Or some other dispatchable low-carbon energy source, which also doesn't really exist at the moment. Load-following nuclear reactors (hooked to thermal storage) could do this physically but a lot has to happen to get the cost down. Meanwhile, various CCS options could play a role if there's a solution for safely storing vast quantities of carbon out of the atmosphere.

I don't know about its efficacy, but my favorite means of mass energy storage is a gravity battery: literally stacking heavy blocks in a huge tower, which are raised and lowered based on energy demands by automated cranes.

https://en.wikipedia.org/wiki/Energy_Vault

At best a 'gravity battery' is as efficient as hydroelectric, since pumped-storage is a gravity battery. Towers, trains, spinning flywheels are high-maintenance by comparison. The technology for pumped-storage requires a supply of water and a high place to pump it to. Used around the world (Europe & US, at least) since 1890s.

Bath County, VA: capacity 24GWh, since 1985

https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricit...

Dinorwig Power Station, Wales: capacity 9.1GWh

https://www.theregister.co.uk/2016/05/16/geeks_guide_electri...

Worldwide, today

https://en.wikipedia.org/wiki/List_of_pumped-storage_hydroel...

Hmm I'm not so sure about that: gravity is weak.

In some news article they say Energy Vault uses 35-ton blocks hoisted up to ~150 m - while it sounds impressive, that's only 14.3kWh (assuming 100% conversion efficiency), or about 1/7 of a single Tesla Model S (100kWh).

When I briefly looked into this, my takeaway was that the energy density (and thus price) is much, much lower than needed to compete with other storage mechanisms.

As another poster said, gravity is relatively weak compared to the other forces.

Yes, when a magnet holds up a nail, what you're seeing is that the force of the tiny magnet on the nail is much much higher than the gravitational force of the entire earth on the nail.

There are many nifty schemes like this, such as compressed air, cryogenics, hydrogen, thermal, different battery chemistries, etc. Efficacy in the form economic scalability is the crucial characteristic.

Seems like a novel take on the same principal behind pumped storage hydro systems. But, at a max of 80MWh of storage, you'd need hundreds of them to match a single pumped hydro facility, which often have 10,000+ MWh of storage.

Storage would be some combination of diurnal storage (batteries, say) and long term storage (hydrogen). The former benefits from high efficiency; the latter from lower capital cost. There is also thermal storage (an order of magnitude cheaper than batteries) for any application involving heating or cooling, including industrial users of heat.

Hydrogen... which can then be put back through a fuel cell to produce electrickery. The germans are looking at this. I believe.