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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?

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The short answer is that pumped hydro is mature technology with a pretty wide range of places it can be implemented (at least in Australia[0], which has a mountain range going down the east coast where everyone lives). A-CAES has three advantages, which are in my opinion aren't very fundamental:

1. It takes up less space on the surface than most PHES. This... is almost always a marginal benefit.

2. It doesn't require building a reservoir or dam. These are very well regulated in Australia and elsewhere, and the downsides are known so they are quite slow to get approval.

3. It's a bit quicker at 2.5 - 3.5 years as opposed to 3-7 years.[1] This is a bigger advantage than it looks if you have some tricky renewable energy targets to hit by 2030 (see our 42% emissions reductions target as well as an 82% renewable energy target)

I can't see this gaining traction outside of a few locations in Australia, at least. I wouldn't be surprised if A-CAES is only briefly viable as a result of subsidies and cheap government financing.

[0]: https://re100.anu.edu.au/#share=g-fa5a20c9c63f6ed6343a7e7573...

[1]: https://www.csiro.au/-/media/Do-Business/Files/Futures/23-00...

> with a pretty wide range of places it can be implemented

How so? Yes, if you dammed up most valleys you could build a lot more capacity than humanity currently has, but that's because we don't really have that much. A factor of two or so might be well within range. But the total amount reasonably buildable simply isn't enough, except for some very local scopes where demand is low in both energy and in other use for the landscape that would be taken over by reservoirs. And that's before you start considering the geological realities required for actually building a dam, you don't just need the geometric shape of a valley, you need the bedrock to brace the dam against and the impermeability of the ground required to not have leakage wash out pathways for ever increasing leakage. Viable sites for pumped hydro are extremely rare and quite a few have actually been given up decades after building the dam, because the geology wasn't quite as cooperative as hoped.

The promise of deep site water head CAES is that you can just throw money at the problem (excavation) and get as much capacity as you want to buy. The price per capacity is higher than that of a low hanging fruit pumped hydro site, but many of those buildable have already been built.

Let's not forget the extreme environmental impact a dam has.

Those are for dams on rivers. PHES doesn't have to be on rivers.

The sources I've seen point out plenty of places. They may be overoptimistic. But excavation is really expensive, and in a competitive market, you cannot just throw money at the problem. It's the same reason nuclear fission is dead in many markets.

Australia loves excavation. If you could double up mining with pumped hydro you'd be the belle of the ball.

It could make a nice option for huge abandoned open cast mines - build a dam across the pit, flood it and then pump water from one side to the other using solar-generated electricity, allowing it to flow back to generate electricity as and when needed.

For example: https://www.news.com.au/technology/environment/abandoned-min...

A big problem is those mines usually leave behind pretty toxic remains so the water you're pumping around is extremely hostile to the people and equipment you're thinking of putting it through/near. Then there's the chance of a dam collapse releasing that toxic water outside the mine.

I've thought for a while that old open-cut iron ore pits could be prime spots for 'inside-out' pumped hydro, like that system that some German researchers were working which pumped water out of submerged concrete spheres instead of up a mountain.

If you have abandoned deep shafts, they may be smaller in volume than an open pit, but they have several hundred metres of head, so every kilogram of water is much more effective than in schemes with less head.

Most viable hydro sites are gone, yes. But you don't need a viable hydro site for pumped hydro. What you need is water at the bottom of a hill. That's common.

If you just have water at the bottom of the hill you'll need to build a giant tank on the top of the hill. The best spots for pumped hydro have a natural depression elevated above a water source, so you can use that instead.

(I agree that this is different from traditional hydro locations, and so there are many good spots still available)

The best spots already have a reservoir up top, or most of one. Pumped hydro is so much cheaper than other long term storage options you can spend a little extra on the reservoir and still come out ahead of competitors.

50 year old example: https://en.wikipedia.org/wiki/Ludington_Pumped_Storage_Power...

Most primary hydro sites are gone. Pumped hydro can work in a much wider variety of places, including in deserts.

https://www.whitepinepumpedstorage.com/

Pumped hydro won't work unless you have a suitable mountain to hand. Compressed air can be built in flat country.

I'd expand on your point (2) with these points.

1. Dams require water. Australia has suffered water supply challenges as long as I've been alive.

2. Dams can be environmental disasters. Both building, maintaining and one day destroying these has a lot of challenges and expenses that we're not great at measuring.

Not having to build and maintain a dam and find an appropriate area to flood is a pretty large upside. Big thing in the US is that we already have a lot of dammed lakes in mountains in some places so we're really talking about making the water level more variable and adding some infrastructure instead of building net new lakes.

Pumped hydro consumes much less water than evaporative cooling of a thermal power plant with the same energy throughput. Pumped hydro doesn't have to be on an existing watercourse, so it doesn't cause the environmental issues that come along with that.

I think you can store energy here not just in the gravitational potential energy of the water, but also in the compression of the air. I _think_ this means that you can get away with a smaller cavern than you could for just pumped hydro.

You might think that you'd need a large amount of water to make the energy release work, but I think it works like this. The force of the water on the air/water interface is dependent not on the reservoir volume, but on the weight of the water in the column (which depends only on the height of the shaft).

By digging a very deep shaft, you can have a very large force of water on the interface, and moving that interface an equal distance hence releases more energy than it would with a shorter shaft.

This way, you can store an arbitrary (up to your ability to compress air to a sufficient pressure, dig a deep shaft, and keep everything from blowing up) amount of energy.

I think if you have a 1 square meter cross section of shaft, and the shaft is 1 kilometer deep, then at the bottom the force of the water above is the weight of 1 square meter * 1 kilometer, or 1000 cubic meters of water, or 1 million kg.

The force then is 9.8×10^6 newtons.

Pressure is force/area, or 9.8×10^6 newtons/square meter here (since we have a unit area).

There's a formula for the energy in compressed air, it's... involved. I downloaded an excel file from here: https://ehs.berkeley.edu/publications/calculating-stored-ene...

It says under this pressure, a 1000 cubic meter tank of air at this pressure stores ~5000 KWh.

The one planned in California is supposed to store 4000 MWh, so I guess they have a tank that is ~a million cubic meters.

A water tank of the same size would store ~2700 MWh of energy (e.g., pumping that much water up a 1KM shaft requires that much energy), so it does seem to be more efficient.

It may also be that hot air turbines are cheaper and easier to maintain than hydropower turbines, but I'm not certain.

>I think you can store energy here not just in the gravitational potential energy of the water, but also in the compression of the air. I _think_ this means that you can get away with a smaller cavern than you could for just pumped hydro.

+ they are storing the heat extracted from compressing the air.

isn't that used when decompressing the air, i.e. closed loop

You just pull that from the atmosphere.

Compress to store [potential] energy, take the heat generated during compression and use it, decompress at time of need and let the [hot?] atmosphere supply energy to the decompressing gas.

I wouldn’t call it a closed loop, but maybe a lack of deficit.

I want to know how this heat storage works. It seems kind of important?

also water tend to be scarse nowaday and valuable nowaday.

they say, writing from an ocean planet

Salt water is a nightmare to include in any sort of mechanical system. It is super corrosive itself, enables galvanic corrosion, and is so “fertile” biologic fouling is a big issue.

You don't want to put salt water in the ground. It will destroy either the soil or the aquifer.

> The force of the water on the air/water interface is dependent not on the reservoir volume, but on the weight of the water in the column (which depends only on the height of the shaft).

Just wanted to highlight this, since it's the key insight which caused this to "click" for me.

The difference between a 1km shaft and a 1km deep reservoir, when both are used for pumped hydro, is the amount of energy stored. Running a turbine at the bottom of the shaft (to where?!) would drain very quickly. But using air, which has very different properties, enables you to use the water as a "piston" to keep the air at a certain constant pressure underground. And you can store a larger volume of (more compressible) air in a space which is easier to access.

Is there any pump efficiency advantage to multiple pressure vessels instead of one large?

You could probably move the compressed air in a GPE gravitational potential energy storage system, or haul it up on a winch and set it on a shelf; but would the lateral vectors due to thrust from predictable leakage change the safety liabilities?

Air with extra CO2 is less of an accelerant, but at what concentration of CO2 does the facility need air tanks for hazard procedures?

FWIU, you can also get energy from a CO2 gradient: "Proof-of-concept nanogenerator turns CO₂ into sustainable power" (2024) https://news.ycombinator.com/item?id=40079784

And, CO2 + Lignin => Better than plastic; "CO2 and Lignin-Based Sustainable Polymers with Closed-Loop Chemical Recycling" (2024) https://news.ycombinator.com/item?id=40079540

From "Oxxcu, converting CO₂ into fuels, chemicals and plastics" https://news.ycombinator.com/item?id=39111825 :

> "Solar energy can now be stored for up to 18 years [with the Szilard-Chalmers MOST process], say scientists"

> [...] Though, you could do CAES with captured CO2 and it would be less of an accelerant than standard compressed air. How many CO2 fire extinguishers can be filled and shipped off-site per day?

> Can CO2 can be made into QA'd [graphene] air filters for [onsite] [flue] capture?

"Geothermal may beat batteries for energy storage" (2022) https://news.ycombinator.com/item?id=33288586 :

> FWIU China has the first 100MW CAES plant; and it uses some external energy - not a trompe or geothermal (?) - to help compress air on a FWIU currently ~one-floor facility.

Did you mean 5000KWh (air) and 2700KWh (water), or MWh for the second?

Also, 1000m3 in air is just a cube of 10m. It seems like a good illustration of the convenience of air vs water.

The same size here is a million cubic meters, what I figure the size of the planned California plant must be in order to store 4000MWH.

The water's not there to directly store gravitational potential energy, it's there to provide a variable sized containment vessel under nearly constant pressure conditions. This has two advantages:

1) You're not losing thermal energy from the main body of air (assuming their thermal recovery at the compressor works well enough) because the change in air pressure only occurs at the compressor.

2) You get constant pressure and constant power output for the entire volume of your compressed air storage, instead of both dropping rapidly as air is released. This gives you far better bang for buck per unit volume of pressurized air.

I haven't done the calcs but I'd guess that if they're bothering doing all this extra stuff the energy stored in a cubic meter of compressed air at these pressures is significantly higher than the energy you'd get from just lowering a cubic meter of water down to the reservoir.

Compressed air and water head are in a balance, my physics instincts strongly suggest that the hypothetical sum is 2x the amount of energy stored by raising the water alone. But just like you I do hope that my instincts are missing something!

(see nephew comment https://news.ycombinator.com/item?id=40273030 )

Hmm.. the more I think about it the more I agree with your intuition. If the air's at constant pressure then the work it's doing as it exits the chamber is coming directly from the water entering the chamber. There's no difference (in terms of energy) between having a turbine at the bottom of the column of water vs. having the water push the air and then the air spin a turbine. The whole thing seems like they started with "let's do compressed air energy storage" and realised half way through that the best way to do compressed air energy storage is for it to actually be pumped hydro.

It's more capacity than just pumped hydro of identical head x volume: even if we ignore the "compressed spring" of the air involved, charging would require at least the energy for lifting the water plus the energy for heating up their heat source. If you close a valve in the water connection, then pop open the air pressure vessel letting all energy still in the compressed gas you to waste, you'd still have the heat storage, and you could let the water rush into the lower reservoir through a turbine, harvesting the entire energy contained in the water (minus inefficiency). So it's definitely more, the heat and the gas discharge we just wasted.

The question is how much more, whether the symmetry suggested by the balanced state implies 2x energy storage or not.

Another mental model to further separate the two storage media (water lift and air compression) is this: you could charge separately, first raising the water with a pump, with the dry cavity at ambient pressure via an open connection to daylight, then seal the cavity an charge it as a compression vessel in the non-isobaric way. I still can't decide between trusting the "balance implies 2x" intuition or not, but it should be easy enough to calculate. At least the thought experiments show that it can't be 1x the energy stored by lifting water alone.

I consider starting with "let's do compressed air energy storage" as a given, the "do it by pumped hydro" does not come into play because pressure vessels are hard, it comes into play because building compressors and turbines that have good efficiency over a wide range of pressure is hard.

You need to get replacement air to the cavern when you pump the water up, and let the air to make room for the water when it's going down. So you need a separate shaft for airflow anyway. I assume the rest is just economics, it's probably a lot cheaper to have all the moving parts easily accessible on the surface.

Edit: Also the fact, that if the pump was at the very bottom and it failed, you'd need to have an alternative way to clear the cavern of water in order to go down and fix the pump.

Put the pump on a chain and hoist it up with a crane, like they do with pumps in lift stations.

The australian outback is not exactly known for its hills and mountains...

Pumped hydro done on a large scale needs a reservoir that is at a considerable elevation gain.

example: https://en.wikipedia.org/wiki/Taum_Sauk_Hydroelectric_Power_...

Have you heard of Snowy 2.0? Massive failure. Pumped storage in AU.

I have not - why is it a massive failure?

They've had problems with tunneling, I believe.

Exactly, they used the wrong kind of TBM to match soil conditions, and land owners are furious over the transmission line easements. Mostly rural property.

I think your answer is the correct one, rather than the replies. By keeping all the machinery at the top, initial build and maintenance costs are cheaper, they can use a cavern of any shape and at any depth, and they only need to use drilling techniques not mining techniques, which will be far cheaper .

I suspect the maximum stored energy for the compressed air solution is significantly higher.

Not only can you store the energy required to lift the water to the surface, but after all the water is lifted you can keep compressing the air in the closed reservoir to store even more energy.

If you only used water you could of course build an open reservoir with more storage capacity instead. But maybe these will be built in areas where that’s not feasible. It’d take much more space on the surface, and you need to deal with evaporation.

I do wonder, regarding the hydrostor approach, if the stored energy (theoretical best, excluding all losses) is 2x the amount that would be stored if it was just the water head (mass x height), with air displacement served by an open connection to the surface, or if there can be more to it:

Constant volume CAES would store energy without any water lift involved, and with water mediated constant pressure CAES the lifted water is added to the amount of energy contained in the "air spring". But that's balanced at equal force in the force x surface area x underground reservoir level system. My bird's eye view understanding suggests that it would come down to 2x the amount of energy stored in either (minus losses, of course) due to the balance. Is that the maximum energy a water-column mediated constant pressure A-CAES could hold, per water displacement volume x head height, or am I missing something? That 2x would surely be an improvement over conventional mineshaft pumped hydro, but it would also define a somewhat sobering limit to the amount of theoretical best case capacity.

Minor but perhaps very relevant detail: afaik, or rather as far as I don't know, hydrostore aims at a shaft depth equivalent to a pressure either right below of where pure CO2 would liquefy, or right above where that happens (no idea about the boiling points of N2, O2 and all those other main components of ambient air). I think the target depth is not quite that deep, as in carefully avoiding the "transliquid" range (as in transonic).

A solution to deep water pumping is to lower the pump(s) into boreholes. Nobody has to go down there.

Yes this is normal even in normal residential wells. Boreholes are just a few inches across so obviously nobody ever goes down them!

I have an injector pump that sits at the top of the borehole and has a pipe that pushes water down to pump water up through a second pipe, but it's more common to have a submersible pump down at the bottom of the bore.

I've wondered how well the inject-pump style works! Do you have experiences to relate? All the electronics are up top where you can get at them. Has the mechanism at the bottom ever needed to be serviced? Can it be retrieved easily?

You can pull the parts out of the hole anytime you want to - special equipment is normally used, but a rope and a tripod to hold a pulley over the hole works (might not be safe).

What is at the bottom of the hole is a "injector" which is basically a U shaped pipe and a small jet to push water back up. If the well water is only 25 feet below the ground a pump at the top along works. This jet system gets down to 60 feet. The pump down the hole gets to 600 feet (check the pump specs - many are rated to only 250). After that you need oil well type pumps where the motor is at the top of the hole but the pump is lowered down.

(I can't find any English lit on it, but my Grundfos Ejektorpump (had to go out and look at it to see what it's called; perhaps the correct translation is ejector pump rather than injector pump?) is in a 85m deep borehole and works great. It's 40 years old, heavily used and never serviced and quite a puzzle how it's still going. I have no idea if they can pump from deeper than that)

Some designs might have no moving parts down there? Nothing to go wrong.

You made me more curious to find out about it.

https://www.vvsbutiken.nu/product.html/ejektorer-grundfos?ca...

Two normal plastic water pipes descend down to the bottom of the borehole. At the bottom is a brass u-shaped fitting with a inlet with a non-return valve.

Most places don't have a suitable mountain where you can destroy the natural beauty with a dam.

In contrast, there is underground everywhere on Earth.

Yeah, I have to wonder how it compares to pumped hydro operationally too. They talk about capital costs but didn't explore what opex would be like at all. I would think with heat, air and water (as opposed to water in pumped hydro), maintenance and operations will be more expensive.

I know of some pumped hydro sites that have been given up due to opex, with no clear change of mind due to changing circumstances in the decarbonisation age yet in sight. What they do have in common, I think, is low head. There's just a lot of water to carry traces of sediment into the mechanism, and so much reservoir ground to keep from leaking for a given amount of capacity.

But you don't arrive at isobaric storage looking at mineshaft pumped hydro wondering if it could be improved upon. You start looking at constant volume (A-)CAES, either in high pressure tanks at/near the surface or in salt caverns, and go from there, what of we did not have to deal with a very wide range of operation pressure.

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