You are talking about a Ground source heat pump. Once you are down about 10m the temperature of the ground is fairly constant at about 10-14 degrees C. You can exploit this temperature difference for either heating or cooling using a heat pump.
Indeed though the ones I've read about uses some type of liquid coolant for heat/cool interchange. Just curious if circulating just air through the piping would suffice as a poor-mans implementation thus reducing the need for the above ground exchanger.
