The device is known as a rocket mass hot water heater and they can be made very cheap as you can see in the video. The only cost was about 50$ worth of copper pipe. The sand and clay mixture was about 50% clay/mud and 50 sand, for most of the build. Some people also call these things rocket stoves. One thing's for sure, this primitive technology is a cheap and effective way to get off grid hot water for free. The ancient knowledge we are taking advantage of in this case is something called thermo siphoning. Some know a similar design known as a cob oven.
Watch the DIY Rocket Mass Off grid Hot Water Heater video