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Comment by grondilu | Hacker News Reader

What determines the depth of the whole?

I mean, since E = mgh, you can get the same energy storage capacity with a less deep hole if you use a heavier weight. And you can get a heavier weight either by using more expensive material (why use a cheap one? it's not like it's going to wear or anything), or a larger hole.

I'm not sure what determines the cost of digging a whole, but I suspect depth matters more than area.

Also, does the shaft has to be vertical? You could dig it with a sharp slope, and put your weight on rails. That would make the shaft longer for the same depth, but it would probably be easier to build and maintain.

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I still not sure about the details, and I don't like how they use the word "cheap" there.

Nevertheless, the density of Lead is 11.35 g/cm3 (the density of water is 1 g/cm3). The densest element in this table is Osmium with 22.6. So replacing if you replace a Lead weight with a more expensive weight you only gain x2.

http://www.lenntech.com/periodic-chart-elements/density.htm

Gold is about 20g/cm^3, and although it sounds expensive, there's a lot of gold just sitting in storage. Why not store it underground here instead? This is in contrast to tungsten which has direct commercial value and would be wasted just being stored.

You have to add the cost of the armed guards, because a big block of gold will be a nice target of a robbery.

Well, yes. To be fair, if there was like a farm of these devices, it might makes sense to swallow that expense.

I wish gold wasn't so rare! Its such an amazing material, and our economy is not based on precious materials anymore (except the commodity markets or course), so it seems like a really sad thing that we can't have more abundant gold just yet

Tungsten is near 20g/cm3 and is much more reasonable as far as price is concerned (Osmium is basically a precious metal). I'm not sure sparing a few hundred meters of hole digging is worth gathering tons of tungsten, but I would not be surprised if it was.

I suspect they plan on recovering the power with a generator. So the input shaft to the generator has to rotate. With a deeper hole, you get more 'clicks' on the generator so you can produce power for a longer period of time before having to pull the weight back up.

Does it really matter?

You should be able to get millions of rotations from a single centimetre hole, given that the mass is large enough and you have a good gear ratio..

I suppose they plan on using a transmission mechanism that will turn one meter down to as many turns as you want anyway, so I guess the depth of the hole is at least partially determined by how good a transmission mechanism you can afford. But since they explicitly say the hole will be the most expensive part, I'm not so sure.

You're losing torque with every gear reduction, and I believe that current out of a generator is a function of torque. So there's some happy medium between hole depth & transmission ratio for the amount of power they want to generate.

You're certainly losing torque with the gearing but that's not a problem. The problem is losing energy. Energy loss can be made arbitrarily small at increasing cost to build the transmission.

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