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Geothermal energy is an alternative clean energy which gained substantial attention because of its abundant reserves. From heating supply to power generation, geothermal energy is widely employed in buildings’ heating, energy generation and hydrogen production. This could be implemented on a small scale to deliver heat for a residential unit by utilizing a geothermal heat pump (GHP), or on a large scale for energy generation by a geothermal power plant. Geothermal energy is known to be cost‐effective, reliable, and an environmentally friendly energy source [21].

Figure 2.7 Biomass installed capacity for energy systems (2010–2025). Ref Num [20].

Geothermal energy resources involve thermal energy from the Earth's interior stored in rock and trapped steam or liquid water. Geothermal systems arise in a number of geological environments where temperatures and depths of the reservoirs change accordingly. A number of high‐temperature hydrothermal systems (greater than 180 °C) are related to present volcanic activities. Intermediate‐temperature (between 100 and 180 °C) and low‐temperature (less than 100°C) systems exist in continental settings, where above‐normal heat production by radioactive isotope decay causes a rise in terrestrial heat flow or where aquifers are charged by water heated by circulation along deeply penetrating fault zones. Under suitable conditions, high‐, intermediate‐, and low temperature geothermal areas could be utilized for energy production and the direct utilization of heat [22, 23].

Geothermal energy sources are characterized as hydrothermal systems, conductive systems and deep aquifers. Hydrothermal systems involve liquid‐ and vapor‐dominated types. Conductive systems entail hot rock and magma over a wide range of temperatures, and deep aquifers consisting of circulating fluids in porous media or fracture zones at depths usually more than 3 km, but they lack a localized magmatic heat source.

Geothermal energy resource utilization technologies may be divided into different types for electrical energy production, direct use of heat, or combined heat and power in cogeneration applications. GHP technologies are a subset of direct use. Presently, the only commercially exploited geothermal systems for energy production and direct use are hydrothermal. Table 2.2 summarizes the resources and utilization technologies [24, 25].

Table 2.2 Types of geothermal resources, temperatures and their applications. Adapted from Ref [24].

Type	In‐situ fluids	Subtype	TemperatureRange	Utilization
Current	Future
Convective systems (hydrothermal)	Yes	Continental	H, I & L	Power, direct use
Submarine	H	None	Power
Conductive systems	No	Shallow (<400 m)	L	Direct use (GHP)
Hot rock (EGS)	H, I	Prototypes	Power, direct use
Magma bodies	H	None	Power, direct use
Deep aquifer systems	Yes	Hydrostatic aquifers	H, I & L	Direct use	Power, direct use
Geo‐pressured	Direct use	Power, direct use

H: High, I: Intermediate, L: Low (temperature range).

Types of traditional geothermal power technologies are as follows: dry steam, flash and binary. In dry steam plants, a high‐pressure steam shoots up from the dry steam reservoir and is used to make the turbines function which then turns on the generator. In flash plants, steam is separated from the high‐pressure and high‐temperature geothermal fluids which includes water and with a high temperature. The steam is guided to a turbine that then turns on the generator. The liquid (condensed from the steam after going through the turbine) and the water are sent into the reservoir. In binary or ORC (i.e. Organic Rankine Cycle) plants, heat is transferred from the high‐temperature water to an organic working fluid that possesses a lower boiling point than water [26].

Approximately 0.4 GW of new geothermal power generating capacity came online in 2016, culminating in total capacities of approximately13.5 GW. Indonesia and Turkey invested in new installations. Kenya, Mexico and Japan also invested in projects during the year, and many other countries had projects under development. The US possesses the biggest geothermal capacity, now just under 3.6 GW, followed by the Philippines (1.9 GW), Indonesia (1.6 GW) and Mexico (1 GW), Figure 2.8 illustrates the installed geothermal electric capacity as of 2019 in the top 10 countries [26]. The United Nations and the International Renewable Energy Agency (IRENA) pledged a fivefold growth in the installed‐d capacity for geothermal energy production and twofold growth or more for geothermal heating by 2030 relative to 2014 levels. Therefore, there are many (short, medium and long) targets for geothermal power generation global installed capacity as: 21 GW (by 2020), 65 GW (by 2030) and 140 GW (by 2050), as shown in Figure 2.9 [12, 26, 27].

Figure 2.8 Cumulative installed geothermal generating capacity by top 10 countries in 2019.

Figure 2.9 Global geothermal installed capacity from 1950 up to 2019 and its forecasting for 2020, 2030 and 2050. Adapted from [12], [26], [27].