Abstract
The contradiction between supply and demand of energy leads to more and more attention on the large-scale energy storage technology; Compressed Air Energy Storage (CAES) technology is a new energy storage technology that is widely concerned in the world. The research of coupled heat transfer and seepage in fractured surrounding rocks is the necessary basis to evaluate the operation safety and effectiveness of CAES. Current studies point to the possibility of cracking in concrete liner seals, but the thermodynamic processes and leakage characteristics of compressed air in the presence of cracking and the heat transfer characteristics of seepage have not been addressed and reported. In order to investigate the leakage, the gas seepage and heat transfer law in fractured rock when the hard rock CAES gas reservoir seal cracks, the COMSOL fracture Darcy module, and the non-Darcy Forchheimer model are used as the constitutive seepage. The global ODE is used to calculate the thermodynamic process of compressed air in gas storage with coupled seepage and heat transfer process. The pressure and temperature of compressed air are obtained as the unsteady boundary of the seepage heat transfer model. A program for calculating the seepage and heat transfer characteristics of fractured surrounding rock in the CAES gas reservoir is established. On this basis, with the proposed Suichang CAES cavern as the background, the seepage and heat transfer characteristics of the fractured surrounding rock of the gas storage are studied. The results showed that when there are fewer cracks in the lining and surrounding rock of the air reservoir, the air pressure decreases due to a small amount of air leakage after 30 operation cycles, and the leakage rate of each cycle is 0.7% of the gas storage capacity, but it still meets the engineering requirements. If the plant is operating under these conditions, the charging rate will need to be increased by 1.2 kg/s per cycle charging stage. In the discharging and power generation phase, the high-pressure air that previously percolated into the rock mass cracks could flow back into the air storage through the lining cracks. Therefore, it is incorrect and unreliable to consider the gas which flows out from the inner surface of the lining as unusable. When the lining crack width is less than 0.3 mm, the seepage flow is Darcy flow and the non-Darcy effect can be ignored; when the lining crack width is greater than 0.5 mm, the non-Darcy effect of seepage cannot be ignored. The gas velocity in the surrounding rock fracture medium is on the order of 0.01 m/s with an influence range of over 100 m, and the gas velocity in the pore medium is on the order of 10-6 m/s with an influence range of 50 m. The findings of this study contribute to a better understanding of the interaction between the thermodynamic properties of compressed air and the seepage heat transfer process in compressed air storage underground reservoirs, as well as the gas leakage process in the event of liner seal cracking.
Highlights
Compressed Air Energy Storage (CAES) is a new technology suitable for large-scale power energy storage; it is one of the key technologies to solve the instability and intermittency of large-scale clean energy such as wind and solar energy [1,2,3,4]
The unsteady boundary of compressed air temperature and pressure coupled with a seepage heat transfer term in the air reservoir was defined by using a custom global ODE method and applied to the inner boundary of the tunnel
(1) After cracking of the concrete lining of the CAES gas storage in the example of this study, under the condition of 3 fractures which is 0.3 mm wide and 50 m long on the lining, 0.7% of the compressed air in the cavern was lost in each operation cycle, which still met the engineering needs
Summary
Compressed Air Energy Storage (CAES) is a new technology suitable for large-scale power energy storage; it is one of the key technologies to solve the instability and intermittency of large-scale clean energy such as wind and solar energy [1,2,3,4]. The seepage and heat transfer characters in the surrounding rock under liner seal cracking failure and the effect of leakage on the thermodynamic process of compressed air in the reservoir have not been studied and reported yet. A non-Darcy correction factor without the flow velocity term is proposed in the theory, which enables any numerical simulation software containing the Darcy model to calculate the non-Darcy seepage process by this correction factor, a mathematical model of compressed air thermodynamics considering seepage and heat transfer is established, and the results of the analytical solution are used as the unsteady state boundary to calculate the seepage heat transfer process in the gas storage reservoir. The dynamic change of fracture width with the process of gas filling and deflation is not considered
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