Also; seems to me if we are transferring heat or cold to soil... it does not matter the medium to carry the heat... what matters is enough soil contact to effectively dissipate the heat/cold. If this is true (suggesting, not stating), then it seems irrelevant which medium is used to carry the heat... what does seem important is enough ground to absorb the heat applied.
Thats not completely true and here is why. water, or antifreeze, doesnt transfer heat rapidly. HDPE is an insulator (R-.6 or .8) so its not the best at getting rid of the heat in the first place...now when your ground temp raises, the heat transfer rate stays the same. as the ground warms, so does the water....since the heat goes from refrigerant to copper to plastic to water to plastic to earth, it imperative that you get the absolute best transfer each time. when the ground warms up it REALLY shows on your water temperatures.
DX on the otherhand only goes from refrigerant to copper to earth...and since copper is a conductor, not an insulator, it doesnt have to over come the heat sink that is the HDPE and the water.
Even if the earth was 90 degrees, a DX system would work fine....would you mind 90 degree liquid temps?
So what you are saying is:
Since DX runs refrigerant through the loop... the temps are wider and as such X-fer works over a wider soil temp?