I have an idea that I haven't been able to find really discussed elsewhere online, so I thought I'd throw it out here.
Basically, instead of using earth tubes to directly condition a space, you would use it to feed cooled air to a 2-hose portable air conditioner.
Potential advantages:
1. Increased efficiency for the air conditioner
2. Eliminates mold concerns associated with earth tubes since heat exchange air and
3. Portable A/Cs are relatively inexpensive ~$400, can easily be upgraded/swapped out.
4. Earth tube design and materials could be simplified since mold isn't so much of a concern.
5. Simplicity/fail-safe nature of air as a fluid. No leaks, special fluids to worry about.
Potential Disadvantages
1. Alternative GSHP, or A/C's could be inherently more efficient with less work.
2. Static pressure in earth tubes, need for booster fans
3. Cost/work involved in excavation for tubes.
Questions:
1. How much of an efficiency gain would the air conditioner see from pre-cooled air?
2. What sort of depth of pipe would be necessary? (a/c only) and in what kind of climates?
3. Thermodynamic effect of condensation in the earth tube?
Cheers!