A number of people replied to my question from last week. Thanks!
The list was not copied, so I'll summarize the responses:

Use EnergyPlus or TRNSYS to simulate earth-to-air heat exchangers.
Use the GAEA program - free trial download here:
http://nesa1.uni-siegen.de/index.htm?/softlab/gaea_e.htm
Designs must allow for automatic removal of condensation (such as by installing earth tubes at an angle of at least 2-3 degrees).
Earth tubes can provide a consistent 3-5?C temperature rise in winter preheating applications.
A list of references is attached.

Regards,
Bill