Stanford’s Global Climate and Energy Project estimates crustal thermal energy reserves at 15 million zetajoules. Coal + oil + gas + methane hydrates amount to 630 zetajoules. That means there is 23,800 times as much geothermal energy in Earth’s crust as there is chemical energy in fossil fuels everywhere on the planet.
The four main next-generation geothermal concepts I will discuss do the same thing. They (1) locate and access heat, (2) transfer subsurface heat to a working fluid and bring it to the surface, and (3) exploit the heat energy at the surface through direct use or conversion to electricity. It is the second step, transferring subsurface heat to a working fluid, that is non-obvious.
Three types -
1) Enhanced geothermal systems - usual hydrothermal features one or more injection wells and production wells from where steam comes out. Mostly relies on rocks and structures for surface heat transfer and ensuring that steam doesn't come out of other sources except the production wells. Fervo startup is doing this.
2) Closed-loop geothermal systems - does this through pipes - smaller surface area more expensive drilling are cons. Pros are non water fluids with lower supercritical temperatures can be used. Startup Eavor from Germany is trying to do this.
3) Heat roots - single vertical shaft. From the base of the shaft, they frack downwards to create a fracture pattern that gives the impression of a root system for a tree. They fill this “root” system with a convective and conductive fluid. Then, using a pipe-in-pipe system, they circulate a separate working fluid from the surface to the base of the shaft and back. At the base of the shaft, a heat exchanger takes the energy concentrated by the heat root system and imparts it to the working fluid. Startup Sage is doing this in Texas.
4) Supercritical EGS - drilling straight to the earth cheaply so that we can get fluids at supercritical temperatures. A huge potential advantage would be the ability to retrofit existing coal plants. With many coal plants shutting down in the next several years, a lot of valuable generator equipment could be lying around idle. These generators take supercritical steam as an input and use it to produce electricity. -Quaise company is doing this
