*Article* **Temperature Regulation Model and Experimental Study of Compressed Air Energy Storage Cavern Heat Exchange System**

**Peng Li 1,2,3, Zongguang Chen 3, Xuezhi Zhou 1,2,\*, Haisheng Chen 1,2 and Zhi Wang 4,\***


**Abstract:** The first hard rock shallow-lined underground CAES cavern in China has been excavated to conduct a thermodynamic process and heat exchange system for practice. The thermodynamic equations for the solid and air region are compiled into the fluent two-dimensional axisymmetric model through user-defined functions. The temperature regulation model and experimental study results show that the charging time determines the air temperature and fluctuates dramatically under different charging flow rates. The average air temperature increases with increasing charging flow and decreasing charging time, fluctuating between 62.5 ◦C and −40.4 ◦C during the charging and discharging processes. The temperature would reach above 40 ◦C within the first 40 min of the initial pressurization stage, and the humidity decreases rapidly within a short time. The use of the heat exchange system can effectively control the cavern temperature within a small range (20–40 ◦C). The temperature rises and regularly falls with the control system's switch. An inverse relationship between the temperature and humidity and water vapor can be seen in the first hour of the initial discharging. The maximum noise is 92 and 87 decibels in the deflation process.

**Keywords:** compressed air energy storage; heat exchange system; thermodynamic response; high pressure; charging process; temperature regulation

## **1. Introduction**

With the gradual development of global carbon emission reduction actions, vigorously developing renewable energy has become an inevitable choice in the new situation. Renewable energy has the advantage of being clean and pollution-free. It has many defects such as instability and difficulty in storage which urgently need corresponding energy storage technology innovation to match. Compressed air energy storage (CAES) is one of the most promising large-scale energy storage technologies. Compared with pumped hydroelectric storage (PHS), CAES is not limited by water source and is a better choice for efficient storage and utilization of clean energy [1].

Today, two existing commercial CAES plants are in operation: a 290 MW unit built in Huntorf, Germany, in 1978, and a 110 MW unit built in McIntosh, AL, USA, in 1991 [2]; the monitoring data of their successful operation bring some valuable validation data for the research related to compressed air energy storage caverns [3]. The research and development progress on energy storage technologies in China has also developed more rapidly [4]. The grid connection of the Feicheng salt cavern advanced CAES plant was realized in 2021 [5]. Other caverns, such as salt caverns [6], abandoned mine caverns [7], underground aquifers [8], and artificial rock-lined caverns [9], can also be used as gas storage design alternatives. Moreover, compared with natural reservoir caverns, artificial caverns with lining, which are more flexible in site selection and more adaptable to the

**Citation:** Li, P.; Chen, Z.; Zhou, X.; Chen, H.; Wang, Z. Temperature Regulation Model and Experimental Study of Compressed Air Energy Storage Cavern Heat Exchange System. *Sustainability* **2022**, *14*, 6788. https://doi.org/10.3390/su14116788

Academic Editors: Luis Hernández-Callejo, Jesús Armando Aguilar Jiménez and Carlos Meza Benavides

Received: 10 May 2022 Accepted: 30 May 2022 Published: 1 June 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

design of large-scale energy storage, are one of the preferred options for achieving energy storage in the future.

Kushnir et al. [10] derived an analytical solution for the temperature and pressure variation of the air in the cavern in adiabatic mode. Then, the theoretical solution for the thermodynamics of the cavern in the heat transfer model was derived based on the air mass and energy conservation equations considering the heat transfer at the cavern wall. It significantly affects the air temperature and pressure variation compared to the adiabatic model [11]. Many scholars also cite this calculation model, and the calculation results are compared with the test data of the Huntorf power station [12]. Kim et al. [13] applied TOUGH-FLAC to study the thermodynamic and mechanical response of lined caverns, and Zhou et al. [14] calculated the heat–flow–solid (THM) coupling process of lined caverns based on COMSOL. Many thermodynamic simulations of CAES caverns show that the temperature field inside the air storage caverns is unevenly distributed and may form extremely high temperatures locally, which poses a significant threat to the lining and surrounding rocks [15]. At the same time, the gas inside the cavern may produce significant temperature fluctuations during the cyclic gas filling and discharging process of the air storage caverns. Under the coupling effect of cyclic temperature and stress, the chambers are prone to thermal stress disasters and safety risks in long-term operation [16]. One of the significant problems of CAES systems is the air temperature rise or fall during the compression or expansion operation, resulting in low efficiency. Some works of literature describe enhancing heat transfer by implementing thermal management measures [17]. Others use numerical and experimental methods to characterize fluid flow patterns and heat transfer behavior at the local level [18–20]. GOUDA proposed a 3D CFD model to simulate the air compression process to achieve near-isothermal operation [21]. It is essential to carry out a thermodynamic simulation of the cavern chamber filling and discharging process and to intervene manually in the possible extreme temperature conditions to realize piezo gas storage power generation [22].

In order to gain insight into the thermodynamic and mechanical response of the operation process of CAES caverns, many countries internationally have carried out experimental cavern tests. Ishihata et al. [23] tested the sealability of a deeply buried underground gas storage reservoir with a test air pressure of 0.9 MPa. However, the test results showed severe cracking of the sealing layer. Swedish scholars GEISSBÜ conducted AA-CAES demonstration plant gas storage adiabatic mode thermal storage test. Due to concrete plug leakage, the air pressure only reached 7 bar, and the thermodynamic response was consistent with the simulation results [24]. An underground lined rock cavern for small-scale pressure gas storage tests as a storage reservoir was tested by Kim. At 100 m underground burial depth, the radius of the cylindrical tunnel designed for gas storage was 2.5 m, and the maximum gas storage pressure was 5 MPa [13]. It is a tremendous challenge for a compressed air energy storage plant to determine whether the test can be conducted for high internal pressure in an underground storage cavern without guaranteeing leakage.

Taking the exploration tunnel of Pingjiang Pumped Storage Plants in Hunan Province, China's first underground gas storage test cavern with a shallowly buried lining of hard rock has been reconstructed to realize the gas storage test with a high internal pressure of 10 MPa. In this paper, we would like to develop a temperature field analysis model for a model underground high-pressure air storage cavern, analyze the temperature fluctuation law of the gas filling and discharging process, and design a heat transfer system in the cavern. Based on thermodynamic, heat transfer, and numerical heat transfer methods, air charging and discharging and heat transfer performance tests in the cavern will be conducted.

#### **2. CAES Cavern Design**

The test cavern established in this study is located in the exploration tunnel (PD4) of the underground powerhouse of the Pingjiang Pumped Storage Power Station in China. The design of the cavern is shown in Figure 1. The buried depth of the testing cavern was about 110 m. The length, the inner diameter, the volume, and the inner surface area

were 5.0 m, 2.9 m, 28.8 m3, and 50.6 m2, respectively. A concrete lining of 0.5 m was set in the testing cavern with a fiber-reinforced plastic (FRP) sealing layer on the surface of the lining. A plug was set at the inlet end of the test chamber to bear the thrust of high-pressure compressed air (maximum design pressure was 10.0 MPa). The test system includes a vehicle-mounted air compressor pressurization system, a charging and discharging pipeline system, cavern gas storage, sealing, and measurement system. The surrounding rock in the flat exploration cave was mainly granite and granite gneiss with a mean value of elastic modulus, deformation modulus, and compressive strength of 63.62 GPa, 35.59 GPa, and 78~130 MPa, respectively. The location of the rock mass was of good quality. The fundamental physical parameters such as density, specific heat, and thermal conductivity of solid materials are shown in Table 1. It was assumed that the changes in solid physical parameters within the calculation temperature range are small and have little effect on the results. The air compressor was designed with a heat storage device to cool down gradually, and the outlet temperature would be cooled down to 30 ◦C.

**Figure 1.** Schematic diagram of the Pingjiang CAES cavern. (Unit: mm).


