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

The temperature of the soil at a depth is measured with the help of extraction soil-depth thermometers. These are planned surveys that are regularly carried out by meteorological stations. Research data serve as the basis for climate atlases and regulatory documents.

To obtain the ground temperature at a given depth, you can try, for example, two simple methods. Both methods involve using reference books:

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

Table 1

1. Table of soil temperatures at various depths from a source "to help a worker in the gas industry" from the time of the USSR

Standard frost penetration depths for some cities:

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

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

The most reliable option for accurate calculations using ground temperatures is to use data from the meteorological services. Some online directories are based on the meteorological services. For example, http://www.atlas-yakutia.ru/.

It is enough to choose here locality, type of soil and you can get temperature map soil or its data in tabular form. In principle, it is convenient, but it looks like this resource is paid.

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

You may be interested in the following material:

The surface layer of the Earth's soil is a natural heat accumulator. Main source thermal energy entering the upper layers of the Earth - solar radiation. At a depth of about 3 m or more (below the freezing level), the soil temperature practically does not change during the year and is approximately equal to the average annual temperature of the outside air. At a depth of 1.5-3.2 m in winter, the temperature ranges from +5 to + 7 ° C, and in summer from +10 to + 12 ° C. With this heat, you can prevent the house from freezing in winter, and prevent it from overheating above 18 in summer. -20 ° C

The most in a simple way The use of the heat of the earth is the use of a soil heat exchanger (PHE). Under the ground, below the level of freezing of the soil, a system of air ducts is laid, which perform the function of a heat exchanger between the ground and the air that passes through these air ducts. In winter, incoming cold air that enters and passes through the pipes heats up, and in summer it cools down. With a rational placement of air ducts, a significant amount of thermal energy can be taken from the soil with little electricity consumption.

A pipe-in-pipe heat exchanger can be used. Internal stainless steel air ducts act as recuperators here.

Cooling in summer

In the warm season, a ground heat exchanger provides cooling of the supply air. Outside air enters through the air intake device into the ground heat exchanger, where it is cooled by the ground. Then the cooled air is supplied by air ducts to the air handling unit, in which a summer insert is installed instead of a recuperator for the summer period. Thanks to this solution, the temperature in the premises decreases, the microclimate in the house improves, and the energy consumption for air conditioning is reduced.

Off-season work

When the difference between the outside and inside air temperatures is small, fresh air can be supplied through the supply grille located on the wall of the house in the above-ground part. In the period when the difference is significant, the supply of fresh air can be carried out through the heat exchanger, providing heating / cooling of the supply air.

Savings in winter

In the cold season, outside air enters through the air intake device into the heat exchanger, where it is warmed up and then enters the air handling unit for heating in the recuperator. Preheating air handling unit reduces the likelihood of icing of the air handling unit recuperator, increasing the effective time of recuperation and minimizing the cost of additional air heating in the water / electric heater.

How air heating and cooling costs are calculated

It is possible to pre-calculate the cost of heating air in winter for a room where air is supplied at a standard of 300 m3 / h. In winter, the average daily temperature for 80 days is -5 ° C - it must be heated to + 20 ° C. To heat this amount of air, you need to spend 2.55 kW per hour (in the absence of a heat recovery system). When using a geothermal system, the outside air is heated to +5 and then 1.02 kW is used to warm up the incoming air to the comfortable one. The situation is even better when using recuperation - you only need to spend 0.714 kW. Over a period of 80 days, respectively, 2,448 kWh of thermal energy will be spent, and geothermal systems will reduce costs by 1175 or 685 kWh.

In the off-season, within 180 days, the average daily temperature is + 5 ° C - it needs to be heated to + 20 ° C. Planned costs are 3305 kWh, and geothermal systems will reduce costs by 1322 or 1102 kWh.

In the summer, for 60 days, the average daily temperature is about + 20 ° C, but for 8 hours it is within + 26 ° C. The costs for cooling will be 206 kWh, and the geothermal system will reduce costs by 137 kWh.

Throughout the year, the operation of such a geothermal system is assessed using the coefficient - SPF (seasonal power factor), which is defined as the ratio of the amount of heat energy received to the amount of electricity consumed, taking into account seasonal changes in air / ground temperature.

To obtain 2634 kWh of thermal power from the soil, the ventilation unit spends 635 kWh of electricity per year. SPF = 2634/635 = 4.14.
Based on materials.

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

Vasiliev G.P., Scientific Director of 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 is today 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-conventional renewable energy sources (NRES) The depletion of reserves of traditional fossil fuels and the environmental consequences of its combustion have resulted in a significant increase in interest in these technologies in almost all developed countries of the world in recent decades.

The advantages of heat supply technologies used in comparison with their traditional counterparts are associated not only with significant reductions in energy consumption in the life support systems of buildings and structures, but also with their environmental friendliness, 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 play a decisive role in the formation of a competitive situation in the market of heat generating equipment.

Analysis of possible areas of application in the Russian economy of energy saving technologies using unconventional energy sources, shows that in Russia the most promising area of ​​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 the widespread use of heat pump heat supply systems (TST) using the soil of the surface layers of the Earth as a universally available low-potential heat source.

Using heat of the earth two types of heat energy can be distinguished - 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 the 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 by means of heat pumps is possible almost everywhere. It is currently one of the fastest growing areas of use. unconventional renewable energy sources.

Low-grade 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 designated as "GHP" - "geothermal heat pumps", ground source heat pumps.

The climatic characteristics of the countries of Central and Northern Europe, which, together with the USA and Canada, are the main regions for the use of low-potential heat of the Earth, determine mainly the need for heating; cooling the air even in summer is relatively rare. Therefore, unlike the United States, heat pumps in European countries they operate mainly in heating mode. IN THE USA heat pumps more commonly used in systems air heating, combined with ventilation, which allows you to both heat and cool the outside air. V European countries heat pumps usually used in hot water heating systems. Insofar as heat pump efficiency increases with a decrease in the temperature difference between the evaporator and the condenser, often underfloor heating systems are used for heating buildings, in which a coolant circulates at a relatively low temperature (35–40 oC).

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

Over the past ten years, the number of systems used for heating and cooling of buildings low grade heat Earth through heat pumps, has increased significantly. The largest number of such systems are in use in the United States. A large number of such systems operate in Canada and the countries of central and northern Europe: Austria, Germany, Sweden and Switzerland. Switzerland is the leader in terms of the use of low-grade thermal energy of the Earth per capita. In Russia, over the past ten years, according to technology and with the participation of INSOLAR-INVEST OJSC, specializing 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, it was actually built for the first time heat pump hot water 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 Association of NP "AVOK" and within the framework of "Long-term energy saving program in Moscow".

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

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

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

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

Table 1. World level of use of low-grade thermal energy of the Earth by means of heat pumps

Soil as a source of low-grade thermal energy

As a source of low-grade thermal energy, groundwater with a relatively low temperature or the 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 falling on the surface and the flux of radiogenic heat from the earth's interior.... Seasonal and daily changes in the intensity of solar radiation and the temperature of the outside air cause fluctuations in the temperature of the upper layers of the soil. The penetration depth of daily fluctuations in the outside air temperature and the intensity of the incident solar radiation, depending on specific soil and climatic conditions, ranges from several tens of centimeters to one and a half meters. The penetration depth of seasonal fluctuations in the outside air temperature and the intensity of the incident solar radiation does not exceed, as a rule, 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 external climate (Fig. 1).

Rice. 1. Graph of soil temperature changes 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 earth's interior differs for different areas. For Central Europe, this value is 0.05–0.12 W / m2.

During the operational period, the soil mass located within the zone thermal influence the register of pipes of a ground heat exchanger of a system for collecting low-grade soil 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 aggregate state of moisture contained in the pores of the soil and in the general case both in the liquid and in the solid and gaseous phases simultaneously. In other words, the soil massif 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 amount 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 they are separated from each other by matter in the mobile phase. The gaps between the solid particles can be filled with mineralized moisture, gas, steam and ice, or both. Modeling heat and mass transfer processes that form the thermal regime of such a multicomponent system is an extremely difficult task, since it requires taking into account and mathematical description of various mechanisms of their implementation: thermal conductivity in an individual particle, heat transfer from one particle to another during their contact, molecular thermal conductivity in a medium filling the gaps. between particles, convection of steam and moisture contained in the pore space, and many others.

Special attention should be paid to the influence of the moisture content of the soil mass and the migration of moisture in its pore space on thermal processes that determine the characteristics of the soil 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 heat propagation process. The 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 bond forces between moisture and skeletal particles, the dependence of the forms of bonding between moisture and material at various stages of moisture, and the mechanism of moisture movement in the pore space have not yet been clarified.

In the presence of a temperature gradient in the thickness of the soil massif, 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 occurs in the liquid phase. In addition, moisture affects the temperature regime of the upper soil layers. atmospheric precipitation as well as groundwater.

The main factors under the influence of which are formed temperature regime the soil massif of systems for collecting 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 a soil massif. In addition to "extracting" the Earth's heat, ground heat exchangers can also be used to accumulate heat (or cold) in the ground mass.

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

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

The main part of open systems are wells, which allow to extract groundwater from aquifers of the 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. Diagram of an open system for the use of low-potential thermal energy of groundwater

The advantage of open systems is the ability to obtain a large amount of thermal energy at relatively low costs. However, the 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 water permeability of the soil, allowing replenishment of water supplies;
• good chemical composition groundwater table (eg low iron content) to avoid problems associated with pipe wall deposits and corrosion.

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-grade thermal energy. This system is located in Louisville, Kentucky, USA. The system is used for heat and cold supply of the hotel-office complex; its capacity is approximately 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 has been adopted, in particular, in the United States. Systems using low-grade heat from water bodies 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 the English-language literature, the terms “ground heat collector” and “horizontal loop” are also used) is usually set up near the house at a shallow depth (but below the level of soil freezing in winter). The use of horizontal ground heat exchangers is limited by the size of the site available.

In the countries of Western and Central Europe, horizontal ground heat exchangers usually represent separate pipes, laid relatively tightly and connected in series or in parallel (Fig. 4a, 4b). To save the area of ​​the site, improved types of heat exchangers were developed, for example, heat exchangers in the form of a spiral, located horizontally or vertically (Fig. 4e, 4f). This form of heat exchanger is common in the United States.

Rice. 4. Types of horizontal ground heat exchangers
a - a heat exchanger of pipes connected in series;
b - a heat exchanger made of parallel-connected pipes;
в - horizontal collector laid in a trench;
d - a 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 for generating 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 the English-language literature, the designation "BHE" - "borehole heat exchanger" is accepted) allow the use of low-potential thermal energy a soil massif lying below the "neutral zone" (10–20 m above ground level). Systems with vertical ground heat exchangers require no space large area 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-family residential building by means of a heat pump installation with a vertical ground heat exchanger is shown in Fig. 5.

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

The coolant circulates through pipes (most often polyethylene or polypropylene) laid in vertical wells with a depth of 50 to 200 m. Usually, two types of vertical ground heat exchangers are used (Fig. 6):

• U-shaped heat exchanger, which are two parallel pipes connected at the bottom. One well contains one or two (rarely three) pairs of such pipes. The advantage of this arrangement 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 pipe with a smaller diameter is located inside another pipe. Coaxial heat exchangers can be of more complex configurations.

Rice. 6. Section of various types of vertical ground heat exchangers

To increase the efficiency of the heat exchangers, the space between the borehole walls 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 sufficient; for large buildings, it may be necessary to install a whole group of 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 USA in the state of New Jersey. The college's vertical ground heat exchangers are located in 400 boreholes 130 m deep. In Europe, the largest number of boreholes (154 boreholes 70 m deep) are used in the heating and cooling system of the headquarters of the German Air Traffic Service (Deutsche Flug-sicherung).

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

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

The ground massif (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, the heat of solar radiation.

There are systems that cannot be unambiguously classified as open or closed. For example, one and the same deep (from 100 to 450 m depth) 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 is returned to the top of the water column in the same well. There is a constant replenishment of the well with groundwater, and open system works like a closed one. Systems of this type in the English-language literature are called "standing column well system" (Fig. 8).

Rice. 8. Standing column well diagram

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 constant water replenishment of the well, which prevents it from freezing. If the aquifer is too deep, a powerful pump will be required for the normal operation of the system, which requires increased energy consumption. The large depth of the well determines the rather high cost of such systems, so they are not used for heat and cooling supply of small buildings. Now in the world there are several such systems 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.

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

During the operation of the ground heat exchanger, a situation may arise when during the heating season the temperature of the soil near the ground heat exchanger decreases, and in the summer period the ground 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 ground temperature, and its temperature potential is further reduced. This forces the design of systems use of low-grade heat of the Earth consider the problem of “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 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-grade heat of the earth the following definition of sustainability is given: “For each system of using the low-grade 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). "

Conducted 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 the temperature of the soil 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 be compensated for in the summer period of the year, and by the beginning of the next heating season the soil comes out with reduced temperature potential. The consumption of thermal energy during the next heating season causes a further decrease in ground temperature, and by the beginning of the third heating season, its temperature potential is even more different from the natural one. Etc. However, the envelopes of the thermal effect of the 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 the periodic one, that is, starting from the fifth year of operation, the long-term consumption of thermal energy from the soil massif heat collection system is accompanied by periodic changes in its temperature. Thus, when designing heat pump heat supply systems it seems necessary to take into account the drop in the 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 established "automatically": in winter (heat supply is required), the soil mass is cooled, in summer time(cold supply required) - heating of the soil mass. Systems that use low-grade groundwater heat are constantly replenishing water supplies from water seeping from the surface and water coming from deeper layers of the ground. Thus, the heat content of groundwater increases both "from above" (due to the heat of the atmospheric air) and "from below" (due to the heat of the Earth); the amount of heat input "from above" and "from below" depends on the thickness and depth of the aquifer. Due to these heat inputs, the groundwater temperature remains constant throughout the season and changes little during operation.

The situation is different in systems with vertical ground heat exchangers. When heat is removed, the temperature of the soil around the ground heat exchanger decreases. The decrease in temperature is influenced by both the design features of the heat exchanger and the mode of its operation. For example, in systems with high values ​​of heat dissipated (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 have measured the temperature of the soil massif, in which a vertical soil heat exchanger with a depth of 50 m is arranged, located near Frankfurt am Main. For this, 9 wells of the same depth were drilled around the main well at a distance of 2.5, 5 and 10 m from. In all ten wells, sensors were installed every 2 m to measure temperature - a total of 240 sensors. In fig. 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 noticeable. 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 “extraction” of heat. The magnitude of this flux, in comparison with the magnitude of the heat flux from the earth's interior in a given area (80–100 mW / m2), is estimated to be quite high (several watts per square meter).

Rice. 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 the relatively widespread use of vertical heat exchangers began to receive about 15–20 years ago, there is a lack of experimental data all over the world, obtained with long (several tens of years) service life 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 Earth's low-grade heat a renewable energy source? What is the "renewal" period for 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 output were at the level of 120-190 W / linear. m length of the heat exchanger.

Since 1986, studies have been carried out on a system with vertical ground heat exchangers in Switzerland near Zurich. A vertical ground coaxial 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-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 soil mass. Annual heat production 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 soil temperature, other parameters were also recorded: the speed of movement of the coolant, energy consumption by the drive of the heat pump compressor, air temperature, etc.

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

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

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

Mathematical modeling showed that the annual decrease in temperature will gradually decrease, and the volume of the soil mass around the heat exchanger, subject to a decrease in temperature, will increase every year. At the end of the operating period, the regeneration process begins: the soil temperature begins to rise. The nature of the process of regeneration is similar to the nature of the process of "extraction" of heat: in the first years of operation there is a sharp increase in the temperature of the soil, and in subsequent years the rate of increase in temperature decreases. The length of the "regeneration" period depends on the length of the operating period. These two periods are approximately 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, heating and cooling systems for buildings that use the low-grade heat of the Earth are a reliable source of energy that can be used everywhere. This source can be used for a sufficiently long time, and can be renewed at the end of the period of operation.

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. AVOK No. 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 heating systems. Journal "Housing and communal services", No. 12, 2002

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

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

14. Vasiliev G.P. An energy-efficient experimental residential building in the Nikulino-2 microdistrict. AVOK No. 4, 2002

Soil temperature continuously changes with depth and time. It depends on a number of factors, many of which are difficult to account for. The latter, for example, include: the nature of the vegetation, the exposure of the slope to the cardinal points, shading, snow cover, the nature of the soils themselves, the presence of suprapermafrost waters, etc. stable, and the decisive influence here remains with the air temperature.

Soil temperature at different depths and in different periods of the year can be obtained by direct measurements in thermal wells, which are laid in the course of exploration. But this method requires long-term observations and significant costs, which is not always justified. The data obtained from one or two wells spread over large areas and lengths, significantly distorting reality so that the calculated data on the soil temperature in many cases turns out to be more reliable.

Permafrost soil temperature at any depth (up to 10 m from the surface) and for any period of the year can be determined by the formula:

tr = mt °, (3.7)

where z is the depth measured from the VGM, m;

tr - soil temperature at depth z, in deg.

τr - time equal to a year (8760 h);

τ is the time counted forward (after January 1) from the moment of the beginning of the autumn freezing of the soil to the moment for which the temperature is measured, in hours;

exp x - exponent (exponential function exp is taken from tables);

m - coefficient depending on the period of the year (for the period October - May m = 1.5-0.05z, and for the period June-September m = 1)

The most low temperature at a given depth will be when the cosine in formula (3.7) becomes equal to -1, i.e., the minimum soil temperature for a year at a given depth will be

tr min = (1.5-0.05z) t °, (3.8)

Maximum temperature soil at a depth z, will be when the cosine takes a value equal to one, i.e.

tr max = t °, (3.9)

In all three formulas, the value of the volumetric heat capacity C m should be calculated for the soil temperature t ° according to the formula (3.10).

C 1 m = 1 / W, (3.10)

Soil temperature in the layer of seasonal thawing can also be determined by calculation, taking into account that the temperature change in this layer is fairly accurately approximated by a linear dependence at the following temperature gradients (Table 3.1).

Having calculated the temperature of the soil at the level of the VGM using one of the formulas (3.8) - (3.9), i.e. putting in the formulas Z = 0, then using Table 3.1 we determine the temperature of the soil at a given depth in the layer of seasonal thawing. In the uppermost soil layers, up to about 1 m from the surface, the nature of temperature fluctuations is very complex.

Table 3.1

Temperature gradient in the layer of seasonal thawing at a depth below 1 m from the earth's surface

Note. The gradient sign is shown towards the day surface.

To obtain the calculated soil temperature in a meter layer from the surface, you can proceed as follows. Calculate the temperature at a depth of 1 m and the temperature of the day surface of the soil, and then, by interpolating from these two values, determine the temperature at a given depth.

The temperature on the soil surface t p in the cold season can be taken equal to the air temperature. In the summer:

t p = 2 + 1.15 t in, (3.11)

where t p is the temperature on the surface in deg.

t in - air temperature in deg.

Soil temperature in non-flowing cryolithozone is calculated differently than when merging. In practice, we can assume that the temperature at the VGM level will be equal to 0 ° C throughout the year. The design temperature of the soil of the permafrost strata at a given depth can be determined by interpolation, assuming that it changes at depth according to a linear law from t ° at a depth of 10 m to 0 ° C at the depth of the VGM. The temperature in the thawed layer h t can be taken from 0.5 to 1.5 ° C.

In the layer of seasonal freezing h p, the soil temperature can be calculated in the same way as for the layer of seasonal thawing of the merging cryolithozone, i.e. in the layer h p - 1 m along the temperature gradient (Table 3.1), considering the temperature at a depth of h p equal to 0 ° С in the cold season and 1 ° С in the summer. In the upper 1 m soil layer, the temperature is determined by interpolation between the temperature at a depth of 1 m and the temperature at the surface.

In vertical collectors, energy is extracted from the ground using geothermal earth probes. it closed systems with wells with a diameter of 145-150mm and a depth of 50 to 150m, through which pipes are laid. A return U elbow is installed at the end of the pipeline. Typically, installation is done with a single-loop probe with 2x d40 pipes (Swedish system), or a double-loop probe with 4x d32 pipes. Double-loop probes should achieve 10-15% more heat extraction. For wells deeper than 150 m, 4xd40 pipes should be used (to reduce the pressure loss).

Currently, most of the wells for extracting heat from the earth have a depth of 150 m. At greater depths, more heat can be obtained, but the costs of such wells will be very high. Therefore, it is important to calculate in advance the costs of installing the vertical collector in comparison with the anticipated savings in the future. In the case of installing an active-passive cooling system, deeper wells are not made due to highest temperature in the soil and at a lower potential at the moment of heat transfer from the solution environment... An antifreeze mixture (alcohol, glycerin, glycol), diluted with water to the required antifreeze consistency, circulates in the system. In a heat pump, it transfers heat taken from the ground to a refrigerant. The ground temperature at a depth of 20 m is approximately 10 ° C, and increases every 30 m by 1 ° C. She is not influenced climatic conditions, and therefore you can count on a high-quality selection of energy both in winter and in summer. It should be added that the temperature in the ground is slightly different at the beginning of the season (September-October) from the temperature at the end of the season (March-April). Therefore, when calculating the depth of vertical collectors, it is necessary to take into account the length of the heating season at the place of installation.

When collecting heat with geothermal vertical probes, correct calculations and design of the collectors are very important. To carry out competent calculations, it is necessary to know whether it is possible to drill at the installation site to the desired depth.

A 10kW heat pump requires approximately 120-180 m of borehole. Wells should be placed at least 8m apart. The number and depth of wells depends on geological conditions, the availability of groundwater, the ability of the soil to retain heat and drilling technology. When drilling multiple wells, the total desired well length will be divided by the number of wells.

The advantage of a vertical collector over a horizontal collector is a smaller area of ​​land to use, a more stable heat source, and independence of the heat source on weather conditions. The downside of vertical collectors is the high cost of excavation and gradual cooling of the earth near the collector (competent calculations of the required power are required during design).

Calculation of the required well depth

Information required for preliminary calculation of the depth and number of wells:

Heat pump power

The selected type of heating - "warm floors", radiators, combined

Estimated number of hours of operation of the heat pump per year, coverage of energy demand

Installation location

Using a geothermal well - heating, DHW heating, seasonal pool heating, year-round pool heating

Using the passive (active) cooling function in the facility

Total annual heat consumption for heating (MW / h)