Enhanced geothermal system

An enhanced geothermal system (EGS) generates geothermal electricity without natural convective hydrothermal resources. Traditionally, geothermal power systems operated only where naturally occurring heat, water, and rock permeability are sufficient to allow energy extraction. However, most geothermal energy within reach of conventional techniques is in dry and impermeable rock. EGS technologies expand the availability of geothermal resources through stimulation methods, such as 'hydraulic stimulation'. In many rock formations natural cracks and pores do not allow water to flow at economic rates. Permeability can be enhanced by hydro-shearing, pumping high-pressure water down an injection well into naturally-fractured rock. The injection increases the fluid pressure in the rock, triggering shear events that expand pre-existing cracks and enhance the site's permeability. As long as the injection pressure is maintained, high permeability is not required, nor are hydraulic fracturing proppants required to maintain the fractures in an open state. Hydro-shearing is different from hydraulic tensile fracturing, used in the oil and gas industry, which can create new fractures in addition to expanding existing fractures. Water passes through the fractures, absorbing heat until forced to the surface as hot water. The water's heat is converted into electricity using either a steam turbine or a binary power plant system, which cools the water. The water is cycled back into the ground to repeat the process. EGS plants are baseload resources that produce power at a constant rate. Unlike hydrothermal, EGS is apparently feasible anywhere in the world, depending on the resource depth. Good locations are typically over deep granite covered by a layer of insulating sediments that slow heat loss. Advanced drilling techniques penetrate hard crystalline rock at depths of up to or exceeding 15 km, which give access to higher-temperature rock (400 °C and above), as temperature increases with depth. EGS plants are expected to have an economic lifetime of 20–30 years.