Here's a post I put in the Heat Pump Manifesto thread, but it also belongs here:
AC, I remember somewhere you made a post in this manifesto considering extra pipe surface area in your ground loops by having the pipe make an inverted U at the top, sending the pipe back down the borehole again to a second U at the bottom of the borehole before returning to the top. I don't think that will work well at all-you will never be able to purge all the air out, and even if you did, any future entrained air or gases that come out of solution will form a bubble that will act like a reverse plumbing trap, stopping or severely limiting flow in that loop! This is how bubble circulation pumps (interesting in their own right-see
HERE ) work.
After realizing just how versatile PEX is while purposely torture testing then reheating it during my radiant floor installation (details
HERE ), I think a better idea is to have a form, such as 2" thinwall PVC, that can be lowered into the borehole. Slip the appropriate return length through the center, heat the pex to make a Ubend at the bottom, then wrap the PEX barberpole style along the outside of the form. Space it out as there is no need to wind it tight-1/2' pex could wrap about 575 times around a 30' length of 2" PVC for a loop length of 480 feet!!! This would be an easy way to avoid pipe welds or connections down the hole while increasing pipe surface area and effective heat transfer due to turbulence in the pipe. Regular 3/4 poly flow is pretty stagnant at the boundary layers because the flow is slower. You can flow almost 1gpm through a 1/2" Pex loop before you get significant head loss if your loops are less than around 300ft.
I feel this will work for depths up to 50 feet before handling the pipe gets unwieldy. Looks like most of us doing boreholes without a professional well driller won't be going deeper than that. Now the only limiting heat transfer issue is the borehole field itself.