What's that bubbling stream of heat gushing underground?

Geothermal energy (geothermal energy) is natural heat energy extracted from the earth's crust, which comes from molten rock inside the earth and exists in the form of heat, the energy that causes volcanic eruptions and earthquakes. Volcanoes, geysers, hot springs, and boiling mud pots are all strong indicators of the existence of large stores of thermal energy in the crust and beneath it. This heat seeps out of the earth's surface, giving rise to geothermal heat. Geothermal energy is a clean, renewable energy source with a very bright future for its development. The famous geologist Li Siguang (1973) has pointed out that: "the earth is a large heat reservoir, the development and utilization of underground thermal energy, is a big thing, just like mankind found that coal, oil can be burned, this is a new energy source opened up in the history of mankind, is also a new field of geological work."

Geothermal resource (geothermal resource), refers to the internal heat resources of the earth that can be economically utilized for human beings, which comes from the heat emitted by the earth's molten magma and the decay of radioactive elements. Geothermal resource is a very valuable comprehensive mineral resources, its function, wide range of uses, not only is a clean energy resources for power generation, heating and other uses, and is a hot brine resources and natural fertilizer resources for the extraction of bromine, iodine, borax, potassium salt, ammonium salt and other industrial raw materials, but also valuable medical hot mineral water and drinking mineral water resources as well as water supply for domestic water supply source.

I, geothermal energy sources and distribution

(A) the source of geothermal energy

Geothermal energy is mainly generated by the decay of radioactive substances within the Earth. At the center of the Earth is the molten core, which can reach temperatures of up to 4,000°C (7,200°F), and is surrounded by the mantle, which consists of semi-liquid material (Figure 4-52). Overlying the mantle is the crust, which averages about 17 km thick. The temperature of the crust increases with depth, rising about 30°C for each 1 km of depth. The temperature at the bottom of the crust (upper mantle) is more stable at about 1000°C, and the temperature increase inside the core is sluggish. If only the average geothermal temperature gradient is considered, all of the heat energy that can be fully utilized is stored below the crustal depth. However, in some zones, mantle lava (magma) is transported to the surface along faults or cracks, forming a "hot spot" 2-3km above the surface, which enriches a large amount of geothermal resources near the surface in localized areas, such as seismic and volcanic zones at the interface of the Earth's tectonic plates. Earth has six major tectonic plates: the Pacific Plate, the Eurasian Plate, the Indian Ocean Plate, the African Plate, the American Plate and the Antarctic Plate. These plates separate, grow, and move outward along the spreading centers (i.e., oceanic ridges) on either side, while the plates move relative to each other, slide past each other, or stagger each other along the horizontal direction (Figure 4-53). Where these plates collide and squeeze each other, the tremendous forces can produce earthquakes or uplift into mountain ranges. At plate junctions, heat energy is rapidly transported from the Earth's interior to surface volcanoes through subterranean magma. As a result, plate margins have become the main distribution zones for high-temperature geothermal fields.

Figure 4-52 Hierarchical structure of the Earth

Figure 4-53 Relative motions of the Earth's six major plates and some smaller plates (arrows indicate the direction of plate motions)

(According to the Institute of Geology and Earth Research, Pisa, Italy, 2004)

1 - Distribution of geothermal fields; 2 - Conversion of trans-oceanic ridges; 3 - Distribution of geothermal fields; 4 - Distribution of geothermal fields; 5 - Distribution of geothermal fields. -Transform faults cutting across mid-ocean ridges; 3-Subduction zones

(ii) Types and distribution of geothermal

The distribution of geothermal resources worldwide is characterized by an obvious regularity. High-temperature geothermal resources are concentrated in the relatively narrow active zone of the earth's crust, i.e., the boundary of the global plate; while low-temperature geothermal resources are widely distributed in the inner plate, but high-temperature geothermal resources may also be distributed in some of the hot spots and hot columns in the inner plate.

Geothermal tropics are divided into two categories: plate-edge (or interplate) geothermal tropics and plate-interplate tropics. Plate margin geotropical refers to the plate boundary along the spread of relatively narrow but extend up to thousands of kilometers of high temperature geotropical. Plate margin geotropical zone because of the global scale, and the first and last, so often also known as the global geotropical zone, it is characterized by:

(1) a clear band distribution;

(2) geographically overlap with the global seismicity zone and active volcanism, or is located in the back edge of the young orogenic zone;

(3) the band of volcanoes more acidic or acidic magma, the magma source is shallow, and with the young orogenic zone, the magma source is shallow, and the magma source is shallow. The magma is of shallow origin and is associated with local remelting activities within the crust, thus constituting a direct heat source for shallow hydrothermal activity;

(4) the intensity of hydrothermal activity is very high, and hydrothermal explosions, intermittent geysers, and most boiling springs are found in the geothermal zone of the plate rim;

(5) the hot springs often discharge water of the sodium chloride type, and often contain some volatile components of the magma;

() (6) Large high-temperature geothermal fields of high economic value often occur.

Tropical zone within the plate generally refers to the widely distributed in the plate internal zone uplift area (folded mountain system, mountain basins) and crustal subsidence area (mainly large Cenozoic sedimentary basins) is relatively small scale of the low-temperature zone. The intraplate tropics are non-volcanic, with no volcanic or magmatic heat sources. In the plate either uplift area or subsidence area, in the tectonic fracture zone or some self-flowing basin, there are rich storage of medium and low temperature geothermal energy resources (150 ℃ or less), geothermal field temperature is generally lower than the local boiling point, mostly between 60 ~ 90 ℃.

Two, geothermal system

(A) the concept of geothermal system

Geothermal system is higher or slightly higher than the normal geothermal temperature gradient of the region, especially at the edge of the plate, the geothermal temperature gradient is significantly higher than the average geothermal temperature gradient. The first type of geothermal system, with lower temperatures, does not exceed 100°C at economic depths; the second type of geothermal system, with a wide temperature span, can exceed 400°C.

What is a geothermal system? What happens in this system? It can be described as "a fluid in the earth's crust, with the entire earth's crust as a radiator, utilizing the earth's crust's own heat storage and the law of upward radiation of heat from the bottom to the top to conduct heat". Geothermal system is composed of three elements: heat source, heat storage and fluid, which are the carriers of heat transfer. A heat source is a high-temperature (600°C) magma intrusion that reaches 5 to 10 km above the surface, or reaches a low-temperature system. A thermal store is a thermally permeable rock formation that stores heat from circulating fluids. Thermal storage overlies a nonpermeable cap layer that is connected to a recharge zone at the surface by atmospheric precipitation recharge and drains fluids through hot springs or boreholes. Geothermal fluids are derived from atmospheric precipitation and are in liquid or gaseous phase due to differences in temperature and pressure. This water is often accompanied by chemical gases such as CO2 and H2S. Figure 4-54 simplifies the depiction of an ideal geothermal system.

Figure 4-54 Schematic diagram of an ideal geothermal system (according to the Institute of Geology and Earth Studies, Pisa, Italy, 2004)

(ii) Mechanisms of geothermal systems

The mechanism of geothermal systems is the convection of a hot body, which is depicted in Figure 4-55 for a mesothermal geothermal system. Convective motion is produced by fluid heating in a gravity field and thermal expansion as a result of that heating; the heat in the fluid convection is the driving force of the geothermal system. The hot, low-density fluid rises and is replaced by a cold, high-density fluid from the system boundary. Under normal conditions, convection tends to increase the temperature in the upper part of the system and decrease the temperature in the lower part. The phenomenon described here seems simple, but a model reconstruction of a real geothermal system is difficult to realize. It requires knowledge of many disciplines and a great deal of experience in reconstructive modeling, especially for high-temperature systems. Geothermal systems occur in nature and different types of geothermal systems arise due to different geological, physical and chemical properties.

Figure 4-55 Model of a geothermal system (according to the Institute of Geology and Earth Studies, Pisa, Italy, 2004)

1-Reference curve for the boiling point of pure water; 2-Typical cyclic roadmap from the recharge point A to the discharge point E

(iii) Artificial geothermal systems

Of all the elements, the heat source is the only natural resource. The other two elements can be artificially provided with favorable conditions. For example, geothermal fluids extracted from a thermal store, after being used to drive the turbines of a geothermal power station into operation, are recharged back into the thermal store through injection wells. Such natural thermal storage is replenished by artificial recharge. Over the years, recharge has become a means of reducing the environmental impact of geothermal development in applications around the world.

Artificial recharge through injection wells can also help replenish and maintain old or depleted geothermal fields. For example, at Geyser, California, USA, one of the world's largest geothermal fields, capacity declined dramatically in the late 1980s due to reduced fluids. The first project was the Southeast Geyser Wastewater Recycling Project, launched in 1997, which fed treated wastewater into a geothermal field 48km deep underground. This project has brought back into use power plants that had previously been abandoned due to lack of fluid. The second project is the Geyser Recharge Project in Santa Rosa, which uses heat pumps to transport 4,150 x 104L of tertiary wastewater per day through a 66km pipeline from the Santa Rosa and other city wastewater treatment plants to the geyser, where it is injected into thermal storage through a specialized borehole. The Dry Heat Rock (HDR) project, first conducted in 1970 at Los Alamos National Laboratory in New Mexico, USA. Both the fluid and the thermal storage for the experiment were artificial. High-pressure water pumped from a drilled well is injected into deep, hot, hard rock, causing communicating fissures to be created. The fluid seeps into the artificial fissures and draws heat from the surrounding rock, which is called a natural thermal store. The thermal reservoir is then drilled into by a second borehole, which is used to extract the hot water. Thus, the system consists of (1) hydraulic fracturing wells, (2) artificial thermal reservoirs, and (3) an injection well-production well system. The entire system forms a closed system with the surface plant (Garnish, 1987) (Figure 4-56).

Figure 4-56 Schematic diagram of the business model of dry heat rock (according to Xinli, 2014)

Three, Geothermal Resource Survey

(1) Contents of Geothermal Resource Survey

Geothermal resources are embedded in the depths of the ground, and the content of the geothermal resource survey mainly includes the following five aspects:

(1) Identify the lithology of thermal reservoirs, spatial distribution, porosity, permeability and its hydraulic connection with normal temperature water-bearing rock layer.

(2) Identify the lithology of the thermal storage cap layer, thickness variations, as well as the regional geothermal warming rate and the planar distribution characteristics of the geothermal field.

(3) Identify the temperature, state, physical properties and chemical components of the geothermal fluid, and evaluate the feasibility of its utilization.

(4) To find out the characteristics of the geothermal fluid power field and the conditions of complementary path discharge.

(5) Under the premise of identifying the geothermal geological background, to determine the formation conditions of hot springs and geothermal resources and geothermal resources can be exploited and utilized in the area and the depth of reasonable exploitation and utilization; calculation and evaluation of geothermal resources or reserves, and put forward proposals on the sustainable development and utilization of geothermal resources.

(2) Geothermal resources exploration technology

From the viewpoint of geothermal exploration technology, at present, there are mainly:

(1) geophysical exploration method represented by surface shallow borehole temperature measurement, electric method, gravity survey and microseismic observation; (2) geochemical exploration method for determining the anomalies of radon, mercury, arsenic, boron, helium and carbon dioxide content in soil; (3) remote sensing interpretation and information of geothermal anomalies in the form of fracture structure; (4) remote sensing method of geothermal resources exploration; and (5) remote sensing method of geothermal resources exploration. (3) Remote sensing methods based on the interpretation of fracture structure and extraction of geothermal anomaly information.

At present, geothermal resources exploration mainly through the geotectonic, hydrogeological and other geological background of geothermal generation research, using comprehensive geophysical, geochemical and remote sensing exploration methods to circle the target area, geothermal exploration work.

1. Geophysical exploration

The role of geophysical exploration is to circle the geothermal field and determine the location of drill holes for mining geothermal fluid. At present, almost all geophysical methods are applied to geothermal exploration, the focus shifts from probing the geological and tectonic environment containing geothermal fluids to probing the fluids themselves.

Electrical exploration is a relatively simple method, its purpose is to detect the location and distribution of the fracture structure with the cause of the relationship between the groundwater, circling the distribution range of underground hot water, to determine the thickness of the cover layer, the location of the heat source and bedrock lithology. Electrical exploration includes frequency-domain detection method (such as MT and CSAMT method, etc.), time-domain method (such as LOTEM method, TEM method and time-domain IP method, etc.), direct current bathymetry and excitation polarization method.

Magnetic method exploration can be divided into aviation magnetic survey, ground high-precision magnetic survey and so on. It is mainly through the measurement of different magnetization intensity of various rock (ore) in the geomagnetic field caused by the magnetic anomalies, and study these magnetic anomalies of the spatial distribution characteristics, laws and the relationship with the geological body, so as to make geological interpretation. In the area of sedimentary rocks, magnetic anomalies are generally a reflection of the presence of intrusive rocks, which in turn is a determining factor in the formation of geothermal heat, the source of heat energy.

In addition to electric and magnetic methods, other geophysical methods such as gravity exploration, seismic exploration and geothermal logging. Gravity exploration is through the measurement of different rock (ore) density differences caused by gravity anomalies to achieve the search for deep large tectonic fractures, bedrock depression in the raised structure of the existence of underground hot water and other favorable parts of the purpose. Seismic exploration is used to solve geological problems by studying the kinematics and dynamics characteristics of artificially excited seismic waves, and this method makes up for the limitations of time-domain electrical exploration in terms of high-resistance shielding and depth. Geothermal logging includes methods such as resistivity, natural potential, and natural radioactivity. Geothermal logging from the means is also divided into drilling logging, high depth digital logging, etc., the method has now crossed the ranks of pure geophysical exploration.

2. Geochemical exploration

At present, geochemical research has formed a set of geothermal geochemical exploration technology series: in the regional scope, the use of aquatic sediments and soil measurements, can be quickly found and circled geothermal prospective area; in the census area, in the covered area with soil measurements, in the outcrop of good (high temperature hot water) area with rock measurements and hydrothermal corrosion research, can be circled Inside and outside the hot field, tectonic geochemical measurements (including Rn, Hg, 210Po, etc.) can indicate shallow or deep tectonic structures controlling the distribution of hot water, and radon (222Rn) contained in strata and rock bodies, which is produced by a series of decays of uranium (235U), can be transported along the tectonic zones, fissures, and groundwater verticals, and enriched at the surface to form radon anomalies. Underground hot water has a good reflection of ground anomalies; in the detailed investigation area, through the soil geochemical detailed investigation and temperature measurement can identify the most favorable sections of hot water storage; in the hot area, using geochemical temperature scale, can estimate the temperature of the deep hot water, predicting the possible temperature of the hot storage, using hydrogen, oxygen isotope study, can identify the source of hot water recharge, determine the nature of the heat source, and so on.

3. Remote sensing survey

S-Bo's law shows that: small changes in the temperature of the earth's surface can be caused by its hemispheric space energy radiation to appear obvious changes. Therefore, surface temperature anomalies formed after the deep Earth heat source is transferred to the Earth's surface by conduction and convection can be easily detected by thermal infrared detectors. Remote sensing geologic interpretation based on multi-band remote sensing data and extraction of geothermal anomaly information based on thermal infrared remote sensing data are the basic research ideas and methods of remote sensing technology to explore geothermal resources.

From the regional point of view, the geothermal anomalies exhibited by jet holes and hot spring points generally reflect the existence of shallow geothermal and heat-controlling structures. Deeper buried underground hot water is usually transferred to the surface by infiltration or convection through vertical fissure systems, forming geothermal anomalies with higher temperatures than the background, and these geothermal resources are influenced by the control of geological structures and the physical properties of stratified rocks. And through the geological interpretation of multi-band remote sensing data, numerous tectonic, lithological and geographic information such as fractures (including hidden fractures), lithology, geomorphology and so on can be obtained in the study area, which undoubtedly provides a technical reference for circling the favorable areas for geothermal resources exploration.

However, the successful application of remote sensing technology in geothermal resources exploration should have the following prerequisites: firstly, there must be thermal anomalies on the surface of the earth, which can be the hot spring points or thermal vents directly exposed on the surface, or the high temperature thermal anomalies formed on the surface by thermal convection or thermal diffusion; secondly, the thermal changes of the ground objects caused by the influence of the geothermal heat are shown on the remote sensing images, such as the mud volcanoes and mud geysers. The emergence of mud volcanoes, mud geysers, changes in vegetation ecology, the emergence of heat-resistant plants, the local melting of snow and ice affected by geothermal, etc.; Finally, it is necessary to have a higher temperature resolution of the thermal infrared detector and more likely to appear in geothermal anomalies of the imaging season, time and good weather conditions, and so on.

(C) geothermal resources exploration technology integrated application

In the process of geothermal resources exploration, only for different geothermal environment, different stages of exploration, the use of different methods of exploration and effective combination of different methods, in order to achieve the purpose of reasonable investment, reduce the risk, and improve the economic benefits. For example, in high-temperature geothermal zones and dry hot rock areas, we should make full use of the existing aeromagnetic, aeroelectric, regional gravity or water sediment measurement data, and at the same time, we should put in the appropriate amount of gravity, magnetic method, such as large-scale fine measurement profile, carry out the area of shallow thermometry, radon and magnetic method, and put in the drilling project in due course. In the middle and low-temperature basin area, we should strengthen the research on the tectonic characteristics of the heat storage basin, the development and evolution history and its relationship with heat storage, and fully interpret remote sensing, aeromagnetic and gravity data, which can provide the basis for evaluating the preferred target area for heat storage, and the geothermal type is suitable for investing in the heavy magnetic fine-tuning profiles, electric method and artificial seismic work.

Four, the utilization of geothermal resources

The utilization of geothermal resources includes power generation and non-power generation utilization. The experience of utilizing geothermal in the world shows that high temperature geothermal resources (above 150℃) are mainly used for power generation, and the hot water discharged from geothermal power generation can be directly utilized; medium and low temperature geothermal resources (below 150℃) are mainly utilized directly. The classic Lindal diagram (Lindal, 1973) shows the possible uses of geothermal fluids at different temperatures (Figure 4-57). Fluids with temperatures below 20°C are rarely used under very special conditions or in heat pump applications.The Lindal diagram emphasizes two important aspects of geothermal resource utilization: (1) the feasibility of geothermal projects can be improved by cascading or combining uses; and (2) the temperature of the resource can limit the possible uses. Existing thermal processes can be modified in some cases to broaden the applications of geothermal fluids.

(I) High-temperature geothermal resources

Based on the characteristics of geothermal resources, high-temperature geothermal resources are mainly used for power generation. At present, the domestic and foreign technologies for the utilization of geothermal resources mainly include dry steam power generation technology, underground hot water power generation technology, combined cycle power generation technology, and dry hot rock geothermal power generation technology.

Figure 4-57 Illustration of geothermal fluid utilization (according to Lindal, 1973)

1. Dry Steam Power Generation Technology

Dry Steam Power Generation System is a simple process, mature technology, safe and reliable, and it is the main form of power generation for high temperature geothermal fields. Dry steam power generation technology is mainly divided into back-pressure turbine power generation technology and condensing steam turbine power generation technology.

Back-pressure turbine power generation technology is the dry steam from the steam wells, first to be purified, through the separator to separate out the solid impurities contained, and then the steam to drive the turbine generator set power generation, steam venting or send heat to the user. Mostly used in geothermal steam in the non-condensable gas content is very high occasions, or comprehensive utilization in industrial and agricultural production and domestic water.

Condensing turbine power generation technology in order to improve the geothermal power plant unit output power and power generation efficiency, after the work of the steam is usually discharged into the hybrid condenser, cooled and then discharged. In this system, the steam can expand to a very low pressure in the turbine, so it can make more work. The system has a simple structure and is suitable for power generation in high-temperature (above 160℃) geothermal fields.

2. Underground Hot Water Power Generation Technology

Flash steam power generation is to send the geothermal wellheads of geothermal hot water to the flash vaporizer for decompression and flash evaporation, so that it produces part of the steam, and then lead to the conventional turbine to do work to generate electricity. The steam from the turbine is condensed into water in a hybrid condenser and sent to a cooling tower. The remaining saline water in the separator is discharged into the environment or pumped into the ground, or introduced as the second stage of the low-pressure flash steam separator, which separates the low-pressure steam and introduces it into the middle of the turbine to expand and do work. This kind of power plant equipment is simple, easy to manufacture, and can be used as a hybrid heat exchanger. The disadvantage is that the equipment size is large, easy to corrosion and scaling, lower thermal efficiency. Because it is directly to the underground hot steam as the work medium, and therefore for the underground hot water temperature, mineralization and non-condensable gas content and so on have high requirements.

Intermediate medium method of geothermal power generation is through the heat exchanger, the use of underground hot water to heat some kind of low boiling point of the work material, so that it becomes steam, and then the steam to promote the turbine and drive the generator to generate electricity. Two types of fluids are used in this type of power generation system, one is the geothermal fluid as the heat source, which is cooled in the steam generator and then discharged into the environment or driven into the ground, and the other is the low-boiling-point working fluid as the working medium (e.g., Freon, isopentane, isobutane, n-butane, chlorobutane, etc.). This work fluid is vaporized in the steam generator due to the absorption of the heat released from the geothermal water, and the resulting low-boiling point work fluid steam is sent to the turbine generator set to generate electricity. After the work is done, the steam is discharged from the turbine and condensed into a liquid in the condenser, and then pumped back to the steam generator by the circulation pump to recirculate the work.

3. Combined cycle power generation technology

Combined cycle power generation technology is the steam power generation and geothermal water power generation two systems into one, its biggest advantage is that it is suitable for higher than 150 ℃ high-temperature geothermal fluid power generation, after a power generation of the fluid, in the conditions of not less than 120 ℃, and then into the duplex power generation system, the second power, making full use of geothermal fluid heat energy, not only to increase the geothermal fluid, but also to improve the efficiency of the power generation system, and to improve the efficiency of the power generation system. The thermal energy of the geothermal fluid is fully utilized, which not only improves the efficiency of power generation, but also reuses the discharged tail water after primary power generation, which greatly saves resources. The whole process of the system, from the production well to power generation and finally recharge to thermal storage, is operated in a totally closed system, so even highly mineralized hot brine can be used for power generation without any pollution to the environment. At the same time, because the system is completely closed, there is no pungent smell of hydrogen sulfide even in the geothermal power plant, making it a 100% environmentally friendly geothermal system. This geothermal power system uses 100% geothermal water recharge, thus extending the life of the geothermal field.

4. Geothermal Power Generation Technology in Dry Rock

Dry rock (HDR), also known as Enhanced Geothermal System (EGS) or Engineered Geothermal System, is a high-temperature body of rock that is generally at a temperature of more than 200°C, buried at a depth of thousands of meters, and has no or only a small amount of internal subsurface fluids. The composition of such bodies of rock can be highly variable, with the vast majority being moderately acidic intrusive rocks from the Mesozoic onwards, but they can also be metamorphic rocks from the Meso-Cenozoic, or even massive sedimentary rocks of enormous thickness. Dry heated rocks are primarily used to extract heat from their interiors, so their main industrial indicator is the temperature inside the rock.

The principle of exploiting dry heat rock resources is to drill a well (injection well) from the surface into the dry heat rock, close the borehole and inject water at a lower temperature into the well at high pressure, which creates a very high pressure. In the case of dense, fissure-free rock, the high-pressure water causes many cracks in the rock in a direction roughly perpendicular to the minimum geostress. If there were already a small number of natural joints in the rock mass, this high-pressure water expands them into even larger cracks. Of course, the direction of these cracks is influenced by the geostress system. With the continuous injection of low-temperature water, the cracks continue to increase, expand, and connect with each other, ultimately forming a roughly faceted artificial dry-heat-rock thermal storage structure. Several wells are drilled at a reasonable distance from the injection wells and penetrate the artificial thermal storage structure. These wells are used to recover high temperature water and vapor and are called production wells. The injected water moves along the fissures and exchanges heat with the surrounding rock, producing high-temperature, high-pressure water or water-vapor mixtures at temperatures of up to 200-300°C. The water is then pumped through the structure, and the water is then pumped to the production wells. High-temperature steam is extracted from the production wells through the artificial thermal storage structure for geothermal power generation and comprehensive utilization. The warm water after utilization is then recharged back into the dry heat rock through injection wells, thus achieving the purpose of recycling.

(2) Medium and low temperature geothermal resources

Medium and low temperature geothermal resources can be used for direct heating of residents and factories. These thermal reservoirs usually contain pressurized underground hot water. This hot water is brought to the surface where a heat exchanger converts the geothermal energy into another liquid. The cooled geothermal energy fluid is then pumped back into the ground through re-injection wells. The heated liquid is used primarily for circulating heating, greenhouses and aquaculture.

Underground hot water is used for heating, which not only saves fuel, but also avoids environmental pollution. Underground hot water can be used in the light textile industry, not only to meet the needs of special processes, but also to improve the quality of products, saving a lot of coal, electricity and salt for water softening. Underground hot water sometimes also contains some special trace components or gas components and a small amount of radioactive substances, in some hot mineral springs near the mineral springs are often accumulated mud, they are beneficial to the human body's physiological functions or have a certain medical effect. According to the local conditions of the underground hot water for the construction of greenhouse breeding seeds, fish farming, irrigation of farmland, breeding fodder and green manure, the development of rural production and economy, for agriculture, forestry, animal husbandry and fishery services, there is a very important significance (Figure 4-58).

(C) gradient utilization of geothermal resources

Geothermal resources development exists in the lower utilization of thermal energy, resource waste, etc., the direct use of the way with 50% to 70% of the heat utilization efficiency, and geothermal power generation is only 5% to 20%, the remaining heat is accompanied by the geothermal water back to the ground or directly discharged into the natural environment, not only a waste of resources but also cause thermal pollution, how to efficiently and comprehensively utilize geothermal resources has been the development of rural production and economy, agriculture, forestry, animal husbandry and fishery services is very important. How to efficiently and comprehensively utilize geothermal resources has become a hotspot of concern at home and abroad.

Figure 4-58 Application of geothermal in indoor heating

(According to the Institute of Geology and Earth Research, Pisa, Italy, 2004)

Gradient utilization of geothermal resources refers to the combination of regional needs, according to the different temperatures of geothermal fluids, geothermal step-by-step utilization. High-temperature geothermal water is first used to generate electricity, then it is used as industrial drying, agricultural seedling farming, building heating, etc., and finally lower-temperature geothermal water is used for bathing. After a series of utilization, the tail water reaches about 20℃, which maximizes the utilization of geothermal resources, so the gradient utilization technology has a broad prospect (Figure 4-60).

V. Development of Geothermal Resources

(I) Problems in the Development of Geothermal Resources

1. Problems of Sustainable Development

With the broadening of the field of utilization of geothermal resources and the increase of social demand, geothermal resources have brought more and more benefits to people's lives, but people's significance in the comprehensive utilization of the value of geothermal resources and the industrialization of the exploitation of geothermal resources is not well understood. However, people do not know enough about the comprehensive utilization value of geothermal resources and the significance of industrialized development and utilization, and confuse geothermal with general mineral resources or water resources. Some geothermal resource-rich areas failed to establish their own characteristics of the geothermal industry, so that the development of valuable geothermal resources to stay at a low level, low-efficiency level, and the phenomenon of serious waste of resources, a considerable portion of the area of natural hot springs are not fully utilized, being wasted; some developers do not understand the characteristics of geothermal resources, resulting in geothermal resources are not reasonably developed and effectively protected.

Geothermal resources are formed under specific geological, tectonic, hydrogeological conditions and hydrogeochemical environment conditions, due to the deep burial, the recharge pathway is far away, regeneration capacity is weak, the amount of resources is limited, not inexhaustible. To maintain the long-term continuous and stable exploitation of its resources, it should be planned and rationally developed and utilized, and to prevent the occurrence of resource wastage and environmental and geological problems caused by blind and disorderly random exploitation, otherwise it will result in the rapid depletion of resources.

In order to achieve the purpose of sustainable development and utilization, in the development, to take effective measures to establish the resource utilization center of high education and low consumption system, we should actively promote the application of high and new technology and facilities, improve the scientific and technological content of geothermal development, the development of conservation, efficiency development and utilization mode, and strive to improve the utilization rate of geothermal, reduce the waste of resources, so that the geothermal to create a higher social, environmental, economic benefits.

Figure 4-59 Gradient utilization of geothermal resources (according to the Institute of Geology and Earth Research, Pisa, Italy, 2004)

2. Environmental Protection Issues

The development and utilization of geothermal resources may produce a variety of environmental problems, the main water pollution, thermal pollution, air pollution, soil pollution, ground subsidence and so on.

(1) In the process of geothermal development and utilization, a large amount of heat is bound to be discharged to the atmosphere and water bodies, causing the temperature of the surrounding air or water bodies to rise, affecting the surrounding environment and the survival and growth of organisms, and destroying the ecological balance of water bodies.

(2) During the development and utilization of geothermal resources, all kinds of harmful gases and suspended matters contained in the thermal fluid will be discharged into the atmosphere, causing air pollution.

(3) Geothermal water with high salt content discharged into farmland will erode the land, destroy the vegetation, and cause serious soil crusting and salinization, while geothermal water contains radioactive elements such as oxygen, uranium and thorium to varying degrees, which is harmful to human health to varying degrees.

(4) Long-term geothermal fluid mining without recharge will lead to subsidence and horizontal displacement of the ground.

So, geothermal development and utilization of environmental problems caused by the process can not be ignored, as long as the correct understanding of these problems, give the necessary attention, and active, serious research, to take a variety of effective technical measures, strict monitoring and prevention and control, can be solved and controlled.

(B) the prospects for the development of geothermal resources

With the traditional non-renewable energy application crisis, the search for new sources of energy has become an important measure to alleviate the energy crisis. Underground storage of geothermal energy is huge (Table 4-8).

Table 4-8 Table of Geothermal Potentials Worldwide (According to the International Geothermal Association, 2001)

With the application of geothermal resources, the large-scale development of geothermal industry and the gradient application of geothermal energy have become the main trend of its development. Promoting the scale of geothermal industry helps to improve the efficiency of geothermal energy application; through the form of gradient application, it can realize the maximum application of geothermal energy and reduce the problem of environmental pollution.

In the 2010 World Geothermal Congress, enhanced geothermal systems were proposed to promote enhanced geothermal systems. Realizing the recycling application of geothermal resources has become an important trend in the development of geothermal resources. Enhanced geothermal system is also known as dry heat rock geothermal, the principle is: from the surface of the ground to the dry heat rock borehole, closed borehole to the well after the injection of lower temperature water, high-pressure water so that the rock body to produce more cracks, with the increase of low-temperature water, cracks gradually develop and expand, and ultimately form a large-scale artificial dry heat storage structure, take this way to realize the heat cycle application. In addition, the existence of shallow geothermal energy is more common, strengthen the development of shallow geothermal energy, realize the scale of shallow geothermal energy application, has a broad development prospect, such as most of the cities and towns in the north of China in the winter need to be heated, the number of days of heating is more than 120 days, the consumption of coal is huge, through the development of shallow geothermal energy, you can effectively reduce the amount of coal application, to realize the comprehensive benefits.