Hard to dig up solid information on the impact of outside air temperature and air conditioner efficiency. The closest study I've been able to find is here:
https://aceee.org/files/proceedings/...193-000015.pdf
Figure 3 & 4 show massive power consumption differences (0.2KW at -5 to 0 deg. F difference to inside building vs 2.5-3KW at 30-35 deg. F difference) Basically, 15 times less power used.
The question is teasing out how much of that is simply the difference in building cooling load.
If the air tubes can cool 90 deg. F outside air to 65 deg. F (pretty average summer soil temp for only a few feet underground in Wisconsin) and the air conditioner reduces that to say 50 deg. F then could we say that the earth tubes are doing roughly 63% of the cooling? My totally uneducated, ignorant calculations would suggest the A/C would have to work about 3x less.
So would a 11 SEER $400 portable A/C combined with earth tubes perform similar to a 33 SEER $1,600 minisplit?