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Comment by syllable_studio | Hacker News Reader

You're thinking too small though ;). You can dig a mile deep. And you can lift enormous weights. As comments show below. Gravity is weak, but cheap. It can scale up to provide 1 GW of storage - enough to balance the load of an entire large city.

Lots of these ideas are popping up right now because it's a very compelling idea which is demonstrated to work. And because climate change is driving massive growth in renewable energy (woot) we will desperately need massive amounts of energy storage in the very new future. The energy storage market is expected to grow massively year over year.

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Sure you can always get the big option but that does not mean it is feasible.

What does it cost to dig and maintain a 1.5km deep shaft?

I found is something around 8000-10000$ per meter. I can buy a whole lot of batteries for that.

This technology just does not scale since mgh always holds true.

Unlike flywheels where you get w^2.

Just making the wheel out of carbon fiber and making them go fast, squares the amount of energy you can store.

$8000 to dig a 1m hole sounds very high - that's months of someone's salary. Surely the best drilling machines can beat the cost of paying someone with a shovel for a month?

But mostly they won't be digging the holes, they will be using shafts left over from earlier mining operations.

Those prices are probably only true once you go deep enough to require permits, ground water management and all the other complications of very deep holes.

That is what i found:

https://minewiki.engineering.queensu.ca/mediawiki/index.php/...

Honestly i would have thought it to be more expensive.

At higher depths the ground pressure is enormous and you need a lot of bracing.

Then there is water management, air, transportation of the dug out material...

When there is rock you usually need blasting or gargantuan drills which i am not sure even exists in the required diameter.

OH come on, I'm sure you can imagine the complexity of digging grows as the depth grows. the first meter would cost 100$ the second 150$, the 3rd 200$, the fourth 500$ the fifth 550$, the sixth 600$, etc.

batteries don’t last as long as a hole. some holes already exist.

Meaningfully deep holes in the ground require some maintenance as well.

One of the biggest is groundwater removal. Behind that would be the maintenance of shoring materials in the face of earthquakes, and ensuring that it remains straight in the face of earth movement.

yup it's feasible. It does scale. Here are two links to research that support the feasibility and cost effectiveness of underground gravity storage.

https://www.storage-lab.com/gravity-based-storage https://www.osti.gov/biblio/6517343

The deeper you dig, the heavier your cable is, and the more energy you waste on accelerating that cable.

And the much bigger issue is, you have to constantly pump out water that will inevitably fill the hole. And that only costs you money.

But the water leaking in is a feature, not a bug! You run pumps during surplus periods and generate hydroelectric in demand periods as the tunnel floods.

No, it's not a feature. It's a massive problem.

1) Pumps that can pump water 150 meters or a kilometer high are very expensive and require constant expensive maintenance.

2) Where will you pump the water to? Now you have to build huge storage tanks.

3) The water never stops flowing in. Sooner or later you will run out of storage space and your expensive hole will fill up with water.

Maybe pump the water to the nearest river? As long as you aren’t digging from the lowest point in the landscape it won’t be a problem to get rid of the water.

seems the water could be a secondary energy source, and the cable energy wouldn’t be wasted, thats just more mass to lift and drop.

Do tell how you generate energy from water at the bottom of a gravity well.

Tidal Power? Though the ocean is not strictly at the bottom of the gravity well. And I assume that's not really what you're asking.

That's from an external system, and won't work in the bottom of a hole.

There is no way, short of fusion that I know of. And fusion is not a valid answer in this context anyway (even if we had the tech for it).

Yeah, my startup, Terrament solves the heavy cable problem by using modules which each contain a motor/generator and they each support their own weight. This allows us to truly maximize height and weight per shaft volume. You can see my other comments for context. (https://www.terramenthq.com/)

> You're thinking too small though ;). You can dig a mile deep. And you can lift enormous weights.

But at that point, why wouldn't you just do a classic pumped-storage reservoir?

Because as noted in other comments here, all the best natural pumped-storage reservoirs are tapped. It's not feasible to build enough dams in enough places to get the job done.

Furthermore, my most optimistic estimates show that underground solid mass gravity storage could compete with pumped hydro on cost anyways. One reason is that solid mass is about 2.5x heavier than water. If excavation is one of your biggest costs, this density is important.

>> the load of an entire large city.

Maybe if you are lifting the entire city. For perspective, take electric vehicles. Each moving EV is a mass being constantly accelerated at say 0.1G. So the weight in the graviticity system to power those EVs would have to be at least 10% of the mass of all the EVs active in a city.

I just mean for load balancing a city running on wind and solar to optimize their costs.

> Maybe if you are lifting the entire city.

This would be a really cool element in a sci-fi world

A long time ago I did the napkin math to check if it'd be worth it to lift a skyscraper up and down just slightly for energy storage. You know, you'd just use adjusting ramps as the height changed ever so slowly. Not surprisingly, it's surely not worth it. The empire state building could run some tea kettles, but, yeah, no.

I calculate a rather better figure for the Empire State Building:

365000 tons × 9.81 m/s2 × 1 m = 1 MWh

which ought to at least be enough to provide short term backup power to the whole building.

Japanese TECOREP brand demolition system generates power as it lowers material down from skyscrapers

https://sipilpedia.com/tecorep-system-high-tech-demolition-s...

This is only used to smooth out power generation with respect to demand. It doesn't have to power an entire city, just the shortfall between demand and current supply. Which, granted, could be substantial in a 100% solar and wind grid.

FTA

Each unit can be configured to produce between 1 and 20MW peak power, with output duration from 15 minutes to 8 hours.

For reference, 20MW is ~4 large aerogenerators. A single nuclear PWR is 500-1000MW.

It can scale up to provide 1 GW of storage - enough to balance the load of an entire large city.

You'd need 50 of these installations for a single city, for 15 minutes of power, 1000 for the 8 hours.

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