<P> The thermal efficiency of geothermal electric stations is low, around 7--10%, because geothermal fluids are at a low temperature compared with steam from boilers . By the laws of thermodynamics this low temperature limits the efficiency of heat engines in extracting useful energy during the generation of electricity . Exhaust heat is wasted, unless it can be used directly and locally, for example in greenhouses, timber mills, and district heating . The efficiency of the system does not affect operational costs as it would for a coal or other fossil fuel plant, but it does factor into the viability of the station . In order to produce more energy than the pumps consume, electricity generation requires high temperature geothermal fields and specialized heat cycles . Because geothermal power does not rely on variable sources of energy, unlike, for example, wind or solar, its capacity factor can be quite large--up to 96% has been demonstrated . However the global average capacity factor was 74.5% in 2008, according to the IPCC . </P> <P> The Earth's heat content is about 1 × 10 TJ (2.8 × 10 TWh). This heat naturally flows to the surface by conduction at a rate of 44.2 TW and is replenished by radioactive decay at a rate of 30 TW . These power rates are more than double humanity's current energy consumption from primary sources, but most of this power is too diffuse (approximately 0.1 W / m on average) to be recoverable . The Earth's crust effectively acts as a thick insulating blanket which must be pierced by fluid conduits (of magma, water or other) to release the heat underneath . </P> <P> Electricity generation requires high - temperature resources that can only come from deep underground . The heat must be carried to the surface by fluid circulation, either through magma conduits, hot springs, hydrothermal circulation, oil wells, drilled water wells, or a combination of these . This circulation sometimes exists naturally where the crust is thin: magma conduits bring heat close to the surface, and hot springs bring the heat to the surface . If no hot spring is available, a well must be drilled into a hot aquifer . Away from tectonic plate boundaries the geothermal gradient is 25--30 ° C per kilometre (km) of depth in most of the world, so wells would have to be several kilometres deep to permit electricity generation . The quantity and quality of recoverable resources improves with drilling depth and proximity to tectonic plate boundaries . </P> <P> In ground that is hot but dry, or where water pressure is inadequate, injected fluid can stimulate production . Developers bore two holes into a candidate site, and fracture the rock between them with explosives or high - pressure water . Then they pump water or liquefied carbon dioxide down one borehole, and it comes up the other borehole as a gas . This approach is called hot dry rock geothermal energy in Europe, or enhanced geothermal systems in North America . Much greater potential may be available from this approach than from conventional tapping of natural aquifers . </P>

Where can a geothermal power plant be used to generate electricity
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