Its a heat exchanger.
The ground is cooler than the air in the hot summer.
Outside the desert you can use a water loop as well and/or run the loop through a water body.
Curious - what sort of trench depth are you digging, what size duct, for what sq ft / area home?
I am not sure yet. It's going to be a 20' circle essentially. I plan to go 3 feet or so. Deeper if needed. Though about using rocks to fill I the space of a tube if some sort. I am open to suggestions
I am curious how you went about choosing these dimensions
I found ceiling joists for free.
The depth I am guessing. I just make stuff up really and it tends to work out. 
Are you using earth bags? Or tires? As your outer walls a la an earthship?
We are short on water for the time being. No plans to use water in it. Good idea though.
quality of life improvement for free..