Energy Storage Caverns Research Articles

Mechanical performance of polyurethane polymer mortar—A novel sealing material for CAES man-made caverns

This work aims to investigate the feasibility of polyurethane polymer mortar (PPM) as a sealing material for compressed air energy storage (CAES) caverns. The effect of polymer content, aggregates ratio (fine sand to medium sand) and filler types (bentonite, clay, and fly ash) on gas permeability, tensile strength, and resistance to high and low temperature of PPM were conducted. The scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) were performed to characterize the microstructure of PPM. Additionally, numerical simulations were performed to analyze the stress distribution characteristics of PPM when used as a sealing structure. The results demonstrate that PPM exhibits low gas permeability (10−21 m2), high tensile strength (up to 2.96 MPa), and significant tensile strain (up to 27.02 %), with a relatively low elastic modulus (11.87–35.05 MPa). Meanwhile, PPM containing fly ash showed the highest strength, followed by those with clay, while PPM with bentonite exhibited the lowest strength. However, only the PPM with bentonite as a filler remained stable and did not collapse under high temperatures. The hoop stresses in the PPM are only compressive and not tensile, and the maximum hoop tensile strain remains well below the ultimate tensile strain of the material, thus preventing the tensile damage and ensuring the gas-tightness. These findings provide a theoretical basis for using PPM in CAES sealing applications, and advance the understanding and application of sealing materials in CAES man-made caverns.

Research of interlayer dip angle effect on stability of salt cavern energy and carbon storages in bedded salt rock

In order to clean and decarbonize energy, large-scale underground energy storage in salt caverns can be utilized. The construction and safety evaluations for these caverns are crucial, especially for salt caverns with significant interlayer dip angles. Aimed at the bedded salt rocks for energy and carbon storage, this study focuses on the impact of different interlayer dip angles on the stability of underground energy and carbon storage caverns in bedded salt rock formations in China. Four salt caverns with varying interlayer dip angles of 0–30° were established for stability comparison. Numerical simulations reveal that caverns with high interlayer dip angles demonstrate low volume shrinkage, general deformation resistance and small equivalent strain, while exhibiting a larger volume of plastic zones compared to caverns with lower interlayer dip angles. It is important to consider the deformation of the cavern wall rock in the down-dip direction for caverns with high interlayer dip angles. The presence of inclined interlayers affects the symmetry of the contour distribution of deformation, safety factor and equivalent strain. Furthermore, recommendations are provided for the construction of salt caverns in salt rock formations with high interlayer dip angles.

Cavern spacing of CAES cavern group in hard rock based on plastic zone and deformation analysis

Existing research on layout parameters of compressed air energy storage (CAES) cavern groups in hard rock has indicated that the key influential factors for group stability include buried depth, diameter, and cavern spacing. Among these, in contrasting to other factors, the research on cavern spacing is limited. This paper focuses on the issue of safe spacing and proposes a numerical method to determine it. By establishing a series of finite element models, the stress state of surrounding rock, changes in shape, equivalent plastic strain of plastic zone, and cavern wall deformation under different spacing have been studied. When a group of air storage caverns in rows is subjected to high internal pressure, for a small spacing, the plastic zone of the surrounding rock between two caverns is connected. Here, the size of the plastic zone, equivalent plastic strain value, and deformation of the cavern wall are relatively large, making the surrounding rock highly susceptible to damage. As the spacing increases, the horizontal width of the plastic zone, the maximum equivalent plastic strain value, the displacement changes of the cavern wall remain stable. This spacing is virtually the same as that when the plastic zone of the surrounding rock is no longer connected and can be selected as a safe spacing. This study explores the design of cavern safe spacing of underground CAES cavern groups and discusses several influencing factors, which can provide references for follow-up energy storage engineering and research.

Influence of spatial variability in surrounding rock on fracture propagation in compressed air energy storage caverns

The expansion of fractures around underground caverns for compressed air energy storage (CAES) has a significant impact on engineering stability. We established a thermo-mechanical coupled random phase field model to investigate the trajectory of fractures in the heterogeneous surrounding rock of underground gas storage caverns. By simulating the different mechanical parameters under different coefficients of variation and autocorrelation distances, we explored the impact of spatial variability of rock mass on the fracture mode of surrounding rock in underground gas storage caverns. The results indicate that when only the spatial variability of elastic modulus is considered, the coefficient of variation primarily affects the inclination and curvature degree of fractures while the autocorrelation distance has a slight impact on the bending shape of fractures. When only the spatial variability of fracture energy release rate is considered, both the coefficient of variation and the autocorrelation distance have a negligible effect on fracture propagation. Compared with the elastic modulus of the rock mass, the fracture energy release rate has a more significant impact on the path of the cavern fractures.

A variable pressure water-sealed compressed air energy storage (CAES) tunnel excavated in the seabed: Concept and airtightness evaluation

For compressed air energy storage (CAES) caverns, the artificially excavated tunnel is flexible in site selection but high in sealing cost. A novel concept of building a water-sealed CAES tunnel in the seabed is proposed in this study, and the airtightness of the system is preliminarily evaluated. Based on the proposed variable pressure water-sealed CAES tunnel excavated in the seabed and combined with the actual operation of the system, a multifield and multiphase coupling calculation model for the airtightness calculation of water-sealed CAES tunnel under pressure changes was constructed, and the rationality of the model was confirmed by a case study. The airtightness analysis of the water-sealed CAES system is carried out. It is an effective method to excavate a tunnel in the seabed, and sealing the CAES system by utilizing the water head pressure in the pores of the rock mass and the low permeability of the rock mass. To satisfy the sealing requirements of the CAES system, the seawater depth of the tunnel should be deeper than 190 m if the gas pressure inside the tunnel is 4.5–10.0 MPa with the rock mass permeability of 1.0 × 10-16 m2 and the overlying rock thickness of 100 m, When the gas pressure inside the tunnel is 4.5–10.0 MPa, the overburden thickness is 100 m and the seawater depth is 50 m; the rock mass permeability should be less than 7.32 × 10-17 m2.

Theoretical and Technological Challenges of Deep Underground Energy Storage in China

Deep underground energy storage is the use of deep underground spaces for large-scale energy storage, which is an important way to provide a stable supply of clean energy, enable a strategic petroleum reserve, and promote the peak shaving of natural gas. Rock salt formations are ideal geological media for large-scale energy storage, and China is rich in salt rock resources and has a major shortage of energy storage space. Compared with the salt domes in other countries, the salt rock formations in China are typical lacustrine bedded salt rocks characterized by thin beds, high impurity content, and many interlayers. The development of large-scale energy storage in such salt formations presents scientific and technical challenges, including: ① developing a multiscale progressive failure and characterization method for the rock mass around an energy storage cavern, considering the effects of multifield and multiphase coupling; ② understanding the leakage evolution of large-scale deep underground energy storage caverns; ③ understanding the long-term performance evolution of large-scale deep underground energy storage caverns; ④ developing intelligent construction technologies for the deep underground salt caverns used for energy storage; and ⑤ ensuring the long-term function of deep underground energy storage spaces. The solution to these key scientific and technological problems lies in establishing a theoretical and technical foundation for the development of large-scale deep underground energy storage in China.

Gas-mechanical coupled crack initiation analysis for local air-leakage of compressed air energy storage (CAES) cavern with consideration of seepage effect

The gas-tightness and stability of compressed air energy storage (CAES) cavern (usually with three-layered struction of sealing layer, concrete lining and surrounding rock) plays an important role for ensuring the power-generation efficiency of CAES system. Since the action of the compressed air with high temperature and high pressure would possibly cause the local damage of the sealing layer and lead to the air-leakage from the sealing layer to the surrounding rock, it is necassary to study the gas-mechanical (GM) coupled crack initiation behavior of the surrounding rock in order to provide a scientific basis for anti-cracking design and stability evaluation of CAES cavern. Currently, the GM coupled crack initiation are studied mainly by experimental and numerical methods but little by theoretical method, in which the gas pressure is taken as a static pressure regardless of the seepage effect. In this paper, the calculation formulea of GM coupled Mode I and Mode II SIFs are derived by the superposition principle with the consideration of seepage effect firstly. And then the criterion of GM coupled crack initiation is established based on our K-ratio criterion in order to analyze its mechanism for local air-leakage of CAES cavern. Finally, a gas seepage device is designed to conduct the tri-axial GM coupled compression tests of pre-cracked sandstone specimens on MTS815 hydraulic servo testing machine combined with PCI-II AE instrument to measure the crack initiation stress σi and initiation angle θC. Research results show both σi and θC are decreased with the increasing of gas pressure PG, while increased with the increasing of crack inclination angle β. All of these pre-cracked sandstone specimens have initiation mechanisms of Mode II. The prediction results of initiation angle with consideration of seepage effect is more closer to the test results than those of initiation angle without consideration of seepage effect, which can prove the validity of both our SIFs calculation formulea and the GM coupled criterion with consideration of seepage effect.

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

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.

Temperature and pressure variations in salt compressed air energy storage (CAES) caverns considering the air flow in the underground wellbore

The flow of compressed air in the wellbore affects the thermodynamic performance in the salt compressed air energy storage (CAES) cavern and this effect is still uncharted. In this study, a coupled explicit finite difference model considering the wellbore flow is proposed to obtain thermodynamic performance of the compressed air in the cavern. It is found that the thermodynamic performance is directly determined by the air temperature and pressure at the wellbore outlet, which are changed by the flow in the wellbore. A sensitivity study shows that the injection mass rate is the key parameter and significantly increases the air temperature and pressure in the cavern. The injection temperature and wellbore length are secondary factors. A high injection pressure is beneficial for energy storage. The inner diameter, roughness and thermal conductivity of the wellbore influence the performance slightly. The developed combined model is a more accurate tool for predicting the thermodynamic performance of a deep CAES cavern.

Study on creep characteristics and constitutive model of typical argillaceous salt rock in energy storage caverns in China

Salt rocks are internationally recognized as a suitable medium for energy storage because of their excellent properties, so the creep behavior of salt rocks has received a lot of attention from scholars at home and abroad. However, the creep behavior of Chinese salt rocks is different from that of high purity salt rocks because they are mostly interbedded, which poses a significant challenge to the safe operation of energy storage salt caverns in China. Therefore, this paper takes a typical muddy salt rock in China as the research object and conducts uniaxial creep tests, considering the accelerated creep phase as a coupled process of loading and creep. A new nonlinear creep damage constitutive model was derived, which was verified based on the creep test data and compared with the traditional classical element model. The model can describe the accelerated creep stage of the specimen well. The model's applicability is also examined, and it is found that the model has a good description for mudstone, high-purity salt rock and mudstone salt rock combination, which provides theoretical support for the safe operation of salt caverns containing interbedded energy storage.

A new theoretical model of local air-leakage seepage field for the compressed air energy storage lined cavern

Current study on air-leakage of compressed air energy storage (CAES) cavern is focused on the global air-leakage seepage field without consideration of the thermo-gas-mechanical (TGM) coupling theory. In this study, a new theoretical model of local air-leakage seepage field for the lined CAES cavern is established based on the thermo-gas-mechanical (TGM) coupling theory and boundary conditions of the non-continuous and non-axisymmetric seepage. Calculation results show that with the increasing of the air-leakage distance, the local air-leakage seepage field p(r,θ,t) is decreased fast at first and then tends to the stable value p(r,θ,t)= 0.1 MPa (atmospheric pressure) at the maximum leakage distance RAA'm (when the air-leakage is stopping). The distribution area of the local air-leakage seepage field (related to the radius r and polar angle θ) is increased gradually from the non-axisymmetric about the center of cavern fan-shape and peach-shape to the axisymmetric circle, which is different from that of the global air-leakage seepage field (circle, independent of θ ). The new theoretical model of local air-leakage seepage field is proved to be valid by numerical simulation results and provides a scientific basis for the safety assessment of the underground geotechnical engineering with local fluid-leakage.

Stability analysis for compressed air energy storage cavern with initial excavation damage zone in an abandoned mining tunnel

Compressed air energy storage (CAES) is a buffer bank for unstable new energy sources and traditional power grids. The stability of a CAES cavern is a key issue to cavern safety. However, the stability of a cavern from an abandoned mining tunnel has not been well studied. This paper investigates the stability of the CAES cavern from an abandoned mining tunnel. Firstly, an initial heterogeneous excavation damage zone (EDZ) in the surrounding rock mass is partitioned based on mining and operation processes. Then, a thermal-hydraulic-mechanical coupling model is formulated for the CAES cavern with a sealing layer and implemented within COMSOL Multiphysics. After verification with available data in literature, this coupling model is used to investigate the influence of partitioned EDZ heterogeneity on the displacement and stability of cavern. It is found that the displacement at the cavern wall increases with operation cycle. The principal stress decreases with operation cycle in the high-damage zone but remains stable in the low-damage zone. The principal stress difference changes first and then becomes stable in the disturbance zone. Larger cavern radius, heat transfer coefficient, initial pressure of cavern, temperature of injected air, and faster gas injection rate induce bigger displacement. These results on the stability of a CAES cavern can be a basis for the repair of an existing cavern.

Construction simulating and controlling of the two-well-vertical(TWV) salt caverns with gas blanket

The two-well-vertical (TWV) cavern-constructing with gas blanket technology is an important alternative to the single-well (SW) with oil blanket technology. This technology has the advantages of achieving larger cavern volume, clean brine, great water flow rate, and high economic benefits. Promoting and developing its theory and technology will help the future development of salt mining and cavern construction for energy storage in China, as it is in the initial stage. In this paper, a visualized physical simulation platform of water dissolving of the TWV caverns with gas blanket was established, and large size salt rock blocks were used to simulate the constructing process of salt caverns. The similarity ratios of size, flow rate, and time between prototype and model were calculated by using the similarity theory and dimensional analysis. A series of physical simulation tests of water-dissolving caverns were carried out, that the research factors including the two-well-spacing, flow rate, and gas/oil blanket. The results show that: (1) The water flow rate can be increased to 200–400 m3/h by increasing the two-well-spacing when using the TWV technology, which is 2–4 times that of the single-well. (2) It is found that 30–40 m is the optimal size for the two-well-spacing, and the two-well-space is much larger than 40 m may exist risks of roof instability. (3) During the initial stage of cavern construction, it will obtain a smaller lateral solution angle and an ideal cavern bottom groove shape with gas blanket. After the cavern construction is finished, it also will obtain better cavern shape, larger cavern volume and clean brine by comparing with oil blanket. So, the technology of the TWV cavern construction with gas blanket is promising for brine leaching and energy storage cavern construction and it is also worth further study.

Investigation on the influences of interlayer contents on stability and usability of energy storage caverns in bedded rock salt

to balance the intermittent of renewable power, large-scale energy storage in underground salt caverns can be utilized. And the construction and safety evaluations for these caverns become necessary. Aimed at the bedded salt rocks for energy storage, this study focuses on the effect of different interlayer content on the stability and usability of the underground energy storage caverns in bedded salt of China. Salt caverns with five different interlayer content of 0–30% are established for stability and usability comparison. The numerical simulations show that the caverns with high interlayer content have better stability performance than that of the cavern with lower or no interlayer content. A usability evaluation is also discussed by evaluating the effective volume of caverns with different interlayer content. Even though the caverns with high interlayer contents have less volume shrinkage, they have smaller available volume because the collapsed sediments of the interlayer expanded in the cavern bottom. Finally, a method to increase the effective cavern volume is discussed. That is to enlarge the bottom cavern in caverns with high interlayer content.

A Modified Creep Model for Cyclic Characterization of Rock Salt Considering the Effects of the Mean Stress, Half-Amplitude and Cycle Period

The fluctuation of the operation pressure during injection and withdrawal is a key factor affecting the long-term stability of surrounding rock in a compressed air energy storage (CAES) cavern. To assess the mechanical behavior of rock salt under different pressure fluctuations and to optimize the operating parameters of CAES caverns, uniaxial cyclic loading tests under the sine condition were carried out by changing three cycle parameters, namely the mean stress, half-amplitude and cycle period. The test results show that the deformation behavior during cyclic loading tests can be divided into transient, steady, and accelerated stages, which is similar to that of creep tests. The strain and steady strain rates increase with increasing cycle parameters, showing a nonlinear deformation property of rock salt under cyclic loading. To describe the nonlinear response that cannot be captured by the Burgers model, a stiffness scaling factor is proposed based on the test results. A modified cyclic creep model of rock salt is established by introducing this stiffness scaling factor into the conventional Burgers creep model. The agreement with the experimental data indicates that the modified cyclic creep model can accurately reflect the influence of the three cyclic parameters on the deformation properties of rock salt.

Shakedown analysis of cavities in cohesive-frictional materials and its application to underground energy storage caverns

In this paper, a shakedown analysis is conducted for both cylindrical and spherical cavities in cohesive-frictional materials subjected to cyclic internal pressures. Based on Melan’s lower-bound shakedown theorem, theoretical shakedown conditions for a two-layered cavity system are developed using the stress and strain solutions obtained from cavity expansion and contraction analyses. The shakedown limits are investigated in relation to the dimensions of the two-layered cavity system and the elastic/plastic material parameters. Numerical validation is conducted through a comparison of the stress path of the critical element and the failure lines of the materials. In addition, closed-form solutions for special cases, namely, the analytical shakedown conditions of cavities in an unbound cohesive-frictional material and in a bound material with different boundary conditions, are explicitly expressed. Finally, the shakedown solutions are applied to solve the thickness design problem of underground compressed air energy storage in lined and unlined caverns.

A viscoelastic, viscoplastic, and viscodamage constitutive model of salt rock for underground energy storage cavern

The underground energy storage in salt rock is of great strategic significance for ensuring the national energy safety and economic development. Due to the complex geological conditions, the construction of underground energy storage cavern in salt rock encounters great challenge and the long-term stability and safety issue of storage cavern in salt rock needs to be solved urgently. Based on the effective stress and the assumption of a small deformation, a unified constitutive model incorporating viscoelasticity, viscoplasticity, and viscodamage is developed by coupling the Kelvin-Voigt model, Duvaut-Lions model, and an improved Darabi viscodamage model to simulate the time-dependent mechanical behavior of salt rock for underground energy storage. The constitutive model is calibrated by experimental data to determine the parameters of viscoelasticity, viscoplasticity, and viscodamage. The model verification and discussion indicate that the constitutive model is able to effectively simulate the mechanical response of salt rock under different loading conditions and provide a safety guarantee for underground energy storage. The constitutive model is numerically implemented and successfully applied in long-term time-dependent deformation and failure analysis of underground energy storage cavern.

A coupled thermo-hydro-mechanical model for evaluating air leakage from an unlined compressed air energy storage cavern

Compressed air energy storage (CAES), a large-scale energy storage technology, is a link between unstable renewable energy and conventional power grids. Air leakage may significantly impact the CAES efficiency. This paper presented a coupled thermo-hydro-mechanical model to evaluate the air leakage from an unlined CAES cavern. This model was validated with field tests, numerical simulations, and an analytical solution. The impacts of air leakage on the variations of temperature and pressure within a CAES cavern and the air seepage in surrounding rock were numerically analyzed. Finally, the effects of rock permeability, mass flow rate of air injection, and cavern radius on air leakage were investigated. It is found that rock permeability is a key parameter to air leakage. For the first cycle and under operational pressure of 5 and 8 MPa, rock permeability should be smaller than 3 × 10−19 m2 to satisfy the tightness requirement for an unlined CAES cavern. That is less than 1% daily air leakage percentage. Larger cavern radius and higher mass flow rate of air injection are helpful to reducing the daily air leakage percentage. These results can provide guidelines for tightness requirements for existing cavern repair or new CAES cavern construction design.

An Analytical Solution for Analyzing the Sealing-efficiency of Compressed Air Energy Storage Caverns

Compressed Air Energy Storage (CAES) is a commercial, utility-scale technology that is suitable for providing long-duration energy storage. Underground air storage caverns are an important part of CAES. In this paper, an analytical solution for calculating air leakage and energy loss within underground caverns were proposed. Using the proposed solution, the air leakage and energy loss under a typical CAES operation pressure were analyzed, and the influences of the permeability of the concrete lining and the lining thickness on the sealing efficiency of underground caverns were investigated. The results showed that under a typical CAES operation pressure, a concrete lining with a permeability of less than 6.36 × 10−18 m2 would result in an acceptable air leakage rate of less than 1%. Air leakage will increase linearly with increasing lining permeability. Increasing the thickness of the concrete lining could enhance the sealing efficiency of the storage cavern.

Numerical modeling of thermally-induced fractures in a large rock salt mass

Numerical modeling of thermally-induced fractures is a concern for many geo-structures including deep underground energy storage caverns. In this paper, we present the numerical simulation of a large-scale cooling experiment performed in an underground rock salt mine. The theory of fracture mechanics was embedded in the extended finite element code used. The results provide reliable information on fracture location and fracture geometry. Moreover, the timing of the fracture onset, as well as the stress redistribution due to fracture propagation, is highlighted. The conclusions of this numerical approach can be used to improve the design of rock salt caverns in order to guarantee their integrity in terms of both their tightness and stability.