Underground Compressed Air Energy Storage Research Articles

Technical feasibility of lined mining tunnels in closed coal mines as underground reservoirs of compressed air energy storage systems

In this paper, four mining levels in a closed coal mine in the Asturian Central Coal Basin (NW Spain) have been selected as a case study to investigate the technical feasibility of underground compressed air energy storage systems. First, in order to determine the suitable level and type of concrete lining, a numerical model has been established to analyze the geomechanical performance considering air pressures of 6, 10, 20 and 25 MPa and three different embodiments of concrete lining. Then, another numerical model is used to study the coupled thermo-mechanical performance at level 3, considering 100 operation cycles between 6 and 10 MPa air pressure using a circular concrete lining with a 20 mm thick sealing layer. The results obtained indicate that the deformations are lower at levels 1 and 3, where the shales are located at the top of the coal seam. Deformations and tensile stresses are significantly reduced when a circular concrete lining is used. As the thermal analysis shows, temperature fluctuations are restricted to sealing layer and concrete lining and do not reach the rock mass itself. Therefore, negligible deformations are produced by the effect of temperature compared to the effect of air pressure. Maximum tensile stress and total displacements during the operation occur at the top of the mining drift and reach 9.5 MPa and 3.6 mm, respectively. A technical feasibility can be achieved using a circular concrete lining with a suitable reinforcement system.

Two-phase Fluid Solid Coupling Modeling: Theory, Numerical Model, and Application

Underground compressed air energy storage (CAES) is seen as an important and promising way for balancing electrical power system. Many large-scale CAES projects (≥300 MW) are being planned in China, some of which have already been permitted for construction. Despite the benefits from CAES, some challenges arise in area of construction technology, operation system, maintenance and monitoring methods. One of the challenges lies in the evaluation of air leakage effect on flow and stress field around the underground cavern. Therefore, considering the actual physical field characteristics, the mathematical formula of mass conservation equations for air and water in porous media, as well as equilibrium equations for solid mechanics are established first and then coupled together for numerical solutions. The constitutive models for solid and porous fluid adopt classical elastic and Darcy’s law respectively, while the air-water interaction adopts classical van-Genuchten model and contributes to solid mechanics by pore pressure. Numerical implementations are based on secondary development to COMSOL software. Then interpretations to simulation results and the comparison with results not taking damage effect into account are made.

Experimental Study on the Mechanical and Permeability Properties of Lining Concrete Under Different Complex Stress Paths

A promising large-scale energy storage is underground compressed air energy storage (CAES) in lined rock caverns. To ensure the safety and stability of storage caverns because of the influence of periodic injection during production, it is crucial to understand the mechanical behavior of lining concrete under different complex stress paths. In this study, three types of uniaxial compressive fatigue test and uniaxial creep test were conducted on concrete. The following conclusions were obtained from the results. 1) The irreversible deformation after the interval was larger than that before the interval in the discontinuous multi-step cyclic loading (DMCL) test. 2) Loading velocity significantly influenced concrete fatigue, and the irreversible deformation in the cycle of low loading velocity was greater than that in the cycle of high loading velocity. 3) The residual strain increased with an increase in stress level. 4) The creep strain increased with an increase in stress level during the multi-step creep loading test; the fractional derivative results were more consistent with the experimental results. 5) The permeability of concrete increased rapidly under the influence of an external force when the stress level exceeded 0.73.

Three-dimensional thermo-mechanical analysis of abandoned mine drifts for underground compressed air energy storage: A comparative study of two construction and plugging schemes

Compressed air energy storage (CAES) is a large-scale energy storage technology that can overcome the intermittency and volatility of renewable energy sources, such as solar and wind energy. Although abandoned mines can be reused for underground CAES of large scale, their feasibility requires further investigations. This study performs a comparative study on the effects of two construction and plugging schemes on the thermo-mechanical responses of an abandoned drift for the 60-MW CAES project in Datong Yunggang Mine, China. The first scheme uses only the drift as the air container, and the shaft is plugged with concrete. The second scheme uses the drift and shaft. A coupled 3D model is established, and the interaction of the temperature field, displacement field, and air state is fully considered. The numerical model is validated via previous analytical and numerical methods. The simulations indicate that air temperature and air pressure evolve “up-down-down-up” in one CAES cycle. The two schemes have roughly consistent air temperature, and relative huge difference exists for the air pressure. During the charging stage, relatively high tensile stress is induced by the air injection into the caverns. The first scheme shows less stress concentration and lower tensile stress, indicating that the concrete plugging at the intersection between Drift 1080 and North-5 Shaft is effective to reduce the stress concentration. This study can serve as reference for CAES design in similar engineering and studies.

Mechanical properties of rock salt under combined creep and fatigue

The mechanical performance of rock salt under combined creep and fatigue plays a critical role in the long-term stability and serviceability of underground compressed air energy storage (CAES). In the present study, laboratory tests were performed to investigate the impact of simultaneous creep and fatigue on the hysteresis behavior, deformability, progressive damage, cycling life, and fracture mechanism of rock salt. The strain-time curves undergo a three-phase (transient, steady-state and tertiary phase) S-shaped evolution, which is similar to that observed in pure creep or fatigue tests. It has been demonstrated that the presence of a stress dwelling period does not change the geometric similarity of the cumulative deformation curve, but may accelerate the accumulation rate of inelastic deformation, leading to a reduction in the cycling life of rock salt. The consistency between acoustic emission (AE) counting rate and the volumetric deformation indicates that micro-cracks are primarily generated during the loading period and grow stably during the stress dwelling period in rock salt within a single loading cycle. The cumulative cyclic damage and time-dependent damage in rock salt promote each other, which results in a notable positive interaction between them. The life of rock salt specimens can be reasonably predicted by employing a nonlinear cumulative damage rule that incorporates an interaction term. The logarithm of the deformation rate in the steady-state phase is linearly correlated with the logarithm of the creep or fatigue life, regardless of the loading history, and serves as an alternative approach for predicting the life of rock salt. It seems that the typical failure pattern tends to transform from multiple shearing to axial splitting as the creep load fraction increases. For rock salt in the combined creep and fatigue test, a mixed microcosmic intergranular and transgranular fracture was observed using a scanning electron microscope.

Converting closed mines into giant batteries: Effects of cyclic loading on the geomechanical performance of underground compressed air energy storage systems

• The feasibility of CAES systems was investigated in a coal mine at 450 m depth. • 3D numerical models were performed to simulate the operating conditions. • Deformations and plasticity states were analyzed at 5, 7.5 and 10 MPa. • The cyclic loading operation was simulated for 10,000 cycles. • An increase of the initial volume of less than 0.5% was observed. There are more than one million abandoned mines around the world. A large number of voids from closed mines are proposed as pressurized air reservoirs for energy storage systems. A network of tunnels from an underground coal mine in northern Spain at 450 m depth has been selected as a case study to investigate the technical feasibility of adiabatic compressed air energy storage (A-CAES) systems. The rock mass in A-CAES plants is subjected on a daily base to mechanical cycling loading during the charge and discharge processes. Therefore, it is essential to analyze the behavior of the rock mass for the entire service life. Two different lining options are analyzed, with 15 cm thick concrete lining and unlined tunnels, both with an internal synthetic seal to avoid air leakage through the lining and rock mass fractures. In this paper, two 3D numerical models have been developed to analyze the geomechanical performance of A-CAES plants. In the first model, deformations and plasticity state are studied assuming pressure values of 5, 7.5 and 10 MPa and considering a storage space of 12,800 m 3 . Then, in the second model, the cycling loading operation is simulated for 10,000 cycles (service life) for lined and unlined tunnels, considering a pressure range between 4.5 and 7.5 MPa. The results obtained show that the rock mass surrounding the tunnels can resist the pressure with moderate deformations and small thickness of plastic zones, while an increase of the initial volume of less than 0.5% has been observed by applying the operating conditions. In addition, no fatigue failure is expected during the operation time.

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.

Long-term stability of a lined rock cavern for compressed air energy storage: thermo-mechanical damage modeling

The long-term stability of a lined rock cavern (LRC) for underground compressed air energy storage is investigated using a thermo-mechanical (TM) damage model. The numerical model is implemented in COMSOL Multiphysics, and TM modeling is verified by the existing analytical solution in the case of no damage. The long-term damage and mechanical responses of the lined cavern are presented. Inhomogeneity and a weak plane are added to the numerical models, and their influences on the stability of the LRC are discussed. Results show that long-term damage occurs within 1 m from the cavern surface. Airtight concrete causes the maximum damage, which occurs in the sealing layer, concrete lining, and host rock, whereas polymer causes the least damage. Compared with the purely thermo-elastic model, the TM damage model obtains larger maximum and minimum principal stresses. In the TM model, the stress of the concrete lining increases and the hoop stress of the host rock decreases with the increase in inclination angle of the weak plane. Moreover, as the inhomogeneity increases, the radial stresses of the concrete and host rock decrease. The maximum hoop stress of the concrete lining in one cycle increases, whereas the maximum hoop stress and maximum hoop strain of the host rock decrease.

Storage system for distributed-energy generation using liquid air combined with liquefied natural gas

This study proposed a storage-generation system for a distributed-energy generation using liquid air combined with liquefied natural gas (LNG). The system comprised three main sites: the renewable-electricity sources (RESs), liquid-air energy storage (LAES), and natural-gas combustion. The low-priced off-peak electricity generated by the RESs was supplied to the LAES. The supplied electricity and previously stored cold energies liquefied the air. At the on-peak time, the liquid air and LNG were pressurized, re-gasified, and burnt immediately after mixing to generate the high-priced electricity while their cold energy was stored in thermal media. The proposed system was evaluated in terms of the thermodynamic, environmental, and economic performances. Its round-trip and storage efficiencies were 64.2% and 73.4%, respectively. The exergy efficiency of the storage-site, the generation-site, and the system was 70.2%, 75.1%, and 62.1%, respectively. The levelized cost of energy (LCOE) ranged from 142.5 to 190.0 $/MWh depending on the sizes and the storage time. The proposed system was compared to the diabatic compressed air-energy storage (CAES) systems and the adiabatic LAES system. The sensitivity analyses compared the systems for the fixed power output and storage time, and for the option to use natural gas. The proposed system showed better storage and round-trip efficiencies than those of comparison systems. Its LCOE was competitive with those of the compared systems except for the under-ground CAES system. The proposed system was an economic and viable option considering the geographical limitations and the environment impacts of the CAES system.

Numerical simulation for the coupled thermo-mechanical performance of a lined rock cavern for underground compressed air energy storage

Compressed air energy storage (CAES) is a technology that uses compressed air to store surplus electricity generated from low power consumption time for use at peak times. This paper presents a thermo-mechanical modeling for the thermodynamic and mechanical responses of a lined rock cavern used for CAES. The simulation was accomplished in COMSOL Multiphysics and comparisons of the numerical simulation and some analytical solutions validated the thermo-mechanical modeling. Air pressure and temperatures in the sealing layer and concrete lining exhibited a similar trend of 'up–down–down–up' in one cycle. Significant temperature fluctuation occurred only in the concrete lining and sealing layer, and no strong fluctuation was observed in the host rock. In the case of steel sealing, principal stresses in the sealing layer were larger than those in the concrete and host rock. The maximum compressive stresses of the three layers and the displacement on the cavern surface increased with the increase of cycle number. However, the maximum tensile stresses exhibited the opposite trend. Polymer sealing achieved a relatively larger air temperature and pressure compared with steel and air-tight concrete sealing. For concrete layer thicknesses of 0 and 0.1 m and an initial air pressure of 4.5 MPa, the maximum rock temperature could reach 135 °C and 123 °C respectively in a 30 day simulation.

Air-Breathing Aqueous Sulfur Flow Battery for Ultralow-Cost Long-Duration Electrical Storage

Summary The intermittency of renewable electricity generation has created a pressing global need for low-cost, highly scalable energy storage. Although pumped hydroelectric storage (PHS) and underground compressed air energy storage (CAES) have the lowest costs today (∼US$100/kWh installed cost), each faces geographical and environmental constraints that may limit further deployment. Here, we demonstrate an ambient-temperature aqueous rechargeable flow battery that uses low-cost polysulfide anolytes in conjunction with lithium or sodium counter-ions, and an air- or oxygen-breathing cathode. The solution energy density, at 30–145 Wh/L depending on concentration and sulfur speciation range, exceeds current solution-based flow batteries, and the cost of active materials per stored energy is exceptionally low, ∼US$1/kWh when using sodium polysulfide. The projected storage economics parallel those for PHS and CAES but can be realized at higher energy density and with minimal locational constraints.

Ambient Temperature Aqueous Sulfur Batteries for Ultralow Cost Grid Storage

Sulfur is an attractive reactant for such concepts due to its exceptionally low cost, high natural abundance, and high specific and volumetric capacity owing to its two-electron reaction. Taking the cost-per-capacity (e.g., in US$/Ah) as a metric, sulfur has the lowest cost of any known electrode-active compound with the exception of water and air. However, in order to take advantage of sulfur’s low-cost potential, all other components must also have low cost. Towards enabling ultralow cost grid storage, we demonstrate an ambient-temperature aqueous rechargeable flow battery that uses low-cost polysulfide chemistry in conjunction with lithium or sodium as the working ion, and an air-breathing cathode. Four different laboratory cell constructions are used to test the half-cell and full-cell reactions, including a pumped air-breathing cell that exhibits stable room-temperature cycling over 960h with a lithium polysulfide anolyte and dissolved lithium sulfate catholyte. In this approach the solution energy density is 30-150 Wh/L, which exceeds current solution-based flow batteries, and the chemical cost of stored energy is exceptionally low, especially when using sodium polysulfide (~1 US$/kWh). Results of techno-economic modeling are also presented, which show that when projected to full system-level, this new approach has energy and power costs that are comparable to those of pumped hydroelectric storage (PHS) and underground compressed air energy storage (CAES), but without their geographical and environmental constraints. This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.

Strength criterion for rocks subjected to cyclic stress and temperature variations

The strength of rocks subjected to cyclic stress and temperature variations must be determined to ensure the stability of caverns for underground compressed air energy storage. This paper proposes a strength criterion for rocks subjected to cyclic stress and temperature variations; the proposed criterion is based on the least energy dissipation principle and the rock damage induced by cyclic stress and temperature effects. The cyclic uniaxial stress-temperature tests and uniaxial compression tests after cycling are conducted to validate preliminarily the proposed criterion. The applicability of the proposed strength criterion and the influence of damage factor selection are discussed. The proposed strength criterion has a clear physical meaning and only involves four parameters to be determined. The predicted rock strengths are in good agreement with the laboratory test results, confirming the applicability of the proposed criterion. The proposed strength criterion can degenerate into certain existing criteria. Moreover, the damage factor is suggested to be calculated with the use of the peak secant modulus when the proposed strength criterion is applied.

Failure Monitoring and Leakage Detection for Underground Storage of Compressed Air Energy in Lined Rock Caverns

Underground compressed air energy storage (CAES) in lined rock caverns (LRCs) provides a promising solution for storing energy on a large scale. One of the essential issues facing underground CAES implementation is the risk of air leakage from the storage caverns. Compressed air may leak through an initial defect in the inner containment liner, such as imperfect welds and construction joints, or through structurally damaged points of the liner during CAES operation for repeated compression and decompression cycles. Detection of the air leakage and identification of the leakage location around the underground storage cavern are required. In this study, we analyzed the displacement (or strain) monitoring method to detect the mechanical failure of liners that provides major pathways of air leakage using a previously developed numerical technique simulating the coupled thermodynamic and geomechanical behavior of underground CAES in LRCs. We analyzed the use of pressure monitoring to detect air leakage and characterize the leakage location. From the simulation results, we demonstrated that tangential strain monitoring at the inner face of sealing liners could enable one to detect failure. We also demonstrated that the use of the cross-correlation method between pressure history data measured at various sensors could identify the air leak location. These results may help in the overall design of a monitoring and alarm system for the successful implementation and operation of CAES in LRCs.

The Fracture Influence on the Energy Loss of Compressed Air Energy Storage in Hard Rock

A coupled nonisothermal gas flow and geomechanical numerical modeling is conducted to study the influence of fractures (joints) on the complex thermohydromechanical (THM) performance of underground compressed air energy storage (CAES) in hard rock caverns. The air-filled chamber is modeled as porous media with high porosity, high permeability, and high thermal conductivity. The present analysis focuses on the CAES in hard rock caverns at relatively shallow depth, that is, ≤100 m, and the pressure in carven is significantly higher than ambient pore pressure. The influence of one discrete crack and multiple crackson energy loss analysis of cavern in hard rock media are carried out. Two conditions are considered during each storage and release cycle, namely, gas injection and production mass being equal and additional gas injection supplemented after each cycle. The influence of the crack location, the crack length, and the crack open width on the energy loss is studied.

Geotechnical Feasibility Analysis of Compressed Air Energy Storage (CAES) in Bedded Salt Formations: a Case Study in Huai’an City, China

The lower reaches of the Yangtze River is one of the most developed regions in China. It is desirable to build compressed air energy storage (CAES) power plants in this area to ensure the safety, stability, and economic operation of the power network. Geotechnical feasibility analysis was carried out for CAES in impure bedded salt formations in Huai’an City, China, located in this region. First, geological investigation revealed that the salt groups in the Zhangxing Block meet the basic geological conditions for CAES storage, even though the possible unfavorable characteristics of the salt formations include bedding and different percentages of impurities. Second, mechanical tests were carried out to determine the mechanical characteristics of the bedded salt formations. It is encouraging that the samples did not fail even when they had undergone large creep deformation. Finally, numerical simulation was performed to evaluate the stability and volume shrinkage of the CAES under the following conditions: the shape of a single cavern is that of a pear; the width of the pillar is adopted as two times the largest diameter; three regular operating patterns were adopted for two operating caverns (internal pressure 9–10.5 MPa, 10–11.5 MPa, and 11–12.5 MPa), while the other two were kept at high pressure (internal pressure 10.5, 11.5, and 12.5 MPa) as backups; an emergency operating pattern in which two operating caverns were kept at atmospheric pressure (0.1 MPa) for emergency while the backups were under operation (9–10.5 MPa), simulated for 12 months at the beginning of the 5th year. The results of the analysis for the plastic zone, displacement, and volume shrinkage support the feasibility of the construction of an underground CAES power station.

Parameters affecting scalable underwater compressed air energy storage

Underwater compressed air energy storage (UWCAES) is founded on mature concepts, many of them sourced from underground compressed air energy storage technology. A fundamental difference between the two systems is the way in which air is stored. UWCAES utilizes distensible boundary, submerged air accumulators as opposed to rigid walled caverns. This paper presents an analysis of the primary design parameters in a basic UWCAES system. The results from the parametric study and first-order sensitivity analysis show the importance and impact each design parameter has on overall system performance and can serve as a first reference guideline in system design. The analysis revealed significant system sensitivities to pipe diameter, expander and compressor efficiencies, and air storage depth. The air compression and expansion processes contributed most to system exergy destruction for all parametric study cases.

Coupled hydromechanical analysis of an underground compressed air energy storage facility in sandstone

The underground storage of compressed air is a favourable and low-cost option for balancing off-peak electricity demands. Large volumes of air can be compressed and stored in the ground during low-demand electricity periods, and this pressurised air can then be released to generate electricity during high-demand times. Excavated caverns (generally in salt rock), depleted gas fields and abandoned/excavated mines are the main options typically considered in the design of a compressed air energy storage (CAES) facility. The storage of compressed air in aquifers is a new alternative that has been studied recently. The natural formations around a CAES facility are subjected to significant perturbations in hydraulic and mechanical fields. This paper presents an analysis of a CAES project in an aquifer using a fully coupled hydromechanical framework with the ability to consider the main physical phenomena that control the behaviour of this kind of system. The case study is based on an actual planned CAES project in Iowa, USA. Particular attention was paid to changes in the hydraulic and mechanical fields induced by the operational phases anticipated in a CAES project and to the impact of porosity heterogeneities on system performance. The analysis confirmed that the site is not suitable for a CAES plant.

An Analytical Solution for Mechanical Responses Induced by Temperature and Air Pressure in a Lined Rock Cavern for Underground Compressed Air Energy Storage

Mechanical responses induced by temperature and air pressure significantly affect the stability and durability of underground compressed air energy storage (CAES) in a lined rock cavern. An analytical solution for evaluating such responses is, thus, proposed in this paper. The lined cavern of interest consists of three layers, namely, a sealing layer, a concrete lining and the host rock. Governing equations for cavern temperature and air pressure, which involve heat transfer between the air and surrounding layers, are established first. Then, Laplace transform and superposition principle are applied to obtain the temperature around the lined cavern and the air pressure during the operational period. Afterwards, a thermo-elastic axisymmetrical model is used to analytically determine the stress and displacement variations induced by temperature and air pressure. The developments of temperature, displacement and stress during a typical operational cycle are discussed on the basis of the proposed approach. The approach is subsequently verified with a coupled compressed air and thermo-mechanical numerical simulation and by a previous study on temperature. Finally, the influence of temperature on total stress and displacement and the impact of the heat transfer coefficient are discussed. This paper shows that the temperature sharply fluctuates only on the sealing layer and the concrete lining. The resulting tensile hoop stresses on the sealing layer and concrete lining are considerably large in comparison with the initial air pressure. Moreover, temperature has a non-negligible effect on the lined cavern for underground compressed air storage. Meanwhile, temperature has a greater effect on hoop and longitudinal stress than on radial stress and displacement. In addition, the heat transfer coefficient affects the cavern stress to a higher degree than the displacement.

Cyclic Thermo-Mechanical Analysis of Wellbore in Underground Compressed Air Energy Storage Cavern

The compressed air energy storage (CAES) method is a viable method of storing surplus energy underground when there is a mismatch between energy generation and demand. Wellbores embedded in rock are an integral part of energy storage structures, and are used for injecting and extracting the compressed air. During injection and production cycles, the storage reservoir and wellbore are subjected to cyclic change in external pressure and temperature, which may cause failure of the wellbore. In this paper, cyclic thermo-mechanical analysis of a horizontal wellbore in an underground CAES cavern is performed using finite element analysis. The rock behavior is simulated using the Mohr-Coulomb constitutive law. The reduction in the yield strength of rock with increase in the number of loading cycles is taken into account in the analysis. Parametric sensitivity studies are carried out to study the effects of dilation and friction angles of rock, the ratio of in situ horizontal and vertical stresses, loading frequency, and the magnitude of the temperature cycles in the cavern on the wellbore performance for different types of rock. The thermo-mechanical cyclic behavior leads to plastic strains that are greater than those obtained by performing mechanical analysis only. Significantly large deformation is generated in rock for large dilation angle and high loading frequencies.