Material Insights · Understanding the Material World

Compressed-air and CO2 storage: Engineering and performance

How cheap caverns make long hours affordable

What this tab covers Engineering and operating performance: round-trip efficiency, whether power and energy can be scaled separately, footprint and siting, and safety.

Adiabatic CAES now reaches about 64 to 70 percent round trip at large scale, below pumped hydro but with cheap extra hours and faster construction.

Round-trip efficiency

Mixed

Efficiency depends on how heat is handled. PNNL's 2023 review estimates 29 to 52 percent for conventional compressed air storage, which throws away the heat of compression. China's newer adiabatic plants are designed for 72.1 percent (Feicheng) and 64 percent rising to 70 percent (Yingcheng), both developer figures; Yingcheng's planned 498 GWh in and 319 GWh out a year match 64 percent (calculated). Hydrostor's president cites about 60 to 65 percent for its plants, and Energy Dome claims over 75 percent. No measured long-run figure has yet been published for any of the new plants. Pumped hydro, for comparison, is around 80 percent.18111315

Conventional CAES (PNNL estimate) 29-52%15 Feicheng / Yingcheng design 72.1% / 64% (up to 70%) (developer design figures)131 Hydrostor A-CAES about 60-65% (company estimate)11 Energy Dome CO2 Battery over 75% (company claim)8

Power and energy decoupling

Favourable

Power is set by compressors and expanders; energy by cavern volume and pressure. DOE/PNNL's 2022 estimate for a 100 MW plant falls from USD 295 per kWh at 4 hours to USD 122 per kWh at 10 hours, and to USD 18 per kWh at 100 hours by 2030, showing how cheap extra storage hours become once the machinery is paid for.6

Installed cost, 100 MW / 4 h USD 295/kWh6 Installed cost, 100 MW / 10 h USD 122/kWh6 Installed cost, 100 MW / 100 h (2030) USD 18/kWh6

Footprint and siting flexibility

Favourable

The store is underground, so the surface plant resembles a gas-turbine station. Salt-cavern plants are limited to salt basins; mined-cavern designs widen the map. China's Yingcheng took about two years to build; its developer compares this with six to eight years for pumped hydro. Energy Dome's dome-and-tank plant is above ground and site-independent, which suits locations without caverns, such as the NTPC Kudgi power station site in Karnataka, India.159

Build time, Yingcheng about 2 years1 McIntosh cavern about 1,500 ft deep, 19.8 million cubic ft5

Safety and thermal runaway risk

Favourable

There is no thermal runaway. Risks are high-pressure equipment failure, cavern instability and, for diabatic plants, normal gas-fired plant hazards. These fall under pressure-vessel, mining and gas-plant rules rather than battery codes.4

Sources

  1. 'World's largest' compressed air energy storage project connects to the grid in China Energy-Storage.news, 2024
  2. US DOE offers US$1.76 billion loan to Hydrostor for A-CAES California project Energy-Storage.news, 2025
  3. DOE's billion dollar bet: the largest-ever loan supporting long duration energy storage Renewable Energy World, 2025
  4. 2022 Grid Energy Storage Technology Cost and Performance Assessment US Department of Energy / Pacific Northwest National Laboratory, 2022
  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. Hydrostor secures grid connection for first Australian CAES project ESS News, 2026
  8. World's largest compressed air energy storage power station launched China Daily (Shandong), 2024
  9. Emerging Technologies Review: Pumped Storage Hydropower (PNNL-34458) Pacific Northwest National Laboratory, 2023