I like the idea of heat pipes.
I want to use a steam to move heat to move heat from an outside rocket stove to an in home water storage tank.
Black iron pipe, copper tubing or the corrugated stainless steel normally used for gas could be used.
In this application a heat pipe would be a lot like a column still with the top wrapped in a water jacket.
Before I try this in the house I will test it in my greenhouse.
The other idea I had involves geothermal.
I would like to drive a shallow well into the ground, then slide a heat pipe into the hole and fill in the gaps with mud.
To keep it cheap, I would want to use pvc or pex pipe.
This means we can't form a partial vacuum in the pipe by heat the working fluid till it evaporates, so I would use ethanol and a vacuum pump instead.
Ideally, I could do this cheap and easily enough that each 5 gallon cloche could have a heat pipe.