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Comment by heisenbit | Hacker News Reader

So 45 cubic meters for 10KW/h means roughly 100 tons concrete. A Tesla power wall with a little more capacity has a weight of maybe 130kg. So roughly a factor of 1000 less capacity.

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Yes, however since they are maintaining the cements structural ability consider that they could replace your houses foundation with a capacitor that could power your house all night long if your solar capacity could both charge it and run your house during the day.

Similarly would building facades providing power storage for lighting or other uses during the "off hours"

Build roads from such?

Shouldnt it just be an inherant regulated requirement to build roads from materials such as this - but require modular units so as upgrades happen...

Having it in the roads would be interesting but difficult to exploit. One idea I heard pitched was to put coils in the roads connected to nichrome (thermal) wire and then have trucks mount magnets under their trailers. As they drove down the road it would warm the pavement which would de-ice it in the winter. (make it hotter in the summer though, so not particularly practical without a switch).

What if you were to do the same, but with subterannian trains travelling in opposite directions (?) - such that the cars north are met with the subway south and they interact for great justice? (Im serious on the idea, but I dont know the outcome?)

(I am talking about huge magnets. if that wasnt clear.)

(Symbiotic asymetrical train propusion through shared interior corridor walls) (easy concept... hard execution)

Roads are tear and wear.

They are everything but not stable.

Also why solar freaking roadways garbage is.

Any large version of this capacitor has to consist of fine layers of material between electrodes and separators. Its structural ability will be very different from bulk cement.

That is precisely correct. The article went to lengths to describe how they maintain cement's compression strength which is the primary stress axis for house foundations. It would definitely make constructing a foundation more time consuming (not the pump concrete out of a truck and fill up the forms that is is today) but there isn't anything in the article or their paper that suggests this wouldn't be a reasonable application. (again, maybe not cost effective, but that is a different axis of trade-offs)

They don't mention = they weren't concerned. Yes normally it's primarily the vertical axis but you don't want these layers slipping sideways in case of light earthquake. Or water from outside getting in not only messing the electrolyte but also causing whole thing to swell. There are plenty of potential problems, needs much more r&d.

The Tesla Powerwall is (despite it's name) NOT a wall! From the sound of things, this new supercapacitor might be. I can very reasonably imagine there being many orders of magnitude more structure vs things that hang from that structure.

I also suspect there may be a slight cost difference between adding some soot to the concrete you were already planning on using vs whatever they put in a Powerwall.

I guess my point is there's probably usages for both.

Energy density is the word you're looking for. It's a important metric for many application (smartphones, EV, etc.) but not for everything: for instance when it comes to grid storage, the TCO trumps everything. And even on cost it doesn't necessarily make sense to compare innovations against the incubant, because lithium-based battery have had lots of R&D over several decades to make them “cheap”, and it's impossible to know in advance who's gonna win in the end: innovation necessarily involves multiple failures for every success.

Slight nitpick: if we're talking about kg then it's specific energy.

"Energy density" = energy per volume

"Specific energy" = energy per weight

They're usually quite correlated though, although confusing them makes hydrogen fuel a pain to talk about.

Thanks, didn't know that.

Thanks. That's the important number. Really hard to find in that overhyped article.

This is a generic problem with battery announcements. I want to see some publication like Electrek publish 1, 5 and 10 years ago in battery PR.

Did you read the article? Nobody is suggesting replacing batteries with this, but 50,000 square miles in the united states is dedicated to roads. What about concrete foundations? Think about how much energy we can store, which is desperately needed for renewable mass adoption.

That's the bullshit part of the article. Cement in an uncontrolled environment is not going to work as a capacitor, even if it works in a controlled environment. Some water will get in. There's expansion and contraction as temperature changes, which causes cracking. The road battery idea belongs in the same category as the road and sidewalk generator idea [1][2], which, in the end, was just an advertising stunt.[3]

The paper jumps directly from showing a tiny effect in a tiny sample to changing the world. This seems to be a disease of battery-related articles.

Also, cement is not the same as concrete. Concrete contains cement as a binder, but is mostly sand and rock. It also absorbs water, which is kind of a problem for a capacitor.

[1] https://www.sciencedirect.com/science/article/abs/pii/S03062...

[2] https://cait.rutgers.edu/generating-power-every-time-you-hit...

[3] https://www.archdaily.com/911965/sidewalks-that-generate-ene...

Touché. I guess I'm cautiously optimistic that it can scale (goodness we need good news), but you bring up plenty of valid points. Thanks for putting me in my place.

> Cement in an uncontrolled environment is not going to work as a capacitor, even if it works in a controlled environment. Some water will get in.

Concrete always has water. It's part of it's composition. What exactly led you to believe it doesn't?

Also, there's a technology that can turn concrete waterproof which is called paint. In harsh environments it's also mundate to coat rebar with epoxy paints to protect against corrosion.

> There's expansion and contraction as temperature changes, which causes cracking.

Thermal stress in monolithic structures does not cause cracking. At best, it can happen in hyper static structures when thermal strain imposes displacements beyond design limits. This is countered by using technologies such as prestress.

What can cause cracking in concrete is delamination from frosting, where water penetrating through pores expands and detaches concrete blades at the surface. This is prevented by applying a technology called paint.

I saw nothing in your comment that put into question the usage of concrete-based super capacitors. You pointed to known failure modes that are addressed in a very mundane way.

The voltage seems key here. From the article:

> they fabricated button-size capacitors capable of holding 1 volt ... As the next step, Ulm says the team is now focused on developing a 12-V supercapacitor using these materials.

If I remember my physics correctly, charge is proportional to voltage, so if they can achieve 12 volts max voltage, shouldn't it result in 12X energy density?

If so, that would reduce the required volume (for 10 kilowatt-hours of storage) from 45 to 3.75 cubic meters.

Apparently concrete slab foundations are typically 4-6 inches (10-15 cm) thick[1], so in a single-story 1000 square foot (92 m^2) house, the volume of the foundation would be 9.4 to 14.1 cubic meters.

Which means you could store around 25 to 35 kilowatt-hours in that foundation.

So it seems like it's in the right ballpark for what they're aiming for.

EDIT: Another article[2] says this:

> Having proved the principle, they now plan to build a series of larger versions, starting with ones about the size of a typical 12-volt car battery, then working up to a 45-cubic-meter version to demonstrate its ability to store a house-worth of power.

Given the main article is not very technical, and both articles mention 12 volts but in different ways, I'm now less sure what the researchers are aiming to do. Maybe they are trying to improve the max voltage of this type of capacitor or maybe not.

---

[1] https://wilsonexteriors.com/a-comprehensive-guide-to-concret...

[2] https://news.mit.edu/2023/mit-engineers-create-supercapacito...

Energy in a capacitor is (1/2)CV^2 - so 12x the voltage would be 144x the energy.

If they can keep C at the same value in a higher voltage version.

Yep. Trivially, one would expect the capacitance to decrease linearly with voltage.

There's also a question about current leakage.

Is the electrolyte concrete? Otherwise I don't see it working too well for building.

What’s the cost difference though? Could it be the foundation of your house? 40x20x2 feet is about 45 cubic meters.

Two feet seems very thick for a foundation. That’s probably thick enough to be an airport runway.

40x20’ is also rather small as far as American houses go

Usual foundations (though it varies in different parts of the USA) would be 8 inches around the perimeter with 6 inches in the "middle" part of the floor. Of course for commercial or multi-story buildings you would see an increase in the thickness.

My house is 80x32 with a six inch poured floor and 9’ poured walls 8” thick. If i punched in my numbers correctly that’s a little over 70 cubic meters.

It seems actually close to classical capacitors. And these are so cheap you can use them as bricks already :)

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