Structural safety of compressed air energy storage sealing materials

The structural safety of sealing materials is one of the important technical problems of compressed air energy storage. FLAC3D is used to analyze and compare various combination schemes using steel lining and flexible material as sealing materials and evaluate the safety of each scheme, which provides a reference for the design of high-pressure gas storage. The contact surface between the concrete and the steel lining is set up to enhance the relative sliding between them, which is conducive to the coordination of steel plate deformation and the reduction of steel plate stress. The stress of steel plate can be significantly reduced by placing some flexible materials in steel plate and lining concrete, and the lower the modulus of flexible materials is, the better the stress of steel plate structure is.


Introduction
Energy storage is a key supporting technology to achieve the dual-carbon goal and the energy revolution, and compressed air energy storage is one of the large-scale power energy storage technologies with the most development potential [1~2].Compressed air energy storage power generation technology is the use of equipment to compress air into high-pressure and high-density gas storage, until the need for power generation to release the high-pressure gas, and then through the equipment to complete the conversion of gas to electricity.Compressed air energy storage has high safety, and has the advantages of large energy storage scale, long discharge time, and long service life, and air can be recycled [3~4].Relevant universities, research institutes, and enterprises at home and abroad attach great importance to the research, development, and application of compressed air energy storage technology.
So far, the structure which is composed of the surrounding rock, concrete lining, and seal layer proposed by Salter et al. [5] and Zhuang et al. [6] is the most used way to store high-pressure air.Among them, the surrounding rock bears the pressure of the cavern, the lining transmits the pressure, and the sealing layer seals the gas stored in the cavern.How to ensure the structural safety of sealing materials is an important technical problems of compressed air energy storage [7].There are many reports on the mechanical stability of underground gas storage, but few studies on the structure of the sealing layer [8~11].Therefore, based on an underground gas storage project, this paper establishes a numerical model to analyze and study the sealing structure of the gas storage under stable high-pressure action, evaluates the tightness and safety of the gas storage, and provides a reference for the design of the high-pressure gas storage.

Scheme
Relying on the site selection of the project, the buried depth is 260 m, the lining thickness is 50 cm, the inner diameter is 10 m, and the operating pressure is 10 MPa.Because of its high strength, reliable performance, and easy processing, steel is one of the preferred materials [12] for compressed air.Relying on the project, steel plates are used as sealing materials, and 15 drums are evenly arranged on the steel plates to enhance the structural safety of the steel plates.Given the possible sealing scheme, its structural safety during operation is calculated and analyzed, and a variety of combination schemes using steel lining and flexible materials as sealing materials are proposed.According to the different sealing structures, sealing materials, and combination types, the following schemes are proposed.

Model
In this paper, FLAC3D is used to establish the overall model, and the calculation model is shown in Figure 1.The calculation parameters are shown in Table 1, in which the linear elastic model is used for the steel liner sealing layer and the ideal elastic-plastic constitutive model based on the Mohr-Coulomb yield criterion is used for the rest materials.
Coordinate system: the center point of the hole is the center of XYZ coordinate, XY is horizontal, the axis of the hole is horizontal X direction, along the height is vertical Z direction, and vertical upward is positive.It conforms to the Cartesian right-hand coordinate system rule.

Stress analysis of the sealing layer
The main stress statistics of steel plates under each scheme are shown in Table 2.
① The principal tensile stress of the steel plate under calculation Scheme 2 (194 MPa) is much less than that under calculation Scheme 1 (723 MPa), and the principal tensile stress under calculation Scheme 4 (57 MPa) is much less than that under calculation Scheme 3 (357 MPa), indicating that the sliding between steel plate and concrete is allowed, and the deformation of steel plate can be coordinated to reduce the stress.
Although steel plate and concrete belong to different materials, the contact surface itself has a certain slippability, but because the internal pressure of compressed air is often relatively large, during the operation period, the steel plate and the concrete surface under the action of internal pressure will be in close contact.Therefore, the design needs special treatment to increase the slippability of the contact surface.
② The main tensile stress of steel plate under calculation Scheme 3 (357 MPa) is much less than that under calculation Scheme 1 (723 MPa), and the main tensile stress under calculation Scheme 4 (57 MPa) is much less than that under calculation Scheme 2 (194 MPa), indicating that certain flexible materials arranged in steel plate and lining concrete can significantly reduce the stress of steel plate.
③ The main tensile stress of steel plate under calculation Scheme 5 (280 MPa) is smaller than that under calculation Scheme 3 (357 MPa), indicating that reducing the modulus of flexible materials can reduce the stress of steel plate.
For support engineering, the steel plate design allows a stress of 290 MPa, and it is recommended to arrange a sealed structure in options 2 and 5.The stress nephogram of the sealing layer of Scheme 2 and Scheme 5 is shown in Figure 2.
Table 2. Stress of the sealing layer in each scheme (unit: MPa).

Stress analysis of flexible materials
In Schemes 3, 4, and 5, some flexible materials are arranged between steel plate and concrete.
According to the analysis in Section 3.1, flexible materials can obviously adjust the stress state of steel plates.The main stress results of flexible structures under each scheme are shown in Table 3. Flexible materials are mainly manifested as compressive stress, basically no tensile stress, and no shear-tensile damage.The maximum compressive stress of recommended solution 5 is -14.96MPa, which meets the stress requirements of flexible materials.As shown in Figure 3, the plastic zone does not appear in the flexible material of Scheme 5. Table 3. Stress of flexible materials in each scheme (unit: MPa).

Displacement analysis
Under the action of 10 MPa pressure, the displacement is outward expansion.The maximum displacement and vertical displacement of the sealing layer appear at the top and bottom of the free surface.The maximum Y-displacement occurs on the left and right sides, and the Y-deformation is basically symmetrical along the Z-axis.The plate displacement statistics under each scheme are shown in Table 4. Table 4. Displacement of the sealing layer in each scheme (unit: cm).2.31 ① The transverse displacement of steel plate under calculation Scheme 2 is larger than that under calculation Scheme 1, and the transverse displacement under calculation Scheme 4 is larger than that under calculation Scheme 3, indicating that the tighter the connection surface between sealing layer and lining concrete is, the smaller the displacement of sealing layer is, and the more unfavorable the self-coordination of steel plate is.
② The transverse displacement of steel plate under calculation Scheme 3 is larger than that under calculation Scheme 1, and the transverse displacement under calculation Scheme 4 is larger than that under calculation Scheme 2, indicating that the arrangement of certain flexible materials between sealing layer and lining concrete increases the circumferential displacement of sealing layer, which is conducive to the coordination of steel plate deformation and the reduction of steel plate stress.
③ The transverse displacement of the steel plate under calculation Scheme 5 is larger than that under calculation Scheme 3, indicating that the softer the steel plate is, the more flexible the material arranged in the lining concrete is, and the larger the displacement of the steel plate is, and the more favorable the deformation coordination of steel plate is.

Conclusions
This paper analyzes and compares various combination schemes using steel lining and flexible materials as sealing materials.According to the conventional design, steel lining is only placed on the lining concrete surface as sealing material, which is not conducive to the structural stress of the steel plate.If a certain contact surface is set between the lining concrete and the steel plate, the relative sliding between the two is enhanced, which is conducive to the deformation coordination of the sealing layer and the stress reduction.The arrangement of flexible materials in steel plate and lining concrete can also significantly reduce the stress of the sealing layer, and the lower the modulus of flexible materials, the better the stress of the sealing structure.For the design of the compressed air seal layer, the design of the connecting surface between the steel plate and the lining concrete is very important.

Figure 2 .
Figure 2. The maximum principal stress of the sealing laye.

Figure 4 Figure 4 .
Figure 4. Displacement cloud image of steel plate in Scheme 5.
Scheme 1: Rock + concrete + 6 mm thick steel plate, fixed design between steel plate and concrete.Scheme 2: Rock + concrete + 6 mm thick steel plate, sliding design between steel plate and concrete.
Scheme 3: Rock + concrete + 1 cm thick flexible material 1+6 mm thick steel plate, fixed design between steel plate and concrete.Scheme 4: Rock + concrete + 1 cm thick flexible material 1+6 mm thick steel plate, sliding design between steel plate and concrete.Scheme 5: Rock + concrete + 1 cm thick flexible material 2+6 mm thick steel plate, fixed design between steel plate and concrete.
(a) Steel plate (b) Global model Figure 1.Calculation model.