[Hippocrates_Garden]

So, basically, a thermal mass. It may look a bit different, but a technique used for centuries variously known as “thermal mass fireplace”, “Russian Fireplace”, or the more recent “rocket mass heaters”. essentially the same idea. Heat up a mass, then let the mass radiate the heat over time. Even “Passive solar” buildings use a version of this, substituting the sun as the heating source onto thermal masses inside the house during winter.