Compressed air energy storage (CAES)
A storage technology that compresses air into caverns or vessels with surplus electricity and expands it through a turbine to regenerate power.
CAES uses electric compressors to pressurise air, typically to 40–100 bar, into salt caverns, aquifers, hard-rock caverns or engineered vessels. On discharge the air is released through an expander or gas turbine. The key engineering problem is heat: compression heats air strongly and expansion cools it.
- Diabatic CAES vents compression heat and burns natural gas to reheat air before expansion. Huntorf, Germany (1978, 321 MW) and McIntosh, Alabama (1991, 110 MW) are the classic examples, with electrical round-trip efficiency around 42% and 54% respectively.
- Adiabatic CAES stores compression heat in a thermal store and reuses it, targeting 60–70% efficiency without fuel.
- Isothermal CAES attempts near-constant-temperature compression; still largely experimental.
Example: 1 m³ at 70 bar expanded isothermally to 1 bar ≈ 7.0 MPa x 1 m³ x ln 70 ≈ 29.7 MJ ≈ 8.3 kWh
China has commissioned several large advanced CAES plants since 2021, including 300 MW-class units in salt caverns in Hubei and Shandong. CAES competes with pumped hydro for long-duration storage where suitable geology exists.
Related terms
All terms →- Long-duration energy storage (LDES)
Storage able to discharge at rated power for roughly 8 hours or more, covering multi-day lulls and seasonal imbalances that 2–4 hour batteries cannot.
- Pumped hydro storage
Energy storage that pumps water to an upper reservoir when electricity is cheap or abundant and releases it through turbines when power is needed; the largest form of grid storage worldwide.
- Round-trip efficiency
The share of energy put into a storage system that can be recovered on discharge, measured at a defined boundary (cell, DC, or AC grid connection).
- Thermal energy storage
Storing energy as heat or cold, in sensible, latent (phase-change) or thermochemical form, for later use as heat or for power generation.
- LCOS (Levelized Cost of Storage)
The average cost per MWh discharged over a storage asset’s lifetime, including investment, operation, charging energy and efficiency losses.