How CO2 could be the future of energy storage?

CO2 is infamous for being the major culprit of climate change, but what if this global warming villain could turn into a green energy hero? Believe it or not, carbon dioxide may be a key component in developing a new green battery that can be quickly deployed around the world. If you think I’m just gasbagging you, you’ll definitely want to check this out. Could giant bladders of CO2 solve our energy storage problem? Let’s see if we can come to a decision on this.

As you might know, carbon dioxide is one of the main greenhouse gasses (GHG) in the atmosphere. To be more specific, CO2 accounts for around three-fourths of global man-made emissions.¹ CO2 and other GHG are naturally present in our atmosphere and act as a sort of blanket for our planet.² It’s essential that we have this blanket, but our blanket is getting metaphorically thicker and holding in too much heat. Clearly, CO2 is the bad guy in this scenario, but there might be room for a redemption story here.

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Renewable energy is one of the top weapons we have to fight global warming by reducing GHG emissions. Yet, its intermittency reduces its climate mitigation potential, which is why we desperately need a cost-efficient and long-lasting energy storage system to manage its ups and downs. Well, carbon dioxide may give us what we’re looking for.

Last June, the Italian startup, Energy Dome, successfully demonstrated the operation of the world’s first CO2 battery³ on a pilot scale in Sardinia. With plenty of sunshine and wind, this isolated area is triggering a sea of renewable opportunities. While the storage capacity of their proven device was only 4 MWh⁴, their first commercial scale plant will have 200MWh and is expected to go live by the end of next year. After only 2 years of testing, the Energy Dome’s battery seems to be ready to store renewable energy and dispatch it over up to 24 hours⁴ anywhere in the world.

Besides the recent $11 million⁵ fundraising to commercialize their utility-scale long-duration batteries, the startup already inked some deals. For instance, they’re going to put together a 20 MW-5h facility for A2A, the second largest Italian utility company. On top of that, Ansaldo Energia signed them up for developing energy storage systems in Italy, Germany, the Middle East, and Africa.

Using CO2 against climate change sounds a bit controversial, right? So, how does that work? Energy Dome technology is based on a closed-loop thermodynamic system.⁶ During the charging process, the plant taps into the grid renewable electricity surplus. This drives a motor drawing CO2 from a massive dome-shaped double membrane gas holder…basically a giant bladder. Here the gas is stored at ambient temperature and pressure. After being pressurized through a turbo compressor, CO2 turns into a highly dense liquid, which is stored in…a sort of fire extinguisher…at room temperature and 70 bar (1015 psi) of pressure.⁶ Essentially, the charging step stores renewable energy into high-pressure liquid CO2. At the same time, the system recovers and stores the compression-induced heat into two thermal units.⁷ When it’s time to discharge the power, the recovered heat is then reused to convert the liquid CO2 back to vapor. After passing through an expander, this gaseous stream drives a turbine to generate the electricity that will be delivered back to the grid. To close the loop, the CO2 is sent back to the dome to start the next charging cycle.

If you’re worried about CO2 leaking out, or needing to top up the system with lots of CO2 over time … don’t worry. Energy Dome said there are no leaks of CO2. I had a chance to talk to Claudio Spadacini, the CEO of Energy Dome, and here’s what he said about how the system is setup.

“We charge the system with CO2 at the beginning of its life, just once over 30 years. So we don’t use CO2 continuously. It’s like the refrigerator or your car air conditioner, that you just charge at the beginning. And the CO2 we use is just the working fluid in our battery.” -Claudio Spadacini

So, at the end of the day, their system will only eat up a few hundred tons of CO2 to begin with, without requiring any more gas once up and running. This is just a few bubbles compared to what our fossil-fuelled plants are emitting every year.⁸ It’s not so much the CO2 that’s used by the system, but rather the CO2 that’s not generated through fossil fuels for power generation that make this a “green” system. But the fact that CO2 is involved at all does make it a little ironic.

CO2 battery: Way more than a gassy speech

If this basic principle sounds familiar, it’s probably because it is. CO2 under pressure is not the only gas you can use as a storage medium. Leveraging a similar approach, you can harness pressurized air instead. Based on how you compress air, you can have either compressed air energy storage (CAES) or liquid air energy storage (LAES) systems. As for the second process, rather than using a compressor to reduce its volume, you cool air down to -196°C, which is why it’s also called cryogenic energy storage.⁹ I have another video on this exact topic if you’re interested.

So, how does CO2 compare to air for storing energy?

Let’s look at energy density, for instance, which gives you an idea of how many kWh you can store per unit of volume. According to the figures reported by Energy Dome on their website, their CO2 battery energy density is up to 11x higher than CAES’¹⁰. I fact-checked this and they’re pretty much right. Even when considering the maximum value of CAES energy density reported in the literature, Energy Dome’s is still 6x greater.¹¹ For this reason, CAES systems need humongous spaces such as underground salt caverns to store the low-energy-density compressed air. These can be found only in specific locations which limit CAES scalability. On the other hand, LAES can pack nearly 2x more kWh per cubic meter compared to the Energy Dome system. Nevertheless, for LAES to work you need to get down to ridiculous low temperatures to liquefy air. Plus, to generate electricity, you’ll have to warm up the liquefied air back to room temperature. This drastic cooling/warming cycle bites into the process round trip efficiency (RTE), ranging between 45 and 70%.¹² Unlike air in LAES, the CO2 used in the Energy Dome system is stored at ambient temperature, which reduces operational costs and energy penalties. Also, Energy Dome’s design is pretty simple as it employs only two steps, meaning compression and evaporation. Combined with lower operational temperatures, this leaner schematic minimizes energy losses, thus leading to a RTE of up to 75-80%.¹³

Aside from being more efficient than air at storing energy, CO2 may also be a solid alternative to metal-based batteries. As claimed by Energy Dome in a press release¹⁴, their device would be 50% cheaper than a similar-sized lithium-ion battery. Apparently, once at full scale, they projected their 25MW/200MWh plant to have a levelized cost of storage (LCOS) as low as $50 per megawatt-hour of energy stored.¹⁵ At this point we’ll have to trust them on that figure, but according to Lazard, they estimated the LCOS of a 4-hour 100MW/400MWh lithium-ion battery system to range between $131-$232/MWh.¹⁶ And LAES is not any cheaper, with an LCOS stretching up to $300/MWh¹⁷.

I can hear you already. How about pumped hydro storage (PHS) systems? Besides being the world’s largest long-duration utility-scale power storage installations¹⁸, they can last up to 150 years.¹⁹ Yet, their LCOS is pegged at $186/MWh²⁰, which is floating well above what Energy Dome is promising. Of course, we’ll have to wait at least until the end of 2023 or 2024 to see whether Energy Dome will go over the predicted budget.

Other than being one of the most wanted fugitives in the world, Energy Dome’s key ingredient is much more sustainable than commercial batteries’ raw materials. For example, extracting 1 ton of lithium drinks up 500,000 gallons of freshwater. Another major cathode component is nickel, which wins the shameful race for most carbon-intensive mining.^{21 22} When it comes to cobalt, about 70% of the world’s supply comes from the Democratic Republic of Congo (DRC)²³, which has a horrible track record with forced labor, child labor, and safety.²⁴

Energy Dome batteries only need steel, CO2 and water, so the CO2 battery can be built using off-the-shelf equipment taken from the existing supply chain. That makes the system easily scalable. For example, their compressor is the current standard used in the oil & gas industry.²⁵ This could give Energy Dome an advantage.

“I don’t see any kind of challenge in the supply chain. The big advantage which we have is that this technology is based on an existing industry, vessels, heat exchanges, a compressor, a turbine, a generator motor, this is an industry which is present everywhere. Is present in North America, is present Europe, and is present in the far east. So I don’t see any kind of constraints like many other technology have. There are many other technology which have bottleneck in the supply chain.” -Claudio Spadacini

Another limitation of lithium-ion batteries is time. From a storage point of view, most installations max out at around 6 hours for the best cost advantage.²⁶ Obviously, that’s not enough to back up the grid during extended periods of low renewable energy generation or overnight. That’s where Energy Dome would come handy as their CO2 battery could churn out energy for up to 24 hours²⁷. Aside from short-term limitations, lithium-ion batteries’ lifespan is 15 years at best as their performance degrades with charging cycles.²⁸ In contrast, Energy Dome system is meant to last for up to 30 years.^{29 30} Flexibility is another plus of the Energy Dome solution. They’re using a modular approach to customize their battery size and match different storage requirements.

What’s bursting the Energy Dome bubble?

Sounds like this Italian startup has killed two birds with one carbonated stone. Creating a promising long-term energy storage option using the same gas that’s causing some of our problems. As promising as it sounds, there are some challenges. Unlike CAES, which stores pressurized air underground, Energy Dome keeps CO2 at atmospheric pressure in an above-ground inflatable gasholder, a.k.a. the dome. Just like a CO2-filled tennis-court bubble as their CEO, Claudio Spadacini, describes it.³¹ The potential issue is space. How well does this system scale size-wise? He did confirm that space is one of Energy Dome’s limitations compared to lithium-ion batteries.

“In terms of dimensions, our system is not the most compact. And this is the only con we have versus lithium ion batteries. The land needed by the CO2 battery is about seven to eight percent the land you need for the solar PV plant which should produce the power that we can store.” -Claudio Spadacini

While space is a minor concern, technical challenges could pop the Energy Dome bubble. As stated by an energy systems researcher at Loughborough University, the current heat exchangers used by the Italian startup may struggle to perform well over the estimated plant lifetime.³² This means they may have to put in some extra work to reengineer and customize the technology, which could delay their scale-up. As mentioned earlier, Energy Dome hopes to reach a RTE of about 75-80%. However, a previous study assessing the performance of a similar plant seemed to be less optimistic. As reported by Chinese researchers, the efficiency of both the compressor and the turbine peaked at ca. 67%.³³ On that note, Claudio had this to say…

“So now we had a lot of people which was not believing in us. It was too fast. So we are claiming 75% efficiency, and there are still many people which doesn’t believe. They say it’s not possible. But now, we promised something. Our idea is that we promise and we deliver. So we really wanted to prove what we say.” -Claudio Spadacini

Energy Dome has been doing just that with the pilot plant. If we assume Energy Dome will hit a RTE of 75-80%, that means they lose 20-25% of the energy they put in their battery. In contrast, lithium-ion battery’s RTE can get up to 90%,³⁴ which means you’re only losing 10% of the energy you put in.³⁵ If you do the math, you’ll see the Italian start-up is losing 2.5x more energy through the RTE. However, this isn’t something we should necessarily worry about. The largest pumped hydro energy storage systems in the world only achieve about 80% efficiency too.³⁶ In other words, for short form energy storage of up to 6 hours batteries still have the edge, but when looking at long duration storage Energy Dome holds its own nicely. However, Claudio raised a point worth remembering: lithium ion batteries slowly degrade over time, so the amount they can store degrades. Energy storage systems like Energy Dome and Pumped Hydro don’t lose any storage potential over time.

When looking at longer-duration technologies, direct competitors worldwide are stepping on the gas and may soon catch up with the Italian rival. For instance, the UK-based Highview Power said their 100MW wind-powered LAES could hit the $50/MWh target by 2030.³⁷ Echogen made exactly the same claim across the pond, except they’ve developed a thermal storage recipe featuring sand, ice and supercritical CO2.³⁸

When it comes to Energy Dome, we’re not going to have to wait too long to see how it performs. They not only spun up their pilot test quickly, but they’re also being very aggressive with their upcoming projects. Some of that confidence comes from their simplified supply chain and parts used.

“…since our product is a standardized product, and we will repeat it, just by economy of scale we will be able in less than a couple of years to cut that delivery time below one year, including construction.” -Claudio Spadacini

While this sounds too fast to be true, it wouldn’t be the first turbo-speed scale-up for Energy Dome. Turning the evil CO2 into a climate savior sounds exciting and Energy Dome seems to know what it’s doing. However, it’s better to slow down the hype machine until we see the real cost of their first commercial-scale plant. It may not be the silver bullet to the climate change dilemma, but it may reduce the need for fossil fuels as a backup source of power for renewables, and it may reduce the mining required for metallic batteries. Regardless, CO2-based storage systems won’t entirely replace lithium-ion batteries, which are still the most cost-efficient option for lower-scale short-run applications. So, an integration of the two technologies may be the best green deal for the years ahead.