Compressed-air and CO2 storage: Science and technology

Why keeping the heat of compression changes everything

Compressed-air storage squeezes air into underground caverns and later expands it through turbines. Older diabatic plants burn gas to reheat the air; newer adiabatic plants store the heat of compression instead, and China is now building them at hundreds of megawatts.

Mechanism

Compressors driven by spare electricity push air into a salt cavern, hard-rock cavern or other sealed store, and the air heats up as it is squeezed. In diabatic CAES (Huntorf, Germany, 1979; McIntosh, Alabama, 1991) that heat is discarded and natural gas is burned to reheat the air on discharge. In adiabatic CAES the heat is kept in a thermal store and returned before expansion, so no fuel is needed; China's 300 MW Feicheng and Yingcheng plants and Hydrostor's designs work this way, with Hydrostor using a surface-fed water column to hold cavern pressure steady. Underwater variants keep air in flexible bags on a lake or sea bed, where water pressure does the same job; Toronto Hydro and Hydrostor piloted six such bags in Lake Ontario in 2015. Energy Dome's CO₂ Battery, as the company describes it, swaps air for carbon dioxide in a closed loop: CO₂ gas sits at roughly atmospheric pressure in an inflatable dome, is compressed until it turns liquid and is held in pressure vessels at ambient temperature, while the heat of compression goes to a thermal store; on discharge that heat re-vaporises the CO₂, which spins a turbine and refills the dome.^14510131417

Material scarcity and toxicity

The storage medium is air, and plants are built from steel, concrete, standard turbomachinery and, for adiabatic designs, heat stores. No critical minerals are needed. The constraint is geology: classic CAES needs salt caverns, though DOE estimates that Hydrostor-style mined caverns could suit about 80 percent of the US subsurface. Diabatic plants burn natural gas, adding fuel cost and emissions. NTPC's chairman cites the absence of lithium, cobalt and other critical minerals, and independence from topography, as reasons for choosing Energy Dome's process.³⁴⁹

Degradation and ageing

There is no capacity fade in the chemical sense; ageing falls on compressors, turbines and heat exchangers, maintained like gas-turbine plant. Huntorf has run since 1979, more than 45 years (calculated). Hydrostor gives design lives of more than 50 years for Willow Rock and Silver City, and Energy Dome now expects more than 30 years for its CO₂ Battery with no efficiency loss over time, up from 25 years in its 2022 statements (developer claims).⁵⁸¹²¹⁷

Variants compared^{13581011131415}

Sources

1. 'World's largest' compressed air energy storage project connects to the grid in China Energy-Storage.news, 2024
2. Hydrostor gets $1.76B DOE conditional loan guarantee for compressed air energy storage Utility Dive, 2025
3. US DOE offers US$1.76 billion loan to Hydrostor for A-CAES California project Energy-Storage.news, 2025
4. DOE's billion dollar bet: the largest-ever loan supporting long duration energy storage Renewable Energy World, 2025
5. Google backs Italian long duration CO2 energy storage technology RenewEconomy, 2025
6. NTPC joins forces with Energy Dome for CO2 Battery deployment Energy Dome via Business Wire, 2025
7. Energy Dome and ENGIE sign pioneering storage offtake agreement Energy Dome via Business Wire, 2024
8. Hydrostor secures grid connection for first Australian CAES project ESS News, 2026
9. Silver City Energy Storage Centre Hydrostor, 2026
10. World's largest compressed air energy storage power station launched China Daily (Shandong), 2024
11. Toronto utility unveils novel underwater energy storage system Utility Dive, 2015
12. Emerging Technologies Review: Pumped Storage Hydropower (PNNL-34458) Pacific Northwest National Laboratory, 2023
13. Italian group taps CO2 for energy storage POWER Magazine, 2022