[danans]

> Potential energy, U = mgh. So the energy required to raise 1m^3 of sand 1400m is 1600 x 1400 x 9.8 = 22MJ = 6.1kWh

If you lowered 10 m^3 of sand (61kWh of potential energy), to generate the minimum 100kW power to participate in grid stabilization markets, you'd have to drop that 16000kg of sand for 61kWh/100kW = 0.61hr = 2196 sec. 1400 meters in 2196 seconds is 0.64 m/sec. That seems reasonable, but you'd need a lot of these (so a wide mineshaft) to generate a more meaningful amount of power (like at least 1 MW). Current grid scale batteries are capable of outputting hundreds of MW of power.

https://en.wikipedia.org/wiki/Battery_storage_power_station

> we can store 3000m^3 of sand, so that's 18MWh.

> I'm sure they've got a lot more space than that, but it just gives some idea of how much sand you're talking about.

They're going to need 3 orders of magnitude more space then because current generation grid scale batteries store GWh of energy, and generally speaking lower cost energy storage competes by offering much higher storage capacity.

[15457345234]

Yeah, my surface level evaluation of these projects is that they're just not going to be viable - you have the height but you don't have the volume - mineshafts are by definition narrow - and some sort of automated load/unload system is going to run into issues of complexity and reliability and ease of access for maintenance to whatever's at the bottom end.

It's always going to be easier to move water around in an automated fashion, though, so I'm immediately skeptical of any system that isn't some variant of two tanks and a pump/turbine.

If you really do want to use gravity as a power source and don't want to go the hydro route you're better off building narrow-gauge train lines up the sides of hills. The lower-impact and lower-output version of that would be aerial ropeways.

[Faaak]

I've participated in grid stabilization in Switzerland (ancillary services), and 100kW is absolutely nothing. As far as I recall, 10 years ago, the minimum needed to become a participant was 10MW

[danans]

Even in the US, I'd bet that few entities bid into the ancillary services markets at only 100kW because the economics don't add up when you are using batteries (due to high fixed costs).

However, having the lower minimum has the advantage of allowing smaller firms to participate (i.e. companies controlling 100s of electric water heaters), instead of just massive utilities.