The conclusion is that it is a niche technology – useful in places, but there is not the capacity to provide a major solution – currently human global energy use is c.13TW but even utilising every bit of land area, geothermal could supply in the order of 2TW.

So useful, and worth exploiting the quick wins – niche technology works well in its niche. But will largely be a bit player.

Perhaps if you can get a free exhausted oil well hole that’s 10,000 feet deep you can extract enough heat to run a 20-kW expander on a sustained basis. The market value of that amount of electricity is about a dollar per hour: $8700/year. But capital costs for the expanders, valves, heat exchangers, pumps, transformers, cables, etc. would likely exceed $200,000 per hole. If operating expenses were zero, the expanders and generators were immortal, there were no decline in heat production, and capital loan service costs were zero, it would only take 23-years to recover the initial investment. That’s why we see geothermal power plants only where very a lot of hot rock is near the surface, and even in those exceptional locations thermal efficiencies are only in the 10-20% range.

Geothermal comfort heating and cooling for homes costs less to install than swimming pools, and I would back government mandates requiring geothermal heating and cooling before granting swimming pool permits.