[WalterBright]

Thanks for the answer. I suppose the answer is a decent insulation on the box. It only has to last for a few hours.

Of course, some engineering would have to go into optimizing the size of the rocks and the surface area / mass for the expected use.

[lazide]

The other issue is ‘heat quality’ (or temperature deltas). It’s easier to recover a given amount of heat from very hot sources than from only luke warm sources, and smaller heat stores will have a very pronounced difference very quickly, resulting in control issues.

Basically, if you only have a small amount of sand, it’s harder to keep it warm for longer (more surface area for the mass, so more insulation required), and harder to deal with effectively (fan needs to blow much harder when it’s only warm, much less when it’s hot), etc.

[mtrycz2]

So like, sand?