Enhanced Geothermal Power in the Great Basin: USGS Puts It at 135 Gigawatts

Almost all geothermal electricity today comes from conventional systems, where hot water already circulates naturally through the rock. Such places are rare. Far more of Earth's crust is hot but impermeable. Enhanced geothermal systems (EGS) engineer that rock, usually by opening connected fractures, so water can circulate through it and carry heat up to drive power plants. EGS resources have been estimated to be able to produce ten times more electricity than conventional ones.

How an enhanced geothermal system works: cool water is injected, flows through fractures in a hot geothermal reservoir, and hot water is pumped to the surface for a power plant. Illustration: U.S. Geological Survey.

EGS projects are still few and largely experimental, but exploration and development have grown over recent decades. The Great Basin of the southwestern United States is a prime target: its high heat flow puts high temperatures within reach of drilling.

A new assessment

The U.S. Geological Survey last assessed EGS power in the western United States in 2008, for rock above 150 °C. The Energy Act of 2020 directed it to update geothermal assessments, including EGS, for the conterminous United States, Alaska, Hawaii and Puerto Rico. For the Great Basin, the USGS and partners built:

• updated maps of heat flow and underground temperature;
• new methods to estimate how efficiently heat can be pulled from different fracture patterns, and how efficiently it becomes electricity.

Heat flow (left) and temperature at 6 kilometers' depth (right) across the Great Basin. Maps: U.S. Geological Survey.

This time the estimate covers all electricity-grade temperatures above 90 °C.

Why "provisional"

Conventional assessments rest on long records of proven, commercial production. EGS technology is still developing and has not been used widely enough to supply such records. So this assessment is provisional: it assumes the technology will evolve and perform as anticipated.

The estimate

For the Great Basin, an area of 633,072 square kilometers, the best provisional estimate is 135 gigawatts electric (GWe). That is just 0.5% of the accessible resource base; the other 99.5% of the heat stays residual. Most of it lies in deeper, hotter rock. Rock heated near active hydrothermal systems, under 1% of the volume, is not included, and depths beyond 6 km, which may need further advances, are left out.

Estimated power density above 6 kilometers across the Great Basin, showing how the best-estimated 135 gigawatts electric are distributed. Map: U.S. Geological Survey.

The figure is consistent with the 2008 assessment, which estimated 518 GWe in the upper 6 km of the much larger western U.S. study area (about 3 million square kilometers), 150 GWe of it in Nevada and Utah alone.

Multiplying efficiencies

Turning hot rock into electricity is a chain of efficiencies multiplied together. If half of the rock's heat can be extracted, and a fifth of that becomes electricity, the output is 10% of the heat available. The assessment weighs five factors, each with its own best estimate and uncertainty:

1. Electrical conversion — electricity generated per unit of heat drawn from the working fluid.
2. Ideal heat extraction — heat recoverable from a fractured reservoir under ideal conditions.
3. Irregularity correction — a reduction for uneven fractures and varied rock.
4. Reservoir spacing — how much of a rock unit can be used; the closest spacing without reservoirs interfering thermally is 74%, and more is possible if some interference is accepted.
5. Viable geology — how much rock can be engineered at all. Few Great Basin sites have yet made successful EGS reservoirs; the share is probably above zero, but this is the least certain factor.

How high or low could it go?

• Upside. Better technology could yield more than ten times the best estimate, a 5% useful resource. Better power-plant conversion could add more, though that was not modeled.

Enhanced geothermal resources above a given depth: the current best provisional estimate, about 135 GWe of useful resources, and how improved technologies could greatly increase them. Chart: U.S. Geological Survey.

• Extremes. Unlikely but possible bounds run from near zero (if almost no rock suits EGS) to 44% useful — about 13 terawatts electric, roughly ten times current U.S. generating capacity.
• Uncertainty. Uncertainty in the temperature and physical models is about 45%, as in 2008. Uncertainty over how much rock can be developed could exceed 1,000%. It comes down to predicting what reservoirs engineering can create, and where favorable rock lies under favorable stress.

As technologies are demonstrated and the three-dimensional geology becomes better known, that uncertainty will shrink — and if the technology keeps improving, the estimates will rise as more heat becomes usable.

Sources

Based on "Enhanced geothermal systems electric-resource assessment for the Great Basin, southwestern United States," U.S. Geological Survey Fact Sheet 2025–3027; a work of the United States government in the public domain. The illustration, maps and chart are reproduced from the fact sheet.