8 ms·

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

JshWright 11y ago
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.
matthiasl 11y ago
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)
infogulch 11y ago
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.
dragonwriter 11y ago
> 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.

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.

matthiasl 11y ago

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)

infogulch 11y ago

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.

dragonwriter 11y ago

> 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.