CO2 Battery

CO₂ Batteries are a class of long-duration energy storage ("LDES") systems that store electricity as thermomechanical energy by compressing, liquefying, and expanding carbon dioxide in a closed-loop thermodynamic cycle. The technology is functionally similar to compressed air energy storage ("CAES") but substitutes CO₂ as the working fluid, exploiting its uniquely favorable thermodynamic properties — particularly its low critical point (31.1°C, 7.4 MPa), which enables liquid storage at near-ambient temperatures without cryogenic equipment. This results in significantly higher energy storage density than air-based systems while using entirely off-the-shelf industrial components (compressors, turbines, heat exchangers, pressure vessels) available from established global supply chains. Energy Dome, founded in Italy in 2020, is the commercial pioneer of the technology, having launched the world's first CO₂ Battery demonstration facility in Sardinia in June 2022 (2.5 MW/4 MWh). The company's first full-scale 20 MW/200 MWh plant in Ottana, Sardinia — funded through the EU-Breakthrough Energy Catalyst partnership and backed by an offtake agreement with ENGIE — was commissioned in Q1 2025, making it one of the first operational 10-hour LDES assets globally with a commercial offtake. A second identical unit is contracted with Alliant Energy in Wisconsin (U.S.), supported by a $30 million DOE OCED award. Academic research shows round-trip efficiencies of 60–76% depending on configuration, with the technology targeting costs at less than half of comparable lithium-ion installations. The CO₂ Battery uses no lithium, no rare-earth elements, and no site-specific geological features, making it deployable anywhere and free from critical mineral supply chain constraints.

Compressed carbon dioxide energy storage ("CCES") emerged as a research concept in the early 2010s, building on decades of experience with compressed air energy storage and thermodynamic power cycles using CO₂ as a working fluid. The foundational insight was that CO₂'s low critical temperature (31.1°C) and moderate critical pressure (7.4 MPa) allow it to be liquefied at near-ambient temperatures — unlike air, which requires cryogenic cooling to approximately -196°C — making CO₂ a far more energy-dense and practical working fluid for compressed gas energy storage. The first CCES system proposed in the academic literature was an adiabatic configuration with liquid CO₂ storage. In 2012, Morandin et al. proposed a transcritical CCES system using water for heat storage and cooled salt water for cold storage, with 50 MW charging and discharging power and a 2-hour discharge duration. This early work established the basic thermodynamic framework. Subsequent research rapidly diversified: He et al. (2018) proposed a supercritical CCES system with turbine inlet temperatures raised to 837 K to improve efficiency; Liu et al. (2019) introduced a trans-critical system with two-stage compression and expansion; and Zhang et al. (2020) used genetic algorithm optimization to demonstrate that a low-temperature liquid CO₂ system could achieve round-trip, thermal, and exergy efficiencies of 41.4%, 59.7%, and 45.4%, respectively. The decisive step from academic concept to commercial technology came with Energy Dome, founded in Milan, Italy in February 2020 by Claudio Spadacini. The company developed a proprietary closed-loop thermodynamic process for its CO₂ Battery — a system that compresses CO₂ gas into liquid form during charging (storing both pressure energy and thermal energy) and reverses the process during discharge to regenerate electricity. Energy Dome's first demonstration facility — a 2.5 MW/4 MWh unit — was completed in Sardinia, Italy in 2022, confirming the technology's performance using entirely off-the-shelf industrial equipment. At COP28 in December 2023, Energy Dome announced funding commitments for its first standard commercial-scale CO₂ Battery in Ottana, Sardinia: a €35 million grant from Breakthrough Energy Catalyst and €25 million in venture debt from the European Investment Bank, through the EU-Catalyst partnership. This was the first European initiative to receive EU-Catalyst funding. Construction reached full notice-to-proceed at the end of 2023. In October 2024, Energy Dome signed its first U.S. supply contract with Alliant Energy for the Columbia Energy Storage Project in Wisconsin — a standard 20 MW/200 MWh unit supported by a $30 million DOE Office of Clean Energy Demonstrations award. In December 2024, Energy Dome and ENGIE signed an offtake agreement for the Sardinia plant, with ENGIE dispatching the stored energy in the Italian power market — the first offtake deal under Energy Dome's "Energy Storage as a Service" model. The Sardinia plant was commissioned in Q1 2025, making it one of the first operational 10-hour LDES assets globally. Google has also backed Energy Dome, committing to fund and validate the technology for potential deployment at data centers as part of its 24/7 carbon-free energy strategy.

Thermodynamic Principle

A CO₂ Battery is a thermo-mechanical energy storage system that converts electrical energy into stored pressure energy and thermal energy during charging, and reverses the process to regenerate electricity during discharging. The system operates on a closed-loop thermodynamic cycle using carbon dioxide as the working fluid. CO₂ never leaves the system and is neither consumed nor produced — it simply cycles between gaseous and liquid (or supercritical) states within sealed equipment.

The technology exploits a critical physical property of CO₂: its critical point sits at 31.1°C and 7.4 MPa. This means CO₂ can transition between gas and liquid phases at temperatures close to ambient, unlike air (critical temperature: -140°C) or nitrogen (-147°C), which require cryogenic conditions for liquefaction. This near-ambient critical temperature is the fundamental reason CO₂ is a superior working fluid to air for compressed gas energy storage — it enables liquid-phase storage without cryogenic equipment, dramatically increasing energy storage density and reducing system complexity.

Charging Process (Electricity → Stored Energy)

During the charging phase, excess electricity from the grid (typically from renewable sources during periods of overgeneration) powers the compression train:

Compression: CO₂ gas is drawn from a low-pressure storage vessel (the "dome" in Energy Dome's design, or a gas holder) and compressed through one or more compressor stages to high pressure. Multi-stage compression with intercooling is typically used to reduce the work input and manage temperature rise. Research indicates that three-stage compression/expansion is optimal for balancing efficiency against system complexity.

Heat Capture: Compression generates substantial heat. In adiabatic and advanced CCES configurations, this heat is captured in thermal energy storage ("TES") units rather than being rejected to the environment. The TES typically uses hot water tanks, molten salt, packed-bed solid stores, or other sensible heat storage media. Capturing and storing this compression heat is essential to achieving high round-trip efficiency, because the same heat will be returned to the CO₂ during discharge to boost turbine performance.

Liquefaction and Storage: After compression and heat removal, the high-pressure CO₂ is cooled below its critical temperature and liquefied. The liquid CO₂ is stored in insulated high-pressure steel vessels (typically above-ground). Because liquid CO₂ is far denser than gaseous CO₂, the stored energy per unit volume is several times higher than in a conventional CAES system storing compressed air in caverns.

Some configurations also include cold thermal energy storage ("CTES"), where the cold energy released during CO₂ evaporation on the discharge side (or generated during cooling stages on the charge side) is stored in cold water or ice tanks and recycled to improve the liquefaction process during subsequent charging cycles.

Discharging Process (Stored Energy → Electricity)

When electricity is needed, the cycle is reversed:

Pressurization and Heating: Liquid CO₂ is pumped from the high-pressure storage tank and vaporized. The CO₂ is then heated using the thermal energy stored during the charging phase (retrieved from the hot TES), raising its temperature and further increasing its pressure.

Expansion: The hot, pressurized CO₂ is expanded through one or more turbine stages, driving a generator to produce electricity. The expansion process converts the stored pressure and thermal energy back into mechanical work and then into electrical power.

Recovery: After expansion, the low-pressure CO₂ gas is returned to the low-pressure storage vessel, completing the closed loop. Any residual cold energy from the expansion process is captured in the CTES for reuse.

System Configurations

Academic literature categorizes CCES systems by the thermodynamic state of the stored CO₂ and the use of external heat:

Transcritical CCES (TC-CCES): CO₂ crosses its critical point during the cycle, transitioning between subcritical (liquid) and supercritical states. This is the configuration used by Energy Dome's commercial design.

Supercritical CCES (SC-CCES): CO₂ remains above its critical temperature and pressure throughout the cycle. Comparative analyses find that supercritical configurations exhibit enhanced thermodynamic properties and simpler system architecture, making them potentially well-suited for large-scale applications.

Liquid CCES: CO₂ is stored entirely in liquid form at high pressure. This configuration offers the highest energy storage density but may have lower cycle efficiency depending on the liquefaction method.

Systems are further classified as adiabatic (no external heat source — all thermal energy is recycled internally) or diabatic (supplemented by external heat sources such as waste heat, solar thermal, or geothermal). Diabatic configurations can achieve higher discharge temperatures and power output but introduce dependency on external energy inputs.

CCES systems can also use isochoric (constant volume) or isobaric (constant pressure) storage. An Energy Conversion and Management study (2024) found that isobaric CO₂ storage significantly reduces exergy destruction compared to isochoric storage, because maintaining constant pressure allows the compressors and turbines to operate at their optimal design points throughout the charge/discharge cycle, analogous to the findings in CAES research.

Energy Dome's CO₂ Battery Design

Energy Dome's commercial implementation uses a patented closed-loop process with CO₂ stored in a flexible above-ground gas holder (the "dome") at near-atmospheric pressure on the low-pressure side, and in steel vessels as liquid CO₂ on the high-pressure side. The system includes hot and cold thermal energy storage to capture and recycle compression heat and expansion cold. The standard commercial unit is rated at 20 MW power / 200 MWh storage (10-hour duration), capable of powering approximately 14,000–18,000 homes.

The entire system is built from off-the-shelf industrial components: centrifugal and reciprocating compressors, axial and radial turbines, shell-and-tube and plate heat exchangers, steel pressure vessels, and water-based thermal stores. Energy Dome emphasizes that this reliance on proven, globally available equipment eliminates supply chain bottlenecks and enables rapid deployment — a claim supported by the short construction timeline from notice-to-proceed (end of 2023) to commissioning (Q1 2025) for the Sardinia plant.

Performance Characteristics

Academic studies report round-trip efficiencies for CCES systems ranging from approximately 41% for basic configurations to 74–76% for optimized multi-stage systems with graded thermal storage. A 2025 Frontiers in Energy study demonstrated a system RTE of 76.4% using a high-temperature graded heat storage structure with multiple heat exchange working fluids. Key parameters affecting efficiency include the compression/expansion ratio, the number of compression and expansion stages, heat exchanger effectiveness, thermal storage temperature differences, and ambient temperature.

The technology offers intrinsic long-duration capability: unlike batteries where energy and power are coupled (and cost scales roughly linearly with duration), a CO₂ Battery can extend its duration simply by adding more liquid CO₂ storage vessels, with marginal cost per additional hour of storage that is very low. This makes it economically suited to 8–24+ hour discharge applications where lithium-ion becomes prohibitively expensive.

CO₂ Batteries offer several structural advantages over the incumbent energy storage technologies: lithium-ion batteries, pumped hydro storage ("PHS"), and compressed air energy storage.

Long Duration Without Degradation

Lithium-ion batteries are optimized for short-duration storage (typically 2–4 hours) and experience capacity degradation over their lifetime through calendar aging and cycle aging. CO₂ Batteries are designed for long-duration discharge (10+ hours) with no electrochemical degradation — the working fluid (CO₂) does not degrade, and the mechanical components (compressors, turbines, heat exchangers) are proven industrial equipment with well-understood maintenance profiles and lifetimes exceeding 30 years. This makes the technology fundamentally suited to the long-duration energy storage market that lithium-ion cannot economically serve.

Geographic Independence

Pumped hydro storage — which provides approximately 95% of global energy storage capacity — requires specific topography (elevation differential) and access to large water resources, restricting it to a limited number of viable sites. First-generation CAES (Huntorf, McIntosh) requires underground salt caverns for air storage. CO₂ Batteries require no site-specific geology: CO₂ is stored in above-ground steel pressure vessels and thermal energy in hot and cold water tanks or dedicated thermal stores. This enables deployment on any flat industrial site, including brownfield locations, and eliminates the geographic bottleneck that constrains both PHS and conventional CAES.

Higher Energy Density Than Air

CO₂ has fundamentally superior thermodynamic properties to air for compressed gas energy storage. Its critical temperature of 31.1°C is close to ambient, allowing it to be liquefied at room temperature under moderate pressure (7.4 MPa) — whereas liquefying air requires cryogenic temperatures (~-196°C). Liquid CO₂ is several times denser than compressed air, resulting in energy storage densities approximately 4–8 times higher than advanced adiabatic CAES. This translates directly into a smaller physical footprint for equivalent storage capacity.

No Critical Minerals

Unlike lithium-ion batteries, which depend on lithium, cobalt, nickel, and graphite supply chains that face geopolitical concentration risk and price volatility, CO₂ Batteries are built entirely from commodity materials: steel, concrete, water, and carbon dioxide. The working fluid is a low-cost, globally abundant industrial gas. The system uses off-the-shelf compressors, expanders, and heat exchangers from mature supply chains serving the oil and gas, petrochemical, and power generation industries.

Closed-Loop Operation

The CO₂ in a CO₂ Battery circulates in a hermetically sealed closed loop — it is never vented to the atmosphere. No CO₂ is consumed or produced during operation. The system has zero direct emissions, zero water consumption (in non-evaporative configurations), and no waste streams.

Despite its promising characteristics, CO₂ Battery technology faces several deployment challenges.

First-of-a-Kind Risk and Limited Operating History

The technology is at a very early stage of commercial deployment. Energy Dome's 2.5 MW demonstration operated from mid-2022, and its first full-scale 20 MW/200 MWh plant was commissioned in Q1 2025. No CO₂ Battery has accumulated multiple years of continuous commercial operation at full scale. While the individual components (compressors, turbines, heat exchangers, pressure vessels) are proven industrial equipment, their integration into a complete thermodynamic cycle optimized for energy storage is novel and lacks the decades of operating experience that CAES (Huntorf has operated since 1978) or PHS enjoy. Investors, utilities, and regulators will require demonstrated reliability data before large-scale commitments.

Round-Trip Efficiency

The round-trip efficiency ("RTE") of CO₂ Battery systems — the ratio of electricity output to electricity input — is lower than that of lithium-ion batteries (85–95% RTE) and pumped hydro (75–85% RTE). Academic studies report RTE values ranging from approximately 41% for early liquid CCES configurations to 74–76% for optimized systems with graded thermal storage and multi-stage compression/expansion. Energy Dome claims competitive RTE for its commercial design but has not published independently verified figures. The efficiency gap relative to lithium-ion is acceptable for long-duration applications where cost per MWh stored is more important than round-trip losses, but it does mean that more renewable energy must be generated for every unit of dispatchable power delivered.

Thermal Management Complexity

The performance of CO₂ Battery systems is highly sensitive to the management of thermal energy generated during compression and consumed during expansion. Optimal systems require hot thermal energy storage ("HTES") to capture compression heat, and cold thermal energy storage ("CTES") to provide cooling for liquefaction. The temperature differences across heat exchangers, the number of compression and expansion stages, and the effectiveness of thermal storage materials all significantly affect RTE. Research shows that multi-stage compression/expansion (typically 3 stages is optimal) with graded heat storage using multiple working fluids can substantially improve efficiency — but this adds system complexity and cost.

Cost Competitiveness

Energy Dome claims its CO₂ Batteries cost less than half of comparable lithium-ion storage facilities, but this claim is based on projected NOAK costs and has not been independently verified at scale. The first-of-a-kind Ottana plant received substantial public funding (€35 million from Breakthrough Energy Catalyst, €25 million from the European Investment Bank, and significant Italian government support). The true LCOE and capital cost at scale will only become clear as multiple identical plants are built and operated. Lithium-ion costs have declined dramatically over the past decade and continue to fall, setting a moving competitive target.

Competitive Landscape

CO₂ Batteries compete not only with lithium-ion but with a growing field of LDES technologies: iron-air batteries (Form Energy), liquid air energy storage (Highview Power), compressed air energy storage (various developers), gravity-based storage, and flow batteries. Each technology has different cost, duration, efficiency, and siting characteristics. The LDES market is still nascent, and it is not yet clear which technologies will achieve the scale and cost reductions needed for widespread deployment. CO₂ Batteries must prove their cost and reliability advantages against these alternatives in competitive procurement processes.

Scale-Up and Manufacturing

While Energy Dome emphasizes that its system uses off-the-shelf components, scaling from a single demonstration plant and a single first-of-a-kind commercial unit to a large fleet of standardized plants requires establishing manufacturing partnerships, optimizing the supply chain for the specific integration of components, and demonstrating consistent quality and performance across multiple installations. The company's modular, standardized approach — where the Sardinia and Wisconsin plants are designed to be virtually identical — is intended to accelerate this learning curve, but the transition from FOAK to reliable NOAK production is not yet proven.