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Comment by crazygringo | Hacker News Reader

I understand what you're saying, but your claim seems misleading to me.

Compressing air absolutely does store energy. It takes work to compress it, and work will be done when it is allowed to expand.

Yes, one way of measuring the stored energy is by the resulting increased temperature. Compressing air necessarily raises its temperature. And then if you want to you can transfer that heat elsewhere, go ahead. That's how air conditioners and refrigerators work, after all.

But in the basic case, it seems entirely accurate to say that compressing air does actually store energy. Just like raising a heavy object against gravity stores energy.

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It doesn't store energy in the sense that all that work you did compressing it doesn't increase its internal energy. It just has the energy it started with, floating around in the atmosphere. Now, some of that energy can be converted to work, if the air is adiabatically expanded, but particularly at high compression it's not large compared to the energy that went into compressing the air (and that was dissipated as heat when the hot compressed air was cooled.)

But it does increase its internal energy. Its temperature goes up.

Obviously if you let the compressed air cool, then you're letting the energy dissipate. So if you want to preserve all the energy, don't do that.

The air that is stored underground has been cooled, so yes, the energy that was added isn't there.

>Compressing air necessarily raises its temperature. And then if you want to you can transfer that heat elsewhere, go ahead. That's how air conditioners and refrigerators work, after all.

The oh-so-clever trick is to transfer heat elsewhere while you're compressing the gas, so you actually reduce back-loading on the piston instead of having it 'fight' the temperature rise. This can be accomplished via water spray, or by compressing a gas bubble that's surrounded by water (eg in the trompe).

By continuously removing heat as you compress the gas, it effectively acts like a train of compressors and intercoolers with an infinite number of stages, ie true isothermal compression. No magic, just physics.

https://news.ycombinator.com/item?id=40278128

I want to note that in this system, where the heat is stored to reheat the gas at discharge time, you want to produce heat during compression. Doing adiabatic compression means you are storing more heat, and therefore can produce more energy at discharge time, than you could with isothermal compression (for a given storage volume and pressure.)

This is unlike traditional CAES, where the air is reheated by burning a fuel. There, fuel use is minimized by isothermal compression.

It’s absolutely misleading. The compressed air absolutely has potential energy. Nail guns, air wrenches, etc convert that potential energy into kinetic energy.

It seems like either this is describing an additional energy storage/extraction method that’s possible with these systems. Either to improve the efficiency or something that’s actually far more efficient than utilizing kinetic energy compressed air can create.

It’s kind of like saying the water in a hydropower plant doesn’t store energy. The turbines aren’t extracting energy from the water itself. They’re extracting energy from gravity acting on water. It doesn’t matter if the water is nearly freezing or nearly boiling, it has roughly the same amount of hydropower available.

Your analogy is nonsense. Water in a hydropower system falls from a higher gravitational potential to a lower. When a pumped hydro system is charged, the water is moved up and energy is stored in its potential.

This is not the case in a CAES system. The internal energy of air, which is the kinetic energy of its molecules, is independent of pressure. It's only a function of temperature. The internal energy of the air is unchanged by a process that increases its pressure without increasing temperature. So, no energy has been stored in the air.

What has been done is reduction of the entropy of the air. So, it is now possible to convert some of that internal energy (which was always there) to work by expanding the air. The air would end up even colder than the initial air.

Is compressed air in itself source of energy ? Or does it depend on the external air pressure as well ? For example I compress air in a canister and move it to outer space or deep underwater, does the energy we can exploit out of it goes up or down ?

Yes, if you take such a container into space, the gauge pressure (pressure relative to ambient atmospheric pressure) goes up. A cold gas thruster is a type of rocket engine that is basically a pressurized tank of gas with a valve connected to a propelling nozzle. The nozzle takes a high-pressure, low-velocity gas and converts it to a low-pressure, high-velocity gas, and directs its output generating thrust (which is a reaction force - Newton's 3rd law). The lower the ambient pressure, the higher the expansion ratio of the nozzle can be, which results in a higher output velocity of the gas, which directly increases the thrust. It would give you a higher specific impulse, which is a measure of fuel efficiency for reaction (thrust) engines.

You can store about 10kWh of heat energy in your water heater at home.

https://www.pvh2o.com/faq

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