I suspect that the cavern will stay at an amazingly constant temperature
all the time. Consider the specific heats involved. Maybe the air will
stay warm longer if it does not wash the walls and has no chance for
convection. It takes hours or days to pressurize something that big. Just
do a rough energy content calculation based on the potential energy of
compressed air stored in a tunnel say 100 meters long and 3 meters
diameter at 100 bar and express it in watts and you'll see what I mean.
What they have in mind seems *much* bigger than the ~2800 cubic meters of
the tunnel in my example.

If the plumbing for the air is somewhat efficient then the heat will
evolve in the compressors and be required in the expanders, not in the
mine.

How about using the compressed air as a giant blower and run a gas turbine
with it, burning methane or hydrogen (in the future). This would make for
better efficiency than just expanding air in a machine. A turbine running
with 100bar inlet and suction on the outlet would be rather good
efficiency-wise I think.

I have a drawing of a mine shaft (horizontal) used as air pump with a
water column that pulls bubbles from a venturi and pushes them down a
considerable depth. There the air over the water is pressurized (by the
water) and lead off to ventilate a mine. Excess water exits by syphon (to
a lower height than the inlet syphon). This is in Canada ? Scale was 100's
of feet in length, depth.

Peter

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