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Comment by montalbano | Hacker News Reader

This is absolutely fascinating.

Interestingly, the cement industry produces ~5% of global c02 emissions [0]. (Cement is an important binding element in concrete [1].)

As this technology is presumably most useful for storing surplus energy from unpredictable renewable sources (e.g. wind, solar) I wonder if there is a conflict of interest? I'd love to know more about the carbon economics involved, perhaps they could use reclaimed (i.e. recycled) cement.

[0] https://blogs.ei.columbia.edu/2012/05/09/emissions-from-the-...

[1] https://www.thespruce.com/difference-between-cement-concrete...

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So it uses concrete which is VERY BAD and also needs a lot of it, but at maybe 1/6th the impact of regular concrete.[1]

It's unclear how this works out in the end. Exactly how much cement do they need to scale this up? What's the CO2 impact per MW of storage, for example? I would say anything that involves significant CO2 emissions is not an ideal candidate for renewable energy storage.

[1] From the article: "Energy Vault would need a lot of concrete to build hundreds of 35-metric-ton blocks... [but they've] developed a machine that can mix substances that cities often pay to get rid off, such as gravel or building waste, along with cement to create low-cost concrete blocks. The cost saving comes from having to use only a sixth of the amount of cement that would otherwise have been needed if the concrete were used for building construction."

I missed that bit in the article, thanks for pointing it out. So they are already developing ways to reduce c02 output of concrete production, nice. This would change c02 balancing estimates greatly.

You may have meant this, but just to clarify - they're developing methods of lowering the CO2 output (and financial cost) of their concrete weights, not concrete in general; they do this by not having to meet the structural strength requirements of regular concrete.

So, how does the CO2 emissions of concrete production compare to that of Lithium ion battery production?

Wouldn't it be mitigated by only being created once and then have a very long lifetime of utilization (vs the battery form of storage)?

Depends on how much the inferiority of the concrete vs. that used for construction affects its durability and longevity. It needs to be able to withstand weight from blocks above, impact whenever it or an adjacent block is moved, and corrosion from any metallic material within.

I could almost imagine these blocks needing to be tougher over time than something made from cement used in construction, to avoid crumbling under those conditions.

Let's investigate this!

For reference, a common electricity mix currently generates about 0.5 tons of CO2 per MWh [0][1].

Now, for the device in question: Creating 1 ton of cement releases about 1 ton of CO2 [2]. A common ratio of cement per mass of concrete is maybe 1/5 [4]. The article says they found a way to reduce that to a sixth, so we are at about 1/30 of the concrete mass in cement. From the article, each block weighs 35 tons. So we'll just assume 1 ton of cement (and CO2) per block.

The graphic in the article shows ~40 layers of no more than 20x6 concrete blocks can be stacked around the tower, each weighing 35 tons. Therefore, each such installation would cost about 5000 tons of cement or CO2.

So its building cost in cement alone is the equivalent of 10000 MWh. Fully "charged", it stores 20 MWh. It would therefore have to complete 500 full cycles of taking energy available for free (and otherwise lost) and feed it back. Roughly assuming it can do that every week (no idea, really depends), that would be ten years to become CO2 neutral, just in terms of cement. So there's your reference value.

However, I haven't seen this kind of calculation for other energy sources. That would be interesting.

Edit: Here is a review of CO2 from lithium-ion batteries: https://www.ivl.se/download/18.5922281715bdaebede9559/149604...

They say 150-200 tons of CO2-equivalent (!) per MWh stored, which is very close to the device in the article (in the above estimate it would be 250 tons of CO2 per MWh, although I did not check whether that is pure or CO2 equivalent).

[0] https://www.eia.gov/electricity/state/ (randomly sampled some states)

[1] https://www.umweltbundesamt.de/sites/default/files/medien/37... (German statistics)

[2] https://blogs.ei.columbia.edu/2012/05/09/emissions-from-the-...

[3] https://sans10400.co.za/concrete-mixes-by-weight/

I’ve also read that concrete absorbs CO2, in significant enough quantities that an isolated ecosystem experiment had to be aborted because plants weren’t getting enough of it.

I'm not doubting it, but on the other hand I don't think that the cement will re-absorb a significant amount of its own mass in CO2 (which it would have to in order to offset this 1-to-1 CO2 footprint).

Portland cement is at least 80% CaO, so in theory it's 63% of the cement's dry mass (CaO + CO2 -> CaCO3)[1]. In practice it's ~43% that value, or 27% of the cement's dry mass.[2]

[1] https://en.wikipedia.org/wiki/Cement_kiln#Carbon_dioxide

[2] https://www.nature.com/articles/ngeo2840

So apparently it depends on the type of cement - non-hydraulic cement hardens as you described, while hydraulic cement does not absorb CO2 but instead hardens in the presence of water. I guess they wouldn't have a reason to use hydraulic cement for this though, so it seems you're right.

It's true that hydraulic cements (eg Portland cement) harden by a different mechanism, but they too will fully carbonatate over time.

https://www.astm.org/DIGITAL_LIBRARY/STP/PAGES/STP39460S.htm

https://nvlpubs.nist.gov/nistpubs/jres/60/jresv60n5p441_A1b....

https://en.wikipedia.org/wiki/Carbonatation

Worth pointing out that if you're patient with setting times, you can use portland cement substitutes (fly ash, ground blast furnace slag) as a substitute for portland cement. Slower strength times (several days longer) but same long term strength and way lower embodied carbon.

The thing i ponder about this is the foundations needed for a setup. Concrete stacks very well (think dams) but you need very strong foundations for tall, solid concrete structures.

Nice work.

From [2]:

Cement manufacturing is highly energy- and emissions-intensive because of the extreme heat required to produce it. Producing a ton of cement requires 4.7 million BTU of energy, equivalent to about 400 pounds of coal, and generates nearly a ton of CO2. Given its high emissions and critical importance to society, cement is an obvious place to look to reduce greenhouse gas emissions.

Ignoring the geographical problems, could using the power source from which energy is stored for these devices also be used to manufacture the concrete, or would you be constrained by the power plants ability to output enough energy intensity to produce concrete?

If feasible to do, then for the geography issue, would the CO2 output from lengthening the trucking route and/or high voltage transmission lines to the storage site possibly overcome the cement issue overall? If the power is ~free, loss in transmission is less important, but then you have to consider the output of manufacturing and installing transmission lines also. Although, perhaps there's spare capacity on some lines.

Actually, once the blocks are manufactured, you can ship them anywhere.

Complicated.

> Actually, once the blocks are manufactured, you can ship them anywhere.

But you won't want to, because they're insanely heavy. At 35 tons a semi-trailer can only carry one.

You really need to build these on-site. The amount of diesel you'd burn to move them long distance is astronomical.

If you build it on site won't you still have to haul in all of the materials to produce it which will weigh more than the final product?

The point is that they would be made locally, not "shipped anywhere". Every city has a bunch of local concrete plants for a reason. If I buy aggregate, it's coming from nearby; it's not being shipped across the country.

Not necessarily. concrete = cement + aggregate; the aggregate might be quarried on site.

You have to haul the materials to make cement in from somewhere though anyways, right?

I know nothing about the cement business, so it's hard to have even a clue how to estimate this, even if you consider the power to be ~free..

>that would be ten years to become CO2 neutral, just in terms of cement

Goodness, this doesn't seem viable at all.

Could the system work with pallets of steel drums filled with water? It looks like 4 layers is usually the height limit.

Why not? I would estimate the lifetime being at least 30 years.

I think the better approach would be to stack limestone (density of 170 lb/ft3 vs. concrete's 150 lb/ft3) and have very little environmental impact.

Does it depend on height to be effective, or could it stack limestone just 30' up or so?

They could make the concrete (the cement) in plants with CO2 capture. Sure, costs a lot more, but then it'd CO2 negative (as curing concrete binds CO2 from the air).

It’s not like they have to make concrete all the time, you would amatorize the concrete capital and environmental costs over the lifetime of the tower.

Would it matter if it was containers filled with water, soil, trash instead?

Way less dense than concrete. So less energy stored.

Sand and water?

Rock.

Aside from mass density, the problem here would be structural integrity. How high can you stack buckets of water before the tower collapses? Probably not 120 meters. It comes as no surprise to me that this would use the same kind of material as a 120 m tall "normal" building.

Considering that water isn't compressible, I'd think you could stack them even higher. Just use cylindrical containers with high strength in the radial direction.

Also, how about compressing plain old air in an array of individual columns? It might be reasonable to improve the efficiency with some sort of heat exchanger.

That would be a lot of steel. 100 meters of water is submarine territory. You need some thick walls to stem the radial pressure alone, probably at least 5 inches or 13 cm of steel (consider [0] and [1], this value also agrees with my intuition from engineering education). Assuming a barrel of 2 m radius and 3 m height (volume ~36 m³), that's 5 cubic meters (or 8 tons) of steel you'd need for the cylindrical hull alone, costing around 8000 USD in material [2] (varies wildly of course, but you're probably not using submarine grade steel). Per ~40 ton barrel. Would make about 40 million in steel costs alone for the number of barrels shown.

I have no idea what your air compression idea entails, but it doesn't sound more realistic. Concrete is pretty good at being stacked (source: look around you).

[0] http://www.madehow.com/Volume-5/Nuclear-Submarine.html

[1] https://en.wikipedia.org/wiki/Submarine_hull#Dive_depth

[2] https://www.wolframalpha.com/input/?i=steel+cost

Compressed air energy storage is also being investigated in a variety of forms:

https://en.wikipedia.org/wiki/Compressed_air_energy_storage

They're not containers, they're structural. They need to be stackable dozens tall. You're basically limited to the kinds of materials you can build tall buildings out of -- concrete or steel. And concrete is much cheaper.

Couldn't they just use recycled concrete (from broken up roads and buildings) or just rocks instead?

Presumably they would need to build a vessel to hold the concrete rubble in, as opposed to casting their concrete directly in the shape they desire.

No, the whole point is that they need to stack these blocks up very high. You need uniformly shaped blocks for this, so no rock, and recycling concrete doesn't let you reuse the cement as binding agent -- it's already inactivated. You just get to use reuse the aggregate, which isn't the environmentally expensive component to manufacture.

People have turned rocks into uniformly shaped objects for thousands of years...

at a huge cost.... https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQlz2Q6...

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