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

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

Rational use of fuel 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 using 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 cleanliness, as well as new opportunities in the field 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, a very effective direction for introducing the technologies under consideration into the practice of domestic construction seems to be wide application 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 as a result of geological processes to a high temperature, which allows them to be used for heating 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, for preventing icing, heating fields in open stadiums, etc. In the English-language technical literature, such systems are designated 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 single 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-potential ground heat is vertical ground heat exchangers of coaxial type, 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 - incident on the surface solar radiation 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 temperature fluctuations upper layers 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- climatic conditions ranges from a few 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 the parameters of the outdoor climate (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 defrosting. At the same time, naturally, there is a change state of aggregation moisture contained in the pores of the soil and located in the general case both in the liquid, and in the solid and gaseous phases at the same time. In other words, the soil mass of the heat collection system, regardless of what 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 and mobile, depending on whether the particles are firmly bound together 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. The modeling of heat and mass transfer processes that form the thermal regime of such a multicomponent system is extremely difficult. difficult task, since it requires taking into account and mathematical description of various mechanisms for their implementation: thermal conductivity in a single particle, heat transfer from one particle to another upon their contact, molecular thermal conductivity 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 noticeable 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 bonding forces between moisture and skeletal particles, the dependence of the forms of moisture bonding with the material at various stages of wetting, 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 reduced 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 are wells that allow extracting groundwater from aquifers soil and return 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 freezing level of the soil 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(in English literature, the designation "BHE" - "borehole heat exchanger" is accepted) 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 sections large area and do not depend on the intensity of solar radiation incident 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 for heating and cooling buildings various sizes. For a small building, one heat exchanger is enough; for large buildings, a whole group of wells with vertical heat exchangers may be required. 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, the largest number of wells (154 wells 70 m deep) 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 as ground heat exchangers building structures, such as 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 borehole diameter is usually 15 cm. lower part a pump is placed in 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, normal functioning the system will require a powerful pump that requires 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 promising directions– use of water from mines and tunnels as a source of low-potential 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 exploiting the source of energy resources. 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, in 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 low 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, long-term consumption of thermal energy from the soil mass 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 mass caused by the long-term operation of the heat collection system, and use the temperatures of the soil mass 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 heat 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), a decrease in temperature will be especially noticeable and can lead to to freezing of the soil mass around the soil 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 coaxial-type ground heat exchanger with a depth of 105 m was installed 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-apartment residential building. The vertical ground heat exchanger provided a peak power of approximately 70 watts per meter of length, which created a significant heat load to the surrounding soil. 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 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 showed 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

13. Energy efficient residential building in Moscow. ABOK №4, 1999

14. Vasiliev G.P. Energy-efficient experimental residential building in the Nikulino-2 microdistrict. ABOK №4, 2002

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 winters and hot summers 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. Geothermal stage in various occasions varies 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 all such cases are not so much rules as 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 etc.

To model temperature fields and for other calculations, it is necessary to know the soil temperature at a given depth.

The temperature of the soil at depth is measured using exhaust soil-deep thermometers. These are planned studies that are regularly carried out by meteorological stations. Research data serve as the basis for climate atlases and regulatory documentation.

To obtain the soil temperature at a given depth, you can try, for example, two simple ways. Both methods are based on the use of reference literature:

1. For an approximate determination of temperature, you can use the document TsPI-22. "Railway crossings by pipelines". Here, within the framework of the methodology for the heat engineering calculation of pipelines, Table 1 is given, where for certain climatic regions, soil temperatures are given depending on the depth of measurement. I present this table below.

Table 1

1. Table of soil temperatures at various depths from the source "to help the worker gas industry» even the times of the USSR

Normative freezing depths for some cities:

The depth of soil freezing depends on the type of soil:

I think the easiest option is to use the reference data above and then interpolate.

The most reliable option for accurate calculations using ground temperatures is to use data from the meteorological services. On the basis of meteorological services, some online directories work. For example, http://www.atlas-yakutia.ru/.

Here it is enough to select the settlement, the type of soil and you can get a temperature map of the soil or its data in tabular form. In principle, it is convenient, but it seems that this resource is paid.

If you know more ways to determine the soil temperature at a given depth, then please write comments.

You may be interested in the following material:

Here is published the dynamics of changes in winter (2012-13) ground temperatures at a depth of 130 centimeters under the house (under the inner edge of the foundation), as well as at ground level and the temperature of the water coming from the well. All this - on the riser coming from the well.
The chart is at the bottom of the article.
Dacha (on the border of New Moscow and the Kaluga region) winter, periodic visits (2-4 times a month for a couple of days).
The blind area and the basement of the house are not insulated, since autumn they have been closed with heat-insulating plugs (10 cm of foam). The heat loss of the veranda where the riser goes in January has changed. See Note 10.
Measurements at a depth of 130 cm are made by the Xital GSM system (), discrete - 0.5 * C, add. the error is about 0.3 * C.
The sensor is installed in a 20 mm HDPE pipe welded from below near the riser, (with outside thermal insulation of the riser, but inside the 110mm pipe).
The abscissa shows dates, the ordinate shows temperatures.
Note 1:
I will also monitor the temperature of the water in the well, as well as at the ground level under the house, right on the riser without water, but only upon arrival. The error is about + -0.6 * C.
Note 2:
Temperature at ground level under the house, at the water supply riser, in the absence of people and water, it already dropped to minus 5 * C. This suggests that I did not make the system in vain - By the way, the thermostat that showed -5 * C is just from this system (RT-12-16).
Note 3:
The temperature of the water "in the well" is measured by the same sensor (it is also in Note 2) as "at ground level" - it stands right on the riser under the thermal insulation, close to the riser at ground level. These two measurements are made at different times. "At ground level" - before pumping water into the riser and "in the well" - after pumping about 50 liters for half an hour with interruptions.
Note 4:
The temperature of the water in the well can be somewhat underestimated, because. I can't look for this fucking asymptote, endlessly pumping water (mine)... I play as best I can.
Note 5: Not relevant, removed.
Note 6:
Fixation error outdoor temperature approximately + - (3-7) * C.
Note 7:
The rate of cooling of water at ground level (without turning on the pump) is very approximately 1-2 * C per hour (this is at minus 5 * C at ground level).
Note 8:
I forgot to describe how my underground riser is arranged and insulated. Two stockings of insulation are put on PND-32 in total - 2 cm. thickness (apparently, foamed polyethylene), all this is inserted into a 110mm sewer pipe and foamed there to a depth of 130cm. True, since PND-32 did not go in the center of the 110th pipe, and also the fact that in its middle the mass of ordinary foam may not harden for a long time, which means it does not turn into a heater, I strongly doubt the quality of such additional insulation .. It would probably be better to use a two-component foam, the existence of which I only found out later ...
Note 9:
I want to draw the attention of readers to the temperature measurement "At ground level" dated 01/12/2013. and dated January 18, 2013. Here, in my opinion, the value of +0.3 * C is much higher than expected. I think that this is a consequence of the operation "Filling the basement at the riser with snow", carried out on 12/31/2012.
Note 10:
From January 12 to February 3, he made additional insulation of the veranda, where the underground riser goes.
As a result, according to approximate estimates, the heat loss of the veranda was reduced from 100 W / sq.m. floor to about 50 (this is at minus 20 * C on the street).
This is also reflected in the charts. See the temperature at ground level on February 9: +1.4*C and on February 16: +1.1 - there have not been such high temperatures since the beginning of real winter.
And one more thing: from February 4 to 16, for the first time in two winters from Sunday to Friday, the boiler did not turn on to maintain the set minimum temperature because it did not reach this minimum ...
Note 11:
As promised (for "order" and to complete the annual cycle), I will periodically publish temperatures in the summer. But - not in the schedule, so as not to "obscure" the winter, but here, in Note-11.
May 11, 2013
After 3 weeks of ventilation, the vents were closed until autumn to avoid condensation.
May 13, 2013(on the street for a week + 25-30 * C):
- under the house at ground level + 10.5 * C,
- under the house at a depth of 130cm. +6*С,

June 12, 2013:
- under the house at ground level + 14.5 * C,
- under the house at a depth of 130cm. +10*С.
- water in the well from a depth of 25 m not higher than + 8 * C.
June 26, 2013:
- under the house at ground level + 16 * C,
- under the house at a depth of 130 cm. +11*С.
- water in the well from a depth of 25m is not higher than +9.3*C.
August 19, 2013:
- under the house at ground level + 15.5 * C,
- under the house at a depth of 130cm. +13.5*С.
- water in the well from a depth of 25m not higher than +9.0*C.
September 28, 2013:
- under the house at ground level + 10.3 * C,
- under the house at a depth of 130cm. +12*С.
- water in the well from a depth of 25m = + 8.0 * C.
October 26, 2013:
- under the house at ground level + 8.5 * C,
- under the house at a depth of 130 cm. +9.5*С.
- water in the well from a depth of 25 m not higher than + 7.5 * C.
November 16, 2013:
- under the house at ground level + 7.5 * C,
- under the house at a depth of 130 cm. +9.0*С.
- water in the well from a depth of 25m + 7.5 * C.
February 20, 2014:
This is probably the last entry in this article.
All winter we live in the house all the time, the point in repeating last year's measurements is small, so only two significant figures:
- the minimum temperature under the house at ground level in the very frosts (-20 - -30 * C) a week after they began, repeatedly fell below + 0.5 * C. At these moments, I worked

One of the best, rational methods in the construction of capital greenhouses is an underground thermos greenhouse.
The use of this fact of the constancy of the earth's temperature at a depth in the construction of a greenhouse gives tremendous savings in heating costs in the cold season, facilitates care, makes the microclimate more stable.
Such a greenhouse works in the most severe frosts, allows you to produce vegetables, grow flowers all year round.
A properly equipped buried greenhouse makes it possible to grow, among other things, heat-loving southern crops. There are practically no restrictions. Citrus fruits and even pineapples can feel great in a greenhouse.
But in order for everything to function properly in practice, it is imperative to follow the time-tested technologies by which underground greenhouses were built. After all, this idea is not new, even under the tsar in Russia, buried greenhouses yielded pineapple crops, which enterprising merchants exported to Europe for sale.
For some reason, the construction of such greenhouses has not found wide distribution in our country, by and large, it is simply forgotten, although the design is ideal just for our climate.
Probably, the need to dig a deep pit and pour the foundation played a role here. The construction of a buried greenhouse is quite expensive, it is far from a greenhouse covered with polyethylene, but the return on the greenhouse is much greater.
From deepening into the ground, the overall internal illumination is not lost, this may seem strange, but in some cases the light saturation is even higher than that of classic greenhouses.
It is impossible not to mention the strength and reliability of the structure, it is incomparably stronger than usual, it is easier to tolerate hurricane gusts of wind, it resists hail well, and blockages of snow will not become a hindrance.

1. Pit

The creation of a greenhouse begins with digging a foundation pit. To use the heat of the earth to heat the internal volume, the greenhouse must be sufficiently deepened. The deeper the earth gets warmer.
The temperature almost does not change during the year at a distance of 2-2.5 meters from the surface. At a depth of 1 m, the ground temperature fluctuates more, but in winter its value remains positive, usually in middle lane the temperature is 4-10 C, depending on the season.
A buried greenhouse is built in one season. That is, in winter it will already be able to function and generate income. Construction is not cheap, but by using ingenuity, compromise materials, it is possible to save literally a whole order of magnitude by making a kind of economy option for a greenhouse, starting with a foundation pit.
For example, do without the involvement of construction equipment. Although the most time-consuming part of the work - digging a pit - is, of course, better to give to an excavator. Manually removing such a volume of land is difficult and time consuming.
The depth of the excavation pit should be at least two meters. At such a depth, the earth will begin to share its heat and work like a kind of thermos. If the depth is less, then in principle the idea will work, but noticeably less efficiently. Therefore, it is recommended that you spare no effort and money to deepen the future greenhouse.
Underground greenhouses can be of any length, but it is better to keep the width within 5 meters, if the width is larger, then the quality characteristics for heating and light reflection deteriorate.
On the sides of the horizon, underground greenhouses need to be oriented, like ordinary greenhouses and greenhouses, from east to west, that is, so that one of the sides faces south. In this position, the plants will receive the maximum amount of solar energy.

2. Walls and roof

Along the perimeter of the pit, a foundation is poured or blocks are laid out. The foundation serves as the basis for the walls and frame of the structure. Walls are best made from materials with good thermal insulation characteristics, fine option - thermoblocks.

The roof frame is often made of wood, from bars impregnated with antiseptic agents. The roof structure is usually straight gable. Fix in the center of the structure ridge beam, for this, central supports are installed on the floor along the entire length of the greenhouse.

The ridge beam and walls are connected by a row of rafters. The frame can be made without high supports. They are replaced with small ones, which are placed on transverse beams connecting opposite sides of the greenhouse - this design makes inner space freer.

It is better to take as a roof covering cellular polycarbonate- popular modern material. The distance between the rafters during construction is adjusted to the width of the polycarbonate sheets. It is convenient to work with the material. The coating is obtained with a small number of joints, since the sheets are produced in lengths of 12 m.

They are attached to the frame with self-tapping screws, it is better to choose them with a cap in the form of a washer. To avoid cracking the sheet, a hole of the appropriate diameter must be drilled under each self-tapping screw with a drill. With a screwdriver, or a conventional drill with a Phillips bit, glazing work moves very quickly. In order to avoid gaps, it is good to lay the rafters along the top with a sealant made of soft rubber or other suitable material and only then screw the sheets. The peak of the roof along the ridge must be laid with soft insulation and pressed with some kind of corner: plastic, tin, or another suitable material.

For good thermal insulation, the roof is sometimes made with a double layer of polycarbonate. Although the transparency is reduced by about 10%, but this is covered by the excellent thermal insulation performance. It should be noted that the snow on such a roof does not melt. Therefore, the slope must be at a sufficient angle, at least 30 degrees, so that snow does not accumulate on the roof. Additionally, an electric vibrator is installed for shaking, it will save the roof in case snow still accumulates.

Double glazing is done in two ways:

A special profile is inserted between two sheets, the sheets are attached to the frame from above;

First, the bottom layer of glazing is attached to the frame from the inside, to the underside of the rafters. The roof is covered with the second layer, as usual, from above.

After completing the work, it is desirable to glue all the joints with tape. finished roof looks very impressive: without unnecessary joints, smooth, without prominent parts.

3. Warming and heating

Wall insulation is carried out as follows. First you need to carefully coat all the joints and seams of the wall with a solution, here you can also use mounting foam. inner side The walls are covered with thermal insulation film.

In cold parts of the country, it is good to use foil thick film, covering the wall with a double layer.

The temperature deep in the soil of the greenhouse is above zero, but colder than the air temperature required for plant growth. The top layer is heated by the sun's rays and the air of the greenhouse, but still the soil takes away heat, so often in underground greenhouses they use the technology of "warm floors": the heating element - an electric cable - is protected by a metal grill or poured with concrete.

In the second case, the soil for the beds is poured over concrete or greens are grown in pots and flowerpots.

The use of underfloor heating can be sufficient to heat the entire greenhouse if there is enough power. But it is more efficient and more comfortable for plants to use combined heating: underfloor heating + air heating. For good growth, they need an air temperature of 25-35 degrees at an earth temperature of about 25 C.

CONCLUSION

Of course, the construction of a buried greenhouse will cost more, and more effort will be required than with the construction of a similar greenhouse of a conventional design. But the funds invested in the greenhouse-thermos are justified over time.

First, it saves energy on heating. No matter how an ordinary ground-based greenhouse is heated in winter, it will always be more expensive and more difficult than a similar heating method in an underground greenhouse. Secondly, saving on lighting. Foil thermal insulation of the walls, reflecting light, doubles the illumination. The microclimate in an in-depth greenhouse in winter will be more favorable for plants, which will certainly affect the yield. Seedlings will easily take root, tender plants will feel great. Such a greenhouse guarantees a stable, high yield of any plants all year round.