Editing Geothermal energy (section)

==Resources==
[[File:EGS diagram.svg|thumb|left|'''Enhanced geothermal system''' 1:Reservoir 2:Pump&nbsp;house 3:Heat&nbsp;exchanger 4:Turbine&nbsp;hall 5:Production&nbsp;well 6:Injection&nbsp;well 7:Hot water to district heating 8:Porous&nbsp;sediments 9:Observation&nbsp;well 10:Crystalline&nbsp;bedrock]]

The Earth has an internal heat content of [[1 E31 J|10<sup>31</sup>&nbsp;joules]] (3·10<sup>15</sup>&nbsp;[[TWh]]), About 20% of this is residual heat from [[planetary accretion]]; the remainder is attributed to past and current [[radioactive decay]] of [[Naturally occurring radioactive material|naturally occurring isotopes]].<ref name="turcotte">
{{Citation |last=Turcotte |first=D. L. |title=Geodynamics |pages=7-8 |year=2002 |edition=2 |location=Cambridge, England, UK |publisher=Cambridge University Press |isbn=978-0-521-66624-4 |author2=Schubert, G.}}</ref> For example, a 5275 m deep borehole in United Downs Deep Geothermal Power Project in [[Cornwall]], England, found granite with very high [[thorium]] content, whose [[radioactive decay]] is believed to power the high temperature of the rock.<ref>{{Cite web |title=United Downs – Geothermal Engineering Ltd |url=https://geothermalengineering.co.uk/united-downs/ |access-date=2021-07-05 |language=en-GB |archive-date=2022-03-08 |archive-url=https://web.archive.org/web/20220308085807/https://geothermalengineering.co.uk/united-downs/ |url-status=dead }}</ref>

Earth's interior temperature and pressure are high enough to cause some rock to melt and the solid [[mantle (geology)|mantle]] to behave plastically. Parts of the [[mantle convection|mantle convect]] upward since it is lighter than the surrounding rock. Temperatures at the [[core–mantle boundary]] can reach over {{convert|4000|°C|°F|abbr=on}}.<ref>{{citation |last1=Lay |first1=Thorne |title=Core–mantle boundary heat flow |journal=Nature Geoscience |volume=1 |issue=1 |pages=25–32 |year=2008 |bibcode=2008NatGe...1...25L |doi=10.1038/ngeo.2007.44 |last2=Hernlund |first2=John |last3=Buffett |first3=Bruce A.}}</ref>

The Earth's internal thermal energy [[heat flux|flows to the surface by conduction]] at a rate of 44.2 [[terawatts]] (TW),<ref name="pollack_et_al">{{cite journal
| last = Pollack | first = H.N.
 |author2=S. J. Hurter |author3=J. R. Johnson
| year = 1993
| title = Heat Flow from the Earth's Interior: Analysis of the Global Data Set
| volume = 30
| issue = 3
| pages = 267–280
| journal = Rev. Geophys.
| doi = 10.1029/93RG01249
|bibcode = 1993RvGeo..31..267P }}</ref> and is replenished by radioactive decay of minerals at a rate of 30&nbsp;TW.<ref name="sustainability">{{cite journal
| last =Rybach
| first =Ladislaus
| date =September 2007
| title =Geothermal Sustainability
| periodical =Geo-Heat Centre Quarterly Bulletin
| location =Klamath Falls, Oregon
| publisher =Oregon Institute of Technology
| volume =28
| issue =3
| pages =2–7
| url =http://geoheat.oit.edu/bulletin/bull28-3/art2.pdf
| access-date =2009-05-09
| archive-date =2012-02-17
| archive-url =https://web.archive.org/web/20120217184740/http://geoheat.oit.edu/bulletin/bull28-3/art2.pdf
| url-status =dead
}}</ref> These power rates are more than double humanity's current energy consumption from all primary sources, but most of this energy flux is not recoverable. In addition to the internal heat flows, the top layer of the surface to a depth of {{convert|10|m|ft|abbr=on}} is heated by solar energy during the summer, and cools during the winter.

Outside of the seasonal variations, the [[geothermal gradient]] of temperatures through the crust is {{convert|25|–|30|C|F}} per km of depth in most of the world. The conductive heat [[flux]] averages 0.1&nbsp;MW/km<sup>2</sup>. These values are much higher near tectonic plate boundaries where the crust is thinner. They may be further augmented by combinations of fluid circulation, either through [[magma conduit]]s, [[hot springs]], [[hydrothermal circulation]].

The thermal efficiency and profitability of electricity generation is particularly sensitive to temperature. Applications receive the greatest benefit from a high natural heat flux most easily from a [[hot spring]]. The next best option is to drill a well into a hot [[aquifer]]. An artificial hot water reservoir may be built by injecting water to [[hydraulically fracture]] bedrock. The systems in this last approach are called [[enhanced geothermal systems]].<ref name="INEL">{{Citation
 |last = Tester
 |first = Jefferson W.
 |title = The Future of Geothermal Energy
 |volume = Impact of Enhanced Geothermal Systems (Egs) on the United States in the 21st Century: An Assessment
 |publisher = Idaho National Laboratory, [[Massachusetts Institute of Technology]]
 |location = Idaho Falls
 |isbn = 978-0-615-13438-3
 |pages = 1–8 to 1–33 (Executive Summary)
 |url = http://geothermal.inel.gov/publications/future_of_geothermal_energy.pdf
 |access-date = 2007-02-07
 |year = 2006
 |display-authors = etal
 |archive-url = https://wayback.archive-it.org/all/20110310030646/http://geothermal.inel.gov/publications/future_of_geothermal_energy.pdf
 |archive-date = 2011-03-10
 |url-status = dead
}}</ref>

2010 estimates of the potential for electricity generation from geothermal energy vary sixfold, from {{gaps|0.035|to|2|TW}} depending on the scale of investments.<ref name="IPCC"/> Upper estimates of geothermal resources assume wells as deep as {{convert|10|km|mi|0}}, although 20th century wells rarely reached more than {{convert|3|km|mi|0}} deep.<ref name="IPCC" /> Wells of this depth are common in the petroleum industry.<ref>{{Cite journal|url=https://www.e3s-conferences.org/articles/e3sconf/abs/2018/35/e3sconf_usme2018_00006/e3sconf_usme2018_00006.html|title=Resource evaluation of geothermal power plant under the conditions of carboniferous deposits usage in the Dnipro-Donetsk depression|first1=Mykhailo|last1=Fyk|first2=Volodymyr|last2=Biletskyi|first3=Mokhammed|last3=Abbud|date=May 25, 2018|journal=E3S Web of Conferences|volume=60|pages=00006|via=www.e3s-conferences.org|doi=10.1051/e3sconf/20186000006|bibcode=2018E3SWC..6000006F|doi-access=free}}</ref> 
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