> Where you can win is if you can get a phase change somewhere
> in your temperature range. You can store 10 or 20 times more
> energy in a solid-liquid or liquid-gas phase change than in the
> whole temperature swing between  0 & 100 degrees C.

At the link I provided they are using solid Sodium Sulphate. This
melts during the day and releases its heat at night

The story with the greenhouse heater was that you'd use the fully
hydrated Sodium Sulphate (Na2SO4.10H20 - not an uncommon
salt), not the anhydrous form. This has obviously been looked into
by whover it was I got the suggestion from, and shown to give
better results.

For such a small box, I think some chemical heat storage is
worth a crack. If the circuit is also giving off some heat that
will help

What's the possibility of using a one-way mirror to collect
heat through that will act as a (mediocre) insulator by internal
reflection at night, like a Thermos flask ?

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