Salt caverns nearly a mile under eastern China hold the compressed air for a 600 megawatt plant that runs without burning any fuel, and close to a third of the electricity pushed in never comes back out

A concrete pad in eastern China with a steel wellhead standing on it, fenced and otherwise unremarkable.

Below it the ground drops away for nearly a mile into a cavity washed out of rock salt.

Nothing is kept down there except air, squeezed until it behaves almost like a liquid.

Storing energy as heat or as pressure keeps reappearing wherever chemistry is too expensive, including inside an empty chamber in an Idaho dam.

When the grid calls for power a valve opens and the cavern breathes out for hours.

No flame is involved at any point.

Which raises the obvious question of where the heat goes.

Why pressure can hold electricity at all

Squeezing air takes work, and that work does not vanish. It becomes pressure and it becomes heat, in roughly equal measure.

Older plants of this type threw the heat away and then burned natural gas on the way back out to replace it.

That worked, and it also meant the machine was half a gas turbine wearing a storage label.

The plant in Jiangsu does the opposite. The heat of compression is captured as it forms and parked in molten salt and in pressurized hot water.

On discharge the cold air rising out of the cavern is reheated by that stored heat before it ever reaches the expander.

That is the whole difference between compressed air storage as a battery and compressed air storage as a power station with an extra step.

So the fuel line is simply absent, and the heat store is what replaces it.

What the cavern actually is

The storage volume is not a tank and nobody dug it out. It was dissolved.

Fresh water goes down a well into a bed of rock salt, the salt passes into solution, and the brine is pumped back to the surface.

What remains is a cavity with walls of salt, which is close to gastight and closes its own small cracks under load.

Here the caverns sit between about 3,770 and 4,920 feet down and hold close to 1.3 million cubic yards between them.

That depth is not decoration. The rock above a cavern is what contains the pressure inside it, and deeper rock contains more.

Salt is also the reason the site can be reused indefinitely, since the cavity is refilled and emptied rather than consumed.

The site was chosen for the salt bed rather than for the grid connection.

The count, the cost and who supplied it

Two units of 300 megawatts each make up the 600 megawatt rating, and the pair hold 2,400 megawatt hours between them.

The first unit reached full load in December, and the second reached the grid and full output on its first attempt weeks later.

Expected annual production is 792 million kilowatt hours, and the build is put at 520 million dollars.

Those figures come from the operator and from the equipment makers rather than from any independent audit.

The turbines, generators, motors and the molten salt tanks were supplied by two large Chinese manufacturers.

Round trip efficiency is quoted at about 71 percent, which is a vendor number rather than a measured season.

What that efficiency figure actually costs

Seventy one percent sounds respectable until the sentence is turned around.

Close to a third of every unit of electricity pushed into this plant never comes back out of it.

A lithium battery of the same rating returns something near nine tenths, and it does that inside a building rather than inside a geological formation.

What the cavern buys instead is duration and cost per hour stored, because enlarging a cavern is far cheaper than enlarging a battery.

Short bursts still belong to chemistry, which is why a developer in Oregon is putting up 82 megawatt hours of cells instead.

The capacity and commissioning figures are set out in the trade report.

Losing a third is an acceptable price only when the hours are long.

What this does not settle

The plant is a demonstration and it is named as one, which means the numbers that matter most are the ones nobody has published yet.

Cycle life is the first of them. Salt creeps under pressure and a cavern slowly shrinks, and how fast depends entirely on the bed.

Availability is the second. A machine that stores its heat in molten salt has a start up sequence and a minimum load, and neither behaves like a rack of cells.

Cost per cycle is the third, and it cannot be worked out from a build cost and a nameplate rating alone.

Nothing in the published material answers any of those three, and none of them can be answered inside a first year of operation.

Storing energy as heat or as pressure keeps reappearing wherever chemistry is too expensive, including inside an empty chamber in an Idaho dam.

The cavern depth, the stored volume and the efficiency claim are laid out in the project account.

What the site proves is a scale rather than a business case.