"Use of low-potential thermal energy of the earth in heat pump systems"

Vasiliev G.P., Scientific director OJSC INSOLAR-INVEST, Doctor of Technical Sciences, Chairman of the Board of Directors of OJSC INSOLAR-INVEST
N. V. Shilkin, engineer, NIISF (Moscow)

Rational use of fuel and energy resources today is one of the global world problems, the successful solution of which, apparently, will be of decisive importance not only for the further development of the world community, but also for the preservation of its habitat. One of the promising ways to solve this problem is application of new energy-saving technologies using non-traditional renewable energy sources (NRES) The depletion of traditional fossil fuels and the environmental consequences of burning them have led in recent decades to a significant increase in interest in these technologies in almost all developed countries of the world.

The advantages of heat supply technologies that use in comparison with their traditional counterparts are associated not only with significant reductions in energy costs in the life support systems of buildings and structures, but also with their environmental friendliness, as well as new opportunities in the field of increasing the degree of autonomy of life support systems. Apparently, in the near future, it is these qualities that will be of decisive importance in shaping a competitive situation in the heat generating equipment market.

Analysis of possible areas of application in the Russian economy of energy saving technologies using non-traditional energy sources, shows that in Russia the most promising area for their implementation is the life support systems of buildings. At the same time, the widespread use of heat pump heat supply systems (TST), using the soil of the surface layers of the Earth as a ubiquitously available low-potential heat source.

Using Earth's heat There are two types of thermal energy - high-potential and low-potential. The source of high-potential thermal energy is hydrothermal resources - thermal waters heated to a high temperature as a result of geological processes, which allows them to be used for heat supply to buildings. However, the use of high-potential heat of the Earth is limited to areas with certain geological parameters. In Russia, this is, for example, Kamchatka, the region of the Caucasian mineral waters; in Europe, there are sources of high-potential heat in Hungary, Iceland and France.

In contrast to the "direct" use of high-potential heat (hydrothermal resources), use of low-grade heat of the Earth through heat pumps is possible almost everywhere. It is currently one of the fastest growing areas of use non-traditional renewable energy sources.

Low-potential heat of the Earth can be used in various types of buildings and structures in many ways: for heating, hot water supply, air conditioning (cooling), heating paths in the winter season, to prevent icing, heating fields in open stadiums, etc. In the English-language technical literature, such systems are referred to as "GHP" - "geothermal heat pumps", geothermal heat pumps.

The climatic characteristics of the countries of Central and Northern Europe, which, together with the United States and Canada, are the main areas for the use of low-grade heat of the Earth, determine mainly the need for heating; cooling of the air, even in summer, is relatively rarely required. Therefore, unlike the United States, heat pumps in European countries they operate mainly in heating mode. IN THE USA heat pumps are more often used in air heating systems combined with ventilation, which allows both heating and cooling the outside air. IN European countries heat pumps commonly used in water heating systems. Because the heat pump efficiency increases with a decrease in the temperature difference between the evaporator and the condenser, floor heating systems are often used for heating buildings, in which a coolant of a relatively low temperature (35–40 °C) circulates.

Majority heat pumps in Europe, designed to use the low-grade heat of the Earth, are equipped with electrically driven compressors.

Over the past ten years, the number of systems that use the low-grade heat of the Earth for heat and cold supply of buildings through heat pumps, increased significantly. The largest number of such systems is used in the USA. A large number of such systems operate in Canada and the countries of central and northern Europe: Austria, Germany, Sweden and Switzerland. Switzerland leads in the use of low-grade thermal energy of the Earth per capita. In Russia, over the past ten years, using technology and with the participation of INSOLAR-INVEST OJSC, which specializes in this area, only a few objects have been built, the most interesting of which are presented in.

In Moscow, in the Nikulino-2 microdistrict, in fact, for the first time, a hot water heat pump system multi-storey residential building. This project was implemented in 1998–2002 by the Ministry of Defense of the Russian Federation jointly with the Government of Moscow, the Ministry of Industry and Science of Russia, the NP ABOK Association and within the framework of "Long-term energy saving program in Moscow".

As a low-potential source of thermal energy for the evaporators of heat pumps, the heat of the soil of the surface layers of the Earth, as well as the heat of the removed ventilation air, is used. The hot water preparation plant is located in the basement of the building. It includes the following main elements:

• vapor compression heat pump installations (HPU);
• hot water storage tanks;
• systems for collecting low-grade thermal energy of the soil and low-grade heat of removed ventilation air;
• circulation pumps, instrumentation

The main heat-exchange element of the system for collecting low-grade ground heat is vertical coaxial ground heat exchangers located outside along the perimeter of the building. These heat exchangers are 8 wells with a depth of 32 to 35 m each, arranged near the house. Since the operating mode of heat pumps using the warmth of the earth and the heat of the removed air is constant, while the consumption of hot water is variable, the hot water supply system is equipped with storage tanks.

Data estimating the world level of use of low-potential thermal energy of the Earth by means of heat pumps are given in the table.

Table 1. World level of use of low-potential thermal energy of the Earth through heat pumps

Soil as a source of low-potential thermal energy

As a source of low-potential thermal energy, groundwater with a relatively low temperature or soil of the surface (up to 400 m deep) layers of the Earth can be used.. The heat content of the soil mass is generally higher. The thermal regime of the soil of the surface layers of the Earth is formed under the influence of two main factors - the solar radiation incident on the surface and the flow of radiogenic heat from the earth's interior. Seasonal and daily changes in the intensity of solar radiation and outdoor temperature cause fluctuations in the temperature of the upper layers of the soil. The depth of penetration of daily fluctuations in the temperature of the outside air and the intensity of the incident solar radiation, depending on the specific soil and climatic conditions, ranges from several tens of centimeters to one and a half meters. The depth of penetration of seasonal fluctuations in the temperature of the outside air and the intensity of the incident solar radiation does not, as a rule, exceed 15–20 m.

The temperature regime of soil layers located below this depth (“neutral zone”) is formed under the influence of thermal energy coming from the bowels of the Earth and practically does not depend on seasonal, and even more so daily changes in outdoor climate parameters (Fig. 1).

Rice. 1. Graph of changes in soil temperature depending on depth

With increasing depth, the temperature of the soil increases in accordance with the geothermal gradient (approximately 3 degrees C for every 100 m). The magnitude of the flux of radiogenic heat coming from the bowels of the earth varies for different localities. For Central Europe, this value is 0.05–0.12 W/m2.

During the operational period, the soil mass located within the zone of thermal influence of the register of pipes of the soil heat exchanger of the system for collecting low-grade ground heat (heat collection system), due to seasonal changes in the parameters of the outdoor climate, as well as under the influence of operational loads on the heat collection system, as a rule, is subjected to repeated freezing and thawing. In this case, naturally, there is a change in the state of aggregation of moisture contained in the pores of the soil and, in the general case, both in liquid and in solid and gaseous phases simultaneously. In other words, the soil mass of the heat collection system, regardless of the state it is in (frozen or thawed), is a complex three-phase polydisperse heterogeneous system, the skeleton of which is formed by a huge number of solid particles of various shapes and sizes and can be both rigid and mobile, depending on whether the particles are firmly connected to each other or whether they are separated from each other by a substance in the mobile phase. Interstices between solid particles can be filled with mineralized moisture, gas, steam and ice, or both. Modeling the processes of heat and mass transfer that form the thermal regime of such a multicomponent system is an extremely complex task, since it requires taking into account and mathematical description of various mechanisms for their implementation: heat conduction in an individual particle, heat transfer from one particle to another upon their contact, molecular heat conduction in a medium that fills the gaps between particles, convection of vapor and moisture contained in the pore space, and many others.

Special attention should be paid to the influence of soil mass moisture and moisture migration in its pore space on thermal processes that determine soil characteristics as a source of low-potential thermal energy.

In capillary-porous systems, which is the soil mass of the heat collection system, the presence of moisture in the pore space has a significant effect on the process of heat distribution. Correct accounting of this influence today is associated with significant difficulties, which are primarily associated with the lack of clear ideas about the nature of the distribution of solid, liquid and gaseous phases of moisture in a particular structure of the system. The nature of the forces of moisture bonding with skeletal particles, the dependence of the forms of moisture bonding with the material at various stages of moistening, and the mechanism of moisture movement in the pore space have not yet been elucidated.

If there is a temperature gradient in the thickness of the soil mass, the vapor molecules move to places with a lower temperature potential, but at the same time, under the action of gravitational forces, an oppositely directed flow of moisture in the liquid phase occurs. In addition, the temperature regime of the upper layers of the soil is influenced by the moisture of atmospheric precipitation, as well as groundwater.

The main factors under the influence of which are formed temperature regime soil mass collection systems for low-potential soil heat are shown in fig. 2.

Rice. 2. Factors under the influence of which the temperature regime of the soil is formed

Types of systems for the use of low-potential thermal energy of the Earth

Ground heat exchangers connect heat pump equipment with soil mass. In addition to "extracting" the heat of the Earth, ground heat exchangers can also be used to accumulate heat (or cold) in the ground massif.

In the general case, two types of systems for the use of low-potential thermal energy of the Earth can be distinguished:

• open systems: as a source of low-potential thermal energy, groundwater is used, which is supplied directly to heat pumps;
• closed systems: heat exchangers are located in the soil massif; when a coolant circulates through them with a temperature lowered relative to the ground, thermal energy is “selected” from the ground and transferred to the evaporator heat pump(or, when using a coolant with an elevated temperature relative to the ground, its cooling).

The main part of open systems is wells, which allow extracting groundwater from aquifers of the soil and returning water back to the same aquifers. Usually paired wells are arranged for this. A diagram of such a system is shown in fig. 3.

Rice. 3. Scheme of an open system for the use of low-potential thermal energy of groundwater

The advantage of open systems is the possibility of obtaining a large amount of thermal energy at relatively low cost. However, wells require maintenance. In addition, the use of such systems is not possible in all areas. The main requirements for soil and groundwater are as follows:

• sufficient permeability of the soil, allowing replenishment of water reserves;
• good chemical composition groundwater (e.g. low iron content) to avoid pipe scale and corrosion problems.

Open systems are more often used for heating or cooling large buildings. The world's largest geothermal heat pump system uses groundwater as a source of low-potential thermal energy. This system is located in the USA in Louisville, Kentucky. The system is used for heat and cold supply of a hotel-office complex; its power is about 10 MW.

Sometimes systems that use the heat of the Earth include systems for using low-grade heat from open water bodies, natural and artificial. This approach is adopted, in particular, in the United States. Systems using low-grade heat from reservoirs are classified as open, as are systems using low-grade heat from groundwater.

Closed systems, in turn, are divided into horizontal and vertical.

Horizontal ground heat exchanger(in English literature, the terms “ground heat collector” and “horizontal loop” are also used) is usually arranged near the house at a shallow depth (but below the ground freezing level in winter). The use of horizontal ground heat exchangers is limited by the size of the available site.

In the countries of Western and Central Europe, horizontal ground heat exchangers are usually separate pipes laid relatively tightly and connected to each other in series or in parallel (Fig. 4a, 4b). To save site area, improved types of heat exchangers have been developed, for example, heat exchangers in the form of a spiral, located horizontally or vertically (Fig. 4e, 4f). This form of heat exchangers is common in the USA.

Rice. 4. Types of horizontal ground heat exchangers
a - a heat exchanger of series-connected pipes;
b - heat exchanger from parallel pipes;
c - a horizontal collector laid in a trench;
d - heat exchanger in the form of a loop;
e - a heat exchanger in the form of a spiral located horizontally (the so-called "slinky" collector;
e - a heat exchanger in the form of a spiral located vertically

If a system with horizontal heat exchangers is used only to generate heat, its normal operation is possible only if there is sufficient heat input from the earth's surface due to solar radiation. For this reason, the surface above the heat exchangers must be exposed to sunlight.

Vertical ground heat exchangers(V English Literature the designation "BHE" - "borehole heat exchanger" is adopted) allow the use of low-potential thermal energy of the soil mass lying below the "neutral zone" (10–20 m from ground level). Systems with vertical ground heat exchangers do not require large areas and do not depend on the intensity of solar radiation falling on the surface. Vertical ground heat exchangers work effectively in almost all types of geological environments, with the exception of soils with low thermal conductivity, such as dry sand or dry gravel. Systems with vertical ground heat exchangers are very widespread.

The scheme of heating and hot water supply of a single-apartment residential building by means of a heat pump unit with a vertical ground heat exchanger is shown in fig. 5.

Rice. 5. Scheme of heating and hot water supply of a single-apartment residential building by means of a heat pump unit with a vertical ground heat exchanger

The coolant circulates through pipes (most often polyethylene or polypropylene) laid in vertical wells from 50 to 200 m deep. Two types of vertical ground heat exchangers are usually used (Fig. 6):

• U-shaped heat exchanger, which are two parallel pipes connected at the bottom. One or two (rarely three) pairs of such pipes are located in one well. The advantage of such a scheme is the relatively low manufacturing cost. Double U-shaped heat exchangers are the most widely used type of vertical ground heat exchangers in Europe.
• Coaxial (concentric) heat exchanger. The simplest coaxial heat exchanger consists of two pipes of different diameters. A smaller diameter pipe is placed inside another pipe. Coaxial heat exchangers can be of more complex configurations.

Rice. 6. Cross section of various types of vertical ground heat exchangers

To increase the efficiency of heat exchangers, the space between the walls of the well and the pipes is filled with special heat-conducting materials.

Systems with vertical ground heat exchangers can be used to heat and cool buildings of various sizes. For a small building, one heat exchanger is enough; large buildings may require a device whole group wells with vertical heat exchangers. The largest number of wells in the world is used in the heating and cooling system of Richard Stockton College in the US state of New Jersey. The vertical ground heat exchangers of this college are located in 400 wells 130 m deep. In Europe largest number wells (154 wells with a depth of 70 m) are used in the heating and cooling system of the central office of the German Air Traffic Control Service (“Deutsche Flug-sicherung”).

A special case of vertical closed systems is the use of building structures as soil heat exchangers, for example, foundation piles with embedded pipelines. The section of such a pile with three contours of a soil heat exchanger is shown in fig. 7.

Rice. 7. Scheme of ground heat exchangers embedded in the foundation piles of the building and the cross section of such a pile

The ground mass (in the case of vertical ground heat exchangers) and building structures with ground heat exchangers can be used not only as a source, but also as a natural accumulator of thermal energy or "cold", for example, solar radiation heat.

There are systems that cannot be clearly classified as open or closed. For example, the same deep (from 100 to 450 m deep) well filled with water can be both production and injection. The diameter of the well is usually 15 cm. A pump is placed in the lower part of the well, through which water from the well is supplied to the evaporators of the heat pump. Return water returns to the top of the water column in the same well. There is a constant recharge of the well with groundwater, and open system works like a closed one. Systems of this type in the English literature are called "standing column well system" (Fig. 8).

Rice. 8. Scheme of the well type "standing column well"

Typically, wells of this type are also used to supply the building with drinking water.. However, such a system can only work effectively in soils that provide a constant supply of water to the well, which prevents it from freezing. If the aquifer is too deep, a powerful pump will be required for the normal functioning of the system, requiring increased energy costs. The large depth of the well causes a rather high cost of such systems, so they are not used for heat and cold supply of small buildings. Now there are several such systems in the world in the USA, Germany and Europe.

One of the promising areas is the use of water from mines and tunnels as a source of low-grade thermal energy. The temperature of this water is constant throughout the year. Water from mines and tunnels is readily available.

"Sustainability" of systems for the use of low-grade heat of the Earth

During the operation of the soil heat exchanger, a situation may arise when during the heating season the temperature of the soil near the soil heat exchanger decreases, and in the summer the soil does not have time to warm up to the initial temperature - its temperature potential decreases. Energy consumption during the next heating season causes an even greater decrease in the temperature of the soil, and its temperature potential is further reduced. This forces system design use of low-grade heat of the Earth consider the problem of "stability" (sustainability) of such systems. Often, energy resources are used very intensively to reduce the payback period of equipment, which can lead to their rapid depletion. Therefore, it is necessary to maintain such a level of energy production that would allow the source of energy resources to be operated for a long time. This ability of systems to maintain the required level of heat production for a long time is called “sustainability”. For systems using low-potential Earth's heat the following definition of sustainability is given: “For each system of using low-potential heat of the Earth and for each mode of operation of this system, there is a certain maximum level of energy production; energy production below this level can be maintained for a long time (100–300 years).”

Held in OJSC INSOLAR-INVEST Studies have shown that the consumption of thermal energy from the soil mass by the end of the heating season causes a decrease in soil temperature near the register of pipes of the heat collection system, which, under the soil and climatic conditions of most of the territory of Russia, does not have time to compensate in the summer season, and by the beginning of the next heating season, the soil comes out with a reduced temperature potential. The consumption of thermal energy during the next heating season causes a further decrease in the temperature of the soil, and by the beginning of the third heating season, its temperature potential differs even more from the natural one. And so on. However, the envelopes of the thermal influence of long-term operation of the heat collection system on the natural temperature regime of the soil have a pronounced exponential character, and by the fifth year of operation, the soil enters a new regime close to periodic, that is, starting from the fifth year of operation, the long-term consumption of thermal energy from the soil massif of the heat collection system is accompanied by periodic changes in its temperature. Thus, when designing heat pump heating systems it seems necessary to take into account the drop in temperatures of the soil massif, caused by the long-term operation of the heat collection system, and use the temperatures of the soil massif expected for the 5th year of operation of the TST as design parameters.

In combined systems, used for both heat and cold supply, the heat balance is set “automatically”: in winter (heat supply is required), the soil mass is cooled, in summer (cold supply is required), the soil mass is heated. In systems using low-grade groundwater heat, there is a constant replenishment of water reserves due to water seeping from the surface and water coming from deeper layers of the soil. Thus, the heat content of groundwater increases both "from above" (due to the heat of atmospheric air) and "from below" (due to the heat of the Earth); the value of heat gain "from above" and "from below" depends on the thickness and depth of the aquifer. Due to these heat transfers, the groundwater temperature remains constant throughout the season and changes little during operation.

In systems with vertical ground heat exchangers, the situation is different. When heat is removed, the temperature of the soil around the soil heat exchanger decreases. The decrease in temperature is affected by both the design features of the heat exchanger and the mode of its operation. For example, in systems with high heat dissipation values ​​(several tens of watts per meter of heat exchanger length) or in systems with a ground heat exchanger located in soil with low thermal conductivity (for example, in dry sand or dry gravel), the temperature decrease will be especially noticeable and may lead to freezing of the soil mass around the ground heat exchanger.

German experts measured the temperature of the soil massif, in which a vertical soil heat exchanger with a depth of 50 m, located near Frankfurt am Main, is arranged. For this, 9 wells of the same depth were drilled around the main well at a distance of 2.5, 5 and 10 m. In all ten wells, temperature sensors were installed every 2 m - a total of 240 sensors. On fig. Figure 9 shows diagrams showing the temperature distribution in the soil mass around the vertical soil heat exchanger at the beginning and at the end of the first heating season. At the end of the heating season, a decrease in the temperature of the soil mass around the heat exchanger is clearly visible. There is a heat flow directed to the heat exchanger from the surrounding soil mass, which partially compensates for the decrease in soil temperature caused by the "selection" of heat. The magnitude of this flux compared with the magnitude of the heat flux from the earth's interior in a given area (80–100 mW/sq.m) is estimated quite high (several watts per square meter).

Rice. Fig. 9. Schemes of temperature distribution in the soil mass around the vertical soil heat exchanger at the beginning and at the end of the first heating season

Since vertical heat exchangers began to become relatively widespread approximately 15–20 years ago, there is a lack of experimental data all over the world obtained during long-term (several tens of years) operation periods of systems with heat exchangers of this type. The question arises about the stability of these systems, about their reliability for long periods of operation. Is the low-potential heat of the Earth a renewable energy source? What is the period of "renewal" of this source?

When operating a rural school in the Yaroslavl region, equipped heat pump system, using a vertical ground heat exchanger, the average values ​​of specific heat removal were at the level of 120–190 W/rm. m length of the heat exchanger.

Since 1986, research has been carried out in Switzerland near Zurich on a system with vertical ground heat exchangers. A vertical ground heat exchanger of a coaxial type with a depth of 105 m was arranged in the soil massif. This heat exchanger was used as a source of low-grade thermal energy for a heat pump system installed in a single-family residential building. The vertical ground heat exchanger provided a peak power of approximately 70 watts per meter of length, which created a significant thermal load on the surrounding ground mass. The annual production of thermal energy is about 13 MWh

At a distance of 0.5 and 1 m from the main well, two additional wells were drilled, in which temperature sensors were installed at a depth of 1, 2, 5, 10, 20, 35, 50, 65, 85 and 105 m, after which the wells were filled with a clay-cement mixture. The temperature was measured every thirty minutes. In addition to the ground temperature, other parameters were recorded: the speed of the coolant, the energy consumption of the heat pump compressor drive, the air temperature, etc.

The first observation period lasted from 1986 to 1991. The measurements showed that the influence of the heat of the outside air and solar radiation is noted in the surface layer of the soil at a depth of up to 15 m. Below this level, the thermal regime of the soil is formed mainly due to the heat of the earth's interior. During the first 2-3 years of operation ground mass temperature, surrounding the vertical heat exchanger, dropped sharply, but every year the decrease in temperature decreased, and after a few years the system reached a regime close to constant, when the temperature of the soil mass around the heat exchanger became lower than the initial one by 1–2 °C.

In the fall of 1996, ten years after the start of operation of the system, the measurements were resumed. These measurements showed that the ground temperature did not change significantly. In subsequent years, slight fluctuations in ground temperature were recorded within 0.5 degrees C, depending on the annual heating load. Thus, the system entered a quasi-stationary regime after the first few years of operation.

Based on the experimental data, mathematical models of the processes taking place in the soil massif were built, which made it possible to make a long-term forecast of changes in the temperature of the soil massif.

Mathematical modeling has shown that the annual temperature decrease will gradually decrease, and the volume of the soil mass around the heat exchanger, subject to temperature decrease, will increase every year. At the end of the operating period, the regeneration process begins: the temperature of the soil begins to rise. The nature of the regeneration process is similar to the nature of the process of "selection" of heat: in the first years of operation, a sharp increase in soil temperature occurs, and in subsequent years, the rate of temperature increase decreases. The length of the “regeneration” period depends on the length of the operating period. These two periods are about the same. In this case, the period of operation of the ground heat exchanger was thirty years, and the period of "regeneration" is also estimated at thirty years.

Thus, the heating and cooling systems of buildings, using the low-grade heat of the Earth, are a reliable source of energy that can be used everywhere. This source can be used for quite a long time, and can be renewed at the end of the operating period.

Literature

1. Rybach L. Status and prospects of geothermal heat pumps (GHP) in Europe and worldwide; sustainability aspects of GHPs. International course of geothermal heat pumps, 2002

2. Vasiliev G.P., Krundyshev N.S. Energy-efficient rural school in the Yaroslavl region. ABOK №5, 2002

3. Sanner B. Ground Heat Sources for Heat Pumps (classification, characteristics, advantages). 2002

4. Rybach L. Status and prospects of geothermal heat pumps (GHP) in Europe and worldwide; sustainability aspects of GHPs. International course of geothermal heat pumps, 2002

5. ORKUSTOFNUN Working Group, Iceland (2001): Sustainable production of geothermal energy – suggested definition. IGA News no. 43, January-March 2001, 1-2

6. Rybach L., Sanner B. Ground-source heat pump systems – the European experience. GeoHeat Center Bull. 21/1, 2000

7. Saving energy with Residential Heat Pumps in Cold Climates. Maxi Brochure 08. CADDET, 1997

8. Atkinson Schaefer L. Single Pressure Absorption Heat Pump Analysis. A Dissertation Presented to The Academic Faculty. Georgia Institute of Technology, 2000

9. Morley T. The reversed heat engine as a means of heating buildings, The Engineer 133: 1922

10. Fearon J. The history and development of the heat pump, Refrigeration and Air Conditioning. 1978

11. Vasiliev G.P. Energy efficient buildings with heat pump heat supply systems. ZhKH Magazine, No. 12, 2002

12. Guidelines for the use of heat pumps using secondary energy resources and non-traditional renewable energy sources. Moskomarchitectura. State Unitary Enterprise "NIAC", 2001

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To calculate what values ​​the pressure inside the Earth reaches, caused by the weight of the rocks that make up the various shells, you need to know the density of the rocks at all depths and the magnitude of gravity also at all depths up to the center.

As we have seen, the density of rocks increases with depth, although unevenly. From 2.5 at the surface, it goes up to 3.4 at a depth of about 100 km and up to 6.0 at 2900 km below the surface. Here, at the boundary of the core, a jump is observed in the density value: it immediately reaches a value of 9.5 (approximately), and then again grows uniformly, reaching 12.5 in the center of the core (according to M. S. Molodensky, 1955) (see Fig. 8).

Rice. 8. Density change inside the Earth.

As for gravity, the following can be said about it. Gravity is the force with which the Earth attracts all bodies to itself. Under the influence of this force, bodies in a free state (for example, in the air) fall to the Earth, i.e., move towards the center of the Earth, gradually accelerating, i.e., receiving “acceleration”. The magnitude of the "acceleration of gravity" can be calculated. At the Earth's surface, the acceleration due to gravity is approximately 9.8 m/s 2; in the depths of the Earth, it first slightly increases, reaching a maximum near the surface of the core, and then rapidly decreases, reaching zero in the center of the Earth (Fig. 9). This is understandable: the point located in the center the globe, is attracted by all surrounding parts, with the same force along all radii, and as a result, the resultant will be equal to zero.

Rice. 9. Change in the acceleration of gravity inside the Earth.

With this information, we can calculate the weight of a column of rocks with a cross section of 1 square. centimeter, and a length equal to the radius of the Earth or any part of it. This will be the pressure exerted by the weight of the overlying rocks on the elementary area (1 sq. cm) deep in the earth. Calculations lead to the following figures: at the "foot" of the earth's crust, i.e., at the base of the sialic shell (at a depth of 50 km) - about 13 thousand atmospheres, i.e., about 13 tons per square centimeter; at the border of the core - about 1.4 million atmospheres; in the center of the Earth - about 3 million atmospheres (Fig. 10). Three million atmospheres is approximately three thousand tons per square centimeter. This is a huge amount. No laboratory has yet been able to achieve such pressures.

Rice. 10. Changes in pressure inside the Earth.

Let's move on to temperature. According to measurements in boreholes, as well as in mines, it was found that the temperature increases with depth, rising by approximately 3 ° for every 100 meters. A similar rate of temperature growth persists everywhere, on all continents, but only in the outer parts of the Earth, near its very surface. With depth, the magnitude of the "geothermal gradient" (geothermal gradient - change in temperature in degrees per centimeter) falls. Calculations based on the thermal conductivity of rocks show that the geothermal gradient known for the outer parts of the globe persists for no more than the first 20 km; below, the rise in temperature slows down markedly. At the sole of the sialic sheath, the temperature is unlikely to be above 900°; at a depth of 100 km - about 1500°; Further, its growth slows down even more. Concerning central parts Earth, in particular the core, then it is very difficult to provide anything with certainty about them. Experts who have studied this issue believe that the interior of the Earth is heated no higher than 2-3 thousand degrees (Fig. 11).

Rice. 11. Temperature change inside the Earth.

It may be interesting to recall for comparison that in the center of the Sun the temperature is estimated at 1 million degrees, on the surface of the Sun - about 6000 °. The hair of a burning electric light bulb is heated up to 3000°.

Interesting data are available on the question of heat sources and the thermal regime of the globe. It was once believed that the Earth retains the "original" heat left to it "inherited" by the Sun, and gradually loses it, cooling down and shrinking in volume. The discovery of radioactive elements has changed previous ideas. It turned out that the rocks that make up the earth's crust contain radioactive elements that spontaneously and continuously emit heat. The amount of this heat is estimated at approximately 6 millionths of a small calorie per 1 cubic centimeter of rock per year, and in order to cover the entire consumption of heat radiated by the earth's surface into world space, it is necessary that the same elementary cube of rock release only three ten millionths of a small calorie per year. In other words, there is no reason to believe that the globe is cooling down. Rather, on the contrary, it can warm up. On this basis, in last years new hypotheses are proposed for the development of the earth's crust and the origin of the movements experienced by it.

Given the presence of high temperature in the bowels of the Earth, we have the right to pose the following question: in what physical (“aggregate”) state are the internal parts of the Earth? In solid or liquid, or perhaps gaseous?

latest version, i.e., the idea of ​​the gaseous state of matter inside the Earth, can be immediately rejected. To turn the minerals that make up the Earth into gas requires a much higher temperature than that which is admissible, judging by the data presented above.

But rocks can be in a liquid state. It is known, for example, that “acidic” rocks melt at 1000°C, “basic” rocks melt at 1000–1200°C, and “ultrabasic” rocks melt at 1300–1400°C. This means that already at a depth of 100–130 km rocks should melt. But there is very high pressure, and the pressure raises the melting point. Whose influence will be greater: high temperature or high pressure?

Here we need to turn again to the help of seismic observations. Longitudinal and transverse waves freely pass through all the shells of the Earth, enclosed between the surface of the Earth and the boundary of the core; consequently, everywhere here matter behaves like a solid. This conclusion is consistent with the conclusion of astronomers and geophysicists, who have shown that the hardness of the Earth as a whole is close to the hardness of steel. According to the calculations of V. F. Bonchkovsky, the hardness of the Earth is estimated at 12 10 11 dynes per square centimeter, which is four times greater than the hardness of granite.

Thus, the totality of modern data suggests that all the shells of the Earth (except for its core!) Should be considered to be in a solid state. The liquid state of matter can only be assumed for quite insignificant areas in the thickness of the earth's crust, with which volcanoes are directly connected.

The biggest difficulty is to avoid pathogenic microflora. And this is difficult to do in a moisture-saturated and warm enough environment. Even the best cellars always have mold. Therefore, we need a system of regularly used cleaning of pipes from any muck that accumulates on the walls. And to do this with a 3-meter laying is not so simple. First of all, the mechanical method comes to mind - a brush. How to clean chimneys. With some kind of liquid chemistry. Or gas. If you pump fozgen through a pipe, for example, then everything will die and this may be enough for a couple of months. But any gas enters the chem. reactions with moisture in the pipe and, accordingly, settles in it, which makes it air for a long time. And long airing will lead to the restoration of pathogens. Here you need a competent approach with knowledge of modern cleaning products.

In general, I sign under every word! (I really don't know what to be happy about).

In this system, I see several issues that need to be addressed:

1. Is the length of this heat exchanger sufficient for its efficient use (there will be some effect, but it is not clear which one)
2. Condensate. In winter, it will not be, as cold air will be pumped through the pipe. Condensate will fall from the outer side of the pipe - in the ground (it is warmer). But in the summer... The problem is HOW to pump condensate out from under a depth of 3 m - I already thought of making a hermetic well-cup for collecting condensate on the condensate collection side. Install a pump in it that will periodically pump out condensate ...
3. It is assumed that the sewer pipes (plastic) are airtight. If so, then the ground water around should not penetrate and should not affect the humidity of the air. Therefore, I suppose there will be no humidity (as in the basement). At least in winter. I think the basement is damp due to poor ventilation. Mold does not like sunlight and drafts (there will be drafts in the pipe). And now the question is - HOW tight are the sewer pipes in the ground? How many years will they last me? The fact is that this project is related - a trench is dug for sewage (it will be at a depth of 1-1.2m), then insulation (polystyrene foam) and deeper - an earth battery). So this system unrepairable in case of depressurization - I won’t rip it out - I’ll just cover it with earth and that’s it.
4. Pipe cleaning. I thought at the bottom point to make a viewing well. now there is less "intuzism" about this - ground water - it may turn out that it will be flooded and there will be ZERO. Without a well, there are not so many options:
A. revisions are made on both sides (for each 110mm pipe) that come to the surface, a stainless steel cable is pulled through the pipes. For cleaning, we attach a kwach to it. Cons - a bunch of pipes come to the surface, which will affect the temperature and hydrodynamic mode of the battery.
b. periodically flood the pipes with water and bleach, for example (or other disinfectant), pumping water from the condensate well at the other end of the pipes. Then drying the pipes with air (perhaps in a spring mode - from the house to the outside, although I don’t really like this idea).
5. There will be no mold (draft). but other microorganisms that live in drinking - very much so. There is hope for a winter regime - cold dry air disinfects well. Protection option - filter at the output of the battery. Or ultraviolet (expensive)
6. How hard is it to drive air over such a structure?
Filter (fine mesh) at the inlet
-> rotate 90 degrees down
-> 4m 200mm pipe down
-> split flow into 4 110mm pipes
-> 10 meters horizontally
-> rotate 90 degrees down
-> 1 meter down
-> rotate 90 degrees
-> 10 meters horizontally
-> flow collection in 200mm pipe
-> 2 meters up
-> rotate 90 degrees (into the house)
-> filter paper or fabric pocket
-> fan

We have 25 m of pipes, 6 turns by 90 degrees (turns can be made smoother - 2x45), 2 filters. I want 300-400m3/h. Flow speed ~4m/s

Temperature change with depth. The earth's surface, due to the uneven supply of solar heat, either heats up or cools down. These temperature fluctuations penetrate very shallowly into the thickness of the Earth. So, daily fluctuations at a depth of 1 m usually no longer felt. As for annual fluctuations, they penetrate to different depths: in warm countries by 10-15 m, and in countries with cold winter and hot summer up to 25-30 and even 40 m. Deeper than 30-40 m already everywhere on Earth the temperature is kept constant. For example, a thermometer placed in the basement of the Paris Observatory has been showing 11°.85C all the time for over 100 years.

A layer with a constant temperature is observed throughout the globe and is called a belt of constant or neutral temperature. The depth of this belt varies depending on climatic conditions, and the temperature is approximately equal to the average annual temperature of this place.

When deepening into the Earth below a layer of constant temperature, a gradual increase in temperature is usually noticed. This was first noticed by workers in the deep mines. This was also observed when laying tunnels. So, for example, when laying the Simplon tunnel (in the Alps), the temperature rose to 60 °, which created considerable difficulties in work. Even higher temperatures are observed in deep boreholes. An example is the Chukhovskaya well (Upper Silesia), in which at a depth of 2220 m temperature was over 80° (83°, 1), etc. m the temperature rises by 1°C.

The number of meters that you need to go deep into the Earth in order for the temperature to rise by 1 ° C is called geothermal step. The geothermal step in different cases is not the same and most often it ranges from 30 to 35 m. In some cases, these fluctuations can be even higher. For example, in the state of Michigan (USA), in one of the boreholes located near the lake. Michigan, the geothermal stage turned out to be not 33, but 70 m On the contrary, a very small geothermal step was observed in one of the wells in Mexico, There at a depth of 670 m there was water with a temperature of 70 °. Thus, the geothermal stage turned out to be only about 12 m. Small geothermal steps are also observed in volcanic regions, where at shallow depths there may still be uncooled strata of igneous rocks. But everything similar cases are not so much the rules as the exceptions.

There are many reasons that affect the geothermal stage. (In addition to the above, one can point out the different thermal conductivity of rocks, the nature of the occurrence of layers, etc.

Great importance in the temperature distribution has a terrain. The latter can be clearly seen in the attached drawing (Fig. 23), depicting a section of the Alps along the line of the Simplon Tunnel, with geoisotherms plotted by a dotted line (i.e., lines of equal temperatures inside the Earth). Geoisotherms here seem to repeat the relief, but with depth the influence of the relief gradually decreases. (The strong downward bending of the geoisotherms at Balle is due to the strong water circulation observed here.)

Temperature of the Earth at great depths. Observations on temperatures in boreholes, the depth of which rarely exceeds 2-3 km, Naturally, they cannot give an idea of ​​the temperatures of the deeper layers of the Earth. But here some phenomena from the life of the earth's crust come to our aid. Volcanism is one such phenomenon. Volcanoes, widespread on the earth's surface, bring molten lavas to the earth's surface, the temperature of which is over 1000 °. Therefore, at great depths we have temperatures exceeding 1000°.

There was a time when scientists, on the basis of the geothermal stage, tried to calculate the depth at which temperatures as high as 1000-2000 ° could be. However, such calculations cannot be considered sufficiently substantiated. Observations made on the temperature of the cooling basalt ball and theoretical calculations give reason to say that the value of the geothermal step increases with depth. But to what extent and to what depth such an increase goes, we also cannot yet say.

If we assume that the temperature increases continuously with depth, then in the center of the Earth it should be measured in tens of thousands of degrees. At such temperatures, all rocks known to us should go into a liquid state. True, there is enormous pressure inside the Earth, and we know nothing about the state of bodies at such pressures. However, we have no data to state that the temperature increases continuously with depth. Now most geophysicists come to the conclusion that the temperature inside the Earth can hardly be more than 2000 °.

Heat sources. As for the heat sources that determine the internal temperature of the Earth, they can be different. Based on the hypotheses that consider the Earth formed from a red-hot and molten mass, internal heat must be considered the residual heat of a body that is melting from the surface. However, there is reason to believe that the reason for the internal high temperature of the Earth may be the radioactive decay of uranium, thorium, actinouranium, potassium and other elements contained in rocks. radioactive elements for the most part are common in acidic rocks of the surface shell of the Earth, they are less common in deep basic rocks. At the same time, the basic rocks are richer in them than iron meteorites, which are considered fragments of the internal parts of cosmic bodies.

Despite the small amount of radioactive substances in rocks and their slow decay, the total amount of heat resulting from radioactive decay is large. Soviet geologist V. G. Khlopin calculated that the radioactive elements contained in the upper 90-kilometer shell of the Earth are enough to cover the loss of heat of the planet by radiation. Along with radioactive decay thermal energy released during compression of the Earth's matter, with chemical reactions and so on.

The temperature inside the earth is most often a rather subjective indicator, since the exact temperature can only be called in accessible places, for example, in Kola well(depth 12 km). But this place belongs to the outer part of the earth's crust.

Temperatures of different depths of the Earth

As scientists have found, the temperature rises by 3 degrees every 100 meters deep into the Earth. This figure is constant for all continents and parts of the globe. Such an increase in temperature occurs in the upper part of the earth's crust, approximately the first 20 kilometers, then the temperature increase slows down.

The largest increase was recorded in the United States, where the temperature rose by 150 degrees per 1000 meters deep into the earth. The slowest growth was recorded in South Africa, the thermometer rose by only 6 degrees Celsius.

At a depth of about 35-40 kilometers, the temperature fluctuates around 1400 degrees. The boundary of the mantle and the outer core at a depth of 25 to 3000 km heats up from 2000 to 3000 degrees. The inner core is heated to 4000 degrees. The temperature in the very center of the Earth, according to the latest information obtained as a result of complex experiments, is about 6000 degrees. The Sun can boast the same temperature on its surface.

Minimum and maximum temperatures of the Earth's depths

When calculating the minimum and maximum temperatures inside the Earth, the data of the constant temperature belt are not taken into account. In this zone, the temperature is constant throughout the year. The belt is located at a depth of 5 meters (tropics) and up to 30 meters (high latitudes).

The maximum temperature was measured and recorded at a depth of about 6000 meters and amounted to 274 degrees Celsius. The minimum temperature inside the earth is fixed mainly in the northern regions of our planet, where even at a depth of more than 100 meters the thermometer shows minus temperatures.

Where does heat come from and how is it distributed in the bowels of the planet

The heat inside the earth comes from several sources:

1) Decay of radioactive elements;

2) The gravitational differentiation of matter heated in the core of the Earth;

3) Tidal friction (the impact of the Moon on the Earth, accompanied by a deceleration of the latter).

These are some options for the occurrence of heat in the bowels of the earth, but the question of complete list and the correctness of the already available open so far.

The heat flux emanating from the bowels of our planet varies depending on the structural zones. Therefore, the distribution of heat in a place where the ocean, mountains or plains are located has completely different indicators.