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Comment by beat | Hacker News Reader

Safety concerns raised... since the maximum amount of power the flywheel can be expected to store is a known quantity, and the amount of kinetic energy that can be stopped by concrete or metal shielding is a known quantity, it's just a matter of burying the flywheel underground, with enough concrete/metal around it (particularly above it) to prevent shrapnel from penetrating to the surface. Easy peasy. It's just a cost problem.

We keep those levels of potential energy in underground gas tanks at every gas station, and no one freaks out about that.

edit: For perspective, a typical gas station has 12,000 to 24,000 gallons of gasoline storage underground. A gallon of gas is about 120 megajoules of potential energy. So there are billions of joules sitting there - much more than these flywheels. Of course, without air, gasoline isn't dangerous. But a 90% empty tank has a lot of air in it, a lot of potential boom.

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NO, an empty gas tank has a lot of gasoline vapors mixed in that air - the concentration is high enough that the vapors will not burn. One of the advantages of gasoline is it will only burn in a fairly narrow window of concentration (IIRC 5-30%, but since I don't feel like looking it up you will have to if you care)

Still, if containment is violated somehow, it's a LOT of power for fire/explosion. These flywheels are much smaller, by comparison.

The tank of gas in a car probably has over 400kwh of energy. How much will these flywheels contain, considering a full charge for an electric car is on the order of 100kwh (electric cars are more efficient than internal combustion)? The energy of two or three tanks of gasoline is enough to get several charges out before needing to spin up the flywheel again.

But a nearly "empty" flywheel is a lot less dangerous than a "full" one. While a full gas tank, even if cracked open and lit on fire, would probably burn, not explode.

I imagine seeing a flywheel leap out of the ground and short through the foundations of a skyscraper...

That gets back to my original point, though - we know from the start the maximum power of the flywheel. Putting enough reinforced concrete above it to keep it from penetrating in case of catastrophic failure is a straight-up engineering problem.

It's not comparable. Gasoline is limited in how fast it can release energy, by how much air it can get. So you get a fire but not an explosion.

In contrast a flywheel can (and will) dissipate 100% of its energy at once, leaving you with white-hot pieces of metal if your containment worked, and supersonic pieces of metal flying everywhere if it did not. (Or some combination of the two.)

It's far more dangerous. The amount of potential energy is irrelevant, it's the power (how fast the energy can be released) that matters.

Surely the danger can be largely mitigated by siting the flywheel underground, surrounded by some energy absorbing walls - assuming the catastrophic failure mode ejects debris in the same plane as the flywheel is turning

Or developing a flexible flywheel that destroyed itself by tangling - like the Velkess project.

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