Lasted edited by Andrew Munsey, updated on June 15, 2016 at 1:08 am.
To build a ground source heat pump using common materials from the hardward store at low cost.
Utilize heat pipe technology commonly found in solar evacuated tubes. Use copper pipe threaded and capped at both ends for the heat pipe. The gravity fed evaporation/condensation cycle requires no motor or pump. Tubes can be placed in holes dug with a post hole auger. The fluid used in the pipe must boil at temperatures below 40 degrees F and should be non-toxic.
IEA Heat Pump Centre Newsletter - in pdf format. See page 6 CO2 Ground-Coupled Probe. A test of using carbon dioxide in a heat pipe as a vertical ground heat exchanger.
Chengdu Onergy Science and Technology - Onergy has invented Double-effective heat transfer technology enabling heat transfer in two ways: heat pipe and fine motion of water, and with high efficiency. It could get the utilization of solar at half the cost of using coal, making the Onergy's heat pipe in the geothermal application competitive. Onergy’s heat Pipe uses carbon dioxide as medium.