So...

What to aim for

By tapping our (by current standards) deep crustal or (by any standards) shallow mantle heat, we could extract energy on a scale to dwarf anything humanity has consumed to date. As a source of energy it would be clean and versatile, and would outlast our requirements on this planet for many millennia at least, longer than we have used any fuel in any systematic way.
The nature of the installations, namely bubbles inflated in hot rock in...

Impossible Temperatures

The temperatures in question certainly are challenging, but they are not beyond practical limits. I doubt that any exit gas temperature beyond about 1200C would be under consideration. Temperatures for melting the way down might be higher, perhaps more like 2000C, but those would be contained at the workface. Perfectly practical alloys could survive such temperatures, and many ceramics could resist them indefinitely. During the process of melting the way down,...

Drilling in Hot Rock

As I already have mentioned, studies show that existing drilling technology can penetrate far deeper into hot rock than we require. All the same I actually am not currently contemplating conventional forms of drilling, except during specific phases where engineering experts might see it as advantageous.
There are several problems with drilling through deep, hot rock with the conventional drill string. One is the sheer difficulty and cost of dealing with...

Blowouts and Pressure Excursions

Oil drillers must be on the alert for blowouts when the shaft structure or the surrounding material cannot contain the pressure of fluids below ground. This is a matter of concern for any deep drilling, but really, it is less of a risk for power bubble drilling than for oil wells.
• Oil wells tend to be drilled where there are likely to be pockets of compressed gas, such that unexpectedly drilling into one could lead to the drill string being...

Objections and Concerns

Adverse Effects of Salt Injection or Disposal

A very reasonable question concerns the ecological hazards of soluble inorganic materials such as salts. Most of them should not be very hazardous under the conditions contemplated. If the working fluids are gases such as air or argon, which are not solvents at temperatures around 700C-1700C, there should be very little discharge and we could ignore pollution problems stemming from their use.
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