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Forget Better Batteries, Nothing That Exists Or is in Development Can Store Energy as Well, And as Cheaply, as Compressed Air (theconversation.com)
An anonymous reader shares a report: The concept seems simple: you just suck in some air from the atmosphere, compress it using electrically-driven compressors and store the energy in the form of pressurised air. When you need that energy you just let the air out and pass it through a machine that takes the energy from the air and turns an electrical generator. Compressed air energy storage (or CAES), to give it its full name, can involve storing air in steel tanks or in much less expensive containments deep underwater. In some cases, high pressure air can be stored in caverns deep underground, either excavated directly out of hard rock or formed in large salt deposits by so-called "solution mining", where water is pumped in and salty water comes out. Such salt caverns are often used to store natural gas. Compressed air could easily deliver the required scale of storage, but it remains grossly undervalued by policymakers, funding bodies and the energy industry itself. This has stunted the development of the technology and means it is likely that much more expensive and less effective solutions will instead be adopted.
There is no “minimum storage pressure” but the economics are poor for anything lower than 50bar. For CAES with tanks, the economics push you towards pressures of 200 - 250 bar. In caverns, the pressure you can use depends on the cavern depth. 120bar is not unusual. For a cavern with 120bar storage pressure that was allowed to swing down to (say) 70bar when “discharged”, you would be storing ~23MJ in each cubic meter of cavern. Thus for 1GWh (3.6 million MJ), you would need 156,000 cubic metres of cavern. That is actually a relatively small salt cavern! If it was a sphere, it would have radius of 33.4m. Surprising as it may seem, most salt caverns in existence are bigger than that!
23MJ of gasoline is about 1 litre, so by volume, gasoline has 1000x the energy density.
https://en.wikipedia.org/wiki/...
Small nukes can create a large cavern with fused glass walls
You're talking about a liquid under pressure. Pressurize liquids store very little energy, because they are largely uncompressable.
You might find it informative to read about the biggest presses. The 1500 HP motors don't make nearly enough power to operate the press directly: they accumulate the energy in pressure accumulator bottles until there's enough in storage to operate the press once. it's a "hydro-pneumatic" system.
It's the only example I can think of where energy is routinely stored and discharged at thousands of PSI, and safely. Scuba tanks store air at a reasonable fraction of that pressure, but they aren't used for power (so limited fill/discharge rate) and they do blow up from time to time.
Socialism: a lie told by totalitarians and believed by fools.
Here's a review of the project you are thinking of and another way of doing underwater compressed air storage. I was looking for this project to post before I came across your message. There's a diagram showing how the system in Toronto works. Unfortunately there's no detailed numbers on how efficient or competitive it is.
They are storing the heat captured during compressing to heat up the air when they decompress the air.
I was curious about efficiency myself, and google and this page (http://energystorage.org/compressed-air-energy-storage-caes) suggests that straightforward energy storage as compressed air is about 42% efficient, increasing to 55% efficient if you can use the waste heat.
If you can store the heat separately to make the process adiabatic, then the efficiency climbs to ~70% - but then you've got the additional cost and complexity of trying to store energy as heat, which is arguably a much more challenging task.
For comparison a Li-ion battery is about 99% efficient, and pumped water is generally in the 60%-80% range, with some claims approaching 90%.
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No, it definitely isn't. A process that sheds heat when done quickly, will still shed heat when done slowly. You might be thinking isothermal (constant temperature), in which case doing it slowly enough is one way to accomplish the goal. There's a lot of confusion between the two, but they're completely different concepts.
Adiabatic basically means "inside a well-insulated container" - it doesn't care how much the temperature changes, so long as no heat enters or leaves. In the case of compressed air storage, it sounds like the normal adiabatic process is to siphon off the heat generated by compression, store it separately in a medium that can store the same amount of heat with a much smaller temperature change and/or volume than the air, and then use it to re-heat the air as the pressure is released.
The Carnot cycle, basis of the internal combustion engine, actually contains two isothermal stages, in which pressure and volume change inversely (PV=constant) as heat is added and removed, and two reversible adiabatic stages where the gas changes temperature while expanding or contracting, without any external thermal transfer.
--- Most topics have many sides worth arguing, allow me to take one opposite you.