Interesting stuff, Paul.  The "geo-thermal" heat pump HVAC system is very popular here in North Central Arkansas.  Here the typical approach is to drill a 300' well with a water well type of rig.  Then they install a PVC or PEX "U", then back fill the hole.  This has the advantage of getting to the water table, so the bottom part of the "U" is immersed in wet back fill.  By about 5' below the surface the temperature of the earth stays at about 50F year around.  This means that heat pumps can work well in the winter as well as in the summer.

I am interested in how your system works out.  It could help me decide what to do for us.  My existing heat pump is at least 20 yrs old and will surely die in another few years.  I am considering about 1 KW of solar panels on the roof of my shop bldg. which faces south and has about a 30 degree slope.  That could protect us against future increases in cost.

How much do your thermal solar bags weigh when they are being used?  How much water do they hold?  That would determine whether my roof structure is adequate.

Looking forward to reports on future progress.

"It is not necessary to have hope in order to persevere."