I think salt would be easier than water, mostly due to water expansion characteristics, but that’s just my opinion.
Viable solutions with sand or rock have been developed and I expect over the next few decades a large number of such projects will be produced.
The sand silo heating projects that I’ve read about are used to feed the excess energy from electricity to central heating. The heat can be stored for months, but converting it back to electricity wouldn’t be very efficient. It’s “only” viable in places with district heating.
There are other power-to-x technologies out there, like splitting water to hydrogen and oxygen, and these are all good ways to use excess energy, but they won’t help on stabilizing the electricity production. Hopefully these technologies can create enough demand for electricity that the prices will always be worth it for the producers, so they can begin (over)producing enough renewable energy to cover the baseload at all times.
The gas production in early 1900s sort of did the same. The gas was produced for heating and light, but the byproducts of gas production lead to all kinds of other very cheap chemicals and products. Similarly we need to think of excess electricity as a very cheap byproduct and invent uses for it, instead of attempting to “balance” it.
In my opinion, this shows why privatization of electricity production is an obstruction. It would have been easier to transition completely to renewable energy if it was a state monopoly setting a fixed price to enable overproduction without regard to price fluctuations.
Oh yeah,I’m no expert. I can see salt being problematic if the system sprung leaks and contaminated the soil which wouldnt be uncommon once you have tens of thousands of houses rigged up. Im pretty sure most water based systems just use water and antifreeze.
Point is that the fundamentals are simple, when theres excess electricity and nobody is home convert it into stored thermal energy that can be used later when people are home, the devils will be in the details.