There are a bunch of other technologies as well. Huge flywheels [1], which are supposed to be used for very responsive power needs (i.e. grid stabilisation), trains loaded with weights which get pulled up a mountain and released [2], redox-flow batteries and so on.

[1]: https://www.energydigital.com/smart-energy/worlds-largest-fl... [2]: https://aresnorthamerica.com/

Also molten salt:

https://www.solarthermalworld.org/news/molten-salt-storage-3...

People talk about making "personal" molten salt units for the home, which seems a little terrifying.

I wonder what the local zoning commission has to say about the guy building a personal toxic metal death ray in his backyard?

That is why I will never live in a HOA neighborhood.

The zoning commission runs at the town/country level, so even if you don't have HOA busybodies you still need to get the permits. The law is usually worded such that if you're doing something abnormal you don't get a "there is no rule that says a dog can't play baseball" loophole, you instead have to bring it up before the county council and justify the project.

Seems the train idea is similar to the old counterbalance system for trollies in Seattle.

Hydro pump storage works because you don't have to pay for the weights. Building a train is worse than batteries in terms of economics.

Usually when you see compressed air storage solutions they're talking about plugging up an old mineshaft or cave and pumping air in.

Doing it under water would seem to have some advantages, like a less catastrophic failure condition and having a ready place to sink all of the heat. Downside is that the water would cool the air much more effectively than solid rock so you lose efficiency.

Are there any combination solar, hydro, fracking/oil plants? I'm joking but not entirely. Not sure how much water would fit in an oil field but it seems like a natural reservoir one could use.

The trick would probably be getting your energy back after you've pumped the old oilfield full of water. Most oil fields aren't gushers, especially after they've had most of the oil sucked out of them.

A lot of these storage technologies have been around for a long time (I have a "Nature" magazine from the 60s that discusses storing excess energy as wind in salt mines and in lakes. This was the 60!).

What people underestimate is just how much energy we need to store. Lakes won't do it and neither will salt mines. Gravity certainly won't do it. We need to think much BIGGER.