Sand batteries are pretty well known in the passive solar greenhouse world. Even in cold northern climates they’ve been proven to store enough heat for year round use to keep tropical fruits producing.
I have used a spare (older) Kelly Kettle as a sand battery while camping up some damp mountains in my neighborhood (Austria) and it has been consistently terrific to charge it up, and bring it inside to heat up the tent. The other (newer) Kelly Kettle serves boiling water purposes too, though, of course .. but otherwise the sand-kettle is real convenient, and easy to recharge as well ..
I do the same thing on canoe/rafting trips as well. Load my metal cook pot full of sand and stick it on the edge of the fire once we’ve finished dinner. Keeping it under the tent fly is enough to keep things a little more toasty all night. Especially nice with how chilly it can get camping next to water.
