by 1 g/kg.	[35,36]
Overall system performance	Refrigeration capacity	$Q_c=m_{\mathrm{a}}(h_{\mathrm{i}\mathrm{n}}-h_{\mathrm{o}\mathrm{u}\mathrm{t}})$	(7)	The system refrigeration capacity.	[37]
	Coefficient of performance (COP)	$COP={\displaystyle \frac{Q_c}{W}}$	(8)	The ratio of refrigeration capacity to total energy consumption.	[34]

presents a multi-dimensional evaluation index system. The system carries out quantitative evaluation from three aspects: cooling performance, dehumidification performance, and overall system performance. It not only covers single-performance indicators, such as cooling capacity and dehumidification capacity, but also realizes the collaborative evaluation of technical feasibility and economic rationality by coupling cooling/dehumidification efficiency with economic cost parameters (such as resource consumption per unit cooling capacity/dehumidification capacity), providing a quantitative basis for the comprehensive evaluation of mine cooling systems.

3.2. Traditional Refrigeration Cooling Technology

Refrigeration cooling technology lowers the temperature of the mine working face via mechanical refrigeration, enhances mine working conditions, and boosts production efficiency and safety. Based on the type of refrigerant used, refrigeration cooling technology can be categorized into water-cooled, ice-cooled, and air-cooled systems.

3.2.1. Water-Cooled Refrigeration Cooling Technology

The fundamental principle of water-cooled refrigeration cooling technology is the vapor compression refrigeration cycle. The refrigeration unit supplies chilled water that is transported via pipelines to an air cooler at the mine working face. Then, it exchanges heat with the high-temperature and high-humidity air, achieving the cooling and dehumidification of the mine environment [38,39]. Based on the installation location of the refrigeration unit, water-cooled refrigeration cooling technology is categorized into the ground centralized refrigeration cooling system (GCRCS), underground centralized refrigeration cooling system (UCRCS), and ground heat removal underground refrigeration cooling system (GHR&URCS) [40].

The GCRCS places the refrigeration unit on the ground, where high-pressure chilled water is converted to low-pressure chilled water via a high-low pressure converter. The chilled water is then transported via pipelines to the mine air coolers, achieving the cooling and dehumidification of the mine airflow. On one hand, the high-low pressure converter reduces the pressure of the high-pressure chilled water generated at the ground and transports it to the air cooler. On the other hand, it uses the potential energy to send the low-pressure hot water after the underground heat absorption back to the ground. This system employs a cooling tower to dissipate the condensation heat on the ground. Figure 5 presents a schematic of the GCRCS. Zhang [41] implemented a GCRCS at the WBM Coal Mine in Indonesia, successfully controlling the mining face temperature below 26 °C and achieving effective mine cooling. The ΔT of the system was 12 °C. To address the heat damage at the Shoushan No.1 Coal Mine in China, the GCRCS achieved ΔT of 11 °C and Δd of 17.5 g/kg, yielding satisfactory cooling results [42]. The GCRCS’s ground-based refrigeration unit facilitates condensation heat dissipation and simplifies subsequent management. However, significant cold loss occurs during transport, leading to high operating costs [43,44].

The UCRCS positions the refrigeration unit underground and directly produces chilled water on-site. Chilled water is transported via pipelines to the working face air cooler, facilitating the cooling of the mine airflow. Figure 6 shows a schematic of the UCRCS. Yan et al. [45] developed an UCRCS for mine applications. Field tests demonstrated that the ambient temperature of the working face was maintained below 26 °C with a relative humidity of approximately 80%. The ΔT of the system was 6 °C, and Δd was 13.7 g/kg. Zhang [46] applied the ZLS-3300 refrigeration unit in the Xiaoyun Coal Mine in China using a UCRCS. The air supply temperature was maintained below 26 °C, and a ground cooling tower was employed to dissipate the condensation heat. The UCRCS features short cooling transmission distances and minimal cold losses. However, the installation of refrigeration units underground complicates condensation heat dissipation. Fresh air, mine water, or return air is commonly used to expel condensation heat from deep mines [47].

The GHR&URCS integrates the advantages of both the GCRCS and the UCRCS. The refrigeration unit is positioned underground. The generated cold energy is transported to the working face air cooler to cool the mine airflow whereas condensing heat is directed to a ground cooling tower for dissipation [48]. Figure 7 shows the principle of the GHR&URCS. This system enables the efficient condensation of heat emissions via ground cooling towers, minimizing cold loss. However, an increased distance between the cooling tower and refrigeration unit can lead to a loss of cooling capacity in the cooling water circuit.

3.2.2. Ice-Cooled Refrigeration Cooling Technology

The ice-cooled refrigeration cooling system utilizes the latent heat of ice melting to cool the mine airflow. Solid ice or ice–water mixtures prepared on the ground are transported to underground ice melting pools for dissolution. The resulting low-temperature water is then conveyed to the working face air cooler to cool the mine airflow [49]. This system comprises four key steps: ice making, ice transportation, ice melting, and ice utilization. The principle of the ice-cooled refrigeration cooling system is illustrated in Figure 8.

Bu et al. [50] implemented natural ice storage technology to mitigate mine heat damage in southwestern Shandong Province by storing ice during the winter for summer mine cooling. Compared with the UCRCS, natural ice cooling systems reduced costs by 19.5% and enabled low-grade energy recovery. Yuan [51] utilized ground-based ice-making and cooling technology to solve heat damage in the Wobei Coal Mine in China. Tests demonstrated that under equivalent cooling loads, the underground ice transport volume was one-quarter of that of water. Ice-cooled refrigeration systems offer higher efficiency owing to the greater latent heat, stronger refrigerating capacity, and ease of ice storage and transportation, which help address refrigerating-capacity supply–demand imbalances. However, ice-cooled systems face challenges such as pipeline blockages and secondary icing during transport, which can compromise system stability.

3.2.3. Air-Cooled Refrigeration Cooling Technology

Air-cooled refrigeration cooling technology employs air as a refrigerant and compresses it into a high-temperature and high-pressure gas in the ground. This gas is cooled to a normal temperature and high-pressure liquid in an air cooler and then transported underground in liquid form. After passing through a downhole expander, the refrigerant is converted to low-temperature air, which delivers cold energy to the mine working face [52]. The principle of the air-cooled refrigeration cooling system is illustrated in Figure 9. Hu et al. [53] applied air-cooled refrigeration cooling technology to coal mines in Bangladesh, achieving a reduction of 3 °C to 4 °C in mine airflow temperature. Air-cooled refrigeration cooling technology offers advantages such as safety, reliability, and environmental friendliness owing to the use of air as a refrigerant. However, the low refrigeration efficiency of air results in high investment and operational costs, necessitating large compressors and expanders and imposing specific requirements on underground space and transportation conditions.

Conventional mine cooling technology can alleviate the problems of high temperature and high humidity in deep mines; however, several notable challenges persist, including high energy consumption, difficulty in condensation heat dissipation, insufficient dehumidification capacity, and significant cold loss during long-distance transportation. This is because mine cooling systems require substantial refrigeration capacity and low evaporation temperatures, which result in diminished energy efficiency and elevated energy consumption [11].

3.3. Green Mine Cooling Technology

Conventional mine cooling technology can mitigate the problem of mine heat damage. However, several notable shortcomings remain. In accordance with the overarching direction of sustainable energy development, it is imperative to investigate novel cooling technologies that not only fulfill the requirements for cooling and dehumidification but also exhibit characteristics of environmental friendliness, energy efficiency, and sustainability.

3.3.1. Deep-Mine Heat Damage Resource Utilization Systems

The heat damage associated with deep mines constitutes a form of geothermal resource, primarily manifested through mine water, exhaust air, and surrounding rock. Integrating heat damage mitigation with geothermal utilization contributes to sustainable energy development and addresses the high-energy consumption challenges of conventional mine cooling systems. Three types of deep-mine heat damage resource utilization systems are introduced below: the mine water heat pump system, mine exhaust air heat pump system, and surrounding-rock heat energy utilization system.

Mine heat pump systems leverage their evaporators to absorb heat for mine cooling and utilize their condensers to release heat, thereby enhancing the thermal energy of the deep mine water inflow and exhaust air. This dual functionality enables the system to supply heat for district heating, domestic hot water, and wellhead ant-freezing, achieving simultaneous cooling and heating supply while significantly improving the energy utilization efficiency.

Deep mine water is a viable geothermal resource. The mine water temperature remains relatively stable throughout the year owing to the constant temperature within a mine. This resource is characterized by a high water temperature, significant heat capacity, and recyclability [54]. He et al. [55] proposed a deep mine cooling system, that is, a high-temperature exchange machinery system (HEMS), which utilizes mine water as a cold source for cooling. The HEMS comprises a cooling unit, pressure conversion unit, heating unit, and additional components [56]. The principle of the system is illustrated in Figure 10. The low-temperature chilled water produced by the refrigeration workstation (HEMS-1) is passed through a plate heat exchanger inside the pressure conversion workstation (HEMS-PT), where it underwent pressure conversion and heat exchange with the high-temperature return water from the cooling workstation (HEMS-2). The chilled water, after temperature reduction, is then used to cool the mine airflow via the air cooler of HEMS-2. The heat extracted by the HEMS serves as a heat source for ground heating and bathing facilities. The HEMS was used for air cooling at the working face of the Jiahe Coal Mine in China. The test results indicated that the working face temperature ranged between 26 °C and 29 °C. The results showed that the K value of the system was 1.5 and the M value was 3.5. Compared with conventional cooling systems, this system represented a temperature reduction of 4 °C to 6 °C and a relative humidity reduction of 3.6% to 17% [57]. This system effectively cools and dehumidifies the air at the working face, thereby enhancing the thermal comfort of miners. Han et al. [58] conducted an extensive investigation of the HEMS and revealed that at the working face of the Zhangshuanglou Coal Mine in China, when the air cooler outlet temperature was maintained at 18 °C with a relative humidity of 80%, some miners still reported discomfort. This study identified that the relative humidity of air within the working face was a critical parameter that influenced thermal comfort. Thermal comfort for all miners was achieved when the relative humidity of the air was maintained between 60% and 75%.

Gao et al. [59] introduced a heat pump integrated system capable of adapting to variable working conditions for both heat storage and cooling. The ground heat exchanger absorbed condensation heat to warm the bathing water while the evaporator of the water-source heat pump (WSHP) supplied refrigeration capacity to regulate the air temperature within the mine working face. The research findings indicated that compared with the GCRCS, the system achieved an energy-saving rate of 26.2% in its refrigeration cycle, corresponding to an energy savings of 321.48 ton of standard coal equivalent. The principle of the WSHP mine cooling system is illustrated in Figure 11. Feng et al. [60] introduced a coupled mine cooling and WSHP system utilizing mine water as both a cooling and heating source. In summer, chilled water was produced for air cooling, whereas in winter, low-temperature hot water was generated for wellbore antifreeze. The measurement results indicated that this system reduced the air temperature by 3–5 °C and relative humidity by 10–15%. The application of heat pump technology in mine cooling not only reduces the air temperature at the working face but also supplies thermal energy, thereby enabling simultaneous cooling and heating services while promoting cascaded energy utilization [61].

Deep-mine exhaust air contains abundant recoverable thermal energy, characterized by a high temperature and high humidity, with significant annual air volume temperatures. The mine-exhaust air source heat pump system cools the inlet fresh air and directs it to the mine working face. Subsequently, the upgraded exhaust air is repurposed for domestic hot water heating in the mining district, thereby enhancing the energy utilization efficiency [62]. Liu [63] proposed using heat pump technology to recover heat from mine exhaust air. This included heat exchange between the mine exhaust air and refrigerant in the condenser to generate high-temperature air for ground-building heating and bathing water. Simultaneously, the system enhanced the heat absorption from cold water using the refrigerant in the evaporator, producing low-temperature water for mine air cooling.

The coordinated cooling technology of surrounding-rock geothermal mining can solve the problem of deep-mine heat damage. Geothermal resources residing within the surrounding rocks of deep mines are abundant and possess significant recyclability. Harnessing the geothermal energy of the surrounding rock can effectively lower the temperature of deep mines, thereby reducing the thermal load on the working face and decreasing the working face temperature. Xu et al. [64] introduced a deep-mine geothermal-energy collaborative mine cooling technology aimed at mitigating mine heat damage while utilizing geothermal resources. The principle of the system is illustrated in Figure 12. The system employs separate injection and regenerative wells. Cold water is used as a cooling medium. Via the injection well, heat exchange occurs between the cold water and high-temperature mine airflow. This process simultaneously reduces the air temperature while increasing the water temperature. The extracted hot water is then conveyed to the ground through a regenerative well for thermal application. To further investigate this technology, Xu et al. [65] conducted experimental tests and numerical simulations focusing on the water injection temperature, time, and rate. Lowering the water injection temperature, extending the injection time, and increasing the injection rate enhanced the refrigeration efficiency. The application of downhole water-injection cooling combined with synergistic geothermal mining technology may destabilize underground production and significantly disrupt the thermo-hydro-mechanical (THM) coupling process. Zhang et al. [66] assessed the impact of geothermal energy extraction on mine temperature and stability. By developing a THM mathematical model to simulate mechanical stability and temperature distribution, the results demonstrated that under favorable geological conditions, optimizing system design parameters enhanced geothermal energy extraction and reduced mine temperatures.

3.3.2. Combined Cooling, Heating, and Power Technology

To address the excessive energy consumption of conventional deep mine cooling systems, CCHP technology has been implemented, enabling simultaneous heating, electricity generation, and cooling. This technology harnesses high-grade fuel energy for electricity generation whereas low-grade residual heat is utilized for heating and refrigeration, thereby enabling cascaded energy utilization [67]. The principle of the CCHP system is shown in Figure 13. During the process of mine extraction, heat damage issues arise from high gas concentrations and temperatures. Following the principles of energy recovery and utilization, gas serves as a fuel for electricity generation, and the generated waste heat drives a lithium bromide absorption chiller to produce cooling energy, thereby reducing the working face air temperature. Additionally, waste heat can be utilized to generate hot water for domestic heating [68]. Yang et al. [69] implemented a CCHP technology to effectively control gas hazards and high-temperature environments in the Huainan mining area in China. The results demonstrated that this approach reduced the working-face air temperature by 5 °C. Relative humidity was reduced by 20% and 0.6 MPa of saturated steam was generated. This steam was used to drive a lithium bromide absorption chiller and provide domestic heating, thereby enhancing the energy utilization efficiency. Yang et al. [70] employed a lithium bromide absorption chiller to cool mine air, with a WSHP system serving as an auxiliary heat source. The heated mine water served as a heat source to warm the diluted lithium bromide solution within the generator. Simulations revealed that, at an air supply velocity of 11 m/s, the heading face temperature reached 24 °C, achieving a more comfortable air temperature.

CCHP technology holds significant promise for applications in deep mines. This technology enables cascaded energy utilization, minimizes energy waste, and reduces greenhouse gas emissions, aligning with the principles of energy conservation, emission reduction, and environmental sustainability.

3.3.3. Deep Dehumidification and Cooling Technology

Deep-mine heat damage manifests not only as high air temperatures at the working faces but also as elevated relative humidity levels. In most working faces, the relative humidity exceeds 80% and approaches 100%. High humidity levels exacerbate the adverse effects of mine working environments [71]. Currently, most mine cooling technologies primarily focus on air temperature regulation. Humidity control predominantly depends on condensation dehumidification. However, these methods have insufficient dehumidification in high-humidity mines, resulting in an excessive air humidity ratio at the working face [72]. Solid and liquid absorption offer advantages such as sufficient dehumidification capacity, recyclability, and broad applicability. These technologies represent a mainstream research direction in air dehumidification and can effectively address the insufficient dehumidification capacity of conventional deep mine cooling systems.

Solid adsorption devices are categorized into two types: fixed adsorption beds and desiccant wheel dehumidifiers. Fixed adsorption beds arrange adsorbent materials within a stationary bed. As wet air passes through the bed, the adsorbent removes the moisture, yielding dry air. Thermal regeneration is required to restore the dehumidification capacity of the adsorbent [73]. In a desiccant wheel system (DWS), wet air passes through the process sector, where the desiccant material adsorbs water vapor, thereby achieving dehumidification. Heated regeneration air is introduced to desorb moisture, after which the desiccant material, now moisture-laden, is rotated via the motor to complete the cycle of dehumidification and regeneration [74,75]. Lin et al. [76] had applied solid adsorption beds to deep mine dehumidification, investigating the impact of adsorbent thickness on dehumidification efficiency. The experimental results indicated that when the adsorbent thickness was less than 5 cm, the dehumidification capacity improved with increasing thickness, reducing the relative humidity from 90.5% to 55.5% within one hour. Chen et al. [77] introduced a deep-mine DWS cooling system driven by a heat storage matrix. The DWS removed moisture from the air. The heat storage matrix supplied thermal energy for air regeneration whereas mine water served as a cooling medium for the air cooler. This system not only cooled and dehumidified the working face air, but also optimized waste heat utilization, thereby reducing energy consumption. The results demonstrated that the system reduced air temperature by 8.9 °C and relative humidity by 52.9%. Ji et al. [78] utilized mine water at varying temperatures to supply heating and cooling energy to the DWS, achieving a humidity ratio difference of over 15.0 g/kg between the inlet and outlet. The principle of the mine-water-driven DWS cooling system is illustrated in Figure 14.

In addition to solid adsorption dehumidification and cooling systems, researchers have applied liquid absorption dehumidification and cooling systems to mitigate deep-mine heat damage. Liquid desiccant systems primarily comprise dehumidifiers and regenerators. In a dehumidifier, the liquid desiccant removes moisture from the air whereas the regenerator uses thermal energy to regenerate the solution [79]. Cao et al. [80] developed a concentrated local solution dehumidification and cooling system for working faces, comprising a dehumidifier, a solution regenerator, and a heat pump. The condensation heat of the heat pump regenerated the solution whereas the evaporator reduced the inlet air temperature. This system reduced the working face temperature and humidity ratio while enhancing energy efficiency. Guo et al. [81] investigated a deep-mine solution dehumidification and cooling system driven by a heat pump. The air at the mine working face was cooled and dehumidified using an evaporator and solution dehumidifier. The solution was regenerated using the condensation heat of the heat pump to restore the dehumidification capacity. The experimental results indicated that the working face air temperature was reduced to 24.7 °C, relative humidity decreased by 23.79%, and the system achieved a COP of 3.43.

Deep dehumidification cooling technology exhibits strong applicability. This technology is highly effective in dehumidifying mines with high-temperature and high-humidity. The dehumidification system is deeply integrated with mine ventilation and refrigeration systems to form a comprehensive cooling and dehumidification system that effectively mitigates deep-mine heat damage.

3.3.4. Liquid-Phase-Change Refrigeration Technology

When conventional deep mine cooling technology operates in high-temperature and high-humidity environments, the evaporation and condensation temperatures of the refrigerant are significantly affected by the ambient conditions, leading to reduced refrigeration efficiency and capacity and insufficient refrigeration performance. To address this challenge, liquid-phase-change technology has been applied to deep mine cooling systems.

Liquid-phase-change refrigeration technology operates on the principle that liquids absorb significant heat during vaporization to achieve cooling. During mine cooling, liquid CO₂ is used as a coolant. Liquid CO₂ is transported via pipelines to underground heat exchangers, where it exchanges heat with high-temperature air at the mine working face. Liquid CO₂ absorbs heat from the air, undergoes a phase change, and cools the air at the working face. The resulting gaseous CO₂ is further injected into the mine goaf to prevent the spontaneous combustion of coal. The principle of applying liquid-phase-change refrigeration technology to deep mine cooling is illustrated in Figure 15.

Song et al. [82] investigated the performance of a liquid CO₂ phase change refrigeration system in a high-temperature mine and conducted tests at the tunneling face of the Banshi Coal Mine in China. While maintaining CO₂ concentrations within permissible limits, the injected liquid CO₂ mass flow rate was 1075.6 kg/h. The measurements showed that the airflow temperature of the working face was 20.3 °C, the airflow humidity ratio was 12.46 g/kg, and liquid CO₂ utilization efficiency reached 77.36%. Compared with the nitrogen injection system, the equipment cost of the liquid CO₂ phase change refrigeration system was reduced by 808,900 CNY. The system demonstrated excellent refrigeration performance and operational cost efficiency. Zhai et al. [83] assessed the refrigeration performance of liquid CO₂ using numerical simulations. At a CO₂ consumption rate of 13.54 m³/h, the air temperature was reduced by 7.72 °C, with the liquid CO₂ phase transition providing 46.68 kW/h of refrigeration capacity. In addition to using liquid CO₂ as a coolant in liquid-phase-change refrigeration systems, some researchers have employed liquid nitrogen–oxygen mixtures for mine cooling applications. Xin et al. [84] utilized the phase change process of liquid nitrogen–oxygen mixtures to cool the working face air in the Zhaolou Coal Mine in China. Theoretical analysis indicated that a liquid nitrogen–oxygen volume ratio of 1:6 satisfied the cooling requirements of the working face.

Liquid-phase-change refrigeration technology in mine cooling applications enables efficient cooling, enhances the system refrigeration capacity, and employs environmentally friendly refrigerants to meet stringent environmental protection requirements. In practical applications, the precise control of coolant storage and transportation is crucial to ensure operational safety and reliability.

3.3.5. Heat Pipe Cooling Technology

Heat pipes are highly efficient heat transfer elements that are characterized by exceptional thermal conductivity. Their operational principle involves energy transfer through a phase change in the internal working fluid. The application of heat pipes for mine cooling is an energy-efficient and environmentally friendly technology. Su [85] implemented gravity-assisted heat pipes for mine cooling applications. Gravity-assisted heat pipes were operated without requiring external power. The evaporation section was positioned underground, where it exchanged heat with water. The resulting chilled water was transported to an air cooler to regulate the air temperature at the working face. The condensation section was located above the ground and connected to a cooling tower for the condensation heat discharge. The refrigerant absorbed heat in the evaporation section, vaporized, and flowed to the condensation section, where it released heat, condensed back to the liquid state, and returned to the evaporation section via gravity. The principle of the separated heat pipe technology for mine cooling applications is illustrated in Figure 16. Zhang et al. [86] evaluated the feasibility of applying separated heat pipes for mine cooling. Separated heat pipes cooled the mine air without external power. However, in deep-mine environments, high temperatures led to elevated heat flux densities in the evaporation section, posing challenges such as heat transfer limitations that required further investigation. Compared with gravity-assisted heat pipe technology, Zhu et al. [87] introduced a composite system combining a heat pump and power heat pipe and experimentally demonstrated its refrigeration performance. This system maintained the mine working face air temperatures between 23 °C and 26 °C, achieving excellent refrigeration performance.

Heat pipe cooling technology achieves heat transfer through phase change without additional energy consumption, offering excellent energy efficiency and environmental benefits. Further optimization of the technology design is essential to enhance its applicability for deep mine cooling.

4. Prospect of Deep Mine Cooling Technology

Existing deep mine cooling technologies exhibit distinct advantages and disadvantages, which are constrained by specific operational conditions. Traditional single-method deep mine cooling technologies suffer from reduced refrigeration efficiency and significantly increased construction costs. For high-temperature and high-humidity deep mines, achieving high refrigeration efficiency, low energy consumption, and cost-effective investment requires the further development of advanced cooling methodologies and equipment.

4.1. Integration of Diverse Technologies

To address heat damage in deep mines, a shift from single-mode cooling to an integrated approach is essential. This approach combines mechanical refrigeration, ventilation system optimization, heat source management (e.g., avoiding high-temperature roadways, dewatering mine hot water, applying thermal insulation materials, utilizing heat pipes, and functional backfilling), and mine cold-source utilization (e.g., low-temperature mine water and seasonal energy storage). By integrating these methods, the thermal environment of deep mines can be effectively regulated. Additionally, the performance of mechanical refrigeration systems can be enhanced through localized precise temperature control, waste heat recovery, and non-mechanical cooling techniques. In mines characterized by significant depth and abundant mine water resources, a WSHP coupled with DWS cooling technology can be used effectively. The dual supply of cooling and heating by WSHPs not only cools and dehumidifies the airflow but also satisfies the regenerative heat requirements of the DWS. While integrated cooling technology, which combines multiple methods, achieves high cooling efficiency, it also results in higher initial investment costs. In practical mine cooling design, it is crucial to balance cooling demands with investment costs based on local conditions.

4.2. Collaboration Cooling and Geothermal Mining

Although elevated ground temperatures can lead to mine heat damage, they are inherently green and environmentally friendly thermal energy resources. By treating mine geothermal resources as co-mined mineral-associated resources and integrating mine ventilation temperature regulation with geothermal extraction, the temperature of the surrounding rock and deposits can be reduced during geothermal utilization. This approach not only regulates the thermal environment of the mine but also aids in heat damage mitigation. Concurrently, geothermal energy can be utilized for bathing, agriculture, heating, and other applications and can be further upgraded to high-grade heat energy via heat pump technology. Mine water inflow and mine exhaust air are both viable resources for utilization. The synergistic integration of these resources with mine cooling technology can enhance the overall system efficiency. Both HEMS and heat pump technologies can utilize mine water inflow as a cooling medium. When mine water inflow satisfies water consumption requirements, it can reduce operating costs and enhance the economic performance of the system.

4.3. Deep Dehumidification and Cooling

Deep dehumidification and cooling is essential to reduce the working face humidity ratio and improve thermal comfort. In mine dehumidification and cooling technology research, efforts should be focused on developing efficient, eco-friendly desiccants, and high-capacity dehumidification and cooling systems. Integrating renewable energy with dehumidification and cooling systems allows the utilization of mine waste heat and geothermal energy to regenerate desiccant materials. In deep-mine heat damage scenarios, single dehumidification technologies struggle to meet deep dehumidification requirements. A multi-stage dehumidification system coupled with integrated multi-technology approaches is necessary to enhance the dehumidification efficiency. For instance, a two-stage DWS or solution dehumidification coupled with a DWS can be employed. Utilizing mine waste heat to drive system regeneration enhances the dehumidification capacity while reducing the energy consumption. A multi-stage dehumidification system enhances both cooling and dehumidification efficiency while leveraging waste heat helps mitigate the energy consumption costs of the system.

4.4. Intelligent Control

Owing to the intricate nature of mine working environments and the rapid advancement of artificial intelligence, the adoption of intelligent control technologies represents a key future development direction. Intelligent control systems can continuously monitor critical parameters, including temperature, humidity ratio, and air speed, in real time. Concurrently, these systems adjust refrigeration system operations based on actual data, optimally allocate refrigeration equipment, prevent resource wastage, and ensure uniform cooling distribution. In addition, intelligent control enhances the overall safety of mines. It is crucial to intensify research on mine intelligent control systems, facilitate intersystem information sharing and coordinated control, and further enhance the overall mine operational efficiency. An intelligent control system is used for mine temperature and humidity control. Although the initial investment cost increases, the safety of mining operations is effectively improved. In addition, intelligent control minimizes the waste of cold energy and reduces the operating costs of the system.

5. Conclusions

To comprehensively address the issue of heat damage in deep mine, this paper has reviewed the relevant literature on deep-mine heat damage and systematically analyzed its causes, effects, and associated cooling technologies. The key conclusions are as follows:

(1)

Deep-mine heat damage poses a significant challenge for mining with complex causes, including heat dissipation from the surrounding rock, production equipment, and air compression. High-temperature and high-humidity conditions exacerbate heat damage, adversely affecting the surrounding rocks, working environments, and supporting structures.

(2)

Conventional refrigeration cooling technologies, categorized by the refrigerant types as water-cooled, ice-cooled, and air-cooled, can reduce the air temperature at mine working faces. However, these methods are associated with high energy consumption, inadequate dehumidification capacity, and a significant loss of refrigeration capacity.

(3)

The deep-mine heat-damage resource utilization system and CCHP technology enable the simultaneous utilization of geothermal resource and mine cooling. These systems optimize energy efficiency by harnessing waste heat from deep-mine water and exhaust air, thereby enhancing overall energy utilization rates. Employing DWS and solution dehumidification technology addresses mine high-humidity-ratio issues and achieves effective cooling and dehumidification at mine working faces. Liquid-phase-change refrigeration technology and heat pipe cooling technology leverage phase transition principles to cool the working face air and enhance the system refrigeration capacity.

(4)

Regarding the future development of deep mine cooling technology, this paper proposes four key directions: the integration of diverse technologies, collaboration cooling and geothermal mining, deep dehumidification and cooling, and intelligent control. By evaluating the strengths and limitations of various cooling technologies, a coupled system can be employed to achieve comprehensive cooling and dehumidification at mine working faces. The integration of green and energy-efficient deep mine cooling technologies is crucial for achieving safe, efficient, and environmentally sustainable deep mining.