1. Introduction
Several mines in China have closed, and others with low production capacity are facing closure or abandonment as they do not meet the safety requirements or are limited by the high mining cost [1,2]. In particular, coal capacity reduction policies in recent years have accelerated the closure of mines with low production capacity or depleted resources and open pits, resulting in a considerable number of abandoned mines. Industrialized societies such as Europe and the United States relied more on the development of mine resources in the early stage, leading to a huge number of abandoned coal mines. It is estimated that Ontario, Canada alone has about 6000 abandoned mines and nearly 7000 mined-out quarry and open pit mines [3]. In 1963, the U.S. built the world’s first abandoned coal mine underground gas storage by using the abandoned Leyden coal mine near Denver (240~260 m from the surface) with a gas storage capacity of 140 million m3. In 1975, Belgium built underground gas storage in an abandoned coal mine in Anderlues, creating a gas storage capacity of 180 million m3 [4,5]. In 1906, Germany excavated a shaft in the Asse salt mine at which mining started in 1908, and the salt mine was acquired in 1964. From 1967 to 1978, a total of 125,000 containers of low–medium radioactive waste were disposed of in the mine [6,7]. Ukraine has opened a hospital for the treatment of asthma patients in and around the rock salt mines in the Solotvyno region, located 206–282 m below the surface, with a statistical cure rate of 84 per cent [8]. According to the Chinese Academy of Engineering’s Strategy Research on Efficient Recovery and Energy Saving of Coal Resource in China, the number of abandoned mines in China will be as high as 15,000 in 2030 [9,10].
Implementation of the strategic goal of carbon neutralization is expected to convert China’s energy use patterns [11,12]. The proportion of clean energy utilization continues to rise, and the randomness, intermittency, and volatility of the electrical system are becoming more apparent [13,14,15,16]. Meanwhile, poor load regulation increases the demand for a new generation of large-scale, high-reliability, electricity storage technology [17].
The construction of pumped storage power stations using abandoned mines would not only overcome the site-selection limitations of conventional pumped storage power stations in terms of height difference, water source, environment, etc. [18,19], but would also have great significance for the smooth availability of green energy, thus improving energy storage capacity and creating an industrial linkage of conventional, non-conventional, and smart power grids [20,21,22].
Pumped storage power plants use water as the carrier and use their own turbines and pumps to convert energy. During the trough period of electricity consumption, the excess electric energy is used to drive the water pump to work, and the water in the lower reservoir is given potential energy and pumped to the upper reservoir for storage. When the power load increases sharply, the water in the upper reservoir is lowered in order to generate potential energy to drive the generator set to run [23]. The surface/underground space of the abandoned mine were converted into an energy storage reservoir, and a water delivery system was put in place to constitute a pumped storage system [24,25]. This can be further developed into a complementary distributed smart energy system that involves water, light, wind, gas, and nuclear energy [26,27,28]. Developing this technology will help convert abandoned mine sites into valuable assets, stimulate the development of the clean energy industry and related industrial chains, and catalyze the conversion of resource-exhausted cities. Focusing on the construction of pumped storage power stations in abandoned mines, this feasibility analysis is carried out from multiple perspectives, the challenges and countermeasures facing the existing technology are proposed, and practical solutions are put forward in combination with specific project sites, so as to provide reference for the construction of abandoned mines.
2. Literature Review of Abandoned-Mine Pumped Storage
Pumped storage technology is currently the dominant large-scale electrical energy storage technology in China [29]. When the power demand is low, the surplus power in the power grid is used to transport water from the lower reservoir to the upper reservoir so that electrical energy is converted to potential energy and stored [30]; when the power demand is high, water in the upper reservoir is released to the lower reservoir in order to drive the hydraulic turbines that convert potential energy into electrical energy in the power grid.
Like a savings bank for electrical energy, a pumped storage power station typically has two storage modes [31]. The first one is integral storage and usage, which uses the power grid to reduce excess power when the requirement is low. The second one is scattered storage and integral usage, which uses wind energy, solar energy, and other unstable and low-efficiency electrical energy sources to pump water and generate electricity as required by releasing this water. Additionally, peak-load regulation by pumped storage is a key tool for managing the operation of the power grid owing to its quick starting and high reliability.
2.1. Mode of Abandoned-Mine Pumped Storage
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A.. Surface subsidence area + underground roadway
The surface subsidence area formed by mining can be converted into the upper reservoir, and the abandoned roadway that extends several kilometers underground is used as the lower reservoir in order to build a pumped storage power station (Figure 1). When the power consumption is low, the water pump is initiated to raise the water in the lower or underground reservoir to the upper reservoir, consuming the excess power in the power grid and converting it into high-level gravitational potential energy for storage. When the power consumption peak of the power grid is reached, the water stored in the upper reservoir could flow into the lower reservoir, and the gravitational potential energy of the water could be converted into electrical energy through the hydraulic turbine to supply power to the power grid and achieve peak shifting. In this mode, the upper reservoir is exposed to the surface, while the lower reservoir was closed underground. Hence, it is also called semi-open abandoned-mine pumped storage.
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B.. Underground roadway + underground roadway
The shallow underground roadway or goaf (depth = −100~−200 m) was converted into the upper reservoir, and the underground roadway or goaf at −500 m or deeper is converted into the lower reservoir. Underground roadways of different depths are used to build the upper and lower reservoirs (Figure 2), and the reservoirs are all closed underground, giving it the name “closed abandoned mine pumped storage”. One of the advantages of this model is the negligible evaporation from the reservoir, making it apt for the reuse of abandoned mines in arid and water-deficient areas.
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C.. Open mode
A pumped storage power station is constructed by utilizing the difference in heights between the abandoned open pits. Since the upper and lower reservoirs are completely exposed to the surface, it is also called open abandoned-mine pumped storage (Figure 3). The construction is similar to that of a conventional pumped storage power station, with mature technology and perfect equipment, while using the existing open pit could greatly shorten the time and cost required for the construction.
2.2. Application of Abandoned-Mine Pumped Storage
This technology has found use across the world. Germany and Australia are in the start-up stage; the USA, Canada, and Ireland are in the industrial demonstration stage; and the UK and China are still in the stage of technological demonstration and development [32,33,34].
The Ruhr group in Germany initiated the 200 MW abandoned Prosper–Haniel Mine pumped storage power station, which adopts a semi-open abandoned-mine pumped storage mode (Mode A) [35]. The Kidston Gold Mine in Australia, an abandoned pit, was the first co-construction project that combined a solar photovoltaic power station and a pumped storage power station. This project involved a 210 MW photovoltaic power station and a 250 MW pumped storage power station (Mode C) [36]. The Asturian Mine in Spain was converted into a semi-open pumped storage power station after abandonment using the mine water as a water source, and a roadway with a length of approximately 6000 m and a section of 30 m2 was constructed (Mode A) [37]. The United States has conducted much research on abandoned-mine pumped storage, and several abandoned-mine pumped storage projects were being planned, including the 260 MW pumped storage project of the Mineville Iron Mine in New York State, which adopted the closed pumped storage model (Mode B) [38].
A proposal to convert the abandoned Bethlehem Mine in Canada into an open-mode 400 MW pumped storage power station has been initiated [39,40]. The conversion of the abandoned Nenagh Silver Mine in Ireland into a 360 MW pumped storage power station is also underway [41]. The conversion of an abandoned deep-well gold mine in South Africa into a large-scale pure underground pumped storage power station is also being planned. The working face of the gold mine is distributed 500–4000 m underground, and a two-stage pumping and storage arrangement will be adopted, with heads of 1200 and 1500 m, respectively [42].
China has also come to the forefront of these studies. In 2010, the Shendong Daliu Tower Mine Demonstration Project was started and 32 underground reservoirs with a total capacity of 31 million m3 have been constructed to date. Indeed, it is the sole coal underground reservoir group globally [43]. In 2015, the Shenhua Group converted an abandoned mine into an underground reservoir in order to verify the feasibility, and laid a foundation and accumulated experience for further research and construction on such power stations [44]. However, there is no detailed research on the scientific evaluation methods and technical and economic calculations for the conversion and utilization of abandoned mining spaces into energy storage power stations.
3. Pre-Feasibility Study of Abandoned-Mine Pumped Storage
The conversion of pumped storage in abandoned mines has risks that arise from many factors, such as the ecological environment, the economy, and society. Since these factors cannot often be fully quantified in the process of treatment, and it is difficult to obtain detailed data for the indicators, a feasibility evaluation index system with natural conditions, mine conditions, safety conditions, and economic benefits as criteria and 29 influencing factors are proposed as index layers (Table 1), in view of the above-mentioned problems [45,46,47].
3.1. Feasibility in Terms of Natural Conditions
3.1.1. Hydrologic Conditions
Since the abandoned-mine pumped storage technology mainly uses the force generated by the water flow to realize the process of discharge, whether the abandoned mine has enough underground water resources to form an underground reservoir is an objective and necessary condition for the mine to carry out pumped storage. Due to the destruction of the underground aquifer by coal mining and the infiltration of ground rainwater, a large quantity of mine water will be produced. Using coal mine underground reservoir technology, it is necessary to store enough mine water to meet the needs of pumped storage. Based on international operating experience, the replenishment of abandoned-mine pumped storage power station usually uses mine water as the replenishment water source, and the storage time can be as long as one year or more. A 50 MW pumped storage power station usually needs to store 200,000–400,000 m3 of water, and a single well water inflow of more than 1000 m3/d can meet the water storage needs [48].
3.1.2. Geographical Conditions
Abandoned-mine pumped storage technology can help the peak shifting of the power grid and improve the operating stability and economy of the power grid, but the construction of the pumped storage power station is restricted by geographic conditions; that is, there must be a large enough drop between the upper and lower reservoirs. Abandoned mines are often located in mountainous and multi-basin areas. Whether the “surface subsidence area and underground roadway mode” or the “underground roadway and underground roadway mode” is adopted, it can enable the construction of the drop between the pumped storage power stations and ground water storage in order to meet the needs of construction of pumped storage.
3.1.3. Power Condition
In addition to connecting the power grid for peak shifting on the power supply, the pumped storage power station can also facilitate the storage of from renewable clean energy such as wind energy and solar energy. Therefore, the insolation and windspeed in the area where the abandoned mine is located should be favorable for the utilization of the pumped storage of the abandoned mine. Existing abandoned mines are often built on mountain slopes, and the surrounding environment can host large-scale wind and solar power stations. At the same time, the mine has complete infrastructure and easy access to the power grid.
3.2. Feasibility in Terms of Mine Conditions
3.2.1. Vertical Drop
The current pumped storage power station mainly uses terrain drop and excess electrical energy to drive the flow of stored water, thereby realizing the sending and receiving of energy. The underground space in an abandoned mine is mainly composed of roadways of mine development and preparation and goaf. These spaces are generally located hundreds of meters below the ground, with a vertical distance of tens of meters to hundreds of meters and have fair conditions in terms of vertical drop [49].
3.2.2. Impermeability of Rock Strata
For the original mining mine, converting it into a pumped reservoir for electricity storage and power generation places higher requirements on the impermeability of the rock formation and rock wall in the mine. For example, the goaf overlying rock contains fractured zones caused by mining, and it has strong conductivity and poor impermeability and stability, which is generally not desirable for a storage reservoir. Current mine shotcrete pouring technology can endow the goaf with impermeability and stability in order to convert it into a good reservoir.
3.2.3. Surface Reservoir
Mining often causes considerable subsidence. Generally, the subsidence depth is between several meters and tens of meters, and it often accumulates water naturally due to rainfall and groundwater flows. These can be directly selected as the construction site of the upper reservoir. In other cases, areas with better anti-seepage and ground conditions can be selected for the construction.
3.2.4. Underground Reservoir
The reconstructed underground reservoir can not only store and protect a large quantity of mine water resources, but it can also purify the water. The vertical distance between the built coal mine underground reservoir and the ground is generally more than 100 m, with water volume reaching 0.2–1 million m3, which has excellent conditions as a pumped storage lower reservoir [50]. A lower reservoir can consist of multiple underground reservoirs located at the same or different mining levels.
3.2.5. Plant System
A plant system typically comprises the main plant, auxiliary plant, main converter room, switch station, etc., all arranged in the underground roadway of the abandoned mine in ample underground space with high safety and reliability, which can reduce the construction requirement of the underground plant, greatly reducing the plant construction cost and construction period.
3.3. Feasibility in Terms of Safety Conditions
3.3.1. Underground Supporting Conditions
Unlike an open pumped storage power station, vertical mining underground space involves different types of rock strata, which are different in terms of water content, hardness, and corrosion resistance. Therefore, the pumped storage reconstruction of the abandoned mine needs to consider the existing rock formations and supporting conditions in the well, and a permanent support roadway or chamber should be chosen to ensure the safety of the project.
3.3.2. Underground Combustible Gas and Coping Strategies
Some abandoned mines contain a large amount of gas, which is flammable and toxic. Therefore, the existence of underground gas is a major uncertainty factor for the utilization and operation of pumped storage in abandoned mines. It is necessary to consider the underground gas concentration, the topology of the underground roadway, and the installation conditions of underground power generation equipment.
3.4. Feasibility in Terms of Economic Benefits
3.4.1. Static Benefits
Static benefits include two aspects. Capacity benefit refers to the reduction in the construction cost of other peak-load regulation power plants. Electricity benefit refers to the benefit brought about by the replacement of conventional power plants by the pumped storage plant during peak load shifting. One of the basic functions of the pumped storage power station is to perform peak-load regulation and shifting on the power grid, exchanging low-priced electricity for high-priced electricity. Once the smart power grid and distributed energy are connected to a wider range of energy storage services, the proportion of nonconventional resources can be increased, and carbon emission by electrical systems can be greatly reduced. To actively respond to the Opinions of the National Development and Reform Commission on Further Improving the Price Formation Mechanism of Pumped Storage issued by the National Development and Reform Commission, it is necessary to further improve the price formation mechanism of pumped storage, form electricity prices in a competitive way, include capacity payment in transmission and distribution price recovery, and improve the cost sharing and grooming of pumped storage power station.
3.4.2. Social Benefits
At present, most mining resource-based cities face many obstacles such as resource exhaustion and conversion, economic growth stagnation, population loss, etc. Through the development and reuse of abandoned-mine pumped storage, and the production of a series of business activities such as the installation, operation, and maintenance of mining facilities, it is beneficial to solve a series of social problems such as re-employment of workers of abandoned mine.
3.4.3. Environmental Benefits
The pumped storage power station uses water to generate electricity and store energy, and there is almost no emission of pollutants. Importantly, it liberates some thermal power units for special purposes, such as peak-load regulation thermal power units and accident backup thermal power units, which makes the thermal power unit more efficient. The output electrical energy is increased for the same coal consumption, and the coal consumption is reduced, thereby reducing the emission of harmful gases, such as CO2, SO2, sulfide, and nitrogen oxides, creating environmental benefits.
4. Challenges in Abandoned-Mine Pumped Storage
4.1. Corrosion-Sediment Wear-Cavitation Synergistic Control Key Technology of Hydraulic Turbine of Underwater Pump in Mine
Most mine water is corrosive [51,52], which can easily aggravate the cavitation effect of the super-high head water pump hydraulic turbine. The solid particles in the water will cause sediment wear at high flow rates, and corrosion, sediment wear, and cavitation will form a coupling effect that deteriorated the structural parts of the water pump hydraulic turbine. Hence, it is an inevitable requirement for the water pump hydraulic turbine of the abandoned-mine pumped storage power station to develop the key technology for corrosion–sediment wear–cavitation coordinated control technology of the water pump hydraulic turbine.
It is urgent to clarify the physical and chemical properties of mine water, analyze the mechanism of action between the chemical composition of mine water and metal materials, and reveal the internal corrosion law of the water pump hydraulic turbine. It is necessary to investigate the solid–liquid two-phase flow inside the water pump hydraulic turbine and reveal the wear law of the solid particles in mine water on the water pump hydraulic turbine at high flow rates. In addition, it is essential to comprehensively study the coupling mechanism of corrosion–sediment wear–cavitation erosion, explore the key technologies of synergistic control of corrosion–sediment wear–cavitation erosion, and propose the screening and development of composite coatings to establish an evaluation system.
4.2. Technology for Underwater Sensing and Mining Cave Health Status Assessment
There are considerable gradients in the surrounding rock in the underground mine group. Popular equipment can measure only unidirectional stresses and show poor coupling with the surrounding rock. The initial bearing capacity is low, which cannot meet the requirements for long-term monitoring of the anisotropic stress of the deep surrounding rock bodies in the lower reservoir of the high-drop pumped storage power station. The sensor stability detection data directly reflects the state of the reservoir in the mine cave. Once the parameters are abnormal, accidents are likely to occur, and there is a great potential safety hazard. Therefore, it is necessary to establish a 3D dynamic model for the underground reservoir based on multi-source time series detection data, to study the influence of ground and water depth and water-gas alternation on the rock wall of the reservoir, to reveal the basic law of variation for the underground reservoir stability, and to determine the sensitive points for rock wall stress in the reservoir, deformation, and seepage. In addition, it is necessary to design multiple communication methods and multi-space scale big data network structures that adapt to the space environment characteristics of the underground and underground mine groups, analyze real-time monitoring data, and establish the health state parameter prediction of the mine group and key equipment model, and propose corresponding health status assessment and early warning methods.
4.3. Seepage Prevention, Support, and Reinforcement Technology of Underground Space for Abandoned-Mine Pumped Storage
The underground roadway of the abandoned mine is intricate and uneven. During the pumped storage process, the underground space is periodically injected with water. To achieve high-efficiency power generation, the flow velocity of the water is often large, and the turbulence phenomenon is severe. Eddy currents are often generated in the local area where multiple roadways meet, the surrounding rock will bear large impact stress, and even the ultra-high-pressure water hammer effect due to cavitation will directly affect the efficiency and durability of abandoned-mine pumped storage. Hence, it is necessary to analyze the stress state of the mine cave group and propose a cyclic fatigue damage assessment algorithm based on the coupling of varying mechanical, impact, and thermal stresses according to the specific morphology of the abandoned mine. In addition, it is necessary to ensure the stability of the underground space support of the abandoned mine, determine the failure mode and location of the roadway based on the mechanical model of fluid–structure interaction, and optimize the seepage prevention, support, and reinforcement technology. Finally, attention should be given to design the water flow pipeline network, build a partitioned and hierarchical support system, and evaluate the support life of the abandoned mine.
5. Feasibility Study of Pumped Storage by the Shitai Mine in Anhui, China
5.1. Resource Conditions of Shitai Mine Pumped Storage
5.1.1. Underground Space Resource
Drawing the relationship curve between the underground space and elevation according to the distribution of rock tunnels in Shitai Coal Mine (Figure 4). In this mine, the space available is about 800,000 m3, but the distribution of the underground space of the mine is significantly different from the capacity distribution of the surface reservoir, which is mainly manifested in the fact that the capacity of the mine underground reservoir has an obvious ladder type, and there is a point where there is a sudden increase in capacity in each mining level (or auxiliary level). For instance, the capacity space of the first mining level is approximately 125,000 m3 at −250 m, the capacity space of the second mining level is about 100,000 m3 at −450 m, the capacity space near the auxiliary level is about 50,000–80,000 m3 at −380 m, and the capacity space at the return air level around −40 m is about 20,000–30,000 m3. Hence, the Shitai Mine has underground water storage space, while it needs to be selected according to the pumped storage system and equipment performance requirements.
5.1.2. Surface Water Source Condition
There are three subsidence areas within the Shitai mining area, with an accumulated water area of about 3.0 km2 and a water depth of 0.5–5 m. The highest water level in the subsidence area is +33.5 m, and the annual rainfall volume is 600~1460 mm (average = 834 mm), with an average relative humidity of 73%. Meanwhile, the Zha River and Lanji River flow through the east of the mining area, with the Longdai River and Shuoli Lake in the west (Figure 5). The water area of Shuori Lake is about 4.5 km2, and the total capacity is as high as 9 million m3 according to the average water depth of 2 m. According to the comparative analysis of local precipitation, surface water evaporation, and the underground water inflow in the Zhahe mining area, the mine has long-term stability and the water inflow can meet the water demand of the power station.
5.1.3. Head Conditions
Shitai Mine adopts the multi-level vertical well development method, so there are better head conditions for pumped storage between the surface and each level, such as the ground and the first level form a head of 250 m, and the ground and the second level form a head of 450 m.
5.1.4. Underground Roadway Surrounding Rock Conditions
The bottom yard of the Shitai Mine and the rock surrounding the main development roadway have good stability, good maintenance, and less deformation and damage. Only a small part of the sprayed concrete layer cracks and falls off, and some roadway steel bars are exposed. Hence, the Shitai Mine has engineering geological conditions for the construction of pumped storage after proper engineering treatment.
5.1.5. Electromechanical Equipment Conditions
The capacity of the underground pumped storage unit is generally 20~100 MW, the head in the mine is generally high, and the installed capacity of the pumped storage power station is small, so a mixed-flow reversible unit can be used. Considering the multi-level grading utilization, separate pumping and turbine units must be used. The number of power station units selected is 2 to 3, which can meet the requirements.
5.1.6. Other New Energy Conditions
Guanshan wind farm is approximately 4 km east of Shitai Mine. As early as December 2013, the first wind turbine in Guanshan Wind Farm, Xiaoxian County of China Wind Power Group, which was constructed by China Energy Construction Group Co., Ltd. (Huainan, China) and Anhui Power Construction Second Company, was officially connected to the grid for power generation. Xiaoxian Guanshan Wind Power Project is constructed in three phases. The total planned installed capacity is 144 MW, with a total of 72 wind power generators. The first phase of this construction project had a single capacity of 2000 kW and 24 wind power generators, with a total of 48 MW wind energy unit. Simultaneously, a 110 kV wind farm substation was constructed.
5.2. Design of Shitai Mine Pumped Storage Project
Figure 6 shows the overall design of the proposed Shitai Mine pumped storage power station.
The construction tasks of the design scheme mainly include [53]: (1) surface reservoir; (2) underground water turbine pump room; (3) underground reservoir; (4) underground unit.
5.2.1. Surface Reservoir
Due to the low terrain of the surface reservoir, the connection between the water transmission pipeline and the reservoir cannot be horizontal, and a vertical connection is adopted to effectively improve the water output rate.
According to Code for Design of Inlet of Hydropower Station (DL/T 5398-2007), it is calculated that the minimum reservoir depth is equal to the minimum submerged depth of the outlet/inlet, so artificial excavation is used to build surface reservoir of dimensions 300 m × 200 m × 6 m inside the industrial plaza.
5.2.2. Underground Water Turbine Pump Room
The water turbine and pump room in this scheme adopts the tail-type layout. Considering the working performance of the water turbine pump unit, it is proposed to adopt a horizontal plant layout, with the first-level hydraulic turbine group installed at the level of −250 m, and the second-level water turbine pump unit installing at the level of −450 m. Specifically, starting from the bottom yard of the first-level reservoir, the excavation is carried out downward to an elevation of −250 m, and a first-level water turbine pump room, main powerhouse, the auxiliary workshop and installation workshop are built in a roadway along this elevation. A large-span roadway is excavated at an elevation of −450 m as a secondary pump room. From this elevation, a water diversion inclined roadway is excavated at an inclination of 55° towards a horizontal main well, and a diversion pipeline is installed inside.
5.2.3. Underground Reservoir
The space at −250 m and −450 elevation is selected for the construction of the underground reservoir. The underground space in the range of −230 m~−250 m elevation is used as the first-level underground reservoir, and the tailpipe inundation depth is controlled within 20 m, with a capacity of about 150,000 m3 and a difference between the upper and lower water levels of 30 m. The underground space in the range of −430 m~−450 m is used as the secondary underground reservoir unit, the lower suction pipe is controlled at the suction head of 50 m, and the submerged depth of the tail pipe is controlled within 20 m with a capacity of approximately 120,000 m3 [54].
5.2.4. Underground Unit
The proposed design can be classified as daily regulation pumped storage power generation. According to the spirit of the relevant documents of the national power grid on charging by time periods, the time for the continuous power generation of the pumped storage power station is determined as: 07:00~15:00 for a total of 8 h, and the remaining time periods are pumping periods with a duration of about 16 h. Shitai Mine has less constraints on the construction of the upper reservoir on the ground. Therefore, the total capacity is limited by the conditions of the underground reservoir. According to the above design and the calculation according to the proportion of stagnant water at 20%, the total installed capacity can be taken as 30 MW. Two-stage mixed-flow hydraulic turbine installation is adopted: the first-stage hydraulic turbine head is 300 m, the flow rate is 120,000 m3 [55], and it is used for 6 h. For the second-stage hydraulic turbine, the head is 500 m, the flow rate is 100,000 m3, and it is used for 4 h.
5.3. Benefits of Shitai Mine Pumped Storage
5.3.1. Economic Benefits
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(1). Investment cost
Compared with the conventional pumped storage power station on the ground, abandoned-mine pumped storage can not only effectively reduce the construction cost of the surface reservoir and the underground reservoir, but it can also save on construction costs such as the land acquisition cost and the amount of earth and rock excavation. The cost saved by this part can be used for isolation dam construction and anti-seepage treatment of the underground reservoir. Therefore, the construction cost of this project should be basically the same as the cost of ordinary pumping and storage units, about 6000–10,000 yuan/kW·h, but the construction period is greatly shortened. After completion, the average annual operation and maintenance cost is 70,000–80,000 yuan/MW. According to the service life of 40 years, the operation cost is about 8 × 0.3 × 40 = 96 million yuan, and the overall investment cost is about 276~396 million yuan.
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(2). Direct benefit
According to the Notice of Anhui Provincial Price Bureau on Matters Related to Reasonable Adjustment of Electricity Price Structure (Anhui Price Business 2017 101), the coal-fired benchmark electricity price in Anhui Province in 2021 will be 0.3844 yuan, the transmission and distribution price, government funds and surcharges will be 0.2286 yuan, and the low valley period (two-part system) industrial electricity price (35 kV) will be 0.3454 yuan/kW·h. Therefore, using the low-trough period to pump water, deducting the transmission and distribution price, government funds and additional comprehensive expenses of 0.2286 yuan, the price of electricity from the pumped storage system would be 0.1168 yuan. The on-grid electricity price is based on the spot market price in Anhui Province in 2021 of 0.3844 yuan. The cost of electricity per kW·h is 0.1557 yuan, and the profit is 0.2287 yuan, calculated based on the comprehensive efficiency of pumped storage of 75%. Therefore, according to the pumped storage power station with a designed installed capacity of 30 MW, the annual power generation is: 0.03 × 6 × 365 × 90% = 59.13 million kW·h, and the total revenue is 59.13 × 40 × 0.2287 = 541 million yuan within the service period of 40 years.
5.3.2. Social Benefits
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(1). Environmental benefits
The construction of the Shitai Mine abandoned-mine pumped storage project has promoted the upgrading and conversion of out-of-capacity coal mines and the problem of re-employment of mining workers. Calculated based on the installed capacity of 30 MW, it can generate about 59.13 million kWh of electricity every year, save 19,700 t of standard coal, and reduce 51,200 t of CO2. In the 40-year service period, it can save 788,000 t of standard coal and reduce 2.048 million t of CO2.
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(2). Energy benefit
The construction of the Shitai Mine pumped storage power station will surely impart the comprehensive benefits of “multiple new energies complement each other, and multiple industries complement each other”, and can play a huge role in the consumption of unstable new energies such as wind energy and solar energy, avoiding the large-scale phenomenon of “abandoning electricity” and promoting the healthy development of the new energy industry.
6. Conclusions and Recommendations
The construction of abandoned-mine pumped storage power stations is feasible, but one still needs to fully consider the mine restoration, conversion, and development of resource-based cities, new energy and energy storage technology, and the placement and employment of mining workers. Construction of abandoned-mine pumped storage power stations will help to eliminate bottlenecks in energy storage links, seize the high-end links and key nodes of new energy and high-end equipment industry chain, and create a new energy–large-scale energy storage–smart power grid innovative industrial cluster. In addition, it promotes high-end technology in nonconventional energy, energy storage, and smart power grid industry, and at the same time solves the resource utilization problems related to abandoned mines, industrial continuity, ecological restoration, personnel resettlement and a series of economic, ecological and social issues.
Despite its relevance, industrial application of this technology is still in its infancy. Due to the urgency associated with large-scale energy storage and mining wasteland management, this technology should be actively developed keeping safety and scalability in mind. Specifically, several mines with stable geological conditions and good underground reconstruction conditions shall be selected for the pilot project of abandoned-mine pumped storage in order to establish the ecological restoration circle, nonconventional energy construction circle, and power grid service circle for the mining wasteland. In this way, the establishment of a national-level underground energy storage cloud connecting abandoned mine energy storage power stations can be achieved, thus contributing to the realization of carbon neutrality in China before 2060.
Conceptualization, X.L. and K.Y.; methodology, J.T.; formal analysis, Y.W.; investigation, X.L.; resources, K.Y.; writing—original draft preparation, X.L. and J.F.; writing—review and editing, X.L. and J.F.; visualization, J.T. All authors have read and agreed to the published version of the manuscript.
Not applicable.
The authors would like to acknowledge the Institute of Energy Hefei Comprehensive National Science Center for the funding and administrative support they provided.
The authors declare no conflict of interest.
Footnotes
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Feasibility indicators for abandoned-mine pumped storages.
| Criteria | Indicator |
|---|---|
| Natural conditions | Hydrologic conditions |
| Average annual sunshine hours | |
| Wind Resource | |
| Geological Conditions | |
| Accessibility of nuclear power equipment | |
| Accessibility of power grid | |
| Accessibility of new energy equipment | |
| Mine conditions | Underground water reserves in the mine |
| Requires space for a upper reservoir | |
| Requires space for a chamber | |
| Requires space for a lower reservoir | |
| Vertical connectivity of upper and lower reservoirs | |
| Spatial connectivity in underground roadway | |
| Suitable high and low vertical distance in goaf | |
| Safety conditions | Good supporting conditions in underground space |
| Low concentration of combustible gas in underground space | |
| Ventilation conditions in underground space | |
| Installation conditions of underground power equipment | |
| Collapse level | |
| Impermeability of rock strata and wall in the mine | |
| Corrosion resistance of rock strata in the mine | |
| Economic benefits | Peak-load regulation and shifting of power grid |
| Saving of operation and construction costs | |
| Re-employment of mining-related workers | |
| New energy power storage level | |
| Treatment cost of sewage in the mine | |
| Emission reduction of hazardous gases | |
| Environmental governance policy support | |
| Surplus electrical energy of power grid |
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; Yang, Ke 2 ; Fang, Juejing 1 ; Tang, Jinzhou 3 ; Wang, Yu 4 1 State Key Laboratory of Mining Response and Disaster Prevention and Control in Deep Coal Mines, Anhui University of Science and Technology, Huainan 232001, China; Institute of Energy, Hefei Comprehensive National Science Center, Hefei 230031, China
2 Institute of Energy, Hefei Comprehensive National Science Center, Hefei 230031, China
3 Institute of Energy, Hefei Comprehensive National Science Center, Hefei 230031, China; Guizhou Provincial Key Laboratory of Rock and Soil Mechanics and Engineering Safety, Guizhou University, Guiyang 550025, China
4 State Key Laboratory of Mining Response and Disaster Prevention and Control in Deep Coal Mines, Anhui University of Science and Technology, Huainan 232001, China