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Comment by dragontamer | Hacker News Reader

I feel like "gravity storage" should take advantage of natural landscapes better.

ARES (rail energy storage) builds a rail-line uphill, for example. Rail cannot handle a very steep slope, but a gentle hill climb will build up potential energy fine.

In the case of vertical-based gravity storage, I'd imagine that lifting blocks to the top of a cliff (or down a valley) would be most efficient.

I mean, Pumped Hydro is gravity storage, and does just that. Pumping water up a mountain and generating energy by dropping it back down. But presumably, we don't want to use water in the Western states (where water is scarce). So Gravity-energy storage WITHOUT water is the goal.

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Lifting things creates hazards for what's below them in the event of catastrophic failure.

For example, all the dam failures that have obliterated entire towns.

I'd rather a failure obliterated a bunch of earthworms and voles.

I expect that the voles disagree with me.

The issue here is that not everyone has that landscape (and as we know, doesn't want to destroy the view). I grew up in the US midwest--no hills for miles on which to build railcars, pump hydro etc.

But high-voltage power lines can run power for hundreds of miles with minuscule losses. Look at the MISO grid: https://upload.wikimedia.org/wikipedia/commons/4/40/2015_MIS...

There's hills somewhere in there, and those hills can form energy-storage solutions that can be transmitted across the entire MISO grid.

High voltage lines are not free. You have to compete with different energy storage that doesn't need special landscapes, e.g. batteries.

High voltage power lines already exist. They are part of the landscape, and connect large parts of the country together. Case in point: a solar panel is Missouri could be providing power to Iowa.

Bonus points: if you want to build a big battery in Iowa, you can take advantage of excess power in Iowa or Missouri. Building tiny batteries here and there will only lower your efficiency.

There's a reason why a lot of discussion here is on large 100s of MW proposals: because anything smaller won't really be a big win economically. And it shouldn't be too hard to fill up 100s of MW capacity because of the nature of our large and reliable power grids here in the USA.

In general more connectivity means you need less storage as it's always windy/sunny somewhere in the world.

The issue with using natural landscapes is that it limits your height. And adding more height is ~exponentially~ (edit: quadratically) better than just adding more installations with an equivalent total height. (You can dig about a mile deep). You can see more details in my comments below - search for the text "It is worth it to dig a hole!"

I was thinking of combining techniques. Best case scenario in USA is something like Owens Valley: 14,000 at its peak, but 4000 feet at the bottom of the valley.

Leading to ~10,000 feet (or nearly 2-miles) of elevation change. From there, you can dig another mile underground, leading to 1-mile (under ground), or -1000 feet elevation, to a peak elevation of 14,000.

If a tower were built on the top of the mountain: you could gain another 2000 feet or so on top: so maybe 16,000 (a 2000 foot tower on top of the mountain peak) to -1000ft (1-mile deep from the bottom of the 4000-ft elevation valley), for a total differential of 17,000 feet.

Ignoring earthquakes and other issues, of course. :-) Just purely from a hypothetical perspective: working with nature and the natural landscape seems like it'd be better than "just" digging a hole.

EDIT: Repurposing abandoned mine shafts might be worthwhile, depending how deep they are.

Wrong, wrong, wrong, so wrong!

Gravitational potential energy is (approximately) linear in height. I say approximately because this assumes constant g (which is a good assumption when h is small compared to the radius of the earth, which it is).

And in fact, a consequence of gravity's 1/r^2 nature is that one is only subject to gravitational acceleration from what is beneath them (shells above cancel out), so mine shafts are less efficient than towers (the effect size is small to the depths we can mine).

So adding more height doesn't help, and if that height is underground it could actually hurt net efficiency.

Efficiency in this context refers potential energy stored per unit height. The field is conservative no matter what you build.

I'm assuming that the quadratic parent refers to comes from the increased potential energy per kg of material multiplied by the ability to store more material due to the additional volume.

If you picture a dense weight like a cannon ball on the end of a string you're right, but if you're digging down n meters, encasing n/2 meters worth of dirt and moving it up and down the free n/2 meters of shaft, the energy storage would indeed be proportional to n^2.

I don't know anything about the field and had the same reaction you did, but considering parent is running a startup in it they're either a lunatic that doesn't know the equivalent of FizzBuzz or there's something we missed on first inspection, and we should charitably assume the latter...

Haha, thanks. It's a little of both of course. You have to be a bit of a lunatic to think you can do these things. But someone's got to get it done.

I read the whole pitch deck and it seems like you've thought it through well. I'm curious about the potential for failure; if one of the last links freezes up somehow (i.e. the disengagement you mention elsewhere fails), is there a process for clearing it?

The single shaft vs multiple parallel approach does seem a bit risky in the early days. If there's a 10% failure rate, and you built one shaft, that's a 10% chance of an existential threat to the company. 10 shorter shafts mean one will likely be inoperable.

Of course over the long term worrying about this doesn't make sense. Once you've scaled, 1k large vs 10k small shafts would not matter from this perspective.

Best of luck mate!

Hey andbberger, Yeah, we're _definitely_ assuming that g is constant for all elevations :)

There's another thread on this comment page talking about why height is important. Please see this slide in our presentation illustrating it. https://docs.google.com/presentation/d/17FI-jrI9RWS3q7Ng44Yh...

I don't think it's fair to call that "scales quadratic". Certainly the energy stored in the total number of blocks is growing quadratic by digging deeper, but it also ignores that costs and effort grow when digging deeper. In effect, adding a block at the bottom comes with substantially higher effort than adding a block right next to it on the same level.

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