Google-born startup thinks its time for a ground-source heat pump renaissance

People which drill water wells use normally bigger diameters, such as 6 inches (150 mm) or even bigger, their drilling rigs are huge truck mounted rigs, but there are a number of much smaller machines suitable for the task of drilling a 100 mm vertical hole up to 120-150 m or so.

The "resonance sonic drill heads to liquify the earth in the drill path" maybe are the "new thing", but it actually dates back to the '80's:

http://www.cpeo.org/techtree/ttdescript/soncdrl.htm

Like more traditional temporary casing it is installed and removed afterwards as actually the casing is made by the actual drilling pipes, of course it is much faster (on the "right" terrains) than having to deal with two pipes (the drilling one and the casing).

The difference in cost of hauling the debris is nothing.

Let's make a 100 m deep hole 10m mm in diamater:

0,05^2x355/113x100=0,78 cubic meters

the same hole but 150 mm in diameter:

0.075^2x355/113x100=1,77 cubic meters

Even if you multiply both by a factor of 2 (terrain "grows" when excavated and the actual hole is bigger than the size of the drill) the difference is between a half empty small truck and a filled up small truck.

If I have to give it a "universal" cost it cannot be more than 100 US$ per cubic meter or 200 US$ for each drilled hole, whichever comes first.

I.e. 0.5 to 1% of the given estimated 20,000 US$ per installation.