Realistic designs for this can achieve 450C delta t. Your energy store can store at ~1000C (slightly below rock melting point), and run steam turbines with a hot side of 600C and a cool side of 150C.

Over the season, your sand drops from ~1000 to ~600 (which is fine since your power needs drop in the spring)

Sorry, but 1000C is wildly unrealistic. There are no bulk materials that can be used to construct heat exchangers for this kind of environment. 600C is already a stretch.

At 1000C steel will react with _water_.

To be clear, I'm not talking about cooling a small crucible but a structure that is at least 20 meters in size.