Heat up molten salt, or shipping containers full of sand. It’s a surprisingly high density and cheap way to store an awful lot of energy. Don’t have sand batteries here yet, but they’re on my todo for deep storage of excess energy, which I currently just dump as heat into the air.
I don't know. Salt (NaCl) is corrosive. Specific heat capacity is not that high (about 1/5th of water per weight). Suppose you have cubic meter of molten salt at 800°C in a dewar, how do you get the heat out again?
