I've wondered if there could be a way to use the Earth's heat by drilling a small hole in your back yard a few km deep and somehow embedding a tiny generator down there to take advantage of the heat differential between various layers of rock. Maybe a Stirling engine? I'm thinking like a long thin tube filled with liquid that boils at low temps. The liquid would boil, expand, rise up through an insulated tube, condense at a cooler layer and circulate back down. A little generator could use the constant motion to power a house.
Deep holes are expensive. Oil drilling, the cheapest kind of drilling since it's very common and in soft rock, costs $100-$200 per vertical foot.* A 2 km hole would cost $650,000 - $1.3M. It would not be economical to do this for one house. Then there are operating costs: operating a power plant on the surface is very likely to be cheaper than operating one underground, and temperature difference is bigger. All in all, a traditional geothermal plant (utility scale, at the surface) looks like that for good reasons.
Residential geothermal heat pumps are commercially available. Generating electricity is possible but less practical. Using it to heat and cool your home without fuel is very doable.
