4 ms·

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 o

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

leggomylibro 9y ago
So...what is that in Megawatt-hours?
- robotresearcher 9y ago
  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.
- oh_sigh 9y ago
  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
  - splittingTimes 9y ago
    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?
    - semi-extrinsic 9y ago
      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%.
Johnny555 9y ago
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.

leggomylibro 9y ago

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

robotresearcher 9y ago
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.
oh_sigh 9y ago
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
- splittingTimes 9y ago
  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?
  - semi-extrinsic 9y ago
    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%.

Johnny555 9y ago

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.