from Hacker News

A company is building a giant compressed-air battery in the Australian outback

by uncertainrhymes on 5/5/24, 10:28 AM with 147 comments

• by pfdietz on 5/6/24, 2:05 PM

  From a thermodynamic point of view, it should be noted that compressed air does not actually store energy! The internal energy of a compressed gas is from the kinetic energy of its molecules, and this is a function only of temperature(*).

  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.

• by greenbit on 5/5/24, 1:31 PM

  I must have missed something. Why not just use the water without the compressed air, i.e., pumped hydro? There must be some advantage, but they didn't seem to say. I'd guess maybe if your lower reservoir were underground, the water-only would require the generator systems to be down there, too, which would mean access for people as well, and being down a mine with a small lake's worth of water overhead seems pretty hazardous. Whereas by forcing air down to push water up, that whole below ground aspect can be almost entirely passive. Maybe?

• by hackerlight on 5/6/24, 6:27 AM

  > The next project would be Willow Rock Energy Storage Center, located near Rosamond in Kern County, California, with a capacity of 500 megawatts and the ability to run at that level for eight hours.

  Their California battery will be 4GWh capacity with a $1.5 billion cost, which is $375/kWh. Their Australian one will be 1.6GWh for $415 million USD, working out to be $260/kWh. Both are more expensive than lithium ion, so I wonder what the case is for it.

• by mikewarot on 5/6/24, 9:00 AM

  I always wondered why compressed air storage systems work against ambient pressure instead of having two tanks at high and higher pressures. This would greatly increase the density of the gas, as well as lowering the temperature differential.

  It would take a long time to get it up to initial pressure, as there would be a lot of heat to dissipate, but then it differential mode, the gradient would be much better.

• by littlestymaar on 5/6/24, 12:05 PM

  How do they work around the ”heat problem” with compressed air storage. When you compress the air, it heats up, and actually there's a big part of the energy that get stored in the form of heat, not pressure. When you want the energy out the pressured air cools down during depressurization.

  If you were able to keep the air hot the whole time the process is almost symmetrical so that's not an issue, the “heat problem” as I call it is how do you store this heat for an extended period of time? At scale, it's much harder to keep than just the pressurized air.

  The prototypes I've seen in the past were not storing the heat, but relied on industrial fatal heat (that was lost anyway) but this also has scale problem as you don't have that much available power except near very specific industries (NPP are an option, as are other heavy industries, but the supply is necessarily limited)

• by gcanyon on 5/6/24, 12:13 PM

  No mention of how efficient the energy cycle is? Without checking sources, I think I've read that batteries and pumped hydro end up in the 80-90% range round trip? Without knowing what this method produces it's almost pointless to consider.

  One advantage this has (I assume) is almost limitless cycle lifespan.

• by tromp on 5/6/24, 7:56 AM

  > the system extracts heat from the air and stores it above ground for reuse. As the air goes underground, it displaces water from the cavern up a shaft into a reservoir.

  > When it’s time to discharge energy, the system releases water into the cavern, forcing the air to the surface. The air then mixes with heat that the plant stored when the air was compressing, and this hot, dense air passes through a turbine to make electricity.

  By "releases water into the cavern", do they mean simply opening the air valve to let the air (pressured by the water) come back out?

• by Pxtl on 5/6/24, 3:17 PM

  I couldn't see in the article - is this using natural caverns or did they excavate? I know the pilot versions of this tech generally use old salt-caverns, like the one in Germany.

  I'm actually pretty excited about this tech - it seems like solar and wind are getting cheap faster than batteries will be able to meet the needs for grid-scale energy storage, so a cheap-but-inefficient energy storage tech is an exciting prospect. Massively overbuild the solar/wind and use these things to defer the overflow.

• by watershawl on 5/6/24, 1:58 PM

  This is a good solution (storing in rock) to get around the heat and corrosion (from water) that above-ground tanks go through when storing compressed air.

• by ourmandave on 5/6/24, 11:54 AM

  Well this changes the whole look and feel of future Bartertown entirely.

  They probably won't even have a Thunderdome. =(

• by foreigner on 5/6/24, 6:48 AM

  How does the air "push the water up"? What mechanism prevents the air from simply bubbling up through the water column? I'm assuming some kind of valve or piston, but would be interested to see what it looks like.

• by masteruvpuppetz on 5/6/24, 10:25 AM

  I found another technique quite fascinating.. When electricity is surplus, spin large disk-shaped rocks levitated by magnets in a vacuumed enclosure. Use this spinning motion to create electricity when required.

• by coryfklein on 5/6/24, 4:26 PM

  What happens when you get a leak in a random seal in the chamber deep under ground, does all the compressed air escape? How do you ensure such a large reservoir stays air-tight for 20 years?

• by ck2 on 5/6/24, 3:36 PM

  Why not lift super heavy but cheap objects like rocks or dirt and then let gravity be your battery?

• by psadri on 5/6/24, 3:37 PM

  While at it… they could try to extract some atmospheric CO2 from the more concentrated air.

• by jimnotgym on 5/6/24, 10:31 AM

  What stops the air from just bubbling up the water pipe?

• by 0xE1337DAD on 5/6/24, 9:51 PM

  Totho, has graduated from snap bows.