We put in a geothermal system (four 200 foot wells under our driveway) when we built our new house five years ago. The AC is essentially free.
Eric, I'm assuming AC means Air Conditioning.
In areas where freezing or snow occurs, a few loops under the driveway to bring the Earth's latent heat up to melt the snow would also be welcome.
I wonder if anyone has tried to put together a low-grade energy generation system utilizing a solar collector for the heat, and a geothermal loop for the cooling? While it would be somewhat weather dependent, the investment costs seem affordable enough to make it worth while for a small plant. Some heat and cooling would be secondary benefits of such a system.
Or to look at it another way, keeping your house cool in the Summertime, and your driveway free of snow and ice in the Wintertime, would be the primary benefits, and a steady stream of low-wattage energy for lights and the refrigerator would be secondary benefits.