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LordHeini

Why do so many of these gravity thingies show up lately?

An easy calculation shows how low the storage potential is.

Lets take a weight of 500 tones of steel, which would be 63.29 cubic meters.

Now sink those 500 tonnes into a hole a 100 meters deep that would make a rather lowly 0.1362 MWh of storage.

https://www.wolframalpha.com/input/?i=500+*+1000+kilograms+*...

Not sure what the cost of digging a 100 meter hole where you can sink 63 cubic meters of steel in would be, specially since water management is needed too.

I am not convinced that it is worth it.

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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.

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 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.

> 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 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/)

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.

I do not know about Gravicity but Heindl Energy (bankrupt as of this year) planned to lift a granite block 500m in radius hydrolicly for an on paper storage capacity of 1614 GWh. source: http://www.eduard-heindl.de/energy-storage/energy-storage-sy...

Wow, I didn't know they went bankrupt! Sad, thank you Heindl for paving the way forward. Innovation is truly a collaborative effort. https://heindl-energy.com/about-us/

How about combining that with a landfill?

Dig a very deep hole with a radius of 500 m, with a 500 m granite block that you can raise and lower in it to store and retrieve energy.

When the block is raised, toss trash into the hole. Then when the block is lowered to retrieve energy it also becomes the world's largest trash compactor.

Eventually, you'll get enough trash in the hole that even massively compacted you won't have enough room to make the energy storage from raising the block worthwhile.

You then have to dig another hole and move the block, leaving behind a full landfill that has more trash in it than a normal landfill of that size would have.

I think that neither is compacting trash that much a problem that you need a huge granite block for that (hydraulic press should be sufficient), nor do you want to move a block of granite 1km in diameter more than necessary. The idea by Heindl was to cut the granite cylinder in the place where the plant is build.

Here's the simplest quick answer I have regarding "is it worth it." This is just one example of independent research vetting that it is indeed promising. The math checks out.

https://www.storage-lab.com/gravity-based-storage

That research is not independent but made by a bancrupt startup of gravity based energy storage.

And it still does not answer how the construction and running costs are calculated.

Only if those are as low as claimed (which I highly doubt) something like this might be feasible.

This tech just does not scale well.

Digging holes is stupid expensive. Moving giant masses is not trivial and to double the storage capacity you need twice the weight or twice the height.

The energy density is way too low.

The only thing run by weights have been those old grandfather clocks. And even there the weights got replaced by springs and in the end batteries.

My understanding is that Dr Oliver Schmidt was hired by Heindl to do this research independently. It explains everything on the link I sent, so I didn't mean to imply that he wasn't hired to do it, but he's a third party. There is other research like the 1984 U.S. PNNL that validates how underground storage is estimated to be cost effective.

https://www.osti.gov/biblio/6517343

Gravitricity is claiming weights as massive as 5,000 tons and shaft depths from 150m to 1,500m

That’s equivalent to ~83 tons of lithium ion battery: http://www.wolframalpha.com/input/?i=5000%20tons%20%2A%209.8...

Ah - so we can measure the specific capacity of a battery in kilometres of altitude. e = mgh, so h = e/mg.

Lead-acid = 0.14 MJ/kg = 14.3 km

LiFePO4 = 0.58 MJ/kg = 59.1 km

LiNiMnCoO2 = 0.74 MJ/kg = 75.5 km

Yes; how high a fully charged battery could lift itself. I guess it’s similar to counting rocket fuel in seconds (lb_force-seconds per lb_weight).

What's the analogous interpretation of the 'seconds' of a rocket fuel? How long you can burn it for?

How long the rocket fuel could offset the force of gravity upon itself, thus suspending itself in-place.

In the real world, you'd need the rocket itself (which in turn contains the rocket fuel) which adds weight, and you'd have a difficult time controlling power output so the rocket doesn't drift and lengthen/reduce the burn time -- but that's not the point.

The idea is that we want a unit that relates the mass of the fuel with its total energy, and one way to approach that is to consider how long the fuel could offset the force of gravity on that fuel, which gives you a cute unit of measure in terms of time.

And uranium = 80,620,000 MJ/kg = 8,226,530,612 km[1], or about 180% the distance to Neptune.

Sometimes I wish we lived in an alternative world with fully developed nuclear power. Think about the possibilities. And no global warming! :/

[1] Not accurate, obviously.

this is the new solar panels on everything, 3d print everything, water from air, spectrophotometer is a tricorder... wave of kickstarter-esque ideas that already have relatively well optimized solutions that they will pick on the drawbacks of while admitting none of their own. In this case, it's the maintenance nightmare something like this would be as an underground project with a falling risk.

This might not work for economic reasons, but not because of falling risk.

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