Depends on the size of the Thermos. The volume scales by the cube while surface-area (I.e. insulation) scales by square, which means it’s exponentially more efficient the larger it is.
Water (or coffee) might be more energy-dense (thanks to its phase-change to steam) which would make the coffee theoretically more efficient (although you’d ruin the coffee when you boil it) but handling fluids like boiled water is expensive (even just the pressure - water expands 1600x when boiling, that’s why steam engines work) where 500°C sand just isn’t. So the cost massively favours sand/rocks.
Is that more or less efficient than filling a giant thermos with coffee?
Depends on the size of the Thermos. The volume scales by the cube while surface-area (I.e. insulation) scales by square, which means it’s exponentially more efficient the larger it is.
Water (or coffee) might be more energy-dense (thanks to its phase-change to steam) which would make the coffee theoretically more efficient (although you’d ruin the coffee when you boil it) but handling fluids like boiled water is expensive (even just the pressure - water expands 1600x when boiling, that’s why steam engines work) where 500°C sand just isn’t. So the cost massively favours sand/rocks.