Compressed carbon dioxide energy storage

Compressed carbon dioxide energy storage can be used to store electrical energy at grid scale. The gas is well suited to this role because, unlike most gases, it liquifies under pressure at ambient temperatures, so occupies a small volume. Energy Storage News reported that it will be "a cheaper form of energy storage than lithium-ion batteries".

Process
A 100MWh store requires about 2000 tonnes of carbon dioxide (CO2). At the start of the process, CO2 gas is stored at atmospheric pressure in a large expandable fabric container, like those used to store biogas, housed within an inflatable protective dome.

To store energy, the gaseous CO2 is compressed to around 70 bar, which heats it to around 400 °C. Passing it through a heat exchanger and a thermal store cools the supercritical carbon dioxide gas enough to liquify it. The liquid CO2 can be stored in this state indefinitely in pressurised cylinders.

When energy is required, the CO2 is passed back through the heat exchanger, where it is warmed by recovering heat from the heatstore and reverts to high-pressure gas. The gas is used to drive a turbine to generate electricity as it passes back into the low pressure store, completing the closed cycle.

Advantages
Liquid carbon dioxide can be stored at ambient temperatures, unlike Liquid air energy storage (LAES), which must keep liquid air cold at −192°C, though the CO2 does need to be kept pressurised.

Liquid CO2 has a much higher energy density (66.7 kWh/m3), than compressed air in typical to compressed-air energy storage (CAES) systems (2-6 kWh/m3), meaning the same energy can be stored in a much smaller volume.

All the components of the system are readily available: gas storage domes are used in biogas production; the compressors and heat exchangers are used in the oil and gas industries; the turbine is similar to a medium pressure steam turbine.

Disadvantages
The gas domes take up land - approximately 6% of the land area of a matching solar farm.

Carbon dioxide is an odourless asphyxiating gas that is heavier than air. A catastrophic rupture of a storage dome in, say, a hurricane would disperse the gas harmlessly, but a rupture in still air, through accident or sabotage, could be more problematic.

Energy Dome
The start-up company Energy Dome built a 2.5 MW/4 MWh demonstrator plant in Sardinia, for the Italian utility company A2A, which came into operation in June 2022. Opportunities are being explored for production of standardised 20 MW/200 MWh energy storage units in conjunction with wind energy engineering group Ansaldo. The company claims a round trip efficiency of 75% and a projected cost of EUR 220/kWh of storage capacity, which is half that of Lithium-ion battery.