Hydraulic Rock Storage – an efficient system for large grid scale storage [pdf] | Hacker News Reader

Hydraulic Rock Storage – an efficient system for large grid scale storage [pdf]

cleanenergybusinesscouncil.com

28 pointsnkurz19 comments

From the article: requires "Possibly compact, homogeneous rock with little tendency to fracture." That's rare geology. It's rare for a tunneling project to find nice hard homogeneous rock through which a tunnel can simply be bored without supports. Both the Gotthard Base Tunnel in Switzerland and the NYC East Side Access, although in areas where such rocks exists, ran into incompetent rock.

In practice, they'd probably have to line both the hole and the plug with something like tunnel lining segments.

I'm not a geologist or a tunnel engineer.

With gravity working downward I suspect it is easier to find a stable column of rock than a stable horizontal bore, i.e. a column isn't going to collapse, the consistency can be very localized, and the person building the storage system gets to choose the location with more freedom than is typical for transportation infrastructure.

> It's rare for a tunneling project to find nice hard homogeneous rock through which a tunnel can simply be bored without supports

Obviously you've never played Dwarf Fortress.

"Rock is dross or nedross. Dross does not crumble; it is rock to trust, grain pure throughout. Nedross rock cannot be trusted, even if it looks solid and pure in grain. It fails. It is -- not truth."

Hydraulic Storage in general: https://en.wikipedia.org/wiki/Hydraulic_accumulator

Similar concept to ARES that has been discussed here before;

http://www.aresnorthamerica.com/

At first this looked a bit weird. It may be a bit more land intensive, but the idea is sound, and would be able to expand to full capacity gradually. I like it a lot better than the hydraulic storage for the near term.

Earlier this was for a much larger cylinder, say kilometers across, where you could leave the nature at the top intact. They have explanation here.

http://www.authorstream.com/Presentation/heindl-958931-hydra...

(Edited to change info it was the same guy, added url)

That is so cool. Now I want to install a hydraulic piston under my house. During the day excess solar production lifts the house, at night as the house slowly sinks it provides power. Zero foot print chemical free energy storage.

Would make for an interesting front walk, that basically would turn into a staircase as the house went up :-)

Install a stationary gatehouse with an elevator.

Plumbing? You're on your own.

Could the same system be implemented with a column of concrete and a tank of water under the surface of the ocean?

interesting idea.

if the seal breaks, then most of the energy stored in the system might over an interval of only a few minutes go into kinetic and potential energy in the form of water flowing over terrain and water gushing high into the air, so at the least it seems like a bad idea to have a road containing through traffic as close to the facility as depicted in the final image in the OP (the PDF).

Why r ^ 4th power? The weight of the rock is ~r ^ 2. Potential energy is weight X height lifted. I'm confused.

> Why r ^ 4th power? The weight of the rock is ~r ^ 2.

No, because, as shown on the fourth page, the length of the cylinder is ~r making the weight ~r^3 (if the length was constant, the weight would be ~r^2.)

I am not a physicist... Why is setting a rock atop a column of water better than an unadorned column of water?

The basic idea is that you are storing energy by lifting something heavy. The more mass you can lift, the more energy it can release as it falls back down. Rock density varies, based on the type of rock, but it is generally 2-3 times heavier than water for a given volume. The limiting factor for water energy storage systems is how much space it takes to store all the water. So the big chunk of rock on top of the water column means you can store more energy in the same space.

From an engineering point of view, I don't know if it is better or not, but the article implies that construction costs scale much better if you leave the rock there.

If you wanted to make an unadorned column of water underground, you'd have to remove all the rock where the column is. Without knowing anything about that sort of construction, I'd guess the cost is proportional to the volume of the cylinder.

The proposed design requires removing a ring of rock plus a disk at the bottom. The article claims that the construction cost is therefore proportional to the cylinder's area.

There are also other reasons which are plausible, but they're not as interesting. Rock is denser, so you get more energy storage per unit volume. (Because: potential energy is given by E = m g h, where m is the mass raised. Adding a rock on top means you get greater mass)

Would water volume itself be an issue here? If the device is built in-place so that just a cylinder edge and the bottom is mined out, the rock can be used immediately without having to mine it out or replace its' volume with millions (?) of gallons of water. Also it should be able to lift higher than water without a tank to hold it.

> I am not a physicist... Why is setting a rock atop a column of water better than an unadorned column of water?

At a minimum, it requires less land area.

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