If he hasn’t already, he should contact the alternative energy institute at West Texas A&M University. They have done some research on this and had some very good information when I asked about it several years ago.
One thing they learned was that if you use straight-edged PVC (normal pipe), the air close to the edges is cooled a lot, but the air that flows through the middle doesn’t cool much. They tried pipe with a corrugated wall and it created turbulence in the air stream, cooling the entire stream.
I’m sure the same thing would happen in a water-based system.
Another thing to consider with an air-based system is that they are humidity traps. If the air is humid upon entry, the water will condense in the pipe and you eventually will have a mold problem.
He’s a smart guy with a lot of technical or engineering knowledge from his years as head of maintinance at a large shop.
He always has some interesting project going on around home.
PVC (compared to corrugated metal pipe) is a smooth-sided, very thick-walled plastic pipe. Compared to most material, the plastic becmoes an effective insulator compared to the low temperature differences available.
Any metal pipe would be many times more effective than the PVC plastic. Though more likely to corrode out over time.