Popular Science featured an article a few years ago about someone that developed a much more compact and therefore affordable residential GeoThermal energy installation:
Instead of having to put long lengths of small diameter tubing through a large area of (relatively shallow) ground and the significant costs associated with it, he developed a system that instead uses a large deep pipe. Think of a well 3'~8' in diameter and 20'~80' long sunk in below the center of your basement. Within this another smaller diameter pipe is located. The inner pipe is almost as big across as the outer pipe or 'well'. A convection current starts within the liquid in the system: as the fluid warms around the outside of the base, it gets less dense and starts to rise between the two pipe walls. Thus there is a natural circulation of water within the well. It gets and stays hotter at the top, and then drops back down the middle of the inner pipe as it cools again. An appreciable amount of energy can be harvested and stored given the large (thermal) mass of the fluid and the Earth that it is coupled to. Down there the temperature of the Earth is stable and warm. Not to hot, not to cold ( 8 This system lends it self to either a closed or open cycle, and even a combination of both. It gives an excellent and free constant heat sink for a heat pump. The warm water can be used for both radiative heating and consumption as well. The inner pipe could be made out of concrete mixed with polystyrene beads added to it to give it some insulation and improve the heat differential, convection and efficiency. Perhaps the inner pipe could just be a cast or molded polystyrene pipe by itself. Insulating the top portion of the well might allow for higher temperatures to be achieved. Though I'm unsure if just leaving the top thermally coupled to the earth and the basement floor wouldn't be as good or better. A bonus would be a basement floor with some built in passive heating.
This one is a winner, on any scale.
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