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Comment by Waterluvian | Hacker News Reader

I think the part that isn't intuitive to everyone is how to "accumulate" potential energy if you don't have batteries/capacitors. Like how I can't fill my car tire with my bike pump. There's... there's magic missing. Something that takes a small amount of pressure and overcomes the huge pressure the other way. Check valves or whatnot.

It's easier to perceive when you think about a massive turbine that can grind out a ton of electricity and power a compressor that by default produces a lot of air pressure.

These tools are really neat to me. I love the idea of things like a hand crank radio, pulleys, and long levers.

Presumably you could fill the tire of a transport truck with the bulb from a blood pressure cuff and a lot of time, but it's not intuitive to me what the mechanics are in between those two things.

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> Like how I can't fill my car tire with my bike pump.

Um. Yes, you can. It just might take a little while. You can fill 100 psi road bike tires with a hand pump. A 30-40 psi car tire is not a problem.

Have done this, can confirm, it does take a while :) like tricept workout kind of while, but it does work... (for expediency I used a floor pump serfas or something like that)....

yeah the pressure isn't a problem, the volume can wear you out though :)

I had to do this once myself. I was surprised by how hot the bike pump was by the end. It was on the threshold between warm and hot. It wouldn't burn you on contact, but you wouldn't want to grip it hard for more than a couple of seconds either.

Was that more due to gas thermodynamics or mechanical friction?

I use bike pumps a lot, but only on bikes, so I've never experienced this.

I assume it was mostly due to ideal gas law reasons. Pumping up a highish pressure truck tire (~65 psi IIRC) did take quite a long time and I was pretty exhausted by the end.

Compress a gas, it warms up. I imagine that's the large part of it. Go grab the cooling fins on a large electrically-driven compressor after it's been running a while. No, wait, don't do that if you like your fingerprints. The cooling fins aren't to cool the motor, they're to cool the compressor housing where the piston goes up and down to compress air.

> how to "accumulate" potential energy if you don't have batteries/capacitors

That's what the compressed gas is - it's potential energy. It's like a battery.

> Like how I can't fill my car tire with my bike pump.

Why not? Because you can't pump hard enough and it's not strong enough to hold the pressure needed? You can build a stronger pump, and you can use a lever or gears (same thing really) to compensate for your strength if you can't pump any more.

> Presumably you could fill the tire of a transport truck with the bulb from a blood pressure cuff and a lot of time, but it's not intuitive to me what the mechanics are in between those two things.

Again that's just a 'lever' problem - you can't squeeze the bulb hard enough, so add a lever (and you may need to make the bulb stronger.)

I personally understand these things. I'm trying (and failing) to communicate that mechanics can be less intuitive than electricity. I regret this post. :)

Interesting question though. I wonder if the answer is more trivial than "mechanics is harder than electricity", perhaps the people you're explaining to don't understand electricity either. If someone is trying to understand "how does air move in only one direction in a compressor", perhaps they also don't understand "how does the electricity move in only one direction in the generator"? Could they explain why the pressure coming from the compressed air canister doesn't cause the electricity generator to run backwards?

You might be alluding to this, but I wonder if people are more prepared to mentally conceptualize electricity as a magic atomic constant: "it just works that way... so given that, of course a huge generator can produce enough power to run a large pump."

Pressure differential is analogous to voltage and mass flow to current, I think we are all on the same page about that.

The sorely missing bit is the equivalent of those amazing DC/DC step-up/down voltage regulators that have revolutionized the application of electricity in recent decades (all the way from tiny electronics to massive HVDC networks), and/or an equivalent of the AC transformers that fulfilled a similar role before. Actually AC is missing in its entirety, there's just no practical way of transmitting power via soundwaves.

The closest pneumatic equivalent to DC-DC converters are coupled turbines, e.g. turbochargers or high bypass turbojets. But those are not just far beyond households appliance scale, they are also limited to the high mass-flow/low pressure differential, but practical applications of pneumatic storage are the opposite.

> DC/DC step-up/down voltage regulators

Interestingly, there is actually a hydraulic mechanism that's the equivalent of a boost converter with inductor, diode, and switching element:

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

It translates a low-pressure fluid with continuous flow into periodic high-pressure impulses. The inertia of the fluid is the hydraulic equivalent of the inductor, and a switching element (valve) closing suddenly causes a spike in pressure as the water decelerates suddenly, equivalent to a voltage spike as an inductor "wants to keep the current flowing".

But it usually operates with water, which has much higher density (hence inertia) than air, so it might not be very efficient (or might require much higher flow rate) in a pneumatic version.

You add a gear to your compressor. There is no need for an air pressure converter (which is effectively just an compressed air powered compressor with a higher maximum pressure).

Substituting pressure with more volume is highly impractical when storage is involved.

You can also lift a car or truck with your hands... and a jack.

Energy is force times distance. If you can only exert a tiny force, you can compensate with enough distance, providing the losses in the system aren't too great.

> I think the part that isn't intuitive to everyone is how to "accumulate" potential energy if you don't have batteries/capacitors. Like how I can't fill my car tire with my bike pump. There's... there's magic missing.

As others have pointed out, you can! If you're rather patient. They've even got compatible valves!

And you can think of a pneumatic tire as a kind of battery -- potential energy is roughly the volume times the pressure differential.

Flywheels are acceptable replacements for batteries. I'm not sure if the Amish use them for energy storage, but the rest of us do.

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