I am not a professional, but designed and installed my own ground-based heat pump system.
1-ton, using a 150′ well, water-water heat pump system that is currently using a copper heat exchanger in a well, about 100′ long in a 160′ well. I have used this to heat my hot water and a super-insulated large room (16×24) for about 10 years, without any problems. I’m thinking of increasing the efficiency of the system so I can use the heat pump to heat the room a little more (right now, I can keep the room at 60F or so in coldest weather, here in upstate NY, near Albany). I want to increase efficiency by doing an open-loop modification: install a pump at about 150′ deep, with return water to just below surface (which is always about 20′ or so below surface).

1) Would an open loop system have considerably more capacity than my closed loop?
2) What is the sodium limit for water in an open loop system? I know the well has a little sodium (don’t remember how much), but is passed inspection for drinking water (15 years ago, we used the well for house supply; not used now for that). My heat pump is cupronickel type.
3) I know my existing copper loop (100′) will ultimate corrode and fail (is over 10 years old); If I DON’T go to an open loop system, but put in pex instead, will that result in considerably less efficiency that I have now?