[hydrox24]

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...

[usrusr]

> 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.

[gregoryl]

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

[pfdietz]

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

[bryanlarsen]

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.

[jefftk]

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)

[bryanlarsen]

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...

[pfdietz]

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

https://www.whitepinepumpedstorage.com/

[olau]

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.

[Intermernet]

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

[taneq]

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.

[psd1]

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.

[globalise83]

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...

[rtkwe]

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.

[verelo]

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.

[rtkwe]

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.

[pfdietz]

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.

[denton-scratch]

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