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Comment by jiggawatts | Hacker News Reader

Compressed air energy storage is already a thing!

The energy stored is limited by the tensile strength of the container. The best capacity for a unit weight is from laminated carbon fibre tanks, but this still doesn’t even approach the energy density of ordinary hydrocarbon fuels.

You’ll find that there’s lots of interesting ways to store energy — like flywheels or chemical cells — but one way or another they’re all inherently limited by chemical bond strengths.

Fundamentally all energy storage is some sort of stored “tension” in chemical bonds that can be released to do useful work.

The reason fuels are so good is that this release needs a second component (oxygen) that is kept separated. This makes high energy densities safe.

No separation — like with compressed gas — means that the energy storage is a bomb waiting to go off. It would be too dangerous to use.

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Excellent analysis, thank you, but this line is a little strange:

> The reason fuels are so good is that this release needs a second component (oxygen) that is kept separated. This makes high energy densities safe.

Don't fuel tanks (for internal combustion engines, for example) typically also contain oxygen? You make it sound like a full tank is safer than a half-empty tank.

I assume you're actually saying that for safety we want the reactants to be stable at common atmospheric temperatures and pressures, and we don't want reactants that spontaneously combust on contact with air or common materials (including each other).

They don't contain nearly enough oxygen to liberate all the stored energy.

Also, especially since gas tanks are no longer directly open to the atmosphere, I wouldn't be surprised if the oxygen concentration was too low most of the time for combustion to be possible.

In principle we could calculate the weight of oxygen in half a fuel tank's worth of air, and work out how much energy that would release if it was used in combustion with the fuel there.

In practice, though, you're right, that we'd have to think about the oxygen concentration, which would immediately start dropping as the fuel burned. Also, it's not easy to calculate what temperature the tank would reach, and for how long, and whether that would cause it to rupture.

For reference, though, the testing process for fuel tanks can include: "the tank being exposed to a pan of raging petroleum coming through specifically designed firebricks to intensify the effect of the naked flame. The tank has to be capable of securely containing diesel during the test for 90 seconds."

https://corillaplastics.co.uk/rotationally-moulded-diesel-fu...

  > You make it sound like a full tank is safer than a half-empty tank.

Back when I was working as a Ford tech, I was told that a full tank is a fire hazard. An empty tank is an explosion hazard.

You can run away from a fire. If the fuel pump can be changed without dropping the tank (e.g. by going under the rear seat or truck bed), then it is preferable to work on a full tank.

A full tank is safer than an almost empty tank.. Also the tank is designed to keep the vapor part of the gasoline too saturated to explode, this is why there is a latch on the fill tube. I found this Quora post: https://www.quora.com/How-come-the-gasoline-in-gas-tanks-nev...

And even if it can explode in the situation where there's almost no fuel left, in that case it's not as bad as liberating the full energy of a full tank, which is what you'd do if you have a flywheel spinning with the same total energy as a full tank of gasoline. That would be madness.

Heh I came to think of the recent demo of the company that wants to spin up satellites on earth and THROW them into orbit. If something goes wrong in that spin-up, they would destroy the entire launch facility and whatever is in the way.

https://youtu.be/JAczd3mt3X0

I once had a CDROM open while the CD was spinning at like 40x or so, it ejected and went into the plaster wall. And that was just a CD..

You may think of it as one key for all energy boxes (a broken valve or a hole in a gas tank) vs multiple keys for every energy box (oxygen molecules). To burn a liter gasoline you have to mix it with the ~same amount of oxygen. It never explodes all at once except in that rare situation. An empty tank is exactly that, because gasoline is volatile.

Energy boxes that can be opened with a single key are more dangerous, cause more prone to accidents. Pressure, kinetic storages, self-sufficient explosives are all much more dangerous than stable chemical bonds.

Also, gravitational potential can be accumulated in a single-key way too. That’s why cliffs are dangerous, but staircases are less so, because energy release is dosed with each step and flight. You have to actively err on stairs to take fatal damage.

There are various forms of energy and various forms of energy storage, including chemical energy and gravitational potential energy.

We probably think of energy as entropy like gases because e.g Maxwell?

Examples of gravitational potential energy:

- A water tower or a pulley with weight suspended a greater distance from the most local mass/graviton centroid.

Potential energy: https://en.wikipedia.org/wiki/Potential_energy

Thanks, yeah that makes complete sense. Plus I would imagine the more energy density you need, the heavier your enclosure has to be, always partially negating the benefit!

Precisely. Any attempt to "optimise" the specific energy of a storage approach will inevitably bring it closer to closer to the point where it explosively disintegrates.

This is why it's a bit hilarious to see how upset people are about lithium batteries occasionally exploding. That's... sort of the point! They've been optimised until the safety margin (=weight) is a low as possible. You want safe batteries? Carry around something the size and weight of a brick!

Ditto with all possible kinetic energy recovery system (KERS) designs. You can have weight-efficient or safe, but not both.

Lithium batteries typically explode because the electrolytes we use are highly flammable. It's actually entirely incidental to the amount of energy stored in the battery. A Li-ion battery with a different electrolyte material could have the same energy density, but a greatly reduced risk of fires and explosion.

Only somewhat - batteries tend to explode when damaged (punctured, squished, etc) when they short internally, or when they get a thermal runaway.

Putting armor, heat sinks, etc. on them does make them safer. Adding battery safety circuitry (charge control, overcurrent protection) also helps. It also adds weight. Unarmored lipo batteries are notoriously dangerous for a reason, and it’s not the electrolyte.

Compressed gas is not too dangerous to use. It is an energy density problem. There are prototype vehicles that fill the same niche as current EVs.

Failure rates for automotive Li batteries are quite low. In the range of 0.001 per 100K vehicles.

I'm aware, and compressed gas storage containers are equally low if not lower. They also fail in a way that is safer than batteries fail. The tanks are engineered to split and the limited, usually zero, shrapnel is redirected downward.

What really limits compressed air as a vehicle energy store is the inability to reasonably run climate control off of it.

And the incredibly low round trip efficiency - heat production then loss to the environment as part of compression is pretty brutal on that front.

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