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.
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 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.
This is unlike traditional CAES, where the air is reheated by burning a fuel. There, fuel use is minimized by isothermal compression.
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.
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.
The effect the prior poster was discussing is most apparent at large scales when someone is doing actual efficiency analysis - compressed air is not a very efficient way of storing or transporting energy in most contexts. No one would use compressed air for a power grid, because it would quickly be apparent how terrible it is.
It is however a great way to transfer and use energy in many industrial contexts, as turbines and pistons using high pressure compressed air can be very compact while also being very powerful, and have built in cooling.
At the level of typical household use, the efficiency effects aren’t notable. No one is going to care or even notice if it cost 1/2 cent of electricity (at the compressor) to use that air impact wrench vs 1/10th of a cent to use an electric one. Especially when the electric one is heavier and has less torque.
do you know how they do it?
This sort of design (a counterflow heat exchanger, basically) keeps the delta-T at each point low, so the system has low entropy production.
A similar heat exchanger could be designed with a liquid thermal storage medium, where the air and the liquid are flowed past each other in opposite directions (separated by solid walls, most likely). The hot storage tank would be insulated. An advantage of this design is the pressure of the liquid storage tanks needn't be high, unlike the vessel containing the pebbles in the earlier described system.
All these may need to dump some waste heat to the environment as a consequence of inefficiencies in the system.
https://www.youtube.com/watch?v=50fJ8Av_g7Q
https://www.youtube.com/watch?v=uvf0lD5xzH0
https://news.ycombinator.com/item?id=27066295
Sad to see people are misinterpreting the cleverness of isothermal compression, and thinking it's "just" thermal storage.
How the conditions are reached makes a big difference in efficiency.
I'm baffled at why you focus on minor implementation details like internal energy at a particular snapshot in time, as opposed to highly salient metrics like overall round-trip system efficiency.
What isothermal CAES (with heat storage) would require is a larger storage volume per unit of energy storage capacity, since less heat is generated during the ideal compression. That heat generation is not inefficiency, it's something you want!
Only if the heat is discarded and you reheat with fuel do you benefit from isothermal compression.
What a compressed gas represents is not stored energy, but stored (negative) entropy. It is a resource that allows low grade heat to be converted to work at high efficiency. This is what's to happen in this facility: the heat of compression is separated out and stored, then used to reheat the compressed air at discharge time. The energy is actually being stored in that thermal store.
But there are other ways to do this that don't involve compressed air storage. Instead, after the heat of compression is removed and stored the compressed air could be reexpanded, recovering some of the work. This would leave the gas much colder than when it started. This cold could be stored (heating the gas back to its initial temperature) and the gas sent around again. To discharge, the temperature difference between the hot and cold stores could be exploited.
This is called "pumped thermal storage". I believe Google/Alphabet has/had a group looking at this (called Malta). It has no geographical limitations.
(*) Highly compressed air will store some energy because the molecules become so crowded some energy is stored in intermolecular repulsion, but that should be a small effect in this system.