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OWOE - Cool Tech - Enhanced Geothermal Systems
  Figure 1 - Enhanced geothermal system (Siemens Pressebild)
Figure 1 - Enhanced geothermal system (Siemens Pressebild)
Figure 2 - US Geothermal Resources (NREL)
Figure 3 - USA Geothermal Resources: Temperature at 10km (ARPA-e)
Figure 4 - Energy per Well (ARPA-e)
Enhanced Geothermal Systems
Topic updated: 2022-11-08

Conventional geothermal technology requires the right combination of rock temperature, a water/steam source, and rock that is permeable enough for the water/steam to flow into the extraction well. There are relatively few locations where combination of these features are adequate for large-scale power generation. However, new technology in the form of Enhanced Geothermal Systems (EGS) provides the opportunity for essentially unlimited power. All that is required is hot rock. EGS uses a technique called "hydro-shearing", which is similar to "hydraulic fracturing" used in the oil business. High-pressure water is injected down a well and into the rock, which fractures the rock and increases permeability. Water travels through fractures in the rock, capturing the rock's heat until forced out of a second borehole as very hot water. After extracting the energy using normal geothermal technology, all of the water is injected back into the ground to heat up again in a continuous closed loop. EGS systems are currently being developed and tested around the world. Figure 1 illustrates the key components of an EGS system. 1:Reservoir; 2:Pump house; 3:Heat exchanger; 4:Turbine hall; 5:Production well; 6:Injection well; 7:Hot water out; 8:Porous sediments; 9:Observation well; 10:Crystalline bedrock

Figure 2 from the NREL shows the geothermal resources in the US based on reaching rock at 150 deg Celsius at well depths up to 10 km. A 2006 study by MITEI concluded that the US could add geothermal electrical generating capacity of 100 GW by 2050, or about 10% of total capacity) by aggressive implementation of EGS.

More recent developments in EGS is being referred to as "Super Hot Rock Energy". Super hot rock is defined as rock at a temperature of 400 degrees Celsius (675 degrees F). At this temperature water becomes a supercritical fluid and transports heat more efficiently than hot water or steam. Rock behavior also becomes consistent across rock types. Water is circulated to absorb heat from the rock and bring it to the surface in the form of superheated steam, which then drives a steam turbine connected to a generator and produces electricity. In some places, Super hot rock is found as shallow as 5 kilometers below the surface and the average depth is 20 kilometers. Figure 3 shows super hot rock energy potential across the US in the form of rock temperature at 10 km depth. A single 8 inch diameter geothermal well is estimated to be able to produce the same amount of power as 320 acres of solar PV. Figure 4 compares the electrical generation potential of a super hot rock well compared to other technologies. It shows a super hot EGS well at about 4 times the electrical potential of a typical shale gas well.

At the forefront of EGS development in the US is AltaRock Energy, which was formed in 2007 to exploit advanced geothermal opportunities. AltaRock spent over a decade performing extensive geological, hydrological, and geochemical studies at their Newberry Volcano leases near Bend, Oregon, to evaluate its potential for EGS development. In 2012 AltaRock demonstrated the value of its "hydro-shearing" technology which opens up fissures in the rock to enhance fluid flow but requires significantly less pressure than "hydro-fracking" used in the oil industry (see OWOE topic: What is hydraulic fracturing (fracking)?). Further testing and subsequent analyses indicated that super hot rock resources could achieve a Levelized Cost of Electricity (LCOE) less than $0.05/kilowatt-hour. AltaRock Energy anticipates formal demonstration of the first super hot rock EGS well system by 2025 at Newberry Volcano, followed with commercial development by 2030.

A 2022 report from the Clean Air Task Force concluded that with significant private and public investment, supportive regulatory policies, and continued technological innovation, superhot rock energy can plausibly be commercialized in the 2030s.

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