Editing IPCC:AR6/WGIII/Chapter-6 (section)

==== 6.4.2.8 Geothermal Energy ====

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Geothermal energy is heat stored in the Earth’s subsurface and is a renewable resource that can be sustainably exploited. The geophysical potential of geothermal resources is 1.3 to 13 times the global electricity demand in 2019 ( ''medium confidence'' ). Geothermal energy can be used directly for various thermal applications, including space heating and industrial heat input, or converted to electricit '''y''' depending on the source temperature ( [[#Limberger--2018|Limberger et al. 2018]] ; [[#Moya--2018|Moya et al. 2018]] ; [[#REN21--2019|REN21 2019]] ).

Suitable aquifers underlay 16% of the Earth’s land surface and store an estimated 110,000–1,400,000 PWh (400,000–1,450,000 EJ) that could theoretically be used for direct heat applications. For electricity generation, the technical potential of geothermal energy is estimated to be between 30 PWh yr –1 (108 EJ yr –1 ) (to 3 km depth) and 300 PWh yr –1 (1080 EJ yr –1 ) (to 10 km depth). For direct thermal uses, the technical potential is estimated to range from 2.7–86 PWh yr –1 (9.7–310 EJ yr –1 ) ( [[#IPCC--2011|IPCC 2011]] ). Despite the potential, geothermal direct heat supplies only 0.15% of the annual global final energy consumption. The technical potential for electricity generation, depending on the depth, can meet one third to almost three times the global final consumption – based on International Energy Agency (IEA) database for IPCC. The mismatch between potential and developed geothermal resources is caused by high upfront costs, decentralised geothermal heat production, lack of uniformity among geothermal projects, geological uncertainties, and geotechnical risks ( [[#IRENA--2017a|IRENA 2017a]] ; [[#Limberger--2018|Limberger et al. 2018]] ). A limited number of countries have a long history in geothermal. At least in two countries (Iceland and New Zealand), geothermal accounts for 20–25% of electricity generation ( [[#Pan--2019|Pan et al. 2019]] ; [[#Spittler--2020|Spittler et al. 2020]] ). Furthermore, in Iceland approximately 90% of the households are heated with geothermal energy. In Kenya, as of July 2019, geothermal accounted for 734 MW effective capacity spread over 10 power plants and approximately one third of the total installed capacity (Kahlen 2019).

There are two main types of geothermal resources: convective hydrothermal resources, in which the Earth’s heat is carried by natural hot water or steam to the surface; and hot, dry rock resources, in which heat cannot be extracted using water or steam, and other methods must be developed. There are three basic types of geothermal power plants: (i) dry steam plants use steam directly from a geothermal reservoir to turn generator turbines; (ii) flash steam plants take high-pressure hot water from deep inside the Earth and convert it to steam to drive generator turbines; and (iii) binary cycle power plants transfer the heat from geothermal hot water to another liquid. Many of the power plants in operation today are dry steam plants or flash plants (single, double and triple) harnessing temperatures of more than 180°C.

However, medium temperature fields are increasingly used for electricity generation or combined heat and power. The use of medium temperature fields has been enabled through the development of binary cycle technology, in which a geothermal fluid is used via heat exchangers. Increasing binary generation technologies are now being utilised instead of flash steam power plants. This will result in almost 100% injection and essentially zero GHG emissions, although GHG emissions from geothermal power production are generally small compared to traditional baseload thermal energy power generation facilities ( [[#Fridriksson--2016|Fridriksson et al. 2016]] ).

Additionally, new technologies are being developed like Enhanced Geothermal Systems (EGS), which is in the demonstration stage ( [[#IRENA--2018|IRENA 2018]] ), deep geothermal technology, which may increase the prospects for harnessing the geothermal potential in a large number of countries, or shallow-geothermal energy, which represents a promising supply source for heating and cooling buildings ( [[#Narsilio--2018|Narsilio and Aye 2018]] ). Successful large-scale deployment of shallow geothermal energy will depend not only on site-specific economic performance but also on developing suitable governance frameworks ( [[#Bloemendal--2018|Bloemendal et al. 2018]] ; [[#García-Gil--2020|García-Gil et al. 2020]] ). Technologies for direct uses like district heating, geothermal heat pumps, greenhouses, and other applications, are widely used and considered mature. Given the limited number of plants commissioned, economic indicators (Figure 6.15) vary considerably depending on site characteristics.

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[[File:326f242c673d51a3849ed67058b073a6 IPCC_AR6_WGIII_Figure_6_15.png]]

'''Figure 6.15 | Global weighted averagetotal installed costs, capacity factors and levelised costs of electricity (LCOE) for geothermal power per year (2010–2020).''' The shaded area represents the 5% and 95% percentiles. Source: with permission from [[#IRENA--2021a|IRENA (2021a)]] .

Public awareness and knowledge of geothermal energy is relatively low ( ''high confidence'' ). Geothermal energy is evaluated as less acceptable than other renewable energy sources such as solar and wind, but is preferred over fossil and nuclear energy, and in some studies, over hydroelectric energy ( ''high confidence'' ) ( [[#Pellizzone--2015|Pellizzone et al. 2015]] ; [[#Steel--2015|Steel et al. 2015]] ; [[#Karytsas--2019|Karytsas et al. 2019]] ; [[#Hazboun--2020|Hazboun and Boudet 2020]] ). Some people are concerned about the installation of geothermal facilities close to their homes, similar to solar and wind projects ( [[#Pellizzone--2015|Pellizzone et al. 2015]] ). The main concerns about geothermal energy, particularly for large-scale, high-temperature geothermal power generation plants, involve water usage, water scarcity, and seismic risks of drilling ( [[#Dowd--2011|Dowd et al. 2011]] ). Moreover, noise, smell and damages to the landscape have been reasons for protests against specific projects ( [[#Walker--1995|Walker 1995]] ). However, with the implementation of modern technologies, geothermal presents fewer adverse environmental impacts. At the same time, people perceive geothermal energy as relatively environmentally friendly ( [[#Tampakis--2013|Tampakis et al. 2013]] ).

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