Engineering360: "How thermal energy storage is enriched molten salts"

Molten salt heat storage is a mature technology, as it is used to recover heat from metal processing, and can find a new application as renewable electricity surpluses become more frequent, as for wind energy in Ireland. I wonder how it compares in cost with "sand batteries". The capacity of either to produce electricity from steam turbines could be the deciding factor in any comparison with batteries and longer duration flow batteries?