I followed a thread along on another board. I believe they were successful. With others who have had success posting. The problem is that pex or pvc doesn't not dissipate heat very well. sure they use these materials, but it takes quite a bit of it to do it. However, titanium, and aluminium as well as copper do dissipate well. I'm sure if you run enough of the pex then you'll be able to achieve the temp drop your looking for. Unfortunately I don't know how to tell you to do the math to know what you need. I don't know if there's anybody on this site who does know how either. Hopefully I'm wrong. However, over on the other big reef message board you can find a thread outlining all this. BTW, I don't believe you can get it too cold. You'll only drop to whatever the earth temp is. You may need to control how much flow goes through it though. A ranco temp controller and a relay on a properly sized pump will solve this for you. Using a ball valve also.
You need to take into consideration the possibility of contamination also. Personally, I would do a open system through the ground source. Now you have the challenge of how to handle the transfer in the DT. Using a titanium coil in a high flow area of sump will work for the actual transfer, a closed system with only fresh water will work for the ground source. You'll likely need to bleed off air from time to time also re prime with H20. Also there may be an issue with a closed system becoming stagnant, leaking and contaminating the DT. Or just diluting if leaking in the DT system.
The thread I was following I believe used two different loops. One the ground source, the other from the DT to a separate container of sorts. To somehow isolate each other or something to protect the DT, or in order to use a copper coil, I cant remember exactly.
BTW, just wanna make sure to say that using a copper or aluminium coil in direct contact with SW is a big ole no-no.
