[dwyerm]

> it's a fairly energetic explosion

So is a lithium battery explosion. So is a gasoline explosion. So is a flour mill explosion. So is a wind turbine explosion.

Energy is energy, and losing control of energy is never great, no matter where it comes from. So while I catch your meaning, it might be better to phrase it with respect to how controllable that energy is.

From what I know of flywheel storage, the problem mostly comes down to keeping the wheel from coming apart, and containing it when it does. The nice thing about using flywheels for grid storage is that you can bury them and make them large. The earth contains your explosion risk and the lack of jostling means that your bearings don't need to take as much stress and limits that failure risk.

[throwaway5752]

I would disagree with you slightly. Energy is not totally fungible. I can punch you in the face or shoot you with a gamma ray (edit: though of course, I would never do either), and it would have very different effects, potentially. Same for a fastball vs a small caliber weapon. Heat, light, and kinetic energy are important distinctions - particularly when you have a bunch of 30k flywheels in close proximity and one of them catastrophically fails. Now, I know that's the first thing someone would think about deploying a bunch of them and they would take precautions. Li batteries are less kinetic and more thermal and that's a bit easier to manage/less likely to cascade.

I thought the trend in flywheels was magnetic suspension and removing mechanical linkages? Admittedly I haven't kept up.

[microcolonel]

> that's a bit easier to manage/less likely to cascade.

What gives you that impression? Seems to me that it explodes if you contain it, and if you don't contain it, it can spout jets of thermal energy at virtually any angle. With flywheels you need to arrest it in bulk heavy objects that don't tend to sustain fire. That seems a lot simpler to me.

The bigger problems with flywheels are cost of manufacture and (depending on the technology used) efficiency for overnight storage.

> I thought the trend in flywheels was magnetic suspension and removing mechanical linkages? Admittedly I haven't kept up.

IIRC flywheels with limited motion gimbals (to reduce the tolerances on the wheel) are becoming more popular, still magnetic bearings.

[throwaway5752]

What gives you that impression?

Insulation is cheap, effective, and very compact. And it's easy to transfer heat quickly, also (either via injecting lots of cold extinguisher or flush lots of hot oxidizing gas)?

Cool, thanks for the update on flywheels. I ... am not trying to create a false dilemna, here. Fuel cells for stranded methane deposits are great. Flywheels have outstanding responsiveness and energy density. Li / compressed air / pumped water et al scale well. They all fit into a more resilient grid storage strategy that permits a transition to periodic sources of input from non-renewable base load.

[wbl]

Except we've had a carbon free baseload tech for decades now.

[throwaway5752]

It's kind of bizarre how nuclear had achieved a sort of counterculture renaissance. It has many, many drawbacks too numerous to go into here that are technical and sociological in nature.

And anyway, you tried to be too clever. I said "non-renewable" and not "carbon free" just to avoid this conversation. Unless you can start synthesizing utility grade quantities of well-behaved fissile material at a net energy surplus then it's not renewable even if we have decades/centuries of supply.

[paulmd]

> Unless you can start synthesizing utility grade quantities of well-behaved fissile material at a net energy surplus then it's not renewable even if we have decades/centuries of supply.

Someday, even the Sun will run out of fuel. In the long run, we are all dead - unless someone figures out how to reverse entropy.

http://www.multivax.com/last_question.html

[strken]

It's not renewable, but it is closer to carbon-neutral than fossil fuels. Advocates generally push it as a stopgap measure for climate change, after we transition away from baseload coal and before we transition to baseload renewables.

[samatman]

Batteries don't like insulation, in fact preferring active cooling.

Which is itself a good thermal-runaway damper, to speak to the second sentence of your second paragraph.

[throwaway5752]

The other kind of firewall: https://en.wikipedia.org/wiki/Firewall_(construction)

[reaperducer]

>So is a flour mill explosion

Having witnesses a corn silo explosion, I was unprepared for the ferocity of that ignition.

>So is a wind turbine explosion

Off to YouTube...

[dwyerm]

The Hornslet collapse[1] is the one I was thinking about in particular. It is especially daunting now that I've toured a site for myself. Those things are huge!

[1] https://en.wikipedia.org/wiki/Hornslet_wind-turbine_collapse

[vvanders]

Ever seen a tire come off of a semi?

Assuming the flywheel keeps its integrity it's much harder to predict the "blast radius" of where that thing is going to go.

[blevin]

Are buried flywheels used in practice? Is the flywheel axis aligned with the Earth rotation axis? Do they pull a vacuum around it?

[dredmorbius]

Yes, in cases, to the first two. Containment is frequently buried concrete vaults.

Not AFAIK for the last, though precessional torque bearing load is a nontrivial consideration.

[blevin]

Thanks for this response. My untrained guess is that precessional torque would cause more loss than air friction, assuming the wheel is cleanly symmetric (ex. no bolt heads sticking out). It's fascinating how many paths there are to push something up various kinds of potential energy gradient in a way that it mostly stays there by itself but is still available to us to access in a controllable way.

Wikipedia claims flywheel loss rate circa 2013 of 5% per day. https://en.wikipedia.org/wiki/Flywheel_storage_power_system

That compares to recent estimates of Tesla li-ion loss rates at under 5% per month -- 0.16% per day.

Amber Kinetics is one company building fixed flywheel storage products. http://amberkinetics.com/

They have one 32 kWh, 5-ton, 98% steel flywheel installation on Oahu; pictures here: http://amberkinetics.com/hawaiian-electric-and-amber-kinetic...

[dredmorbius]

Frictionaal losses are part of the consideration, but just plain bearing wear is a bigger one AFAIU.

Angular momentum in 100 kWh - multi MWh rotational systems is large.

[ballenf]

Also flywheels don't give off noxious fumes during failure.

[naravara]

For now. If you want to use one for long term energy storage, though, you're going to need to invent some kind of room temperature superconductor and frictionless surfaces. Who knows whether those would be noxious or not.

[smaddox]

Why? Magnetic bearings and copper work just fine...

[jhayward]

Are you sure? Metal fumes and vaporized epoxies are usually pretty toxic.

[tossandturn]

...or during production (relatively speaking).

[Florin_Andrei]

> So is a lithium battery explosion. So is a gasoline explosion. So is a flour mill explosion. So is a wind turbine explosion.

No, they're not all the same.

Compressed air, flywheel, flour mill - very dramatic events.

Lithium battery - much more mild, typically.

Gasoline - it depends.