[dzhiurgis]

> you can't really do -600C sand (or anything)

You can if you stagger AC/HP or even peltier elements.

[myrmidon]

You misunderstood-- temperature is physically limited to -273°C, this is not an engineering problem. You have a smaller usable temperature range in a "cold storage" than with heat from fundamental physics alone.

[dzhiurgis]

Damn you got me there!