China's 700 MW compressed-air plant is now the world's largest grid battery
China has completed the second phase of the Huaneng Jintan compressed air energy storage (CAES) plant in Jiangsu Province, bringing it to a total capacity of 700 MW and 2.8 GWh of storage — making it the world's largest facility of its kind by every meaningful measure. The plant uses salt caverns roughly a kilometre underground to store electricity as pressurised air, not lithium cells. For grid planners wrestling with how to balance wind and solar output over many hours, this is the clearest proof yet that the technology works at industrial scale.
How it works
When electricity demand is low — overnight, or on a windy afternoon — surplus power drives large compressors that push air into natural salt caverns at pressures above 130 atmospheres. Salt rock is ideal: it is dense, largely impermeable, and can hold gas without leaking for years. When demand spikes, valves open, the compressed air expands through turbines, and power flows back to the grid.
The engineering challenge is heat. Compressing air generates a lot of it, and dumping that heat wastes energy. Jintan captures it in 16 spherical water tanks totalling 56,000 cubic metres, then uses that stored heat to warm the air again during generation. That heat-recovery loop pushes round-trip efficiency above 70% — and critically, the plant burns no fossil fuel at any stage, unlike older CAES designs that used gas burners to reheat the air before expansion.
The scale
Each of the two units delivers 350 MW, a new world record for a single CAES machine. The Jintan Phase II project can handle around 330 full charge-discharge cycles a year, with each discharge lasting roughly four hours. The main borehole moves up to 2.16 million cubic metres of air per hour. Germany's Huntorf plant — built in 1978 and previously the world's second-largest at 290 MW — is now 2.4 times smaller.
Why it matters outside China
Lithium-ion batteries dominate short-duration storage, but they struggle to compete economically beyond about four hours of discharge. That gap is exactly where CAES and pumped hydro sit, and it becomes more pressing as renewable penetration on grids rises. As the Hydrostor CEO on long-duration storage has noted, a grid that needs ten or more hours of backup simply cannot rely on li-ion alone.
The UK's Cheshire Basin has been identified as a viable CAES site. US developers including Hydrostor and Cavern Energy are pursuing similar projects. Jintan does not prove those projects will succeed on their own terms, but it does remove the biggest doubt: whether non-fuel CAES at grid scale is technically viable. It is.
Huaneng says the plant will eventually reach 1,000 MW. Whether that timeline holds, and what the full capital cost works out to per kilowatt-hour, has not been disclosed.
