The idea is actually not new. Greenhouses have been running underground air loops for a decade at least. I first found the idea online and then in a solar greenhouse building book called "The Year-Round Solar Greenhouse: How to Design and Build a Net-Zero Energy Greenhouse" by Lindsey Schiller:
https://www.amazon.com/gp/product/0865718245
He has examples of it up and running. There are a couple of YouTube videos out there that likewise show private and commercial cooling projects based on direct earth cooling of air.
As far as bacteria, viruses and mold - that's why the pipes are to be laid out on a grade so condensation can run down into the ground trap. You're right though, it needs to be cleaned or disinfected to avoid problems over years of use. There are some AC units that use chlorine or bromine in their drain pans but there are other detergents that can be used too. Basically, the system would need to be dried out and flushed once a year when it's not being used (winter).
As far as cost- it's the rental on the digging equipment and the cost of PVC pipe and the blower fans. Estimate is ~ $400 for PVC, $150 for the fan and $950 for the excavation. So $1500. But the cost over time is primarily the fan power usage at 200W.
Since I'm renting the excavation equipment anyway to install the water cooling coils, it's not entirely fair to count all the cost against the air coils. Also need to trench for power, water, etc... from the house to the sunroom.
But it's not free, you're right. Just cheaper.
On fresh air: totally agree! I do plan on having low cool air intake vents on the southern face and have louvre'd windows against the north wall to create a draft in and up for nice days. I thought of adding vent fans up over the north wall but fans in roof glass got complicated. May need to review that idea.
The main door is south facing too so it can add to the venting. Dallas wind in nearly always south to north. I do plan on using fresh air as long as humidity is under 60% and temp is under 80F.
