energies Article Design of a New Compressed Air Energy Storage System with Constant Gas Pressure and Temperature for Application in Coal Mine Roadways Kangyu Deng 1, Kai Zhang 1,2,* , Xinran Xue 1,2 and Hui Zhou 3 1 State Key Laboratory for Geomechanics & Deep Underground Engineering, China University of Mining & Technology, Xuzhou 221116, Jiangsu, China; [email protected] (K.D.); [email protected] (X.X.) 2 School of Mechanics & Civil Engineering, China University of Mining & Technology, Xuzhou 221116, Jiangsu, China 3 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan 430071, Hubei, China; [email protected] * Correspondence: [email protected]; Tel.: +86-185-014-10088 Received: 13 August 2019; Accepted: 31 October 2019; Published: 2 November 2019 Abstract: 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 effectively 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. Keywords: compressed air energy storage (CAES) system; constant gas pressure; abandoned coal mine space; roadway; pumped-hydro storage (PHS) 1. 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, Energies 2019, 12, 4188; doi:10.3390/en12214188 www.mdpi.com/journal/energies Energies 2019, 12, 4188 2 of 14 Energies 2019especially, 12, x in terms of the altitude difference. Therefore, the sites of PHS plants may not2 of be 14 consistent with those of renewable energy [5,6]. As for CAES,As for CAES,there are there several are several technical technical soluti solutions,ons, such such as as liquid liquid airair energyenergy storage storage [7], [7], underwater underwaterconstant constant air pressureair pressure energy energy storage storage [8], Isothermal [8], Isothermal CAES CAES [9], supercritical [9], supercritical CAES CAES [10], and [10], advanced and advancedadiabatic adiabatic CAES CAES [11]. There [11]. There are now are two now operating two operating large-scale large-scale CAES plants,CAES plants, which arewhich in America are and in AmericaGermany and Germany [12,13]. Both[12,13]. plants Both use plants salt caverns use salt to caverns store the to compressed store the compressed air. In China, air. some In China, companies are some companiesalso planning are also to construct planning CAES to construct plants using CAES salt plants caverns using [14 ].salt However, caverns the[14]. geographical However, the distribution geographicalof suitable distribution rock salt of issuitable a problem rock for salt these is a projects. problem Alternatively, for these projects. using cavernsAlternatively, in hard using rock for CAES caverns inmay hard become rock for an CAES effective may method become in an China. effective The method reason isin thatChina. a lot The of reason coal mines is that are a nowlot of closed to coal minesoptimize are now coal closed production to optimize capacity coal production in China. Forcapacity example, in China. over 500For coalexample, mines over were 500 closed coal in 2018. mines wereThere closed are manyin 2018. roadways There are in many the closed roadways coal mines, in the whichclosedare coal suitable mines, forwhich storing are suitable compressed for air after storing compressedsome modification air after [15some,16]. modification [15,16]. ResearchersResearchers have studied have CAES studied in CAEScaverns in of caverns hard rock, of hard and rock, have and proposed have proposed some possible some possible scenarios.scenarios. At present, At the present, main theproblem main problemwith CAES with in CAEShard rock in hard caverns rock is caverns the tightness is the tightness and the and the stability ofstability the surrounding of the surrounding rock [17–19]. rock The [17– underg19]. Theround underground cavern of cavern the CAES of the station CAES is station large-scale is large-scale (105–106 m(103),5 –10has6 highm3), air has pressure high air (5–10 pressure MPa (5–10 or more), MPa orand more), strict and sealing strict requirements sealing requirements (leakage (leakagerate rate in 24 h shouldin 24 h not should be more not bethan more 0.5%–1.0%) than 0.5%–1.0%) [20–24]. Thus, [20–24 the]. Thus, inner the wall inner of the wall gas of storage the gas cavern storage cavern generallygenerally needs to needsbe lined to and be lined sealed, and as sealed, shown as in shown Figure in 1. Figure 1. Sealing Layer Lining Surrounding Rock Figure 1. TheFigure structure 1. The of structure a compressed of a compressed air energy air storage energy (CAES) storage cavern. (CAES) cavern. The distributionThe distribution and geological and geological conditions conditions of roadways of roadways in coal mines in coal is minesdifferent is difromfferent other from other caverns. caverns.Some particular Some particular spaces in spacescoal mines, in coal such mines, as vertical such as shafts, vertical can shafts, also be can used. also beTherefore, used. Therefore, the the current scenarioscurrent scenarios mentioned mentioned above for above hard rock for hard cave rockrns may caverns not be may the not best be choice the best for choice coal mines. for coal mines. In the currentIn the research, current research, a new CAES a new scheme CAES scheme is proposed is proposed for coal for mine coal roadways. mine roadways. The new The scheme new scheme has has two specialtwo special features. features. Firstly, Firstly, PHS PHSand andCAES CAES are combined, are combined, and andthe thepressure pressure of the of thecompressed compressed air is air is keptkept constant constant by bypumping pumping water water through through the the vertical vertical shaft. shaft. Secondly, Secondly, the compressed airair isis stored in stored inflexible flexible bags, bags, and and the the leakage leakage of airof air can can be wellbe well controlled. controlled. The The control control equations equations of the of CAES the system CAES systemare proposed, are proposed, and the and key the parameters key parameters are analyzed. are analyzed. 2. Design of a New Compressed Air Energy Storage System in Hard Rock 2. Design of a New Compressed Air Energy Storage System in Hard Rock 2.1. Methodology of the Research 2.1. Methodology of the Research The configuration of a CAES system is complicated. The present research focuses on the possible 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 application of the system in underground coal mines. Some specific characteristics of the CAES are investigated. The methodology of the research is as follows: system are investigated. The methodology of the research is as follows: (1) Investigate the conditions of different roadways in underground coal mines. 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. Energies 2019, 12, 4188 3 of 14 (2) Describe the typical configuration of a CAES system and propose a new CAES system for Energiesapplication 2019, 12, x in coal mine roadways, which features a combination of pumped-hydro3 andof 14 compressed-air methods, and utilization of flexible bags to store compressed air. (3)3) DeduceDeduce the the governing governing equations equations of of the new CAES system to investigateinvestigate thethe responseresponse ofof thethe systemsystem during during release release of of compressed compressed air. air. (4)4) CarryCarry out out case case studies to investigate the powerpower generatorgenerator capacitycapacity andand operationoperation timetime withwith roadwaysroadways of of different different depths.