They try really hard not to explain how the system works. It doesn’t mention it at all in this news story, or anywhere easy to find on their site. I crunched some numbers, and it seems like this device has about double the heat capacity for the mass than just plain water would have. That means it probably has to be a phase change material. All of the “smarts” and app control is nice, but at its core, it’s really simple concept. I think this kind of thing would sell better if instead of trying to make it sound high-tech, they just admitted it was a low tech solution that they’ve made user friendly.

I did find this mention in the site q and a:

Q: What is the storage medium made of?

A: Proprietary mix of high-density, inert, non-toxic and low impact materials