Crystallization of sodium acetate - GIF

[nova_robotics]

A yes, like those "instant heat" packs you can get got cold weather activities etc. A plastic pouch with liquid "stuff" in it. You click s little disk that is inside the pack, and it solidifies, giving off heat to warm your hands etc.

Yup, that's them. I've been wanting to build a thermal battery for my house for ages. I have geothermal and evacuated tube solar to produce heat. But the solar is intermittent, and I live in the frozen winter hellscape of Canada. You can use bricks or water to store energy, but that's linear in its cooling. The temperature goes down as they lose heat energy. So there's almost no heat coming out of the stupid thing by morning. Thermal batteries are not linear. They maintain a roughly constant high temperature until they're pretty much depleted, then the temperature crashes. This is way more useful. Both parafin and sodium acetate are interesting candidates for a thermal battery.