Enhanced geothermal system (EGS)
A geothermal reservoir created by stimulating hot, low-permeability rock, usually by hydraulic fracturing or shearing, so that water can be circulated between wells to extract heat.
Conventional geothermal needs three things together: heat, fluid and natural permeability. EGS removes the last two requirements by engineering the reservoir, potentially opening hot dry rock almost anywhere at sufficient depth.
The method: drill an injection well into rock at roughly 150–250°C (typically 3–6 km deep, depending on the geothermal gradient), stimulate fractures by pumping water at high pressure, then drill one or more production wells into the fractured zone. Cold water is injected, heated as it flows through the rock, produced at the surface, used in a binary or flash plant, and reinjected in a closed loop.
Thermal output of a well pair is:
A flow of 60 kg/s cooling from 190°C to 70°C gives about 30 MW thermal, or roughly 3–5 MW electric.
Milestones include Soultz-sous-Forêts (France, about 1.7 MW since 2016) and the US DOE FORGE field laboratory in Utah. Fervo Energy's Project Red in Nevada (2023) used horizontal drilling and multi-stage stimulation adapted from shale oil, reporting about 3.5 MW per well and flows of about 60 kg/s; its Cape Station project in Utah targets 500 MW. NREL ATB and DOE see large potential, with the US EGS resource estimated in the hundreds of GW, if drilling costs fall. The main risks are induced seismicity (Basel 2006 and Pohang 2017 projects were halted after felt earthquakes), short-circuiting of flow paths and cost.
Related terms
All terms →- Geothermal gradient
The rate at which temperature increases with depth in the Earth's crust, averaging about 25 to 30 °C per kilometre away from tectonic plate boundaries.
- Binary cycle power plant
A geothermal power plant in which hot brine heats a separate low-boiling-point working fluid that drives the turbine, enabling generation from resources of roughly 100 to 180 °C.
- Capacity factor
The ratio of the energy a power plant actually produces over a period to the energy it would produce running at full nameplate capacity for the whole period.
- Baseload
The minimum level of electricity demand on a grid over a day, season or year, historically served by plants that run continuously at high output.
- Dispatchable generation
Generation whose output can be scheduled, raised or lowered on request by the operator, independently of weather.