Exploring Porous Media for Compressed Air Energy Storage: Benefits, Challenges, and Technological Insights

Abstract

The global transition to renewable energy sources such as wind and solar has created a critical need for effective energy storage solutions to manage their intermittency. This review focuses on compressed air energy storage (CAES) in porous media, particularly aquifers, evaluating its benefits, challenges, and technological advancements. Porous media-based CAES (PM-CAES) offers advantages, including lower costs and broader geographical availability compared to traditional methods. This review synthesizes recent advancements in numerical modeling, simulation, and experimental studies, which have enhanced the understanding of air–water–heat flow interactions and improved efficiency in these systems. Field studies demonstrate that using existing idle and abandoned wells can minimize infrastructure costs and environmental impact. This review underscores the potential of CAES in porous media to support the growing demand for sustainable and reliable energy storage solutions.

1. Introduction

The global energy landscape is gradually shifting towards renewable energy sources, even though the consumption of fossil fuels remains significant and has not yet shown a marked decrease. This transition reflects the growing global commitment to sustainable energy solutions, despite the current reliance on traditional fuels [1,2]. Renewables, particularly solar and wind, are increasingly contributing to electricity generation [3,4]. However, unlike traditional fossil fuel power plants, these renewable sources are characterized by their intermittency and fluctuation, which affect their efficiency and reliability. Consequently, energy storage has become crucial [5,6].

There are various energy storage methods, such as pumped hydro, capacitors, and batteries [7,8]. Over the past decades, advancements have been made in electrical energy storage. Compared to these methods, using fluids as storage media has gained considerable attention due to its broader applicability and lower environmental impact [9,10].

While batteries and pumped hydro storage have traditionally supported grid stability, they exhibit limitations when scaled to meet the demands of increasing renewable energy integration. Batteries, while advantageous for short-term storage due to their rapid discharge and recharge capabilities, face challenges at larger scales. The cost of batteries escalates when capacities above 500 MW are required, limiting their practical application in long-term energy storage scenarios [11,12]. Furthermore, environmental concerns regarding the lifecycle and disposal of batteries add to their drawbacks, making them less sustainable as a large-scale, long-term storage solution [13,14].

Pumped hydro storage (PHS) has been the backbone of energy storage for decades, offering substantial capacity and relatively high efficiency [15,16]. However, its deployment is constrained by geographical and environmental factors [17]. Pumped hydro facilities require specific topographic conditions and substantial water resources, restricting its feasibility to certain locations. Additionally, the environmental impact of constructing large-scale reservoirs and the associated regulatory hurdles can be prohibitive [18].

Compressed air energy storage (CAES) is one such fluid-based method. CAES operates by using electric compressors to inject high-pressure air into storage during periods of low electricity demand and releasing it through turbines to generate electricity when needed [19,20]. CAES can be categorized into systems using tanks, caverns, and saline aquifers. Tank-based systems are small-scale, while cavern-based systems, like those in Germany and the US, have achieved commercial success. However, these systems are geographically limited, and tank systems require stringent land and safety regulations. CAESA, which refers to CAES applied in aquifers, broadens the potential locations for deployment and minimizes the environmental disruption typically associated with constructing new facilities. CAESA has been practiced globally due to its lower cost and more convenient cyclical injection and withdrawal processes. In addition, the operational efficiency of CAESA is enhanced by the natural properties of aquifers, which aid in the thermal management of stored air, potentially leading to improved energy recovery efficiency compared to the 40% to 70% typically seen in other CAES systems.

Traditional CAES systems, typically utilizing salt caverns, have demonstrated their effectiveness over decades of operation. However, the potential of PM-CAES, such as aquifers and depleted hydrocarbon reservoirs, offers a promising alternative due to the vast availability of these geological formations.

Oldenburg and Pan laid the theoretical groundwork for PM-CAES [21], focusing on the coupled wellbore–reservoir system and highlighting the unique challenges posed by using porous media for energy storage. Their simulation studies demonstrated that, while PM-CAES is feasible, it presents significant differences in energy storage dynamics compared to traditional cavern-based systems. Building on this foundational work, Liu et al. provided a comprehensive review of the latest technological advancements in PM-CAES, addressing efficiency improvements and the scalability of these systems, which are critical for their broader adoption [22].

A microfluidic study by Zhang et al. [23] emphasized the importance of pore network and wettability in determining the efficiency and storage capacity of these systems. These findings underscore the need for detailed site-specific analyses to optimize PM-CAES performance. Additionally, Mouli-Castillo et al. highlighted the environmental benefits of PM-CAES, particularly its potential to integrate with renewable energy sources and contribute to reducing greenhouse gas emissions, making it an attractive option for future energy systems [24].

PM-CAES offers a cost-effective and scalable solution for energy storage. By utilizing existing idle or abandoned wells, PM-CAES reduces capital expenditure and leverages existing infrastructure, lowering both environmental impact and operational costs. Its economic advantages, combined with its capacity to store large energy volumes efficiently, make PM-CAES an attractive option for integrating renewable energy into the grid.

Previously, Jankowski et al. provided a review of mechanical CAES systems, including diabatic (D-CAES), adiabatic (A-CAES), and isothermal (I-CAES) technologies, focusing on their industrial applications [25]. Menéndez and Loredo wrote an editorial review about CAES; they discussed various types of underground energy storage systems that are critical for enhancing the integration of intermittent renewable energy sources like wind and solar photovoltaics into the energy grid, including underground energy storage technologies that utilize disused mines or other subsurface spaces, which can be adapted to store energy in the form of water, compressed air, or hydrogen [26]. The discussions are centered around how these technologies can effectively support the grid, manage energy supplies from renewable sources, and help achieve reductions in greenhouse gas emissions adapted to store energy in the form of water, compressed air, or hydrogen. While these reviews provide valuable insights into the broader applications and benefits of CAES, there remains a gap in the literature regarding holistic reviews, specifically focusing on CAES in porous media or aquifers. This review aims to address this gap by providing a detailed examination of the potential of aquifer-based CAES systems, evaluating their technical feasibility, economic viability, modeling progress, and practical implications.

This review is structured to explore PM-CAES, with a focus on aquifers. Section 2 discusses the geological and technical criteria for selecting suitable CAES sites. Section 3 examines the characteristics, cost-effectiveness, and global examples of CAES systems. Section 4 delves into advancements in numerical modeling and their impact on system performance. Section 5 evaluates the economic aspects, including cost comparisons and the use of idle wells. Section 6 addresses practical challenges, geomechanical impacts, and potential alternatives like compressed carbon dioxide energy storage (CCESA). Section 7 concludes with key findings and future research directions for optimizing CAES technology.

2. CAESA Site Criteria

Selecting an ideal site for CAES operations requires a detailed evaluation of geological criteria, mirroring the practices established in the mature natural gas storage industry, which has over a century of operational experience. Studies by Giramonti et al., Allen et al., and Succar and Williams provide comprehensive frameworks for assessing the suitability of PM-CAES (

Table 1. Essential reservoir and cap rock characteristics for CAES operations [29].

Score	Score Interpretation	Permeability (mD)	Porosity (%)	Total Reservoir Volume (VR/VS)	Total Closure Rating (h/H)	Depth to Top of Reservoir (m)	Reservoir Pressure (MPa)	Type of Reservoir	Residual Hydrocarbons (%)	Cap Rock Leakage	Cap Rock Permeability (mD)	Cap Rock Threshold Pressure (MPa)	Cap Rock Thickness (m)
1	Unusable	<100	<7	<0.5	<0.5	<137 or >760	<1.3 or >6.9	Highly discontinuous	>5%	Leakage evident	>10−5	2.1–5.5	<6
2	Marginal	100–200	7–10	0.5–0.8 or >3.0	0.5–0.75	140–170	1.3–1.5	Moderately vugular limestone and dolomite	1–5%	No data available	<10−5	>5.5	>6
3	OK	200–300	10–13	0.8–1.0 or 1.2–3.0	0.75–0.95	170–260 or 670–760	1.5–2.3 or 6.1–6.9	Reefs, highly vugular limestone, and dolomite	<1%	Pumping tests show no leakage
4	Good	300–500	13–16	1.0–1.2	0.95–1.0	260–430 or 550–670	2.3–3.9 or 5.0–6.1	Channel sandstones
5	Excellent	>500	>16			430–550	3.9–5.0	Blanket sands

), outlining specific geological properties and threshold values [27,28,29]. These frameworks include key parameters such as porosity, permeability, reservoir volume, and structural integrity, which are critical for ensuring the efficient and safe operation of CAES systems [30].

The primary criteria for selecting a CAES site include the presence of a high-quality reservoir characterized by porosity and permeability. A suitable reservoir should exhibit porosity greater than 10% to ensure ample space for compressed air storage, and permeability should exceed 300 millidarcies (mD) to facilitate sufficient flow rates for effective CAES operation. The geological structure of the reservoir also plays a critical role; it should be laterally homogeneous to allow for predictable and efficient behavior of the stored air. Additionally, the presence of a structural closure, such as a dome, is crucial. This structure acts to naturally trap the compressed air due to buoyancy, enhancing storage efficiency. The quality of the closure is measured by the closure rating, defined as the ratio of the thickness of the reservoir above the lowest closing contour to the total thickness at a specific point. A higher closure rating is desirable, as it indicates that a larger portion of the reservoir can be utilized for air storage [31,32].

The physical depth and pressure conditions of the reservoir are equally important, as they determine the operational parameters for the CAES system, including the design of the surface turbomachinery. Optimal conditions typically involve pressures between 3.9 and 5 MPa. Additionally, the reservoir should contain minimal residual hydrocarbons—less than 1%—to prevent the formation of damaging compounds that could affect the performance of both the reservoir and the machinery [33,34].

In addition, an effective CAES operation requires a robust cap rock or an impermeable layer to prevent the escape of stored air. The ideal cap rock should cover the entire reservoir area, exhibit a thickness greater than 6 m, have a high threshold pressure above 5.5 MPa, and possess very low permeability (less than 10⁻⁵ mD). The integrity of the cap rock should be confirmed through pump tests that show no leakage, ensuring that the compressed air remains contained within the reservoir layer [35,36].

Evaluating various geological and mechanical properties of potential sites is also important. Critical to this assessment is the geological stability and the integrity of the surrounding rock formations, which should be capable of withstanding the cyclical stresses associated with the repeated compression and decompression cycles of CAES operations. Geological formations should not only be homogenous and exhibit specific mechanical properties but also be free of faults and fractures that could compromise the containment of high-pressure air. Thorough geotechnical surveys are also important to ascertain the mechanical properties of the subsurface structures to ensure they meet the operational demands of CAES systems [37,38].

Moreover, the environmental implications of selecting a site for CAES are also essential [39,40]. The chosen site must minimize environmental disruption, particularly in ecologically sensitive areas. The presence of an impermeable cap rock is vital to prevent air leakage and ensure the integrity of the storage system over its operational lifespan [41,42]. Additionally, the site should allow for the development of infrastructure necessary for CAES operations without causing environmental damage. Regulatory and community acceptance plays a crucial role in the feasibility of developing a CAES facility, as public opposition or stringent environmental protection laws can impede project development. Therefore, the site selection process should include a comprehensive environmental impact assessment to facilitate regulatory approvals and community support, ensuring the sustainability of the CAES project in the chosen location [31].

In short, the integration of these geological criteria, supported by empirical data and modeled projections, forms the basis for evaluating and selecting potential CAES sites. Choosing sites should not only meet the specific operational needs of CAES systems but also optimize energy storage capacity and efficiency, aligning with long-term sustainability and operational goals.

3. CAES Characteristics and Examples

3.1. CAES Characteristics

3.1.1. Advantages and Cost-Effectiveness

Compared to underground caverns, aquifers offer two primary advantages for CAES. First, aquifers are more widely distributed globally, providing more potential storage sites. Second, the cost of storage in aquifers is lower. As mentioned in the literature [33], the cost of storing energy in underground caverns ranges from 6.0 to 10.0 per kilowatt-hour, whereas aquifer storage is approximately 2.0 to 7.0 per kilowatt-hour, and when increasing capacity, the cost can drop as low as $0.11 per kilowatt-hour. However, this low cost may not fully reflect all operational expenses such as maintenance, monitoring, and potential environmental restoration costs.

3.1.2. Technological Comparative Analysis and Innovative Challenge

Besides CAES, other forms of underground energy storage like underground pumped hydro power (UPSH) and suspended-weight gravity energy storage (SWGES) also merit attention [43,44]. CAES differs from UPSH and SWGES not only in terms of energy storage mechanisms, capacity, and infrastructure requirements but also in fluid type. CAES stores energy by compressing air in an underground reservoir and releasing it to drive turbines during discharge; UPSH involves using two water reservoirs at different elevations, with energy stored by pumping water to the upper reservoir and released through turbines when the water flows back to the lower reservoir; SWGES stores energy by lifting a suspended weight in a vertical mine shaft and releases energy by allowing the weight to descend, driving a generator. While CAES offers higher energy density and potential for large-scale storage, it requires suitable geological formations, which may limit site availability, and thermal efficiency can be affected by heat loss during air compression and expansion.

Other forms of energy storage in subsurface pore space include underground pumped storage hydropower (UPSH) and suspended-weight gravity energy storage (SWGES) [43,44]. CAES differs from UPSH and SWGES in terms of energy storage mechanisms, capacity, and infrastructure requirements, in addition to the fluid type. CAES stores energy by compressing air in an underground reservoir, releasing it to drive turbines during discharge, while UPSH involves using two water reservoirs at different elevations, with energy stored by pumping water to the upper reservoir and released through turbines when the water flows back to the lower reservoir. SWGES, on the other hand, stores energy by lifting a suspended weight in a vertical mine shaft and releases energy by allowing the weight to descend, driving a generator. The capacity for CAES and UPSH is primarily dependent on the underground reservoir capacity, whereas SWGES relies on the depth of the mine shaft and the mass of the suspended weight [44,45].

3.1.3. Environmental Impact and Technological Innovation

One major selling point of CAES is its relatively low environmental impact compared to that of constructing new reservoirs. However, such an assessment should include long-term environmental monitoring data and potential ecological disturbances. Moreover, utilizing idle mines or salt caverns as existing infrastructure can reduce initial construction costs but might introduce long-term geological stability issues [15,46]. Additionally, SWGES, being an innovative technology that can repurpose existing vertical mine shafts with a simple mechanical system potentially having low operational costs, currently offers lower energy storage capacity than CAES and UPSH due to limitations in mine shaft depth and suspended mass [45].

In short, CAES holds potential for energy efficiency and environmental sustainability as an energy storage solution. However, its practical application needs to consider multiple factors, including technological readiness, geographical constraints, and long-term economic and environmental costs. Future research should focus on integrating these complex factors comprehensively and exploring strategies to optimize the design and application of energy storage systems for broader sustainable development goals.

3.2. CAES Examples

Feasibility studies for compressed air energy storage (CAES) date back to the 1970s, with the first field CAES project conducted in Pittsfield, Illinois, United States, led by the Electric Power Research Institute (EPRI) in the early 1980s. The results of this early project were analyzed using relatively simple models [47]. This initial attempt laid the groundwork, but the development of CAES remained stagnant for a significant period, largely until the 2006 Iowa Project led by the Iowa Association of Municipal Utilities. This pause in progress could be attributed to the established maturity of salt cavern CAES technology, which may have diverted focus from alternative approaches. The Iowa Project, although marking a renewed interest, also faced challenges.

A subsequent initiative in 2015, a 270 MW project in Dallas Center, Iowa, led by the Iowa Stored Energy Park Project Agency (ISEPA), involved extensive geophysical research for site selection. However, despite its ambitious scope, it was halted due to economic constraints [48]. This illustrates the economic challenges that can hinder even well-researched projects, despite the growing demand for energy storage and the need for robust mathematical models to simulate the impacts of geological conditions, well design, and gas injection processes. Additionally, modern CAES processes have begun integrating geothermal processes and utilizing CO₂ as a cushion gas, representing a shift towards more innovative and sustainable approaches.

In contrast to the U.S. projects, a comprehensive CAES evaluation was conducted in Gotland, Sweden, within the Middle Cambrian Faludden sandstone reservoir by several Swedish institutions [32,49]. This study involved a thorough analysis of over 2300 km of seismic reflection profiles and data from more than 300 wells, leading to the creation of detailed maps of the reservoir’s thickness and structural topography beneath Gotland (Figure 1). The evaluation revealed that the Faludden sandstone’s properties, such as a porosity of 14.9% and permeability of 559 mD, were conducive to CAES. Seven potential structural closures in the eastern and southern parts of the island were identified as promising for development. However, a notable disadvantage of this site was the lack of an obvious structure, raising uncertainties about whether the non-dipping geology scenario is favorable for air storage, particularly regarding storage volume and flow rate. These concerns underscore the need for dynamic reservoir simulations to validate the site’s feasibility for CAES, contrasting with the more straightforward geological settings in some U.S. projects.

Further west, in the United States, CAES projects with industrial and utility partners were conducted in inland Washington and Oregon, led by the Pacific Northwest National Laboratory (PNNL) and Bonneville Power Administration (BPA). The initial screening narrowed down five potential sites to two—Columbia Hills and Yakima Minerals—based on criteria such as geological suitability, proximity to infrastructure, and economic viability. The Columbia Hills site was ideal for a traditional CAES plant capable of generating 207 MW with substantial storage capacity, allowing for continuous operation without recharging. This site benefitted from existing natural gas supplies and infrastructure, making it a practical choice for implementation. In contrast, the Yakima Minerals site, facing limited natural gas availability, led to an innovative design for a no-fuel hybrid geothermal CAES plant. This design, which combines geothermal energy with compressed air storage, represents a novel and environmentally friendly solution aligned with the region’s renewable energy goals. Economically, while the Columbia Hills site offers energy at costs comparable to conventional gas turbines, the Yakima Minerals hybrid plant, though more expensive, provides a sustainable alternative to peaking power plants. This comparison highlights the trade-offs between traditional and innovative approaches, with each offering distinct advantages and challenges depending on local resources and infrastructure.

In another notable project, Guo et al. reported a CAES test in Pike County, Illinois, United States, sponsored by the Department of Energy (DOE) [33]. This test used a compressor capable of injecting air at high pressures to simulate the operational conditions of a CAES system. The field test, which involved injecting approximately 74 million cubic feet of dry air into the St. Peter sandstone formation over several months, demonstrated the formation’s high permeability and porosity, making it an ideal candidate for CAES. Numerical simulations further validated the field results, showing that the aquifer could withstand the cyclic pressure stresses from repeated air injections and withdrawals. This contrasts with the Gotland project, where dynamic simulations are still needed to confirm feasibility, highlighting the varying levels of certainty in different geological settings (Figure 2).

In China, a field test and numerical simulations were conducted in Dezhou, Shandong, led by several Chinese institutions. This project represents a significant step in understanding the practical dynamics and feasibility of CAES technology in deep aquifers. The field setup included an adapted wellbore originally drilled for geothermal exploitation, targeting the Neogene Guantao formation. The test demonstrated the formation’s suitability for compressed air storage, with high porosity and permeability observed in the sandstone layers. Numerical simulations using the T2Well/EOS3 model provided further insights into the stability of the air bubble and its efficiency in maintaining pressure over time. Compared to other projects, this Chinese initiative showcases the adaptability of CAES technology in different geological and operational contexts, addressing challenges such as water coning and air bubble replenishment [50].

4. CAES Numerical Modeling Progress

4.1. Base CAES Modeling and Parameter Analysis

Yang et al. investigated the impact of different working cycles on the CAESA system’s performance, focusing on daily, weekly, and monthly cycles [51]. Using a three-dimensional numerical model with TOUGH3/EOS3 based on the Pittsfield aquifer field test, they analyzed parameters such as pressure, gas saturation, and temperature. Their model demonstrated that simulation results aligned well with observational data (Figure 3), particularly wellhead pressure after the initial 30 days of air injection. The study found variations in parameters across different cycles, with the daily cycle showing the smallest pressure drop due to shorter periods, resulting in the smallest energy losses and the highest remaining pressure in the aquifer after the first cycle (Figure 4). Gas saturation decreased gradually as the air bubble dissipated, with minimal differences in gas remaining among the three cycles after one month. The daily cycle resulted in higher temperature increases due to more frequent high-temperature air injections, leading to energy recovery efficiencies of 96.96%, 96.27%, and 93.15% for daily, weekly, and monthly cycles, respectively (Figure 5), indicating that higher air temperatures in the daily cycle produced more energy [51]. This study highlights the trade-off between cycle frequency and energy efficiency, with daily cycles providing higher energy recovery but potentially increased thermal stress on the system.

Li et al. utilized T2Well/EOS3 simulation software to model the underground processes of CAES in aquifers, focusing on the initial gas bubble formation and cycling processes [50]. Unlike Yang et al. [51], who concentrated on cycle duration, Li et al. emphasized the impact of layered heterogeneity on air–water–heat flow (Figure 6) [50]. Their research revealed that heterogeneity affects air distribution and energy efficiency, leading to multi-fingering flow patterns in the aquifer. Systems with high-porosity and -permeability layers on top performed better, exhibiting more stable pressure, less temperature fluctuation, and higher energy recovery [52]. Compared to the uniform cycle analysis by Yang et al. [51], Li et al.’s study underscores the importance of geological heterogeneity in determining the effectiveness of CAES operations, showing that the structural composition of the storage site can significantly influence performance outcomes [50].

Prado et al. [53] combined analytical models developed in MATLAB with three-dimensional computational fluid dynamics (CFD) simulations conducted using ANSYS Fluent 16.2 to investigate CAES performance. Their models were developed for abandoned mines rather than aquifers, but they offer insights into the impact of sealing materials on system efficiency. Unlike the studies by Yang et al. [51] and Li et al. [52], Prado et al. [53] focused on the thermal efficiency of different sealing layers, analyzing variations in air temperature, density, and pressure, as well as heat transfer through materials like fiber-reinforced plastic (FRP) and steel. The study found that steel, with its higher thermal conductivity, reduced air temperature fluctuations and conserved more thermal energy compared to FRP, making it a more effective material for sealing in CAES systems (Figure 7) [53]. This finding contrasts with the earlier studies by emphasizing the importance of material properties in maintaining thermal efficiency, suggesting that the choice of sealing material can play a crucial role in optimizing energy storage and retrieval.

Zhang et al. [54] explored the geomechanical implications of CAESA in salt domes, an area less frequently studied compared to aquifers. Their research involved both analytical calculations and numerical simulations using COMSOL Multiphysics to determine the stability of storage cavities under varying internal pressures and geological conditions. Unlike previous studies that focused primarily on thermal and fluid dynamics, Zhang et al. highlighted the critical role of geomechanical factors, such as the ratio of horizontal to vertical geological stresses and the material properties of the surrounding rock, in maintaining the structural integrity of the storage site [54]. This approach adds another layer of complexity to CAES modeling, demonstrating that successful implementation requires not only efficient thermal management and fluid flow but also a thorough understanding of the mechanical stresses within the geological formation.

4.2. Coupled Process and System Integration

Coupled simulations for CAES have been relatively well developed; for example, integrated systematic thermodynamic models were conducted with concentric diffusion heat transfer models for cylindrical packed-bed latent thermal energy storage (LTES) in tank-based CAES systems [55,56]. However, fewer studies focus on aquifer-based CAES—the paragraphs below present several examples.

Pfeiffer et al. [57] performed coupled simulations for CAESA, integrating a power plant and storage site to evaluate the performance of adiabatic and diabatic CAES systems (Figure 8). Their simulation model coupled TESPy, an open-source thermal energy system simulator, with the ECLIPSE reservoir simulator. This integration ensured accurate data exchange and convergence controls, accurately representing the power plant and geostorage interactions. The simulations assessed the impact of storage pressures and mass flow rates on the storage capacity and efficiency of the CAES system. Results showed strong feedback between storage rates and capacities, confirming the benefits of the integrated modeling approach. They found that adiabatic systems met discharging targets but faced reduced charging rates over cycles due to increased storage pressure, while diabatic systems maintained consistent charging but exhibited decreased discharging power over time. The efficiency of storage systems varied with power plant topology, with diabatic systems showing higher initial efficiencies that decreased over cycles [57]. This comparison illustrates the trade-offs between different CAES configurations, where adiabatic systems excel in energy storage but may struggle with consistent performance over time, while diabatic systems offer steadier operation but with efficiency trade-offs as cycles progress.

Guo et al. provided another perspective by integrating T2Well and EOS3, combining aquifer thermal energy storage (ATES) with CAESA to explore how different injection air temperatures influence overall energy recovery efficiency [58]. Their simulations varied injection air temperatures (20 °C, 40 °C, 60 °C, and 80 °C) and analyzed key parameters such as pressure distribution, temperature variation within the aquifers, and energy flow rates in the injection well. The study found that higher injection air temperatures led to greater total energy storage capacity, with an 80 °C injection storing about 10% more energy than a 40 °C injection (Figure 9). This result suggests that thermal management strategies, including controlling injection air temperature, are crucial for optimizing CAES performance. However, the study also highlights that while higher temperatures increase storage capacity, they also introduce additional thermal stresses that must be managed to maintain system efficiency [58]. Compared to Pfeiffer et al.’s [57] focus on system integration, Guo et al. emphasize the importance of temperature control in achieving high energy recovery, adding another dimension to the complex interplay of factors that determine CAES effectiveness [58].

5. CAESA Economics

The economic factors influencing CAESA are categorized into capital and operational costs, efficiency, scalability, and levelized cost of storage (LCOS) [59,60,61,62]. Capital costs during the charging phase involve surface facilities for air compression, with operational costs tied to the efficiency of this process. Storage costs are minimal due to the near-zero marginal costs of utilizing subsurface aquifers once initial well facility expenses are covered, making CAESA favorable for large-scale, long-term storage. In the discharging phase, costs involve converting stored air back to electricity, with efficiency directly impacting operational costs. The use of existing idle oil and gas wells for storage reduces capital costs and mitigates environmental liabilities. CAESA aims for a competitive LCOS by achieving a capital cost of $2/W, operating expenses under $0.10/W per annum, and a roundtrip efficiency of 68%, targeting an overall cost of $0.05–0.08/kWh, comparable to fossil-generated power [63].

Idle and long-term idle wells present significant potential for CAESA projects [64]. Repurposing these wells can save the abandonment and remediation costs, typically between $80,000 and $250,000 per well, while generating new revenue streams. However, abandoned wells carry risks such as brine or greenhouse gas leakage, which may necessitate costly re-abandonment [65].

CAESA offers a cost-effective alternative to traditional CAES in salt caverns or tanks [66]. Construction costs for a CAESA power plant range from $2 to $7 per kilowatt-hour, lower than the $6 to $10 per kilowatt-hour for salt caverns. Incremental storage volume expansion costs are also lower at $0.11 per kilowatt-hour compared to $2.00 per kilowatt-hour for salt caverns [67].

6. Synthesis of Key Findings and Implications

6.1. Technological and Environmental Implications

Based on the current development of CAES evaluation and operation, aligning storage capacities with the demands of renewable energy sources such as wind and solar is critical, given their variability. Geological formations offer a viable solution for the long-term storage necessary to manage energy surpluses during peak production periods. However, the operational implementation of CAES systems requires the rigorous assessment of potential geological and environmental impacts such as land subsidence, alterations in groundwater flow and composition, and the integrity of geological formations. These concerns underscore the importance of developing predictive tools and monitoring systems that can effectively manage and mitigate risks [68,69].

6.2. Challenges in Applications and Modeling

The competitive use of subsurface spaces, which may already be allocated for activities like geothermal energy production, mineral extraction, or drinking water supply, poses challenges for the deployment of CAES systems. Strategic subsurface spatial planning is required to reconcile these competing demands, necessitating a quantitative approach that can allocate specific subsurface areas to different uses based on their spatial demands.

From another perspective, the most fundamental question during CAES application is whether pore space can provide large-scale energy storage. While CAES can store energy, pressure gradients across the reservoir lead to spatially variable energy storage density. This variability is critical because it implies that the effectiveness of energy storage can differ across different parts of the reservoir, depending on their proximity to the injection site.

The efficiency of storing energy is another critical issue. Properly designed cap rock and hydrologic seals prevent excessive pressure diffusion, which could otherwise undermine storage efficiency. Ideally, with high efficiency, the injection and production of air should not significantly mobilize residual liquid water, thus mitigating potential efficiency losses. Energy loss due to air–water interactions also needs consideration. Fortunately, current simulations show that once gravity equilibrium is well established during air injection, air will not significantly move water around, maintaining energy efficiency. However, pressure diffusivity needs careful management, as it affects the pressure change rate in the reservoir. Simulations indicate that high-pressure diffusivity could slow down air recovery and limit injection rates, potentially preventing sustained pressurization in open reservoirs. To prevent the negative impacts of pressure diffusion, pressure gradients need to be well maintained.

In the simulation process of compressed air energy storage in aquifers (CAESA), several challenges must be addressed to optimize system performance and reliability. First, geological selection remains a critical issue. Effective site selection requires a comprehensive understanding of the geological structure, including properties such as porosity, permeability, and the mechanical integrity of the rock formations. For instance, the Iowa Stored Energy Plant Agency project case highlighted the complex interplay between geological suitability and project feasibility. Selecting a suboptimal site due to inadequate geological characteristics, such as insufficient closure or unfavorable permeability gradients, can lead to inefficiencies or project failures.

From another perspective, the general assumption of isentropic processes for compressors and turbines may not accurately reflect the real operational conditions of these components. In reality, CAES systems involve complex thermodynamic processes that are influenced by various factors such as heat transfer, friction, and leakage. These factors can significantly affect the performance and efficiency of the system. For instance, during compression, the air temperature increases due to the work done on it, leading to irreversible losses in the form of heat. Similarly, during expansion, the air cools down, causing additional irreversible losses. Moreover, the presence of pressure drops and flow distortions within the components can also impact their performance.

To address these issues, more advanced models need to be developed that take into account non-isentropic effects and other real-world considerations. These models should incorporate empirical data or experimental results to capture the behavior of compressors and turbines under different operating conditions. They should also consider the interactions between the components and the surrounding environment, including heat exchange with the atmosphere and mechanical losses in the system.

Also, future directions for CAESA technology should focus on improving the predictive accuracy of geological assessments and enhancing material resistance to operational stresses. Advances in materials science could yield new composites or coatings that better withstand the harsh conditions of high-pressure air storage, thereby enhancing system durability and reducing maintenance needs. Moreover, the development of more sophisticated simulation tools would allow for a more accurate prediction of both physical and chemical interactions within the aquifer, facilitating better design and operational strategies to mitigate the identified risks. Addressing these challenges requires a multidisciplinary approach combining geology, materials science, and mechanical engineering. By focusing on these areas, the feasibility and efficiency of CAESA can be enhanced, contributing to its viability as a sustainable energy storage solution.

6.3. Potential Geotechnical and Environmental Issues

Bauer et al. [70] pointed out several issues associated with CAES and underground pumped hydro storage that can induce several specific geotechnical and environmental impacts that require detailed technical considerations. One issue is the alteration of subsurface pressure regimes due to the cyclic injection and extraction of air or water. This cyclical loading can lead to geomechanical disturbances such as induced seismicity, where the stress changes in the rock mass might activate existing faults or create new fractures. Additionally, subsidence or uplift may occur, depending on the volume and pressure of the material injected and the mechanical properties of the subsurface formations.

Technical details concerning the integrity of geological formations are crucial. For example, the integrity of cap rocks—impermeable layers that seal the storage formations—must be rigorously evaluated. This involves assessing their mechanical strength, thickness, and permeability to ensure they can withstand increased pressures without fracturing, which could lead to the unintended migration of stored gases or fluids. Hydrological impacts are also significant; changes in the pressure and chemical composition of injected fluids can alter the groundwater flow, potentially leading to the contamination of aquifers with saline or polluted waters from deeper strata or the mobilization of naturally occurring substances like arsenic or radon.

Moreover, the thermal effects of injected air or water at different temperatures from the native groundwater can induce thermal stress in the geological medium, affecting its mechanical properties and potentially leading to further geomechanical instability. The continuous monitoring of these effects through seismic sensors, subsidence measurements, and groundwater quality assessments is essential for managing these risks. Advanced numerical modeling tools are also employed to simulate the complex interactions within the subsurface, allowing for better predictions and management of the induced effects, thereby ensuring the long-term sustainability and safety of subsurface energy storage schemes. These technical assessments and ongoing monitoring form a critical part of the regulatory and environmental compliance processes, ensuring that the deployment of geological energy storage technologies does not adversely impact the surrounding environments or communities [70].

6.4. Exploring Compressed Carbon Dioxide Energy Storage

In terms of stored fluids, carbon dioxide may also be an ideal choice. Li et al. compared compressed CCESA with CAESA. Simulations of these two fluids were performed under consistent geological, structural, and operational conditions [71]. These simulations revealed that while CO₂ energy storage absorbs heat from its surroundings, compressed air energy storage continuously loses heat. Notably, the reservoir pressure for the CO₂ system was observed to be substantially lower (by 1.91 MPa) than that of the air system, resulting in a decreased risk of cracking in the geological structure. Additionally, within a 200-day period, CO₂ and air were shown to migrate up to distances of 167 m and 487 m, respectively, indicating a reduced spatial requirement for CO₂, which occupies only about 11.8% of the floor space required for compressed air.

Further findings highlighted that the mass fraction of gas produced from CO₂ storage was 38.81% higher than that from compressed air storage, reflecting a higher retention of stored gas. In terms of energy efficiency, CO₂ storage not only demonstrated an increase in daily energy efficiency but also maintained an average energy efficiency that was 20.15% higher than that of air storage (Figure 10). These results underscore the enhanced performance of compressed carbon dioxide energy storage compared to air, particularly when a local source of CO₂ is readily available, making it a more advantageous option for large-scale energy storage solutions [71]. In addition, Liu et al. found that when using CO₂ for a hybrid energy storage system, the cost could reduce to $0.23 per kilowatt-hour, and the efficiency is 60.5% [67].

6.5. Future of CAES

The future development of compressed air energy storage (CAES) is closely linked to its scalability and ability to reduce costs. The U.S. Department of Energy’s (DOE) strategic initiative, Storage Innovations 2030, emphasizes the goal of reducing the levelized cost of storage (LCOS) to $0.05/kWh by 2030. Achieving this ambitious target will necessitate significant advancements in CAES technology and substantial cost reductions across various system components. Investments in advanced manufacturing techniques, alternative thermal storage solutions, and innovations in pressure regulation and heat exchanger technologies are critical to driving these cost reductions. By focusing on these areas, the industry aims to make CAES a more economically viable option for large-scale energy storage [72].

In tandem with cost reduction, improving the efficiency of CAES systems is a primary focus for the future. Enhancing roundtrip efficiency (RTE) through the integration of advanced components, such as lower-temperature turbines and more effective thermal energy storage systems, is crucial. These improvements aim to optimize energy conservation, minimize losses during the compression and expansion phases, and boost overall system performance. As the technology matures, these efficiency gains will be essential in making CAES more competitive with other energy storage solutions.

Technological innovation is another driving force behind the future of CAES. The DOE report underscores the importance of developing advanced materials and refining system designs to enhance durability, efficiency, and operational lifespan. This includes the use of advanced alloys, novel materials for well linings, and organic phase change materials. Moreover, there is a growing interest in hybrid systems that combine CAES with other energy storage technologies, such as hydrogen storage. These hybrid systems offer the potential to increase flexibility and reduce the reliance on fossil fuels, further enhancing the sustainability and appeal of CAES.

Geological and environmental considerations will play a significant role in the future of CAES. As development progresses, there will be a greater emphasis on understanding and utilizing geological formations for air storage, including porous rocks, saline aquifers, and depleted gas wells. The DOE highlights the need for advanced technologies to evaluate subsurface conditions and assess reservoir suitability, ensuring safe and efficient storage operations. Additionally, environmental sustainability remains a key priority, with ongoing research focusing on reducing greenhouse gas emissions through the integration of geothermal energy and the use of alternative gases like CO₂.

Deployment and integration strategies are expected to evolve as CAES technology advances. The future will likely see a greater emphasis on deploying demonstration projects to validate new system configurations, such as isothermal and adiabatic processes. These projects will be critical in demonstrating the economic viability and performance of CAES in various real-world scenarios. Furthermore, co-locating CAES with renewable energy sources and industrial sites is emerging as a strategic trend. This approach can enhance system efficiency and reduce operational costs by leveraging existing infrastructure and energy resources.

Finally, the role of policy and market support will be pivotal in the widespread adoption and scaling of CAES technologies. Government policies and regulatory frameworks will be essential for creating a conducive environment for investment, providing incentives, establishing long-term contracts, and reducing market uncertainties. The DOE also stresses the importance of standardizing component sizing and manufacturing processes to achieve economies of scale. Such standardization will help drive down costs and facilitate the broader adoption of CAES. In conclusion, the future of CAES is promising, with substantial potential for innovation in technology, materials, and system integration. These advancements, supported by favorable policies and market conditions, will be crucial in positioning CAES as a key technology in the global energy storage landscape.

7. Conclusions

This study has explored CAES systems within porous media, PM-CAES. By synthesizing recent advancements in numerical modeling, simulation, and experimental studies, our paper has enhanced the understanding of air–water–heat flow interactions and improved efficiency in these systems. Field studies have demonstrated that utilizing existing idle and abandoned wells can minimize infrastructure costs and environmental impacts.

Our findings indicate that CAES in aquifers offers broader geographical availability and lower storage costs compared to traditional methods such as those used in salt caverns or tanks. Moreover, the use of compressed carbon dioxide as a working fluid in CAES systems has shown potential for increased energy storage efficiency. CO₂’s ability to absorb heat from its surroundings results in a higher retention of stored gas and increased daily energy efficiency compared to compressed air storage.

Despite the effectiveness of CAES technology as an energy storage solution, it is essential to address potential geological and environmental impacts during implementation, such as ground subsidence, alterations in groundwater flow and composition, and the integrity of geological formations. To mitigate these concerns, future research should focus on improving the predictive accuracy of geological assessments and developing materials capable of withstanding the operational stresses of high-pressure air storage.

In summary, the application of CAES in porous media, particularly aquifers, has revealed its potential to support growing demands for sustainable and reliable energy storage solutions. With further research and technological advancements, CAES is poised to become a crucial link in integrating renewable sources such as wind and solar power.