[squirtle24]

Thinking out loud here, but why sand and not water? Yes, water is only 1000kg/m^3, but probably much easier to transfer around using pumps and pipes. Water can make good use of the horizontal space within the mine, unlike sand. (The graphic in the article shows them using conveyor belts or trucks to move the sand horizontally which seems silly.) Thus the mine could basically be a mini pumped hydro power station, build a new reservoir at the top and use the mine as the bottom reservoir, then pump water water between the two.

[usrusr]

When I thought about that silently, my answer was that storing water underground is a challenge when unprotected walls can turn into sludge. Water would require expensive preparation of the existing mine volume, whereas dry material could just be filled in, even stabilising the mine a little in the process. Water and sand sit at different points in the cost per W vs cost per Wh spectrum.

A machine for lifting/lowering loose material is more complicated than a pump, no doubt about that. But a deep shaft would mean that you don't build one machine per shaft, you build a few dozen smaller ones with a good handover mechanism and start getting small serial production benefits right from the first installation. Capacity would be virtually infinite, because with excess energy you could just mine more of whatever stuff is down there.

I guess if it's expensive to switch the dry mass mechanism between directions or speed states, it might be worthwhile to prepare some basin volume up and down and run a small capacity pumped storage in parallel at the same site for higher frequency load changes and short peaks. You might even find yourself discharging the wet battery while charging the dry one or the reverse if there is a sufficient delta in dispatchability.