I came across an interesting video on Cheap Geo Thermal Air Conditioning.
The idea is you use antifreeze in buried 1inch water pipes, a pump, a car radiator and a fan.
It's an alternative to air filled pipes and a heat exchange.
No doubt it is significantly cheaper. But could it work? What are the draw backs? If it's so good, why can't I find any examples? I guess I don't know the correct word for the tech and there's probably lots of information out there.
Ground loop systems are well known. Usually they are paired with a heat pump that provides heating and cooling. The radiator and fan setup will work to cool a small room or tiny house. I am highly skeptical of its performance for greenhouse heating.
So yes, this can work. The question is capacity -- how many BTUs/kJ the system can move. That's where it gets complicated. Subsoil temperature, flow rate, fluid type, and the length of buried piping need to be matched to the need. It's a complex calculation.
If the system is too small, it won't perform worth a damn.
Memory is fuzzy, but I recall that heating my house with a ground loop and heat pump would have required something like a kilometer of buried piping. I can't swear to that; others here can hopefully chime in.
The other thing is redundancy. If I was disturbing that much ground, I would likely include spare pipe in case a failure occurred in the active pipe.
Small room or house sounds perfect. I'm renting and my options for 2000 sq feet is on or off. I can't cool one room. I can't do anything about it here but good to know it's a potential option in the future.
As for greenhouse, I'm guessing stopping frost is the primary objective.