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Comment by elektropionir | Hacker News Reader

The energy density for gravity is just immensely small, that's why you need dams holding back rivers to use them to generate electricity. For a 1km hole (that's in the middle of their 500m - 1500m range) you have an energy density of 10kJ/kg of the weight that stores the energy. The energy stored in a Tesla roadster battery pack is around 50kWh which is 180MJ. This means that you need a 18,000kg weight in a 1km deep (immovable) hole to have the equivalent of a Roadster battery pack. I'd go with the battery pack.

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You just need more mass. Have a look at this concept: http://eduard-heindl.de/energy-storage/index-e.html (unfortunately the english version is very sparse, the german version has some more images and videos: http://eduard-heindl.de/energy-storage/ )

The idea is to cut a 1km diameter cylinder into the ground (solid rock), and then lift it up up to 500m by pumping water underneath it. The power stored can power a country for a day.

I am somewhat sceptical, but this guy is totally serious about it.

But presumably you'd need more than a day's worth of power to lift the damn thing? That's probably doable, but still, I suspect that'd be the main problem.

The point is to have a way to store the excess energy we generate at off peak times like the middle of the night.

The efficency is said the be 80%, just like any other pumped storage system. It just uses considerably less space than a water reservoir (actually close to none, as the top of the cylinder remains untouched) and is scalable by adjusting the radius.

I guess the main problem is to keep the water confined (200bar of pressure) while keeping the mass movable. Maybe someday they build a small testing facility, so we know for sure.

18,000 kg is just 1.58 cubic meters of lead.

You can have a much much heavier weight.

It becomes viable when you use a 500,000kg weight...about 1.2MWh

This is not unachievable with some lifting systems currently exceeding this lifting weight.

Also lifting 500 tons is a lot easier when you don't have to move it horizontally. 1.2MW isn't as much as a wind turbine though which is a pity because they conceivably could be a good pairing.

In trying to ascertain how much weight is practical to lift, I came across this list of the strongest cranes, which can lift millions of kilograms (thousands of metric tons): http://enr.construction.com/products/equipment/2012/0925-the...

That's a devastating critique. The idea seemed a bit too good to be true, but I couldn't quite put my finger on why. Thanks for posting that.

I did a little more research, and I'm not sure anymore that it's a complete deal-breaker. But it's definitely going to put limits on how far this can scale.

That's true. Unfortunately, current Tesla battery packs use lithium, which isn't super-abundant in Earth's crust.

That's why companies like Ambri are looking at using other materials in the batteries they're developing for grid energy storage.

Other materials, like lead and antimony?

http://en.wikipedia.org/wiki/Abundance_of_elements_in_Earth%...

  Lithium: 33
  Lead: 37
  Antimony: 62

Well they are more common

That table uses a mass-based unit, so wouldn't that tend to create the misleading impression that lighter elements are more scarce? Energy storage capacity depends on the number of atoms, not mass (as you can intuit by comparing lithium batteries to lead or nickel-cadmium batteries -- heavier doesn't mean more powerful).

Well, if Pb and Sb are more scarce than Li by mass, then they're even more scarce when you're counting atoms.

If Ambri's technology does pan out, it will be due to a cheaper manufacturing process, not because they're avoiding lithium.

I thought those numbers were showing that lead and antimony are more common than lithium.

In 2011, we mined 0.2% of known lithium reserves.. This would provide over 350 years worth of lithium at current rates. Obviously usage is dramatically increasing but so will our known reserves as the overall level of demand increases. This is also without developing a method to pull the relatively abundant lithium from sea water and without recycling any lithium we're currently disposing of.

Ambri is interesting from a lifecycle and cost perspective but I don't think a lithium shortage is going to have any real impact on prices.

"The greatest shortcoming of the human race is our inability to understand the exponential function."

That's a fine saying but even at 20% YoY increases in lithium usage, we'd still have 35 years or known reserves -- without any increase in exploration or recycling.

As oil & gas have shown, if there's exponential demand for a naturally occurring element, we'll definitely find ways to pull more of it out of the ground.

Increases in known reserves can also be exponential.

The known reserves are 13 million tonnes. However lithium is the 25th most abundant element and you get 20mg of it for every kg of crust, so there is a lot more to find.

"Everybody who has ever claimed that something is doomed because of an exponential function was wrong."

"... or dead."

"The greatest shortcoming of exponential forecasting is the S-curve."

Except all those dire predictions turned out be wrong. We didn't run out of food or resources, and birth rates have now declined in almost every country in the world. Population growth is basically now a complete non-issue. In retrospect, ignoring the Paul Ehrlichs of the world turned out to be the right move - a far cry from mankind's greatest shortcoming.

If we run out of lithium we'll figure something else out, just like we would for any other resource.

Except that then we have run out of available lithium, which means that solutions that require lithium will be inaccessible. Running out of chemical elements is a problem. The helium shortage is a serious problem. High-energy physics is made from helium.

If those physicists hurried up with cold fusion that helium shortage wouldn't be such a problem.

Why is helium found underground with natural gas? My theory is that the helium underground comes from radioactive decay and gets trapped along with methane.

-- I guess there's no need to speculate when you have Google: https://www.google.com/search?q=where%20does%20helium%20come...

We will never "run out" of anything, including oil or lithium. We'll stop wasting these resources the minute their prices rise to a level where something else becomes more economical. Regardless of when that happens, there will be plenty of lithium left for any potential fusion sources that might be developed later.

For peak storage, vanadium accumulators are well established. Lithium cells give you an advantage in density, but for peak storage you are looking at capacity first, and then you consider the footprint. With vanadium you are only limited by the size of your electrolyte tank. The additional advantage is that vanadium reserves are much greater than lithium reserves.

He's not suggesting that we use lithium battery packs. He's suggesting the idea is just plain ludicrous.

I think it's more likely we'll see an asteroid mining economy bringing lithium for batteries on Earth than millions of 1km holes with 20 ton weights storing energy for single charges of electric vehicles.

Whatever local (i.e. small scale) energy storage we have in the future you can probably bet (from first principles) with 99% certainty it will be based on the electromagnetic force. Gravity is too weak to be practical for smaller scale storage and cannot give mobile storage units since you have to deal with huge weights. For large scale see the "pumped storage" system already mentioned in this thread. Just for illustration, the "Taum Sauk Hydroelectric Power Station" can produce 175MW of continuous power. If you dropped the 18T weight in the 1km shaft you'd get about 13MW averaged over the 14s fall. Then you have to lift it up again if there are still enough pieces left.

Even if you found a way to store energy in nuclear interactions (i.e. "charge a nuclear battery") you don't want to have a bunch of containers full of radioactive material all over the place. If one thought it through, I wouldn't be surprised if there are in-principle issues for the charging part similar to the ones with gravity.

Since we don't know of any others, this leaves only the electromagnetic force to store energy with sufficient mass (or volume) density in practical ways. Whatever it ends up being (chemical batteries, supercapacitors or something else) the future local energy storage unit will separate and hold charges apart.

Okay, so what if you built a huge artificial dam way above sea level? To store energy you would pump water uphill. Usable work could then be extracted when the water is allowed to travel back downhill.

This is what is currently done.

http://en.wikipedia.org/wiki/Pumped-storage_hydroelectricity

Well known as "pumped storage". They're competing with it, but dry and upside down. Is it more economical? Maybe. Is it safer? See http://en.wikipedia.org/wiki/Taum_Sauk_Hydroelectric_Power_S...

At least if this contraption breaks, there's nothing important below it.

I live near a twin-lake hydro system that does just that.

Every night the lights dim for a bit when the grid starts being used to pump water back up.

http://en.wikipedia.org/wiki/Smith_Mountain_Dam

I would think you could find an elevated lake that replenishes naturally to avoid pumping the water up. But then maybe that is just "energy".

This doesn't make sense. Gravity doesn't have an energy density. The energy density of this system is going to be directly related to the density of the material being raised/lowered.

Isn't that the math that the GP posted...?

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