Energy storage by batteries - or compressed air?

I've been thinking about renewable energy storage using compressed air.  As you do.

I started looking at the topic because of a recent article in the normally techno optimist MIT Technology Review which talks about the limited role, due to expense, that massive lithium ion battery banks can play in providing large scale grid storage.   Sure, they have their place in providing short term power when needed - as in the South Australian Tesla battery case - and the article doesn't argue against their effectiveness in that role.  But it argues that for very large scale storage as you increase renewable energy generation, they are just going to be too expensive.

(It doesn't talk about the benefits of household lithium ion batteries, but that is a different issue, even if important in its own right.)

Bill Gates and many others are looking into alternative forms of grid batteries, and we hear of potential new flow batteries and such like, but it seems that there is some way to go in terms of cost.

Which made me think - how is the idea of compressed air storage holding up?

There seem to be various companies promoting their ideas for compressed air energy storage, but the fundamental issue appears to be - where to store the air?   Many companies are suggesting underground storage, perhaps in salt caverns or former natural gas wells.   But this seems a pretty limiting idea as far as siting is concerned.

However, one idea from Canada by a company called Hydrostor has caught my eye as a clever proposal:  store compressed air in deep enough water in bladders that take advantage of the surrounding water pressure:

The concept is simple enough: When the energy bag is anchored underwater—at least 25 meters deep and ideally 100 meters or more—the weight of the water naturally pressurizes the air, allowing more air, and thus energy, to be stored in a given volume. (The pressure increases roughly 1 atmosphere, or about 100,000 pascals, every 10 meters.) At depths greater than 500 meters, says Garvey, “the cost of the containment becomes negligible compared with the costs of the power-conversion machinery.”

In the Toronto system, the bags (or “flexible accumulators,” as Hydrostor calls them) will be deployed at a depth of 80 meters, and they should be able to supply about a megawatt of electricity for 3 hours or so. The company will also be testing fixed-wall accumulators, in which the compressed air will displace water inside the vessel. “This is the smallest size we would contemplate,” says VanWalleghem. A more typical capacity, he says, would be 20 to 30 megawatts that can be discharged over 10 to 20 hours. Eventually, the company will aim for an efficiency of about 60 to 70 percent. The technology easily scales up, he adds. “We just make the air cavity bigger, so there really is no upper limit.” By year’s end, the company plans to build a bigger and deeper underwater energy storage facility in Aruba.

In an interview, the President of the company goes into more detail about the depth at which this should work best (my bold):

Cameron Lewis: We have an interesting twist on it because we do underwater CAES. For the roundtrip that we do, we’ll take electricity and run it through a specialised compressor, and we capture and store the heat generated out of that compression. We’ll add that back in later and increase our efficiency. So we store the heat and then the air is sent underwater to depths of 80m, 100m, 200m and put into flexible accumulators. You could say that they look an awful lot like a hot air balloon – the balloon will expand and hold the air there. So just like traditional underground fixed-wall caverns do, we store the air at pressure, but the pressure is a result of the depth. Now, when we reverse the flow, the accumulator will collapse and it will push the air back to the surface at pressure, and we will then add back in the heat that we’ve stored. We then run it back through a generator and put the power back into the grid. We get about a 70% roundtrip efficiency on this, but without needing to use natural gas and with several benefits. You’re dealing with an underwater environment so it can be a bit tricky at depths like that, but the advantage is that you get a very low cost cavern in which to store the air. The other advantage is that unlike a fixed-wall cavern, you get out every drop of energy that you put in, because it’s not a ramp up power curve.

Matthew Wright: So is the material for the accumulator – a buoyant bag, or whatever – something special that needs to be able to handle pressure or is it just the water pressure that’s holding all the air in?

Cameron Lewis: It’s the water that’s doing it. When we pump the air down, it’s at the same pressure that you would find hydrostatically that you’re at. When you look at the fabric that the accumulator is made of, it doesn’t hold much pressure at all – maybe one or two psi.

Matthew Wright: I noted that on your website you’re talking about an example that’s at a depth of around 80m, I think that’s about 1 atm per 10 m. What is the minimum depth at which you can operate? Some of the bays around cities in Australia are not that deep.

Cameron Lewis: The minimum is about 60m, but the range is roughly between 60-500m in depth. In this case, what depth really affects is the cost. The deeper you go, the cheaper it becomes. The reason is that you hold more power per cubic meter at a higher pressure at a greater depth than you do at a lower depth. At a lower depth, you’ll need many more cubic meters to hold the same amount of energy as you do at a greater depth.

Well, there's a problem - how far off, say, Brisbane or Sydney do you need to go to get to water more than 60 m deep?   Let me Google that for you.  The images below from this website show depth contours of 20, 40, and 100 m:

It would seem that for both of these cities, there are points of land where it would be under 10 km to get to 100 m depth (and of course it would be less if working at 80 m).

I wonder - does having a compressed air pipe 10 km long possibly work, or introduce its own inefficiencies?   I don't know the answer to that, but it is the only way it would work unless you get wind turbines out to sea at such distance - which then has the issue of getting the power back to land across 10 km.  :(    (Incidentally, I see there is talk of using floating wind turbines that don't need to sunk into the sea bed, and could work out to sea scores of km from land.  But to use the benefit of compressed air storage, you need a regular turbine too.)

Anyway, apart from getting your spare renewable energy from wind turbines, there is always solar, as long as it is coastal.

It's not as simple as I would like, but still,  the idea of using water pressure to do a lot of the work is clever.  We just need deeper water nearby...