Storing energy using CO2
Harnessing the cause of global warming to help prevent it
A couple of weeks ago I published an article about emerging energy storage technologies. The article was the most popular article I’ve ever published (I mean, I’ve only published 6 including this one but still…). My tweet about the article received more likes than all my other tweets put together.
But then a few days ago an article about CO2 batteries popped up and I realised, I’d missed one! There’s another energy storage technology out there that I hadn’t covered. I needed to dive into it.
(When I started writing on Medium, I also committed to publishing an article every 2 weeks. The one I’m working on right now is taking longer than usual as it’s a big thorny topic that’s new to me. So finding something that I can cover quickly also allows me to stick to my “article every 2 weeks rule”.)
Hence this article, covering a single energy storage technology, the CO2 battery.
Before I get started, another plug for my previous article which I reference a fair bit below. Now… on to CO2 batteries!
What is a CO2 battery?
The CO2 battery is being developed by Italian company Energy Dome. It uses carbon dioxide to store energy in a way that seems to meld elements of liquid air energy storage (LAES) and compressed air energy storage (CAES). But instead of air, it uses carbon dioxide. And there’s something poetic about using the cause of our climate issues to solve one of the key climate change mitigation challenges — long duration energy storage.
Energy Dome describes their technology as using “a closed thermodynamic process” to store energy. When electricity is abundant, it is used to power pumps and compressors that take gaseous CO2 from a low pressure tank (or dome) and store it in a second tank as a liquid at very high pressure. In this way, the system is “charged”.
When electricity is needed again, the CO2 is evaporated and expanded. The pressure created is used to drive a turbine that generates electricity.
How does it stack up against other storage technologies?
The obvious comparisons here are to CAES and LAES. Energy Dome’s CO2 battery involves compressing a gas so that it’s at a very high pressure (like CAES), and then storing it in tanks (like LAES). Similar to both of these technologies, Energy Dome’s CO2 battery uses off-the-shelf components that are tested and can be sourced through well-established supply chains. The technology doesn’t rely on elements like lithium and cobalt, nor does it suffer from degradation like a lithium-ion battery.
Energy Dome argues that CO2 is a better medium than air for storing electricity. This is because CO2 becomes a liquid at high pressure, even at ambient temperature. This allows compressed CO2 to store more than 10 times as much energy as the same volume of compressed air. Whilst a compressed air system might need a pressure vessel as large as a salt cavern to store a meaningful amount of energy; a compressed CO2 system can make do with a large tank. This allows CO2 batteries to be installed wherever there’s a connection to the electricity grid and enough space to build a CO2 “dome”. In comparison, CAES systems can only be installed where there’s favourable geology.
Liquid air systems convert air to a liquid, but require temperatures of -200°C to do so. Liquid air stores significantly more energy per unit volume than a standard compressed air system (20–50 times more by Energy Dome’s estimates) allowing the use of tanks rather than salt caverns. However, making something as cold as -200°C is a complex, energy intensive process, increasing the initial capital costs as well as the running costs associated with an LAES system. Energy Dome’s CO2 battery avoids this complexity and cost, allowing claimed roundtrip efficiencies of >75% (compared to 50–75% for CAES and LAES).
Energy Dome’s hope is that their CO2 battery can achieve comparable energy density to LAES, but without the requirement for cooling and the giant thermos flasks. To quote Energy Dome, their CO2 battery technology “has the same benefits of LAES and CAES (high energy density and storing energy at ambient temperature, respectively) but without their associated drawbacks relating to efficiency, cost and site dependency”.
What stage is the technology at?
Similar to CAES and LAES, Energy Dome’s CO2 battery is still in its early stages. In June 2022, the company announced the launch of a demonstration project in Sardinia. This 2.5 MW facility is capable of storing 4 MWh of electricity (enough to power 500 homes for a single day).
In a press release, Energy Dome described the Sardinia project as having “confirmed the performance of the CO2 Battery and its capability of storing energy for a long duration, all while maintaining highly competitive round-trip efficiency, without degradation”.
In partnership with Italian energy company A2A, Energy Dome is working on a 20 MW / 100 MWh project to be deployed on the Italian grid. Energy Dome expects to have completed this plant by the end of 2023.
More recently, Energy Dome announced that it is working with Danish renewables giant Ørsted to assess the feasibility of a 20 MW / 200 MWh project. This project would be delivered in continental Europe with construction starting in late 2024. Assuming this first project is delivered successfully, Ørsted and Energy Dome have left open the possibility of deploying multiple facilities across Europe.
Conclusion
Like nothing in the energy sector, CO2 batteries are not going to be a silver bullet. We may not see any of them delivered at all (beyond the Energy Dome facility in Sardinia).
But as long as there’s the possibility of CO2 batteries delivering long duration storage at a competitive cost, I’m happy to see another energy storage option entering the fray. We need all the help we can get if we want to get to an energy system that is 100% renewable.
