Hi all Hope to find someone here, that knows more than me :)

So I have a friend who has been asked to build 100 non-permanent student houses. He has come up with a cheap solution and is not planning to earn anything on the project, other than the time he spends building.

So far so good.

The units will be 25 square meters or 269 square foot big.

But during the summer, the temperature is about 30C or 86F, so the small rooms will be fairly hot very fast. Winter is very mild in the area.

As the area is very big - 10.000 square meters or 107.639 square foot - I was thinking:
By digging 5 meters (196 inches) below ground, you could create a loop, whereby by forcing air through a pipe, the air would cool.

I have done this in my greenhouse, and it cools during summer and heats during winter ( along with the sun ).

As he is trying to help out, the project needs to be as cost-efficient as possible, and I am trying to see if I can give him an easy and co2-friendly way to address this issue.
Adding heat pumps to each free-standing student house would probably kill the budget.

If it worked in my greenhouse, should it not also work in a well-isolated house?

I know the ground temperature at 54 cm (21 inches) below ground, right now is 11C/51F, while the air temperature today was high 18c/64f.

If a room can be cooled to 22C/71F or 24c/75F during summer, that would be quite ok.

Perhaps I'm just waaaaaaay to simple-minded to understand thermal dynamics :)

Yep, you need to work out the thermal dynamics... You will need a lot of buried pipe to cool for more than minutes... And then there is the condensation and mold issue that could potentially kill you. This was tried in the 60s with very little success. But it certainly could be made to work...at least to some extent. However, if you have a low humidity, diurnal climate, a whole house fan and high interior thermal mass gets the job done far better and more economically too. That's what we used to eliminate any need for AC.

Gayle