This giant bubble on the island of Sardinia holds 2,000 tonnes of carbon dioxide. But the gas wasn’t captured from factory emissions, nor was it pulled from the air. It came from a gas supplier, and it lives permanently inside the dome’s system to serve an eco-friendly purpose: to store large amounts of excess renewable energy until it’s needed.

Developed by the Milan-based company Energy Dome, the bubble and its surrounding machinery demonstrate a first-of-its-kind “CO2 Battery,” as the company calls it. The facility compresses and expands CO₂ daily in its closed system, turning a turbine that generates 200 megawatt-hours of electricity, or 20 MW over 10 hours. And in 2026, replicas of this plant will start popping up across the globe.

We mean that literally. It takes just half a day to inflate the bubble. The rest of the facility takes less than two years to build and can be done just about anywhere there’s 5 hectares of flat land.

The first to build one outside of Sardinia will be one of India’s largest power companies, NTPC Limited. The company expects to complete its CO2 Battery sometime in 2026 at the Kudgi power plant in Karnataka, in India. In Wisconsin, meanwhile, the public utility Alliant Energy received the all clear from authorities to begin construction of one in 2026 to supply power to 18,000 homes.

And Google likes the concept so much that it plans to rapidly deploy the facilities in all of its key data-center locations in Europe, the United States, and the Asia-Pacific region. The idea is to provide electricity-guzzling data centers with round-the-clock clean energy, even when the sun isn’t shining or the wind isn’t blowing. The partnership with Energy Dome, announced in July, marked Google’s first investment in long-duration energy storage.

“We’ve been scanning the globe seeking different solutions,” says Ainhoa Anda, Google’s senior lead for energy strategy, in Paris. The challenge the tech giant has encountered is not only finding a long-duration storage option, but also one that works with the unique specs of every region. “So standardization is really important, and this is one of the aspects that we really like” about Energy Dome, she says. “They can really plug and play this.”

Google will prioritize placing the Energy Dome facilities where they’ll have the most impact on decarbonization and grid reliability, and where there’s a lot of renewable energy to store, Anda says. The facilities can be placed adjacent to Google’s data centers or elsewhere within the same grid. The companies did not disclose the terms of the deal.

Anda says Google expects to help the technology “reach a massive commercial stage.”

Getting creative with long-duration energy storage

All this excitement is based on Energy Dome’s one full-size, grid-connected plant in Ottana, Sardinia, which was completed in July. It was built to help solve one of the energy transition’s biggest challenges: the need for grid-scale storage that can provide power for more than 8 hours at a time. Called long-duration energy storage, or LDES in industry parlance, the concept is the key to maximizing the value of renewable energy.

When sun and wind are abundant, solar and wind farms tend to produce more electricity than a grid needs. So storing the excess for use when these resources are scarce just makes sense. LDES also makes the grid more reliable by providing backup and supplementary power.

The problem is that even the best new grid-scale storage systems on the market—mainly lithium-ion batteries—provide only about 4 to 8 hours of storage. That’s not long enough to power through a whole night, or multiple cloudy and windless days, or the hottest week of the year, when energy demand hits its peak.

A series of large cylindrical tanks of different diameters, each as tall a multistory building After the CO2 leaves the dome, it is compressed, cooled, reduced to a liquid, and stored in pressure vessels. To release the energy, the process reverses: The liquid is evaporated, heated, expanded, and then fed through a turbine that generates electricity. Luigi Avantaggiato

Lithium-ion battery systems could be increased in size to store more and last longer, but systems of that size usually aren’t economically viable. Other grid-scale battery chemistries and approaches are in development, such as sodium-based, iron-air, and vanadium redox flow batteries. But the energy density, costs, degradation, and funding complications have challenged the developers of those alternatives.

Researchers have also experimented with storing energy by compressing air, heating up blocks or sand, using hydrogen or methanol, pressurizing water deep underground, and even dangling heavy objects in the air and dropping them. (The creativity devoted to LDES is impressive.) But geologic constraints, economic viability, efficiency, and scalability have hindered the commercialization of these strategies.

The tried-and-true grid-scale storage option—pumped hydro, in which water is pumped between reservoirs at different elevations—lasts for decades and can store thousands of megawatts for days. But these systems require specific topography, a lot of land, and can take up to a decade to build.

CO2 Batteries check a lot of boxes that other approaches don’t. They don’t need special topography like pumped-hydro reservoirs do. They don’t need critical minerals like electrochemical and other batteries do. They use components for which supply chains already exist. Their expected lifetime stretches nearly three times as long as lithium-ion batteries. And adding size and storage capacity to them significantly decreases cost per kilowatt-hour. Energy Dome expects its LDES solution to be 30 percent cheaper than lithium-ion.

China has taken note. China Huadian Corp. and Dongfang Electric Corp. are reportedly building a CO₂-based energy-storage facility in the Xinjiang region of northwest China. Media reports show renderings of domes but give widely varying storage capacities—including 100 MW and 1,000 MW. The Chinese companies did not respond to IEEE Spectrum’s requests for information.

“What I can say is that they are developing something very, very similar [to Energy Dome’s CO2 Battery] but quite large in scale,” says Claudio Spadacini, Energy Dome’s founder and CEO. The Chinese companies “are good, they are super fast, and they have a lot of money,” he says.

Why is Google investing in CO2 Batteries?

When I visited Energy Dome’s Sardinia facility in October, the CO₂ had just been pumped out of the dome, so I was able to peek inside. It was massive, monochromatic, and pretty much empty. The inner membrane, which had been holding the uncompressed CO₂, had collapsed across the entire floor. A few pockets of the gas remained, making the off-white sheet billow up in spots.

Meanwhile, the translucent outer dome allowed some daylight to pass through, creating a creamy glow that enveloped the vast space. With no structural framing, the only thing keeping the dome upright was the small difference in pressure between the inside and outside air.

“This is incredible,” I said to my guide, Mario Torchio, Energy Dome’s global marketing and communications director.

“It is. But it’s physics,” he said.

Outside the dome, a series of machines connected by undulating pipes moves the CO₂ out of the dome for compressing and condensing. First, a compressor pressurizes the gas from 1 bar (100,000 pascals) to about 55 bar (5,500,000 pa). Next, a thermal-energy-storage system cools the CO₂ to an ambient temperature. Then a condenser reduces it into a liquid that is stored in a few dozen pressure vessels, each about the size of a school bus. The whole process takes about 10 hours, and at the end of it, the battery is considered charged.

To discharge the battery, the process reverses. The liquid CO₂ is evaporated and heated. It then enters a gas-expander turbine, which is like a medium-pressure steam turbine. This drives a synchronous generator, which converts mechanical energy into electrical energy for the grid. After that, the gas is exhausted at ambient pressure back into the dome, filling it up to await the next charging phase.

Workers in hard hats and a series of connected pipes and tanks with the quilted-looking exterior of the dome in the background Energy Dome engineers inspect the dryer system, which keeps the gaseous CO₂ in the dome at optimal dryness levels at all times.Luigi Avantaggiato

It’s not rocket science. Still, someone had to be the first to put it together and figure out how to do it cost-effectively, which Spadacini says his company has accomplished and patented. “How we seal the turbo machinery, how we store the heat in the thermal-energy storage, how we store the heat after condensing…can really cut costs and increase the efficiency,” he says.

The company uses pure, purpose-made CO₂ instead of sourcing it from emissions or the air, because those sources come with impurities and moisture that degrade the steel in the machinery.

What happens if the dome is punctured?

On the downside, Energy Dome’s facility takes up about twice as much land as a comparable capacity lithium-ion battery would. And the domes themselves, which are about the height of a sports stadium at their apex, and longer, might stand out on a landscape and draw some NIMBY pushback.

And what if a tornado comes? Spadacini says the dome can withstand wind up to 160 kilometers per hour. If Energy Dome can get half a day’s warning of severe weather, the company can just compress and store the CO₂ in the tanks and then deflate the outer dome, he says.

If the worst happens and the dome is punctured, 2,000 tonnes of CO₂ will enter the atmosphere. That’s equivalent to the emissions of about 15 round-trip flights between New York and London on a Boeing 777. “It’s negligible compared to the emissions of a coal plant,” Spadacini says. People will also need to stay back 70 meters or more until the air clears, he says.

Worth the risk? The companies lining up to build these systems seem to think so.

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Grid-Scale Bubble Batteries Will Soon Be Everywhere

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