I would agree.

Pex/Al/Pex drilled closed loop into a radius with outer radius @ 45 deg and middle radius drilled @ 30 deg and inner radius drilled @ 15 deg would be most optimal at say 100' per 4"borehole. CLOSED LOOP GEO-THERMAL CLUSTER.In process of doing the first one now at 9879' elevation INDIAN MOUNTIAN , COLORADO.Topmanifold area fits in a 10' radius w/ a 20' diameter.Further enhance this conductivity as you call it by using a thermal enhancing super grout.Also if at all possible drill this cluster first right under basement slab.I have patented this idea Jan 1,2009.It will and is going to work come hell or high water.Now the Germans say they case thier boreholes w/ black coregated plastic pipe before they send loop and tremie downhole to further enhance.I don't know about it because in order for the colour black to conduct it seems it must be exposed to direct sunlight.I could be wrong and this is only my opinion.William Wainwright/Advanced Geo-Thermal Drilling LLC./Owner Operator

Another pipe in the bore hole besides the one carrying the fluid would seem to be counter productive. 

Mack - That is what I am thinking - I am presently moving foward without it and leaving borehole uncased.

Soil temp is generally related to the average daily air temp of about a month prior. It varies less as you go deeper.