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Comment by splittingTimes | Hacker News Reader

I would add gravity storage [1].

"The fundamental principle is based on the hydraulic lifting of a large rock mass. [...] Water is pumped beneath a movable rock piston, thereby lifting the rock mass. [Later] the water which is under high pressure from the rock mass, is routed to a turbine [...] and generates electricity using a generator."

"[For certain pistons] the storage capacity increases with the fourth power of the radius, r^4. The construction costs however only increase with the square of the radius, r². [...] Strictly speaking, the price per kilowatt hour of storage capacity decreases with 1/r². This is the outstanding competitive advantage of this storage concept."

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[1] http://www.heindl-energy.com/gravity-storage/gravity-storage...

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I think I recall seeing on Hacker News a system they were building where they sent train cars filled with rock up a mountain. That seems like a good way to move a lot of mass. (I'm not sure if it is better than water...)

I read that as well... good spot being left out of this article. Here is the link: https://www.wired.com/2016/05/forget-elons-batteries-fix-gri...

I just thought of a concept of using a water fall to fill a tank that gets lifted up and then when gravity makes the lift fall to the bottom, water is dumped by opening a gate. Something near the top blocks the water from refilling it while the lift moves back up. This is more efficient than moving a rock up and down, given the gate opening and water blocking is energy efficient. You can put this under hydro dams for extra energy after it passes the turbines. Why waste right?

Interesting concept, but lifting rock "only" gives you a ~3x advantage over lifting water, which can be lifted much higher.

I saw a talk of the inventor a couple of years back. The test site they where exploring they wanted to build a rock piston with a diameter of 250 and a height of 500-1000m (cannot remember exactly, but it was in this order). Since they can lift the piston to half its height that would be 250-500m altitude difference. Not sure what the limiting factor of the height of the piston is, so it might be even bigger.

For comparison the Three Gorges damn has an altitude difference of ~100m [1].

Also, it is much more compact then a traditional dam [2].

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[1] https://en.wikipedia.org/wiki/Three_Gorges_Dam#Composition_a...

[2] For anybody wondering why ~3x: density of water 1 g/cm^3, density of rock ~ 3 g/cm^3

I wonder what the cost would be to excavate a 250m diameter, 1km cylinder of rock versus building a large reservoir on top of a 3km mountain.

Granted, there are places in the world where it's going to be impossible to find a nearby mountain, but I'd imagine that it's similarly difficult to find places where you can dig out a stable 1km cylinder. Bedrock is not always a single hunk of rock, it can have seams that separate it into separate chunks of rock. And while in some places bedrock reaches the surface, in others it may be hundreds of meters below the surface.

So...what is that in Megawatt-hours?

potential energy = HAM = Height * Acceleration(due to gravity) * Mass.

Height = 250m

Acceleration(on earth) = 9.8 m/s/s

Mass = pi * r^2 * h = pi * 125m * 125m * 500m * 3 g/cm^3 = 7.36e13

HAM = 250 * 9.8 * 7.36e13 = 1.8e17

Compared to the potential energy of lifted water mass in a dam, isn't the water also highly pressurized under that rock mass? Would that not need to be taken into account as well?

Water is very incompressible, so you don't really get anything in addition to the potential energy. If you decompress water from 300 bar (equivalent to 1 kilometer height of rock above it) to 1 bar (atmospheric pressure) it only expands by 1.5%.

Lifted rock is 3X whatever the number for lifted water is. The KWH is proportional to how much was lifted and (approximately) how high it was lifted.

It can be lifted much higher if you have the right topography.

I also wonder about using something much denser, but still reasonably cheap (lead is what, $1/lb?)

you where faster than me :)

The seal is great: http://www.heindl-energy.com/gravity-storage/idea-function/s...

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