General Information on Geothermal Energy

Abstract

The use of natural resources in a more responsible and comprehensive manner has become more relevant in recent years. The energy crisis and climate change have targeted the development of technologies that allow the use of renewable energies with greater performance, efficiency, and results. Geothermal energy plays an important role since it is available 24 hours a day, 365 days a year however, it represents a great challenge since its extraction is not a trivial fact, that is, every day it is necessary to further improve exploration techniques and exploitation to access increasingly deeper resources and greater energy potential. This section addresses the different applications that have been developed throughout the world and that serve as parameters and guides for their replication in Latin America, since due to its geothermal potential it has various opportunities to develop technology and agro-industrial production processes that are of Vital importance for human development in the coming decades.

1. Introduction

Geothermal energy is used worldwide for the generation of electricity directly from geothermal heat through the so-called direct uses; if it is used properly, it is possible to obtain an integral use of the Geothermal and its resource. Mexico and Latin America have enormous potential for Geothermal resources; however, successful projects for the comprehensive use of geothermal resources, such as the geothermal food dehydrator in Mexico, are just beginning to materialize.

Currently, the use of geothermal energy applied to direct uses has been an alternative to be used in industry, in general in other parts of the world, obtaining multiple benefits, and making geothermal projects sustainable [1, 2, 3, 4, 5, 6].

The opportunities for exploiting medium and low-temperature geothermal resources (<180°C) have great potential in Mexico and Latin America. These opportunities range from low-tech balneology to air conditioning applications and industrial services. In 1959, Mexico was a pioneer in Latin America in the exploitation of geothermal energy, however, it has lagged significantly in terms of the comprehensive use of direct uses of geothermal energy. For this reason, the iiDEA Group of the Institute of Engineering of the National Autonomous University of Mexico (UNAM) developed a methodology to achieve projects for the geothermal energy. Including several areas of specialization to obtain a successful and sustainable project, giving greater importance to the social components of the projects, and directing them to the benefit of local communities. This article describes the method to integrate technical, environmental, and legal frameworks with economic, social, and political characteristics. This method offers a guide to mitigate the barriers and challenges that prevent the development of direct use of geothermal projects, according to the specific needs of each geothermal resource and the local community that surrounds it. For example, technical issues such as temperature and mass flow of the resource; commercial issues such as the product’s marketing channel; political issues such as government support and the lack of specialized technical advice; and most importantly, include the local community in the operation and business of the projects.

2. Principles of geothermal energy

The thermal energy coming from the interior of the Earth is manifested indirectly through volcanism, thermal gradients in the ground, displacement of tectonic plates, and superficial geothermal emanations: lava, boiling mud pools, fumaroles, geysers, and thermal waters. Geothermal energy, as indicated by its etymological origin (lati. Geo-earth and thermo-heat), has already been explained and can be defined as the heat stored inside the earth. Its origin is associated with four sources:

1. Proto-Earth: Stage of formation of the planet Earth 6400 million years ago. Thermal energy is generated by shocks or impacts associated with meteorites that shaped the planet Earth. Heavy and light elements separate to form the core, mantle, and crust. The heat has been preserved by the insulating effect of the rock, gradually manifesting toward the surface.

2. Radiogenesis: Radioactive elements such as uranium, thorium, rubidium, and potassium generate 60% of the heat in the crust, where they are present in abundance compared to the mantle because their large atomic radii are less compatible with structures, minerals of said terrestrial layer; however, it is estimated that about 47 MWt of the mantle are generated by radiogenesis [7].

3. Gravitational pressure: Similar to Charles’s Law, where the temperature of a gas is increased by its compression, something similar happens in solids, except that the change in volume is not as evident as in gases. The heat is generated by the internal gravitational attraction or compression of the rocks, accumulating the heat in the depths.

4. Earthquake fault friction: Manifestation of local frictional heat along earthquake faults; the heat generated is enough to partially melt the rock (pseudotachylite). Most geothermal power plants harness the energy produced at these active faults [7].

2.1 Geothermal systems

They are different geothermal systems, of which hydrothermal systems are of great relevance because they are commercially exploited through geothermal fields with particular characteristics between each of them. The rest of the systems found in the earth’s crust, which are expected to be exploited once the technology are perfected, are geopressurized systems, magma chambers, and hot dry rock. The main characteristics of each of them are presented below.

2.1.1 Hydrothermal systems

They are very unique cases, considered as rare hydrothermal places; they represent less than 10% of the geothermal systems around the world. They are characterized by the accumulation of water, steam, or both, and this is what allows the conduction of heat from the most deep settlement toward the surface. Such a system has permeable formations that contain the fluid and is located within the economic range of drilling platforms so that some of the fluids can be brought to the surface for useful applications. The geothermal fields, through which the resource is exploited, can be:

1. Semi-thermal: Produces hot water up to 100°C at a depth of 1 or 2 km

2. Hyperthermals: They can be wet or dry, and are sometimes called water-dominant and steam-dominant fields, respectively.

   • Wet: They produce water under pressure at more than 100°C, while its pressure is lowered and removed, a fraction evaporates and turns into steam.

   • Dry: They produce dry saturated steam or slightly superheated at pressures above atmospheric.

2.1.2 Geopressurized

These fields are filled with pressurized hot water, its pressure exceeds 40 to 90% of the hydrostatic pressure corresponding to its depth. They are found at great depths (up to about 6000 m in some places), so drilling costs are high.

Magma chambers: Located at different depths, there are pockets of magma with temperatures between 600 and 1300°C, which is why they are considered manifestations with high energy content. However, they are not the usual ones and the technology allows them to handle high temperatures.

2.1.3 Hot dry rock at moderate depths

About 50 to 60% of the Earth has these manifestations of heat (between deep or superficial), from can already be extracted through methods that have been successfully tested, but the process still needs to be perfected to it to the level a commercial. The first research works were carried out in the 1970s and were called enhanced geothermal systems (EGSs). It consist of creating an artificial reservoir of hot water for its extraction in the form of steam. The Soultz project in France was a pioneer in the development of EGS and of which the technical feasibility was demonstrated; however, it was only possible to produce 50% of the expected steam.

2.1.4 Hot dry rock at great depths

It is a resource with greater abundance than the previous ones. Due to the great depth at which they are located, they are currently economically unfeasible. Surely derived from the improvement of the EGS systems, the first studies for their use can be developed.

2.1.5 Hydrothermal vents

There are submarine hydrothermal systems, with temperatures of up to 300°C, they can be shallow manifestations (depth < 200m) or deep; examples of surface manifestations in the world are found in Planty Bay, New Zealand; Ambitle and Lithir hydrothermal system, Papua, New Guinea; Cala Karaternaya, Russia; Milos Island, Aegean Sea; Eolian Islands, Italy; Punta Banda to the west of the Baja California Peninsula, the western part of the Gulf of California in Bahía Concepción and the central part of the Pacific coast of Mexico. Its main characteristics are the discharges that apparently have a considerable effect on the adjacent biological communities (benthic, planktonic, and phytoplanktonic), presenting a good adaptation to this type of habitat, with high concentrations of nutrients, heavy metals, and traces. They form along underwater fractures, so they align with them, forming groups in a row with separations of up to 20 m between each one. Currently there is no technology that allows its commercial use, and the way in which it would affect the biotic system that depends on these underwater manifestations has not been tested. Several prototypes have been designed and tested to generate electricity, developing organic Rankine cycle (ORC), Seebeck effect electricity generation systems, as well as water desalination processes, mineral extraction, and biofuel generation.

3. Installed potential and current energy consumption in the world

Variables in the properties of matter at high depths and pressures are not easily replicable in laboratories, so they are subject to estimation. H. Cristopher and H. Armstead (1989), cited in Ref. [7], estimated usable energy of 79 PJ. To arrive at this estimate, the approximate averages of the specific heat and thermal conductivity of the material under these conditions, the local differences in the thickness of the crust and the density of the rock were considered. It is worth mentioning that in this calculation only the energy contained in the crust was considered, so all the chemical energy present in the form of fuel, all the energy emitted by radioactive rocks, and all the heat conducted from the hot mantle were discarded.

The total heat of the crust could satisfy a large part of man’s energy needs for a long time, coming to be considered, in a certain sense, so abundant as to be a practically infinite source of energy; however, on a local scale, a field can be depleted by prolonged exploitation. It has been documented that for a field to maintain its production, it depends on drilling more wells in an area that is always expanding, reaching the limits of the geothermal system and then gradually decreasing its production. Therefore, it is important to continue with the development of technology that allows free access to 90% of the geothermal energy of the crust that is currently not exploited. EGS for heat extraction from hot and dry rock systems is being researched and developed in various parts of the world, the most representatives are presented in Table 1.

Country	Project	Developer/consortium	I + D	Pilot and demo
Electricity Generation
Korea	Pohang	DESTRESS		✓
Germany	Groß Schönebeck	DESTRESS		✓
Swiss	Haute-Sorne	DESTRESS	—	—
	Bedretto	DESTRESS	✓
	Etzwilen	Geo-Energie Suisse AG	—	—
	Triengen	Geo-Energie Suisse AG	—	—
	Pfaffnau	Geo-Energie Suisse AG	—	—
	Avanches	Geo-Energie Suisse AG	—	—
France	Soultz-sous-Forêts	DESTRESS		✓
Australia	Embalse Habanero	Geodynamics Ltd.		✓
	Depósito Parlana	Petratherm Ltd.		✓
USA	FORGE	Geothermal Technologies Office of the USDOE	✓
European Comission	GEOFIT	Consortium	✓
	MEET	Consortium		✓
Mexico	GEMex	SENER - CONACYT - UE	✓
Spain	Proyecto Güimar	Arllen Development** y ayuntamiento Güimar	✓
District Heating/Greenhouses
Lithuania	Klaipėda	DESTRESS		✓
Hungary	Mezőberény	DESTRESS		✓
Netherlands	Westland	DESTRESS		✓
	Middenmeer	DESTRESS		✓
Iceland	Geldinganes	DESTRESS	✓
France	Rittershoffen	DESTRESS	✓

Table 1.

EGS projects in the world.

^**

Slovak multinational.

— Canceled.

USDOE: Department of Energy EE. UU.; SENER: Secretary of Energy, Mexico; CONACYT: National Council for Science and Technology, Mexico; EU: European Union; DESTRESS: Demonstration of soft stimulation treatments of geothermal reservoirs. (Consortium).

The most exploited geothermal systems are hydrothermal and, to a lesser extent, EGS. The electrical generation cycles are by condensation, back pressure, ORC cycles, and kalina. The installed capacity in the world for the generation of electricity grows an average of 11% per year.

However, the production of electrical energy was not the first activity that was developed for the use of the heat of the earth; the first application was balneology, later the cooking of food and therapeutic applications. There is currently a record of 997 GWt installed for different applications, all of them cataloged under the generic name of direct uses.

4. Use of geothermal energy

Direct uses (DUs) are applications for thermal use in residential, commercial, and industrial sectors; Where before the heat from the burning of L.P. gas, natural gas, firewood, coal, or other was used, it is now replaced by the heat of the Earth. However, there is the development of electricity generation systems with moderate temperatures, up to 200°C, for local consumption and/or applied to the aforementioned sectors. Geothermal energy is classified into three groups, high (t > 200°C), medium (90≤ t < 200°C), and low enthalpy (30 ≤ t < 90°C). It is important to mention that the medium and low enthalpy scales can be relative, and the above depends on the environmental temperatures of each region; for very cold countries with temperatures below zero, having water at 10 °C from the Earth can be considered geothermal, but in very hot regions with temperatures up to 45°C, it may not be classified as such.

Based on this definition, the use of the resource for its various applications is regulated and controlled. Currently, the Lindal diagram is well known in honor of Baldur Lindal [8], who documented a series of geothermal applications based on the amount of energy/temperature/enthalpy of the resource, but over time it has been updated along with the evolution of industrial activities.

In search of more efficient use of medium and low enthalpy energy, a series of DUs are included that operate sequentially in the order of the energy level they require. The first applications are those that need more temperature, then those of intermediate temperature, and finally those of low temperature, until all the available thermal energy is extracted, with ambient temperature being the lower limit; quality and direction of energy are concepts explained by the second law of thermodynamics and that are perfectly exemplified through this practice called cascading uses (CU) (Figure 1).

Figure 1.

Cascading usage concept.

In 1995, only 28 countries used geothermal energy; in 2000, 58; 2005, 72; 2010, 78, 2015, 82, and finally in 2020, the figure reported was 87, with the participation of Bolivia, Burundi, Cyprus, Faroe Islands, Malawi, Malaysia, and Nigeria. With this growth in the use of geothermal energy (see Table 2), energy savings are 24.4 million tons of oil equivalent (TOE) per year (167 million barrels), leaving 36 million tons burning coal (96 million tons of CO₂). Worldwide, there are 279 cases of DU, and some of them integrate an entire CU system; the distribution of said DU can be seen in Table 3.

4.1 Direct uses, background, and current situation

Despite the fear generated in the world by the most violent manifestations of geothermal energy, such as volcanoes, due to the destruction of Pompeii and Herculaneum [7], it did not take long for humanity to explore the benefits of thermal waters, since the appearance of the belief in drinking water as a prophylactic remedy for its healing and laxative properties, spreading and popularizing these practices, thus giving rise to balneology.

4.1.1 Balneology

It flourished at the time of the Roman Empire; however, it was a practice that came from the Greeks. Baths, as they were known, were established as meeting places, in some ways comparable to the coffee houses of the eighteenth century London. In North America, Paleo-Indians used minerals for medicinal purposes and hot springs were neutral zones where members of warring nations should bathe together in peace.

Spas, hydro-treatments, and jet baths spread throughout Europe and elsewhere, frequented by invalids, hypochondriacs, and those who simply flocked to them for pleasure. Subsequently, the concept of SPA emerged, which comes from the Latin abbreviation S: sanitas, P: per, A: aquas [12], or health through water. In the nineteenth century, SPAs in Europe were very sophisticated and elegant centers, like the famous spas in France, Germany, and Burma.

During World War II, in Rotorua, New Zealand, the Queen Elizabeth Hospital used mineral waters and mud from hot springs to help soldiers in the Pacific wars recover from war wounds. Currently, in European countries and Japan, they have specialist doctors who attend spas, with the aim of treating or preventing different diseases [13].

Yasuhiro Ishikawa, better known as Dr. Bath, in Japan, states that the typical bath is necessary to maintain proper hygiene, but it is not enough to improve the immune system, prevent diseases and maintain physical and mental health as is achieved with baths, hot springs baths enriched in salts. The effect of these salts has been studied, and the adequate retention of heat has been verified by means of a thermographic camera, activating the cells that increase heat-shock proteins that delay the production of milk acid in the muscles.

Currently, of the 87 countries that take advantage of low enthalpy geothermal energy, 72 do so through authorized thermal centers, mainly for tourism. Between all of them, there is an installed potential of 24,190 MWt.

4.1.2 Mineral extraction

After the development of balneology and some culinary practices such as cooking fish, eggs, and some vegetables in geothermal fumaroles, the extraction of minerals and salts present in the hot water was developed. In 1818, in Larderello, Tuscany (Italy), boron salts were exploited for the first time for industrial purposes; the evaporation was done inside a brick dome, which was known as Covered Lagoon. Near the dome, deep wells were drilled to access hot water with high concentrations of boron. In 1827, the founder of this industry, the French Francois Larderel, developed a system to take advantage of the heat of the fluids in the evaporation process, leaving aside the combustion of wood. It was considered the most important industry in Europe, and by then sulfur, vitriol, alum, and boric acid were already being extracted. Over time, however, electricity generation became more valuable than geothermal mining.

Currently, a new industry is growing driven by the new energy economy, and it is due to the great demand for lithium that has been increasing at an unprecedented rate. For the next decade alone, an increase in demand of just under 10 times the current one is expected (in 2018, the demand was 150 thousand tons, by 2028 an increase to 1.5 million tons is expected), a demand that cannot be satisfied with conventional lithium extraction processes, since they are expensive and cannot generate the production volumes necessary for a future in which the vehicle fleet will be mostly electric.

Unlike current extraction methods, geothermal lithium extraction is a closed-loop system that returns spent brine to its original source; and by virtue of the fact that it is an activity derived from electricity generation, it benefits from the electrical energy produced in these plants, so it is a process that operates with 100% renewable energy. In a matter of hours, not months, compared to traditional technology, it produces high-purity lithium; its environmental advantages are its small carbon footprint, close to zero, it is not dependent on weather or water, it does not require open pit mines or large evaporation ponds, and it operates 24/7.

Currently, lithium production is concentrated in Australia, China, and South America (Argentina, Chile, and Bolivia). According to Benchmark Mineral Intelligence, the United States currently produces 1% of the world’s raw lithium materials and only 7% of its lithium chemicals, while China, Japan, and South Korea produce 85% of the chemicals of lithium needed to power electric vehicles. For this reason, the US has recognized the need to develop a strategy around its critical mineral security.

4.1.3 Domestic service: Heating and hot water supply at the district level

The Greeks and, later, the Romans were the ones who left examples of applications with geothermal energy, as already mentioned, for which hot water was distributed through an open network that connected to the basements of buildings. These practices were spread by the Romans and eventually reached Japan, America, and Europe.

In 1332, in Chaudes-Aigües, France, a new distribution system with hollow logs was installed, serving 30 houses, as well as activities related to the washing of wool and fur. Also in France, but in 1833, in the Grenelle district, Paris, the first borehole began, through an artesian well 548 m deep, which took 8 years to build and from which drinking water at 30°C was extracted.

The first modern district heating network powered by geothermal energy was installed in Reykjavík, Iceland, in 1930. Since then, this innovative heating system has spread throughout the world, with plants installed in France, Italy, Hungary, Romania, Russia, Turkey, Georgia, China, the United States, and Iceland, in Iceland itself, where currently 95% of its inhabitants have heating through a network of 700 km of insulated pipes that transport hot water. In 1947, Kemler, E.N., in his publication “Methods of earth Heat Recovery for the Heat Pump” already showed the diagrams of the different connection methods of heat pumps.

After the Second World War and due to the expansion of other cheaper energy sources, mainly petroleum derivatives, this system was left aside. District heating networks continued to be installed in Europe during this period, mainly in the Nordic countries due to the shortage of natural gas and electricity. In the 1970s, with the oil crisis, district heating networks regained their importance, especially in the United States, as well as in Northern Europe, Russia, Japan, China, and Korea, initiating an intense activity of exploration and investigation of geothermal resources in order to use them for the production of electrical energy or for heating and hot water. In this way, the development of geothermal energy was stimulated, and global geothermal production increased from 400 Wt in 1960 to 15,847.2 MWt in 2020.

These heating systems, widely used in Europe, represent 30% of the total energy used for conditioning spaces and water, benefiting 75% of all buildings that have heating [14]. The main countries with high installed capacity for this geothermal use are listed in Table 4.

	Country	1995; MWe	2000; MWe	2005; MWe	2010; MWe	2015; MWe	2020; MWe
	Total	6866.80	7974.10	9067.10	10,716.70	12,635.10	15,847.2
1	USA	2816.7	2228.0	2544.0	3093.0	3450.0	3700.0
2	Indonesia	309.8	589.5	797.0	1197.0	1340.0	2289.0
3	Philippines	1227.0	1909.0	1931.0	1904.0	1870.0	1918.0
4	Turkey	20.4	20.4	20.4	82.0	397.0	1549.0
5	New Zealand	286.0	437.0	435.0	628.0	1005.0	1064.0
6	Mexico	753.0	755.0	956.0	958.0	1017.0	1005.8
7	Italy	631.7	785.0	790.0	843.0	916.0	916.0
8	Iceland	50.0	170.0	322.0	575.0	665.0	755.0
9	Kenya	45.0	45.0	127.0	167.0	594.0	1193.0
10	Japan	413.7	546.9	535.0	536.0	519.0	550.0
11	Costa Rica	55.0	142.5	163.0	166.0	207.0	262.0
12	El Salvador	105.0	161.0	151.0	204.0	204.0	204.0
13	Nicaragua	70.0	70.0	77.0	88.0	159.0	159.0
14	Russia	11.0	23.0	79.0	82.0	82.0	82.0
15	Papua New Guinea	0.0	0.0	39.0	56.0	50.0	11.0
16	Guatemala	33.4	33.4	33.0	52.0	52.0	52.0
17	Germany	0.0	0.0	0.2	6.6	27.0	43.0
18	Portugal*	5.0	16.0	16.0	29.0	28.0	33.0
19	China	28.8	29.2	28.0	24.0	27.0	34.9.0
20	France**	4.2	4.2	15.0	16.0	16.0	17.0
21	Ethiopia	0.0	8.5	7.0	7.3	7.3	7.3
22	Austria	0.0	0.0	1.0	1.4	1.2	1.3
23	Australia	0.2	0.2	0.2	1.1	1.1	0.6
24	Argentina	0.6	0.0	0.0	0.0	0.0	0.0
25	Romania	0.0	0.0	0.0	0.0	0.1	0.0
26	Taiwan	0.0	0.0	0.0	0.0	0.1	0.3
27	Thailand	0.3	0.3	0.3	0.3	0.3	0.0

Table 2.

IGA data [9, 10].

CODE	Capacity MWt
	2020	2015	2010	2005	2000	1995
GHP	82,319.61	50,258.00	33,134.00	15,384.00	5275.00	1854.00
H, & D	6113.02	7602.00	5394.00	4366.00	3263.00	2579.00
G	9124.37	1972.00	1544.00	1404.00	1246.00	1085.00
F	949.67	696.00	653.00	616.00	605.00	1097.00
A	256.66	161.00	125.00	157.00	74.00	67.00
I	852.77	614.00	533.00	484.00	474.00	544.00
B	24,190.42	9143.00	6700.00	5401.00	3957.00	1085.00
C & S	435.07	360.00	368.00	371.00	114.00	115.00
O & K	110.99	79.00	42.00	86.00	137.00	238.00
Total	124,352.58	70,885.00	48,493.00	28,269.00	15,145.00	8664.00

Table 3.

Installed capacity in TJ/year per application developed in the world [3, 11].

I = Industrial process heat; H = Individual space heating (other than heat pumps); C = Air conditioning (cooling); D = District heating (other than heat pumps); A = Agricultural drying (grain, fruit, vegetables); B = Bathing and swimming (including balneology); F = Fish farming; G = Greenhouse and soil heating; K = Animal farming; O = Other (please specify by footnote); S = Snow melting; GHP = Geothermal Heat Pumps.

No	Country	Capacity (MWt)
1	Iceland	1650.00
2	Turkey	1453.00
3	France	510.00
4	Germany	349.54
5	China	346.00
6	Hungary	300.60
7	Italy	225.00
8	Russia	220.00
9	Japan	203.34
10	USA	179.03
	Others (30 countries)	676.52

Table 4.

Top 10 countries with the largest installed capacity for heating, 2020.

Distinto a GHP.

On the other hand, the heating of spaces through geothermal heat pumps (GHP) is presented independently because they normally work with a fluid that is not necessarily geothermal; however, because the heat transfer is carried out with a constant temperature from the earth, somehow it is still considered as energy coming from the Earth. A GHP consists of a closed system of high-density polyethylene pipes through which water (not geothermal water) flows; This fluid transfers or absorbs energy from the earth depending on the season of the year. The leading countries in the implementation of this type of system are listed in Table 5.

No	Country	Capacity (MWt)
1	China	26,450.00
2	USA	20,230.00
3	Sweden	6680.00
4	Albania	4497.00
5	Germany	4400.00
6	Finland	2300.00
7	Switzerland	2172.00
8	France	2015.00
9	Canada	1822.50
10	Ukraine	1600.00
	Others (44 countries)	10,153.11

Table 5.

Top 10 countries with installed capacity in GHP.

4.1.4 Aquaculture and agricultural products

Aquaculture is defined as the farming or rearing of aquatic species, such as catfish, tilapia, sturgeon, largemouth bass, shrimp, tropical fish, crustaceans, molluscs, aquatic plants, and even alligators. This activity is carried out under controlled conditions with the aim of favoring the development of the specimens. The use of geothermal energy directly or indirectly in heating the habitat of the species depends on the quality of the water since normally the geothermal water or brine has dissolved salts and minerals that can be harmful. A heat exchanger is generally used to transmit this energy to the water in the ponds. The rearing of species in controlled warm environments affects chemical and biological processes (such as metabolism) favoring larger and more developed specimens in less time, an ideal practice in ectothermic¹ organisms. Table 6 shows the top 10 countries with aquaculture development.

No	Country (MWt)	Ref.	Project
1	China; (420.00)	[15, 16]	There are 14 units that use geothermal water to raise left mouth fish, grouper, tilapia, white-backed turtles, crabs, and prawns. The reported aquaculture area is 6.47 × 105 m^2. A total of 300 aquaculture farms are reported and represent 4.45 × 106 m^2 throughout the country. It saw strong growth in Beijing, Tianjin, Fujian, and Guangdong, as well as 20 other provinces, mainly Guangdong and Fujian. Of the total geothermal water used in the country, only 5.7% is used for aquaculture.
2	Italy; (130.00)	[17, 18]	An installed capacity of 1425 MWt for direct uses is reported, of which 4.6% corresponds to aquaculture. The projects are located in Orbetello (Tuscan coast) and the Apulian coast. It is considered the most important aquaculture industry in Europe. The production of Orbetello alone is estimated at 900 tons/year with a staff of 60 workers.
3	USA; (122.13)	[19, 20, 21, 22, 23, 24]	Liskey Farms Inc.; It is a company dedicated to the breeding of tropical fish. The temperature of the 37 ponds is 23°C (74°F).; Idaho Fish Breeders Inc.; It is dedicated to the breeding of catfish for its commercialization. Their activity began in the early 1970s. They maintain a constant temperature in the ponds between 27 and 29°C.; Oregon Institute of Technology.; Research began in 1975 with the idea of harnessing the remaining thermal energy from the University’s heating system. Giant freshwater prawns (Macrobrachium rosenbergii) are farmed. The goal is to have a constant temperature in the ponds around 27 ± 1°C.; Fort Indian Community, California.; They market catfish, mainly in San Francisco. They use geothermal water at 40°C, mix it with fresh water, and heat the ponds to 27°C.; Various projects.; Throughout the country, 27 projects are reported with an installed capacity of 122.13 MWt.
4	Iceland; (110.00)	[25, 26, 27]	In 2013, there were 70 aquaculture parks that together produced 70 thousand tons of salmon, mainly arctic char and trout. Of those plants, only 20 were powered by geothermal energy. Currently, the main producers are two. The first is the Íslandslax plant, belonging to Samherji company. It has 2000 m^2and a water volume of 1500 m^3. For its process, 10 kg/s of geothermal water at 90°C is required. The other plant is Silfurstjarnan located in the north of the country in Öxarfjördur. It annually produces thousand of tons.
5	Israel; (31.40)	[28, 29]	Aquaculture in Israel is a unique practice in that it has been developed throughout the Negev desert; there are about 15 aquaculture centers, established since 1980. These aquaculture centers produce fish for human consumption (barramundi (Lates calcarifer), red croaker (Sciaenops ocellatu), European sea bass (Dicentrarchus labrax), North African catfish (Clarias gariepinus), Nile tilapia (Orechromis niloticus), ornamental fish and some crustaceans. Only ornamental fish are exported.; Aquaculture is developed in two regions: Jordan Valley at Hammat Gader Springs, where four springs of different temperatures provide warm water (27°C) for fish and shrimp farming. It is found along the Mediterranean coast, where numerous ponds have been established. The hot water (26°C) comes from different shallow geothermal wells (30 m deep).
6	New Zealand; (17.00)	[30]	The first aquaculture farm was established in Taupo, Wairakei in 1987. Fresh water from the Waikato River is conditioned with geothermal hot water at a constant temperature of 28°C. The study began with 20 males and five females imported from Malaysia; a year later, 25 males and 30 thousand post larvae were imported again from the same region.
7	Argelia; (15.00)	[31, 32, 33, 34]	Meanwhile, a production of 1700 tons/year of tilapia is estimated. The geothermal water used (from the Sahara desert) has a temperature of 40°C. The projects are 80% financed by the government as an initiative of the National Aquaculture Development Plan.
8	France; (9.40)	[35]	Take advantage of the remaining energy from the heating of greenhouses (5 hectares) for the cultivation of eels and sturgeons. From a flow of 200 m^3/h of hot water at 75°C, cold water from the L’Eyre river is heated to condition it up to 17°C. Annual production of 10 tons of eels and 20 tons of sturgeons is estimated.
9	Japon; (7.61)	[36]	Tilapia farming in a cold area of eastern Hokkaido, shrimp in Fukushima, and tiger puffer fish in Tochigi.
10	Argentina; (7.03)	[37]	It has two aquaculture projects, one in the Greater region and another in Bahía Blanca.

Table 6.

Top 10 countries with aquaculture development.

Regarding the development of agriculture through heated greenhouses with geothermal energy, either directly or through the use of GHP, it represents a great opportunity for countries that do not have the right climatic conditions to produce certain foods throughout the year, or simply it would be impossible. An example of this, which at the time caused euphoria among Icelandic farmers, was in 1930, when the cultivation of tropical fruits, such as bananas, Began; Iceland currently has more than 200 thousand m² dedicated to greenhouses that supply fruits and vegetables (mushrooms, tomatoes, strawberries, flowers, and bananas) to the country’s supermarkets, even allowing some export sales [38].

This practice has allowed the saving of 77 million m³ of gas in the Netherlands, where the dependence of horticulture (greenhouses) on fossil fuels denotes the energy risk that can be solved with this type of energy [39].

The dehydration of food is another novelty, which consists of eliminating the moisture that food has. This can be done by different methods, and one of them is by hot air. This process consists of extracting heat from geothermal water through the use of a radiator (water–air heat exchanger), in this way the air is heated, which is then used to dry food, ranging from cereals, tubers, crops oilseeds, vegetables, spices, cocoa (Theobroma cacao L.) and coffee (Coffea L. Rubiaceae), fruits, and medicinal plants.

4.1.5 Industrial

The activities can be as varied and even as particular as enhanced oil recovery, mineral or rare earth leaching, metal mining (such as gold), mushroom cultivation, pulping for paper, drying of wood, and the tanning of wool or leather. Currently, the developments in this line and the new trend toward cogeneration processes or simply the creation of new production processes in order to optimize energy, agricultural, and natural resources in general, projects of singular interest have been developed (Table 7).

Country	Ref.	Project
Italy	[40, 41, 42]	Cheese factories, nurseries, and craft beer.; Vapori di birra is a brewery that harnesses geothermal energy. Its two characteristic beers are Geyser and Solfurea e la Magma, each with its own characteristics and peculiarities.
Iceland	[43]	The hot water that flows into Urriðavatn is the only certified drinking hot water in Iceland, so it has been used to produce a special beer as well as geothermal tea.
Kenya	[44, 45]	The development of an industrial park is sought in Mai Mahiu, Naivasha, where government capital projects offer alternatives in new jobs and with productive activities that represent important income in the state. The GDC company is testing prototypes for: milk pasteurization greenhouses aquaculture laundries cereal dryer
New Zealand	[46, 47, 48, 49, 50, 51]	Imanaka (Japanese company) in association with a group of Maori entities has created the company Waiu Dairy (67% of the shares belong to 11 Maori entities and 33% to the Japanese company). Its objective is to produce 8 thousand metric tons of powdered milk with geothermal steam (4 to 6 tons/h at 16 Barg). Oji Fiber Solutions (OjiFS) is a pulp mill in Tasman, Kawerau. It consumes from 20 to 50 tons/h of geothermal steam at 12 Barg. With this project, greenhouse gas emissions have been reduced by 10,000 tons of CO2/year.
EUA	[52]	Klamath Basin Brewing Company opened in 2005 after renovating the historic Crater Lake Creamery building is located in Klamath Falls, Oregon. Currently, 10 different beers are brewed.

Table 7.

Examples of industrial development with geothermal energy.

4.1.6 Electricity generation with low-temperature geothermal resources

4.1.6.1 Historical panorama of electricity generation with geothermal energy

With the development of populations toward a modern civilization, geothermal energy was used to meet the energy needs that said civilization had been demanding. At the beginning of the twentieth century, Prince Piero Ginori Conti experimented with geothermal steam to produce electrical energy. After several years of experimentation, in 1904, he managed to turn on five light bulbs; he used a piston coupled to a 10 kWe dynamo. The system was powered by steam that was produced in a heat exchanger, which was fed with steam from the geothermal field near Larderello. The temperature of the resource was generally 150°C, with which pure water evaporated. Currently, it is possible to take advantage of geothermal resources with temperatures from 60 to 150°C using organic Rankine cycles, so the Principe Piero plant can be classified as the first binary cycle, water–water.

In 1908, a 20 kWe generating plant was installed, supplying power to the main industrial and residential buildings in Larderello. Five years later, the first commercial plant was built in Larderello, consisting of a 250 kWe turbine, manufactured by the Electromecánica Tosi company, which was designed to work with dry steam at 3 bar pressure at the wellhead. The steam was generated in a heat exchanger fed with geothermal steam at a temperature of 200 to 250°C. In 1923, the installed capacity in electricity generation was equalized with hydroelectric power plants; it was achieved with the generation of 3.5 MWe, and generated with the first direct geothermal power plant, that is, the steam produced was entered directly into the turbine, improving the use of the energy. Energy, without having to evaporate pure water. In 1930, there was an installed capacity of 12.15 MWe, of which 7.25 were generated in indirect cycles and 4.9 came from direct cycles.

The Larderello region was a strategic region, it provided electricity to the entire railway network of central Italy, for which it was bombed in the spring of 1944; together with all the geothermal power plants, chemical plants in the area and the production wells, with the exception of the 23 kWe well, which has served as a school well for the training of technical personnel from 1925 to date. After World War II, installed capacity recovered and by 1950 there were about 300 MWe. Until now, the technology developed only served to generate electrical energy with the dry steam that was produced, but in fields such as those in New Zealand, wet steam was available. In November 1958, the first groups of turbines were installed, five high pressure and two intermediates pressures, which used the wet steam that characterized the Wairakei geothermal field. The biphasic mixture was led to a cyclone separator, the resulting water was evaporated with pressure reduction, evaporating between 15 and 20%.

Over time, the steam that fed the high pressure turbines decreased, and currently only the intermediate pressure turbines are working, and three more low-pressure turbines have been installed. Other New Zealand fields, Ohaaki, Rotokawa, Mokai, Kawerau, Ngatamariki, Tauhara, and Ngawha, have been developed and between them have an installed capacity of 15,854 MWe.

4.1.6.2 Organic Rankine cycles

One of the technologies that has gained the most interest in recent years are ORCs [53], systems capable of generating electricity from low-temperature energy sources (less than 180°C), using working fluids whose evaporation temperatures are lower than that of water.

The main systems that make up an ORC are shown in Figure 2. To extract the geothermal resource, a pump is required, which is responsible for making the fluid reach the heat exchanger (evaporator), to later be returned to the geothermal reservoir or to be used in another process.

Figure 2.

Schematic diagram of the ORC diagram [by the authors].

The path of the working fluid begins in a storage tank, from where it is pumped (normally with a centrifugal pump) to the evaporator, where the heat transfer from the geothermal resource to the organic fluid will take place. Once the desired temperature is reached, the fluid will pass to the axial turbine, to rotate its blades and thus obtain electricity through an electric generator. Finally, the fluid is sent to the second heat exchanger (condenser), where the temperature of the working fluid is lowered using cooling water. At the end of the cycle, the fluid returns to the storage tank and the process is repeated.

The cooling water is sent to the condenser to obtain the heat from the organic fluid, so it leaves the exchanger with a higher temperature. Therefore, a cooling tower is required to lower the temperature using fans and a pump to send the fluid back to the condenser. It should be noted that this step is omitted if the heat exchanger is replaced by an air condenser, since the working fluid will be cooled directly with air, so cooling water would not be required. This equipment is generally used when there is no water available at the installation site.

Regarding the working fluids, these can be selected from a long list of candidates, including hydrocarbons, hydrofluorocarbons, siloxanes, and mixtures of these components [54], each with different thermodynamic properties.

Among the first commercial ORC-type plants were the following (Bronicki, 2017):

1. In 1952 in Kiabukawa, Congo, a small 200 kW unit was installed that used water at 91°C.

2. 1966, Ormat (Mali, Africa), creates a 0.6 kW solar turbogenerator, using dichlorobenzene.

3. 1967 in Russia 500 kW geothermal plant using R12.

4. 1979 at Kawasaki, Japan with 2900 kW using Freon 11.

5. 1979, Ormat built a 150-kW solar pond.

6. 1979 McCabe (California) builds a 12.5 MW ORC in cascade with two fluids, isopentane, and isobutane.

7. 1982, Ormat installed 15 kW in Mexico using freon 113.

5. Methodology for projects of direct use of geothermal energy, sustainable development

The implementation of direct use projects in Latin America has had an incipient growth with respect to other places in the world, however, it should be noted that with respect to electricity generation, there is a greater area of opportunity [55, 56, 57]. This methodology is intended to develop projects for direct use that generate a positive impact on society, the economy, and the environment. This seeks the understanding and acceptance of these projects by the communities, which would serve as a spearhead for the development of larger projects, for example, electricity generation.

Sustainable development can be defined as a dynamic process, or an action plan or road map, toward a desirable future state for human societies in which living conditions and resource use continue to meet human needs without undermining the integrity, stability, and beauty of natural biotic systems. The efficient use of resources through saving and reuse provides an opportunity for each human being to develop freely, in balance with society and in harmony with the environment; that is, to avoid the loss, change, deterioration, impairment, adverse effect, or modification of the habitat, ecosystems, elements, and natural resources, of their chemical, physical, or biological conditions, of the interaction relationships that exist between them, as well as the environmental services they provide [58].

Therefore, it is considered that sustainable development is built on three fundamental pillars, which work in harmony for the gestation of true sustainable development, all with the aim of guaranteeing the right of every person to live in a healthy environment for their development, health, and wellness, see Figure 3.

Figure 3.

Venn diagram of sustainable development at the confluence of the three pillars that characterize it.

5.1 Characterization of geothermal zones of low enthalpy

Currently, there are many works to estimate geothermal potential, where medium and low enthalpy resources stand out. This information is essential for the spatial location of the future project, information that will be concatenated with another set of data on its viability and type of project.

5.2 Hierarchy of areas with the highest probability of success

The hierarchization of zones of greater probability is a job that requires secondary sources, to carry out the analysis of the social, environmental, legal, and productive conditions of the geothermal zones of interest. This ranges from analyzing the number of men and women in the region, the immigration rate, and the distribution of the population in the educational level of 15 years and over. It is important to verify if the area of interest is not within an ecological reserve or one with a high environmental impact. The activities and products that are already carried out in the area to support them instead of inserting new ones.

With this analysis, it is possible to prioritize the areas that do not have environmental barriers, that have agro-industrial activities with thermal processes below 150°C and with a target population that wants to participate in the projects, with which a preliminary study is carried out feasibility by zones and in this way the target population is identified, that is, the one that has the resource and the area of opportunity for the development of direct use projects.

5.3 Measurement of the interest-acceptance of the population

This section of the methodology consists of generating instruments and tools that allow working directly with the communities, and these tools focus on the exploratory march and social mapping. These visual tools will help to engage the community with the project and it will be possible to detect the way in which the inhabitants perceive their space.

5.4 Choose lines of business model

The possibilities of successful productive projects are wide; however, projects that are understandable and accepted by the surrounding community are recommended, so the activities already carried out before the project must be taken into account so that their introduction is more natural [59].

Once said analysis is completed, the proposal will be presented to government entities or private investors in order to obtain the economic resources that contribute to the execution of the project.

5.5 Realization of the project

Finally, the project is developed, and in many cases, it is intended to be a turnkey project, which means that a 100% functional and operational production process is delivered to the community/client/investors, which implies the training of the personnel that plant or unit will be in operation. The steps to follow are grouped into four sequential steps and their acronym is called IDEA Development, an acronym that defines Identify, Develop, Evaluate, and Advance².

6. Conclusion

The use and efficient use of energy is a very popular topic in recent years to date; due to the energy crisis, the exhaustion of the main oil reserves, the growing demand for energy in the world, the reforms to the law in countries like Mexico, which speak of the right to a healthy environment, and last but not least, climate change.

Most DUs use geothermal energy left over from a previous process, this is known as cogeneration, and local geothermal development has many benefits, including social ones. The comprehensive use of resources accelerates the rates of return on investments, lowering the cost of energy, and consequently also increasing profits from the development of new comprehensive projects. However, the generation of jobs is the key factor that allows the integration of communities in the development of geothermal energy, leaving the doors open to the development of large projects such as geothermal power plants.

It is worth mentioning that most of the projects that have an agri-food purpose are aligned with the 17 sustainable development goals published by the UN, of which goals 2) Zero Hunger, 5) Gender Equality, 7) Affordable and non-polluting energy, 8) Decent work and economic growth, and 13) Climate action [58].

On the other hand, the production of electrical energy with binary cycles offers a great opportunity in the field of energy efficiency by being able to take advantage of the residual heat of industrial processes such as medium and low enthalpy geothermal energy to produce electricity, and recently, in applications of microsystems for the generation of heat, cold, and electricity (in so-called trigeneration applications) in homes or small commercial units, from the point of view of smart networks, which although they are in the demonstration phase, the investment and maintenance costs they are perceived as affordable and capable of saving primary energy with low GHG emissions [60].

Considering complementing the stationary electricity generation scheme with an on-site generation scheme, also known as distributed generation, through technologies such as binary cycles, will allow, among other aspects, to make the electricity supply more efficient, reduce transmission problems, decongest the electrical systems of each country, increase the efficiency of industrial processes through cogeneration schemes, thus reducing internal electricity demand, and as users, become independent from electricity companies, with economic benefits from the sale of electricity in those cases where there is great recovery potential.

Distributed generation is a relatively new concept that has been developed to reduce the operational problems and generation costs of electricity generation and transmission systems in a country.

The main characteristics of distributed generation are the following: 1) it reduces losses in the network by reducing energy flows to remote consumption areas, 2) the energy generated normally goes to the consumption centers and does not reverse flows in the transmission lines, 3) generation capacity generally ranges from a few tens of kW to 10 MW, and 4) for rural areas, distributed generation sources are generally mini or micro-hydroelectric, geothermal, and/or cogeneration plants that take advantage of waste from industries or agricultural to generate electricity on a small scale.

Some recommendations to encourage the development of electricity generation projects with low enthalpy resources are: 1) carry out a reassessment of the potential of the country/site of this resource with modern remote sensing technologies, supported by terrestrial measurements, 2) develop and adapt technology to quantify in detail the punctual resources (assessment of small shallow reservoirs), 3) develop and adapt technology to drill small geothermal wells, 4) technology to pump very hot water, but at a shallow depth, 5) develop or adapt the technology for small generation plants (<1 MW), and 6) include small geothermal energy in the legislation for the promotion of renewable energies.

Finally, to promote the development of small low enthalpy geothermal fields, it is required: 1) a good understanding of the size of the resource (how much hot water can be extracted without depleting it), 2) technology to extract hot water (drilling with preventers; extraction with deep well pumps), 3) economical and reliable technology for generating electricity (Turbines and associated equipment), and 4) legislation to market the energy generated (own uses, connection to the network).

Appendices and nomenclature

UNAM

Universidad Nacional Autónoma de México

EGS

Enhance Geothermal Systems

ORC

Organic Rankine Cycle

DUs

Direct Uses

CU

Cascade Uses

TOP

Tons of Oil Equivalent

GHP

Geothermal Heat Pumps