Design of a New Compressed Air Energy Storage System with Constant Gas Pressure and Temperature for Application in Coal Mine Roadways

: Renewable energy (wind and solar power, etc.) are developing rapidly around the world. However, compared to traditional power (coal or hydro), renewable energy has the drawbacks of intermittence and instability. Energy storage is the key to solving the above problems. The present study focuses on the compressed air energy storage (CAES) system, which is one of the large-scale energy storage methods. As a lot of underground coal mines are going to be closed in China in the coming years, a novel CAES system is proposed for application in roadways of the closing coal mines. The new system combines pumped-hydro and compressed-air methods, and features constant air pressure and temperature. Another speciﬁc character of the system is the usage of ﬂexible bags to store the compressed air, which can e ﬀ ectively reduce air leakage. The governing equations of the system are derived, and the response of the system is analyzed. According to the equations, for a roadway with depth of 500 m and volume of 10,000 cubic meters, the power generation capacity of the CAES system is approximately 18 MW and the generating time is 1.76 h. The results show that the new CAES system proposed is reasonable, and provides a suitable way to utilize the underground space of coal mines.


Introduction
Renewable energy (wind and solar power, etc.) are developing rapidly in China and around the world. For example, the newly installed capacity of wind power in China in 2017 is 188.4 GW, which is 35% of the newly installed wind power in the world [1,2]. However, compared to traditional power (coal or hydro), renewable energy has the drawbacks of intermittence and instability. As a result, approximately 10% of the capacity of wind power is wasted in China.
Energy storage is the key to the above problems. There are many methods of energy storage, such as pumped-hydro storage (PHS), compressed air energy storage (CAES), flywheel energy storage, superconducting magnetic energy storage, and battery and super capacitor energy storage [3,4]. Among these methods, PHS and CAES are suitable for large-scale energy storage. Regarding the high efficiency of PHS, until the end of 2017, the capacity of PHS in China is 28.7 GW and another 38 GW worth of plants are under construction. However, the PHS plant has critical requirements of the geoconditions, As for CAES, there are several technical solutions, such as liquid air energy storage [7], underwater constant air pressure energy storage [8], Isothermal CAES [9], supercritical CAES [10], and advanced adiabatic CAES [11]. There are now two operating large-scale CAES plants, which are in America and Germany [12,13]. Both plants use salt caverns to store the compressed air. In China, some companies are also planning to construct CAES plants using salt caverns [14]. However, the geographical distribution of suitable rock salt is a problem for these projects. Alternatively, using caverns in hard rock for CAES may become an effective method in China. The reason is that a lot of coal mines are now closed to optimize coal production capacity in China. For example, over 500 coal mines were closed in 2018. There are many roadways in the closed coal mines, which are suitable for storing compressed air after some modification [15,16].
Researchers have studied CAES in caverns of hard rock, and have proposed some possible scenarios. At present, the main problem with CAES in hard rock caverns is the tightness and the stability of the surrounding rock [17][18][19]. The underground cavern of the CAES station is large-scale (10 5 -10 6 m 3 ), has high air pressure (5-10 MPa or more), and strict sealing requirements (leakage rate in 24 h should not be more than 0.5%-1.0%) [20][21][22][23][24]. Thus, the inner wall of the gas storage cavern generally needs to be lined and sealed, as shown in Figure 1. The distribution and geological conditions of roadways in coal mines is different from other caverns. Some particular spaces in coal mines, such as vertical shafts, can also be used. Therefore, the current scenarios mentioned above for hard rock caverns may not be the best choice for coal mines. In the current research, a new CAES scheme is proposed for coal mine roadways. The new scheme has two special features. Firstly, PHS and CAES are combined, and the pressure of the compressed air is kept constant by pumping water through the vertical shaft. Secondly, the compressed air is stored in flexible bags, and the leakage of air can be well controlled. The control equations of the CAES system are proposed, and the key parameters are analyzed.

Methodology of the Research
The configuration of a CAES system is complicated. The present research focuses on the possible application of the system in underground coal mines. Some specific characteristics of the CAES system are investigated. The methodology of the research is as follows: 1) Investigate the conditions of different roadways in underground coal mines. 2) Describe the typical configuration of a CAES system and propose a new CAES system for application in coal mine roadways, which features a combination of pumped-hydro and compressed-air methods, and utilization of flexible bags to store compressed air.

Sealing Layer
Lining Surrounding Rock The distribution and geological conditions of roadways in coal mines is different from other caverns. Some particular spaces in coal mines, such as vertical shafts, can also be used. Therefore, the current scenarios mentioned above for hard rock caverns may not be the best choice for coal mines. In the current research, a new CAES scheme is proposed for coal mine roadways. The new scheme has two special features. Firstly, PHS and CAES are combined, and the pressure of the compressed air is kept constant by pumping water through the vertical shaft. Secondly, the compressed air is stored in flexible bags, and the leakage of air can be well controlled. The control equations of the CAES system are proposed, and the key parameters are analyzed.

Methodology of the Research
The configuration of a CAES system is complicated. The present research focuses on the possible application of the system in underground coal mines. Some specific characteristics of the CAES system are investigated. The methodology of the research is as follows: (1) Investigate the conditions of different roadways in underground coal mines. (2) Describe the typical configuration of a CAES system and propose a new CAES system for application in coal mine roadways, which features a combination of pumped-hydro and compressed-air methods, and utilization of flexible bags to store compressed air. (3) Deduce the governing equations of the new CAES system to investigate the response of the system during release of compressed air. (4) Carry out case studies to investigate the power generator capacity and operation time with roadways of different depths. (5) Summarize the advantages of the proposed CAES system.

Distribution of Roadways in Underground Coal Mines
The coal mines mentioned here are for underground mining. In these coal mines, at least two vertical or inclined shafts are first excavated for ventilation and transport. There are three types of underground roadways: (1) development roadways, which serve the entire mine or a mining level, and are mainly distributed in hard rocks (such as sandstone); (2) preparation roadways, which serve a specific mining area, and the surrounding rock is mainly rock mass; (3) mining roadways, which serve one or two mining faces and are located in coal seams [25,26]. Among these roadways, the support strength of development roadways is the highest. After the mining progress, the mining roadways collapse and the preparation roadways may undergo large deformation. Therefore, only the development roadways are suitable for implementing a CAES system.
The section area of development roadways is generally over 10 m 2 , and the length may be several kilometers. Therefore, the possible volume for storing compressed air can be several tens or hundreds of thousands of cubic meters, which is sufficient for a large-scale CAES plant.

Brief Introduction of a Compressed Air Energy Storage System
A typical CAES system without heat storage has three parts, as seen in Figure 2a, i.e., air compressing (electromotor and compressor), air storage, and the power-generating unit (turbine and generator). In the air compressing unit, the air is compressed to a specific pressure [27][28][29]. If the required air pressure is high (for example, 10 MPa), a multistage compression procedure should be used to increase the efficiency. During the compressing process, the air temperature increases, which makes it difficult to compress the air further. Therefore, the air should be cooled during compression. 3) Deduce the governing equations of the new CAES system to investigate the response of the system during release of compressed air. 4) Carry out case studies to investigate the power generator capacity and operation time with roadways of different depths. 5) Summarize the advantages of the proposed CAES system.

Distribution of Roadways in Underground Coal Mines
The coal mines mentioned here are for underground mining. In these coal mines, at least two vertical or inclined shafts are first excavated for ventilation and transport. There are three types of underground roadways: (1) development roadways, which serve the entire mine or a mining level, and are mainly distributed in hard rocks (such as sandstone); (2) preparation roadways, which serve a specific mining area, and the surrounding rock is mainly rock mass; (3) mining roadways, which serve one or two mining faces and are located in coal seams [25,26]. Among these roadways, the support strength of development roadways is the highest. After the mining progress, the mining roadways collapse and the preparation roadways may undergo large deformation. Therefore, only the development roadways are suitable for implementing a CAES system.
The section area of development roadways is generally over 10 m 2 , and the length may be several kilometers. Therefore, the possible volume for storing compressed air can be several tens or hundreds of thousands of cubic meters, which is sufficient for a large-scale CAES plant.

Brief Introduction of a Compressed Air Energy Storage System
A typical CAES system without heat storage has three parts, as seen in Figure 2a, i.e., air compressing (electromotor and compressor), air storage, and the power-generating unit (turbine and generator). In the air compressing unit, the air is compressed to a specific pressure [27][28][29]. If the required air pressure is high (for example, 10 MPa), a multistage compression procedure should be used to increase the efficiency. During the compressing process, the air temperature increases, which makes it difficult to compress the air further. Therefore, the air should be cooled during compression. The air storage unit is usually the most costly part in the CAES system, and it can utilize caverns in rock mass, steel containers, flexible bags, etc. [30].
The power-generating unit is composed of multistage turbines and a generator, and extracts energy by expanding the compressed air. The compressed air flows through the turbines and changes into cold air with low pressure. To increase the efficiency of the system and prevent the temperature of the export air from being too low, the compressed air should be heated by burning natural gas or electric heating [31].  The air storage unit is usually the most costly part in the CAES system, and it can utilize caverns in rock mass, steel containers, flexible bags, etc. [30]. The power-generating unit is composed of multistage turbines and a generator, and extracts energy by expanding the compressed air. The compressed air flows through the turbines and changes into cold air with low pressure. To increase the efficiency of the system and prevent the temperature of the export air from being too low, the compressed air should be heated by burning natural gas or electric heating [31].
As the air should be cooled during compressing and heated before flowing into the turbines, a lot of energy is wasted, and the efficiency of the CAES system is low (approximately 42% for the plant in Germany). Thus, an advanced CAES system, as seen in Figure 2b, has been proposed with heat storage [32]. In the advanced system, the heat during compression is stored in a heat-exchanging unit, and the final compressed air is at approximately room temperature. When generating electricity, the compressed air first flows into the heat-exchanging unit and is heated to high temperature, then expands in the turbines. The advanced scheme has a much higher efficiency (over 60%).

Design of a New Compressed Air Energy Storage System for Application in Coal Mine Roadways
For an efficient CAES system, several principles should be followed. (1) The air pressure should be kept as constant as possible, as this will cause the turbines to work efficiently around the rated state.
(2) The thermal energy of the compressed air should be properly treated. When the air is compressed, it will emit heat. If the heat is not stored, the efficiency of the system will inevitably decrease. (3) Leakage of compressed air should be controlled.
Based on the current technology of CAES and the conditions of roadways in coal mines, a new CAES system is proposed here. According to the principles mentioned above, the remarkable solutions in the new system are as following: (1) The pump-hydro and compressed air methods are combined. This makes the air pressure constant during the electricity generation. First, a reservoir is constructed in the ground. Then, during the electricity generating operation of the CAES system, the water in the reservoir flows into the roadway through the pipes in the vertical shaft and pressurizes the air. As the variation of water pressure is small, the air pressure can be maintained. Finally, during the air charging period, the water in the roadway is pumped into the reservoir. (2) Flexible bags are used to store compressed air in the roadways. The bags may be produced using rubber or other cheap materials. There are two reasons for this scheme: the first is to separate the air from the water; the other is to prevent the air from leaking into the surrounding rock, as the flexible bags have much lower permeability than concrete or rock. (3) Heat storage is applied. During the compression process, the heat will be stored and the compressed air temperature will be reduced to that of surrounding rock of the roadway. By doing this, the heat exchange between the stored air and the surrounding rock can be substantially reduced, and thermal energy loss is controlled. During the electricity generation process, the air will be heated using the stored thermal energy before flow into the turbines.
The layout of the new CAES system is shown in Figure 3. Firstly, the maximum pressure of the compressed air is calculated based on the pressure head of the reservoir. Secondly, the surrounding rock of the roadway is reinforced according to the air pressure. In practice, the surrounding rock can be treated with grouting to strengthen the damage zones, and then supported using a concrete lining. Thirdly, the flexible bags are designed to have the same cross section shape as the roadway, but with slightly larger size. When the compressed air is stored in the bags, there will be no tensile stress distributed in the bag wall, which makes it quite simple to maintain the integrity of the bags. Fourthly, the parameters of the turbines are designed to make the system work around the rated state based on the thermodynamic state of the compressed air at the outlet.

Fundamental Assumptions
For the new CAES system, some assumptions are made to simplify the analysis, which are: 1) The compressed air has been cooled to the temperature of the roadway before being stored in the flexible bags. Therefore, there will be no thermal exchange between the compressed air and the environment during the storage period.
2) The thermal exchange between the compressed air and the water during air release is neglected.
Thus, the temperature of the compressed air in the flexible bags is constant.
3) The reservoir is large enough, and the variation of the pressure head during air release is neglected. Therefore, the pressure head at the bottom of the water pipe is constant. 4) The pressure loss of water in the roadway is neglected, as the flow is slow. 5) During air release, the distribution of air pressure in the flexible bags is uniform. That is to say, the transmission of air pressure between the two plugging walls is assumed to be instantaneous. In reality, the air pressure at the plugging wall around the compressed air pipe is lower than that around the water pipe.

Equations of the New Compressed Air Energy Storage System
In the present study, only the response of the air storage part of the system is analyzed, and the equations of the compressor, turbines, and heat storage unit are not included. For the air storage part, two kinds of variables are involved, i.e., water-related and compressed air-related variables, which are denoted by a subscript "w" or "a", respectively, in the following.
Based on the assumptions listed above, the variables of the system can be reduced. The main variables are shown in Figure 4. During the storing air period, the compressed air is pumped into the flexible bags until the air pressure reaches a specific value. In this period, the water does not flow and the considered variable of the system is only the air pressure in the bag. However, in the process of releasing air, the air and water will move simultaneously, and the response of the system is more complicated. Therefore, here, we only analyze the process of releasing air.

Fundamental Assumptions
For the new CAES system, some assumptions are made to simplify the analysis, which are: (1) The compressed air has been cooled to the temperature of the roadway before being stored in the flexible bags. Therefore, there will be no thermal exchange between the compressed air and the environment during the storage period. (2) The thermal exchange between the compressed air and the water during air release is neglected.
Thus, the temperature of the compressed air in the flexible bags is constant. (3) The reservoir is large enough, and the variation of the pressure head during air release is neglected.
Therefore, the pressure head at the bottom of the water pipe is constant. (4) The pressure loss of water in the roadway is neglected, as the flow is slow. (5) During air release, the distribution of air pressure in the flexible bags is uniform. That is to say, the transmission of air pressure between the two plugging walls is assumed to be instantaneous. In reality, the air pressure at the plugging wall around the compressed air pipe is lower than that around the water pipe.

Equations of the New Compressed Air Energy Storage System
In the present study, only the response of the air storage part of the system is analyzed, and the equations of the compressor, turbines, and heat storage unit are not included. For the air storage part, two kinds of variables are involved, i.e., water-related and compressed air-related variables, which are denoted by a subscript "w" or "a", respectively, in the following.
Based on the assumptions listed above, the variables of the system can be reduced. The main variables are shown in Figure 4. During the storing air period, the compressed air is pumped into the flexible bags until the air pressure reaches a specific value. In this period, the water does not flow and the considered variable of the system is only the air pressure in the bag. However, in the process of releasing air, the air and water will move simultaneously, and the response of the system is more complicated. Therefore, here, we only analyze the process of releasing air.  According to the configure of the turbines, a constant mass flow rate is set to operate at the rated power, which is represented by . Then, based on the mass conservation of the compressed air at the inlet and outlet of air pipe, we get: where va1 and a1 are the velocity and density of the compressed air at the inlet of the air pipe, respectively. va2 and a2 are the velocity and density of the compressed air at the outlet of the air pipe, respectively. d is the diameter of the air pipe. The density a1 and a2 relate to the air pressure according to the gas state equation, which are: where pa1 is the pressure of compressed air at the inlet of air pipe. pa2 is the pressure of compressed air at the outlet of air pipe. Z is the compressibility factor, which depends on temperature and pressure, and is 1 for an ideal gas. R is the gas constant, which is 287 J/(kgK). T is the air temperature in Kelvin. When the flexible bag is discharged under constant pressure, due to the influence of friction and gravity, the air will undergo a pressure drop in the air pipe. The relation between the air pressure at the inlet and outlet of the air pipe is [33,34]: According to the configure of the turbines, a constant mass flow rate is set to operate at the rated power, which is represented by . m a . Then, based on the mass conservation of the compressed air at the inlet and outlet of air pipe, we get: where v a1 and ρ a1 are the velocity and density of the compressed air at the inlet of the air pipe, respectively. v a2 and ρ a2 are the velocity and density of the compressed air at the outlet of the air pipe, respectively. d is the diameter of the air pipe.
The density ρ a1 and ρ a2 relate to the air pressure according to the gas state equation, which are: where p a1 is the pressure of compressed air at the inlet of air pipe. p a2 is the pressure of compressed air at the outlet of air pipe. Z is the compressibility factor, which depends on temperature and pressure, and is 1 for an ideal gas. R is the gas constant, which is 287 J/(kg·K). T is the air temperature in Kelvin. When the flexible bag is discharged under constant pressure, due to the influence of friction and gravity, the air will undergo a pressure drop in the air pipe. The relation between the air pressure at the inlet and outlet of the air pipe is [33,34]: where b is a parameter and equals to 2g RT , and g is the acceleration of gravity. c is a parameter and equal to λ 16ZRT π 2 d 5 , where λ is the friction coefficient along the pipe. h is the height from the roadway to the reservoir.
For the water, according to the Bernoulli equation, the relation between the pressure head at the lower end of water pipe and reservoir is as follows: where p w is the water pressure at the bottom of the water pipe, and is equal to p a1 according to the assumption in Section 3.1. ρ w is the density of water, which is 1000 kg/m 3 . v w is the water velocity in the pipe. h f is the major head loss along the water pipe, and h j is the minor head loss at the junction of water pipe and roadway, which can be determined by: where ξ is the local friction coefficient, which is 1 here for the water flowing from pipe to roadway. D is the diameter of the water pipe.
As the volume flow rate of water equals to that of compressed air at the inlet of the air pipe, v w has a relation with v a1 as: Combining Equations (1)-(6), we get the governing equations of the new CAES system as follows: In Equation (7), there are six variables that need to be solved, i.e., p a1 , p a2 , v a1 , v a2 , v w , p w . The equations can be solved, given the air mass flow rate, . m a and the pressure head of the reservoir, h. The power of the electric generator is a key factor for determining the scale of the CAES plant. The main factors affecting generator power include air pressure and temperature, air mass flow rate, turbine efficiency, and generator efficiency. For the CAES system, the output power of the electric generator is [35,36]: where P g is the output power of the electric generator. T a3 is the air temperature at the inlet of the turbines. P atm is atmospheric pressure. η t and η g are the efficiency of turbines and electric generator, respectively, which are approximately 0.8 to 0.9.

Response of the System during Air-Releasing Operation
At present, the depth of coal mines in China is generally between 300 m and 1000 m. Two possible CAES systems at the depth of 500 m and 1000 m are analyzed here, and the volume of the roadway is assumed to be 10,000 m 3 . The values of the system parameters are listed in Table 1. m a is 100 kg/s. Equation (7) is solved according to the parameters in Table 1, and the results are listed in Table 2. It can be seen that the pressure loss in the water pipe is approximately 0.13 MPa, while that in the air pipe is 0.33 MPa. The pressure loss depends on the parameters of the pipes, i.e., the diameters and friction coefficient. The relations between the air pressure at the inlet (p a1 ) and outlet (p a2 ) and the pipeline parameters are analyzed and shown in Figure 5a,b. The results show that smaller cross sections of the water and air pipes will induce larger pressure loss of the final air pressure (p a2 ), especially when the diameter of water pipe (D) is less than 0.8 m and that of air pipe (d) is less than 0.5 m for the roadway with 500 m depth. From Figure 5c, it can be seen that the friction coefficient has a clear effect on the final air pressure, i.e., the higher the friction coefficient, the lower the final air pressure. Therefore, according to the acquirement of the CAES system and the economic factors, the optimal diameters for water and air pipes are 0.8 m and 0.5 m, respectively, and the friction coefficient of the pipes should be decreased as much as possible. According to the parameters in Table 1, the power generation capacity of the system is calculated by using Equation (8) and listed in Table 2. For the given air mass flow rate, the power generation capacity of the CAES system is approximately 18.0 MW, which is equivalent to the capacity of eight general wind turbine generator sets. For the 10,000 m 3 space volume, the power generation duration of the CAES system can be calculated according to Equation (9), which is approximately 1.76 h, which is practical for power grid peak shaving. According to the parameters in Table 1, the power generation capacity of the system is calculated by using Equation (8) and listed in Table 2. For the given air mass flow rate, the power generation capacity of the CAES system is approximately 18.0 MW, which is equivalent to the capacity of eight general wind turbine generator sets. For the 10,000 m 3 space volume, the power generation duration of the CAES system can be calculated according to Equation (9), which is approximately 1.76 h, which is practical for power grid peak shaving.
where is the air density in the flexible bags. is the volume of the roadway.

Compressed Air Energy Storage System at Depth h = 1000 m and = 100 kg/s
For comparison, a CAES system at the depth of 1000 m is analyzed. The same parameters listed in Table 1 are used. The results are given in Table 2. It can be seen that the pressure loss in the water pipe is approximately 0.11 MPa, while that in the air pipe is 1.19 MPa. The output power of the CAES system is approximately 19.52 MW, which is 1.5 MW higher than that at the depth of 500 m. That is to say, the mass flow rate has a critical effect on the output power.
For the 10,000 m 3 space volume, the power generation duration of the CAES system at the depth of 1000 m can be calculated according to Equation (9) Table 1 are used. The results are given in Table 2. It can be seen that the pressure loss in the water pipe is approximately 0.11 MPa, while that in the air pipe is 1.19 MPa. The output power of the CAES system is approximately 19.52 MW, which is 1.5 MW higher than that at the depth of 500 m. That is to say, the mass flow rate has a critical effect on the output power.
For the 10,000 m 3 space volume, the power generation duration of the CAES system at the depth of 1000 m can be calculated according to Equation (9), and is approximately 3.49 h.

Maximum Output Power of the Compressed Air Energy Storage System
For water pipes, the maximum water flow rate is limited due to the water hammer effect. For a steel pipe with wall thickness of 5 mm and diameter of 1 m, if the yield strength of the steel is 235 MPa, the restricted speed in water pipe is approximately 3 m/s [37,38]. For the roadway with a depth of 500 m, the maximum output power of the CAES system is calculated, considering of the water hammer effect in the water pipe. The parameters in Table 1 are used, and the results are listed in Table 3. It can be seen that the capacity of the system can be increased to 26.0 MW.  (8), it can be seen that there are other factors that influence the capacity of the system, such as the temperature and pressure of air at the inlet of turbines and the efficiency of turbines. By optimizing the factors, the capacity of the system can be improved further.

Extension of the New Compressed Air Energy Storage System
In the new CAES system, the air pressure is determined by the pressure head of the reservoir. Therefore, for shallow roadways, the air pressure may be too low. Then, as the scale of the stored energy depends on the mass of the compressed air in the bags, larger storage volume should be used for shallow roadways, which will increase the investment of the system. Alternatively, the air pressure can be increased over the water pressure. Taking the roadway with 500 m depth as an example, the water pressure is 5 MPa. In practice, the air pressure in the flexible bags can be over 5 MPa, for example 8 MPa. The discharge of the flexible bag from 8 MPa to 5 MPa is a nonisobaric and nonisothermal process. The pressure and temperature decrease during releasing air process. The initial gas mass in the flexible bag is as follows: where m a0 is the initial air mass in the flexible bag; p a0 is the initial air pressure, which is 8 MPa; T a0 is the initial air temperature, which is equal to the surrounding rock temperature and assumed to be 283.15 K here; Z is the air compression coefficient, which is 0.97; R is the gas constant and equals to 287 J/(kg·K). V storage is the volume of the roadway, which is 10,000 m 3 . During the air releasing process from air pressure of 8 MPa to 5 MPa, the air pressure at time t is: where . m is the air mass flow rate and assumed to be 100 kg/s; p a is the air pressure at time t, and k is the adiabatic index, which is 1.4.
According to Equations (10) and (11), the exhaust time can be calculated. The air pressure takes approximately 0.8 h to decrease from 8 MPa to 5 MPa. When the air pressure is lower than 5 MPa, the water flows into the roadway and keeps the air pressure constant. Then, the following air releasing process is isobaric and isothermal. Therefore, during the releasing air process, the variation of air pressure in the flexible bag is shown in Figure 6.
It should be noted that according to the initial in situ stress, there may be tensile stress in the surrounding rock if the air pressure is too high. Then, some mechanical analysis on the surrounding rock of the roadway should be carried out for safety evaluation, which is beyond the scope of the present study. rock of the roadway should be carried out for safety evaluation, which is beyond the scope of the present study.

Advantages of the New Compressed Air Energy Storage System
From the analysis above, we can see that the roadways of underground coal mines can be used to implement large scale CAES systems. Meanwhile, according to the conditions of the coal mines, the pump-hydro and compressed air methods can be combined to realize a new CAES system with constant pressure. Compared with other CAES systems applied in salt caverns or steel tanks, the new system has the following advantages.
(1) Compared to the salt caverns, the roadways can be easily maintained. The surrounding rock can be reinforced by grouting, and lined with concrete, while the salt caverns are usually inaccessible. Meanwhile, when flexible bags are used, the air leakage can be well controlled in the roadways, while that in the salt caverns is a general risk. (2) Compared to steel tanks, the investment is much lower and the capacity of the system can be much larger.
(3) Around the coal mines, there are generally power stations, and the generated power of the CAES system can be easily transferred to the power grid. (4) As the air pressure and temperature are almost constant in the new CAES system, the design of the system is simpler, and the efficiency of the system will be higher. (5) The underground coal mines generally have different mining levels, and the roadways have different depths. Therefore, the scale of the CAES system can be enlarged easily as required.

Conclusions
The present study focuses on the application of CAES systems in rock caverns. According to the special conditions of underground coal mines, a new CAES scenario was proposed. The new CAES system combines pumped-hydro and compressed-air methods, and features constant air pressure and temperature. The governing equations of the storing part of the system are derived. The main findings of the study are as follows: (1) The compressed air is cooled to the temperature of the roadway. Therefore, there will be little thermal loss during storage of the air. Besides, the compressed air is stored in flexible bags, and the leakage of air is avoided. (2) During release of the compressed air to generate electricity, the air pressure at the inlet of turbines is constant due to the pressure compensation of the pumped water. The value of the air pressure has a close relation to the diameters and friction coefficient of water and air pipes. The optimal diameters for water and air pipes in the present research are 1.0 m and 0.5 m, respectively. (3) Compared to other CAES systems, the proposed system has several advantages, and may provide a reference for the application of CAES in other rock caverns.

Advantages of the New Compressed Air Energy Storage System
From the analysis above, we can see that the roadways of underground coal mines can be used to implement large scale CAES systems. Meanwhile, according to the conditions of the coal mines, the pump-hydro and compressed air methods can be combined to realize a new CAES system with constant pressure. Compared with other CAES systems applied in salt caverns or steel tanks, the new system has the following advantages.
(1) Compared to the salt caverns, the roadways can be easily maintained. The surrounding rock can be reinforced by grouting, and lined with concrete, while the salt caverns are usually inaccessible. Meanwhile, when flexible bags are used, the air leakage can be well controlled in the roadways, while that in the salt caverns is a general risk. (2) Compared to steel tanks, the investment is much lower and the capacity of the system can be much larger.
(3) Around the coal mines, there are generally power stations, and the generated power of the CAES system can be easily transferred to the power grid. (4) As the air pressure and temperature are almost constant in the new CAES system, the design of the system is simpler, and the efficiency of the system will be higher. (5) The underground coal mines generally have different mining levels, and the roadways have different depths. Therefore, the scale of the CAES system can be enlarged easily as required.

Conclusions
The present study focuses on the application of CAES systems in rock caverns. According to the special conditions of underground coal mines, a new CAES scenario was proposed. The new CAES system combines pumped-hydro and compressed-air methods, and features constant air pressure and temperature. The governing equations of the storing part of the system are derived. The main findings of the study are as follows: (1) The compressed air is cooled to the temperature of the roadway. Therefore, there will be little thermal loss during storage of the air. Besides, the compressed air is stored in flexible bags, and the leakage of air is avoided. (2) During release of the compressed air to generate electricity, the air pressure at the inlet of turbines is constant due to the pressure compensation of the pumped water. The value of the air pressure has a close relation to the diameters and friction coefficient of water and air pipes. The optimal diameters for water and air pipes in the present research are 1.0 m and 0.5 m, respectively. (3) Compared to other CAES systems, the proposed system has several advantages, and may provide a reference for the application of CAES in other rock caverns.