Pumpless Geothermal Cooling

[edited to simplify design of, and cooling of, the geothermal field]

Design and build an in-wall radiant cooling system, with good "upward drainage", so that if any gas enters the system, it can rise to the highest point, without being trapped anywhere by the shape of the piping.

Design and build a geothermal loop. No special properties will be necessary, except that it will need to be constructed of materials that won't collapse due to the sub-atmospheric pressure of the liquid within.

Place several heat pipes in the geothermal field, leading upwards to the surface, and ending above the frost line. The working fluid should be some substance whose freezing point is below typical winter time lows... ethanol would be a good choice.

Add a gas/liquid separator, and a direct contact condenser, as would be used in a solar bubble pump water heating system.

The radiant system, geothermal system, separator, and condenser are all connected in a manner directly analogous to the solar bubble pump -- the radiant system corresponds to the solar collector, and the geothermal system corresponds to the hot water heat exchanger.

When cooling is needed, a thermostatic valve allows cool liquid from the condenser into the radiant system. The combination of low pressure in the system and the heat of the house causes the liquid to boil. The pipe diameter is selected so that the boiling produces a vapor pump effect, moving liquid to the gas/liquid separator. The liquid from the separator descends (via gravity) into the ground heat exchanger, and back up into the condenser (whose height is below the separator). Gaseous working fluid from the separator descends into the condenser, joins with the cool liquid from the ground heat exchanger, and flows back into the radiant panels.

Separate rooms can, of course, have separate thermostats.

Whenever the temperature of the ground near the tops of the heat pipes is cooler than the temperature of the ground near the bottom of the heat pipes, the heat pipe working fluid condenses at the top of each pipe, drips downwards, and evaporates at the bottom, transferring heat upwards, and cooling the geothermal field. This heat transfer is entirely passive, and unidirectional (upwards).