First post here, let me know if i break some rules. Also i'm not an english native speaker.

I have a small electronic device that i need to keep above a certain operating temperature. The device works for 10 mins and rests for 6 hours approximately. I need a reaction to produce heat for a short time and i need the reaction mixture to go back to the initial state during the 6 hours of "sleep". This would make for a good 0-energy-cost heater.

The first thing that came to my mind is the reaction used in hand warmer: a supersaturated solution of sodium acetate crystallizes and gives out energy. This is perfect BUT a hand warmer can be reused only after being boiled. I need something that "regenerates" without human intervention, with just time and solar rays basically.

Does such a reaction exist? Thank you!