Imagine a home that is always at a comfortable temperature, with no heating or cooling system in sight. This system works efficiently, but does not require complex maintenance or special knowledge from the owners.

Fresh air, you can hear the birds chirping and the wind lazily playing with the leaves on the trees. The house receives energy from the earth, like leaves, which receive energy from the roots. Great picture, isn't it?

Geothermal heating and cooling systems make this a reality. A geothermal HVAC (heating, ventilation and air conditioning) system uses the ground temperature to provide heating in winter and cooling in summer.

How geothermal heating and cooling works

The ambient temperature changes with the seasons, but the underground temperature does not change as much due to the insulating properties of the earth. At a depth of 1.5-2 meters, the temperature remains relatively constant all year round. A geothermal system typically consists of internal processing equipment, an underground pipe system called an underground loop, and/or a water circulation pump. The system uses the earth's constant temperature to provide "clean and free" energy.

(Do not confuse the concept of a geothermal NHC system with "geothermal energy" - a process in which electricity is generated directly from the heat in the earth. In the latter case, a different type of equipment and other processes are used, the purpose of which is usually to heat water to a boiling point.)

The pipes that make up the underground loop are usually made of polyethylene and can be laid horizontally or vertically underground, depending on the terrain. If an aquifer is available, engineers can design an "open loop" system by drilling a well into the water table. The water is pumped out, passes through a heat exchanger, and then injected into the same aquifer via "re-injection".

In winter, water, passing through an underground loop, absorbs the heat of the earth. The indoor equipment further raises the temperature and distributes it throughout the building. It's like an air conditioner working in reverse. During the summer, a geothermal NWC system draws hot water from the building and carries it through an underground loop/pump to a re-injection well where the water is released into the cooler ground/aquifer.

Unlike conventional heating and cooling systems, geothermal HVAC systems do not use fossil fuels to generate heat. They simply take heat from the earth. Typically, electricity is only used to run the fan, compressor and pump.

There are three main components in a geothermal cooling and heating system: a heat pump, a heat exchange fluid (open or closed system), and an air supply system (pipe system).

For geothermal heat pumps, as well as for all other types of heat pumps, the ratio of their useful action to the energy expended for this action (EFFICIENCY) was measured. Most geothermal heat pump systems have an efficiency of 3.0 to 5.0. This means that the system converts one unit of energy into 3-5 units of heat.

Geothermal systems do not require complex maintenance. Properly installed, which is very important, the underground loop can serve properly for several generations. The fan, compressor and pump are housed indoors and protected from changing weather conditions, so they can last many years, often decades. Routine periodic checks, timely filter replacement and annual coil cleaning are the only maintenance required.

Experience in the use of geothermal NVC systems

Geothermal NVC systems have been used for more than 60 years all over the world. They work with nature, not against it, and they don't emit greenhouse gases (as noted earlier, they use less electricity because they use the earth's constant temperature).

Geothermal HC systems are increasingly becoming attributes of green homes as part of the growing green building movement. Green projects accounted for 20 percent of all homes built in the US last year. An article in the Wall Street Journal says that by 2016 the green building budget will rise from $36 billion a year to $114 billion. This will amount to 30-40 percent of the entire real estate market.

But much of the information about geothermal heating and cooling is based on outdated data or unsubstantiated myths.

Destroying myths about geothermal NWC systems

1. Geothermal NVC systems are not a renewable technology because they use electricity.

Fact: Geothermal HVAC systems use only one unit of electricity to produce up to five units of cooling or heating.

2. Solar energy and wind energy are more favorable renewable technologies compared to geothermal NVC systems.

Fact: Geothermal NVC systems for one dollar process four times more kilowatts / hours than solar or wind energy generates for the same dollar. These technologies can, of course, play an important environmental role, but a geothermal NHC system is often the most efficient and cost-effective way to reduce environmental impact.

3. The geothermal NVC system requires a lot of space to accommodate the polyethylene pipes of the underground loop.

Fact: Depending on the terrain, the underground loop can be located vertically, which means that a small surface area is needed. If there is an available aquifer, then only a few square feet of surface is needed. Note that the water returns to the same aquifer it was taken from after it has passed through the heat exchanger. Thus, the water is not runoff and does not pollute the aquifer.

4. HVK geothermal heat pumps are noisy.

Fact: The systems are very quiet and there is no equipment outside so as not to disturb the neighbors.

5. Geothermal systems eventually wear out.

Fact: Underground loops can last for generations. Heat exchange equipment typically lasts for decades as it is protected indoors. When it comes time to need to replace equipment, the cost of such a replacement is much less than a new geothermal system, since the underground loop and well are its most expensive parts. New technical solutions eliminate the problem of heat retention in the ground, so the system can exchange temperatures in unlimited quantities. There have been cases in the past of miscalculated systems that actually overheated or subcooled the ground to the point where there was no longer the temperature difference needed to operate the system.

6. Geothermal HVAC systems work only for heating.

Fact: They work just as efficiently for cooling and can be designed so that there is no need for an additional backup heat source. Although some customers decide that it is more cost effective to have a small backup system for the coldest times. This means that their underground loop will be smaller and therefore cheaper.

7. Geothermal HVAC systems cannot simultaneously heat domestic water, heat pool water, and heat a house.

Fact: Systems can be designed to perform many functions at the same time.

8. Geothermal NHC systems pollute the ground with refrigerants.

Fact: Most systems use only water in the hinges.

9. Geothermal NWC systems use a lot of water.

Fact: Geothermal systems do not actually consume water. If groundwater is used for temperature exchange, then all water returns to the same aquifer. In the past, some systems were indeed used that wasted water after it passed through the heat exchanger, but such systems are hardly used today. Looking at the issue from a commercial standpoint, geothermal NHC systems actually save millions of liters of water that would have been evaporated in traditional systems.

10. Geothermal NVC technology is not financially feasible without state and regional tax incentives.

Fact: State and regional incentives typically amount to 30 to 60 percent of the total cost of a geothermal system, which can often bring the initial price down to near the price of conventional equipment. Standard HVAC air systems cost approximately $3,000 per tonne of heat or cold (homes typically use one to five tons). The price of geothermal NVC systems ranges from approximately $5,000 per ton to $8,000-9,000. However, new installation methods significantly reduce costs, down to the prices of conventional systems.

Cost savings can also be achieved through discounts on equipment for public or commercial use, or even large orders for the home (especially from big brands such as Bosch, Carrier and Trane). Open loops, using a pump and a re-injection well, are cheaper to install than closed systems.

Source: energyblog.nationalgeographic.com

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

Temperatures of different depths of the Earth

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

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

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

Minimum and maximum temperatures of the Earth's depths

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

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

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

The heat inside the earth comes from several sources:

1) Decay of radioactive elements;

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

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

These are some options for the occurrence of heat in the bowels of the earth, but the question of the complete list and the correctness of the existing one is still open.

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

In vertical collectors, energy is taken from the earth using geothermal earth probes. These are 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. Usually 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 pressure loss).

Currently, most of the wells for extracting ground heat are 150 m deep. 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 cost of installing a vertical collector in comparison with the expected savings in the future. In the case of installing an active-passive cooling system, deeper wells are not made due to the higher temperature in the soil and the lower potential at the time of heat transfer from the solution to the environment. An anti-freeze mixture (alcohol, glycerin, glycol) circulates in the system, diluted with water to the desired anti-freeze consistency. In a heat pump, it transfers the heat taken from the ground to the refrigerant. The temperature of the earth at a depth of 20 m is approximately 10°C, and rises every 30m by 1°C. It is not affected by climatic conditions, and therefore you can count on high-quality energy extraction 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 installation site.

When extracting heat with geothermal vertical probes, the 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.

For a heat pump with a power of 10kW, approximately 120-180 m of wells are needed. Wells should be placed at least 8m apart. The number and depth of wells depends on geological conditions, the presence of groundwater, the ability of the soil to retain heat and drilling technology. When drilling multiple wells, the total desired length of the well is 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 from the weather. The downside of vertical collectors is the high cost of earthworks and the 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 the preliminary calculation of the depth and number of wells:

Heat pump power

Selected type of heating - "warm floors", radiators, combined

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

Place of installation

Use of 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 (MWh)

temperature inside the earth. The determination of the temperature in the Earth's shells is based on various, often indirect, data. The most reliable temperature data refer to the uppermost part of the earth's crust, which is exposed by mines and boreholes to a maximum depth of 12 km (Kola well).

The increase in temperature in degrees Celsius per unit of depth is called geothermal gradient, and the depth in meters, during which the temperature increases by 1 0 C - geothermal step. The geothermal gradient and, accordingly, the geothermal step vary from place to place depending on the geological conditions, endogenous activity in different areas, as well as the heterogeneous thermal conductivity of rocks. At the same time, according to B. Gutenberg, the limits of fluctuations differ by more than 25 times. An example of this are two sharply different gradients: 1) 150 o per 1 km in Oregon (USA), 2) 6 o per 1 km registered in South Africa. According to these geothermal gradients, the geothermal step also changes from 6.67 m in the first case to 167 m in the second. The most common fluctuations in the gradient are within 20-50 o , and the geothermal step is 15-45 m. The average geothermal gradient has long been taken at 30 o C per 1 km.

According to VN Zharkov, the geothermal gradient near the Earth's surface is estimated at 20 o C per 1 km. Based on these two values ​​of the geothermal gradient and its invariance deep into the Earth, then at a depth of 100 km there should have been a temperature of 3000 or 2000 o C. However, this is at odds with the actual data. It is at these depths that magma chambers periodically originate, from which lava flows to the surface, having a maximum temperature of 1200-1250 o. Considering this kind of "thermometer", a number of authors (V. A. Lyubimov, V. A. Magnitsky) believe that at a depth of 100 km the temperature cannot exceed 1300-1500 o C.

At higher temperatures, the mantle rocks would be completely melted, which contradicts the free passage of transverse seismic waves. Thus, the average geothermal gradient can be traced only to some relatively small depth from the surface (20-30 km), and then it should decrease. But even in this case, in the same place, the change in temperature with depth is not uniform. This can be seen in the example of temperature change with depth along the Kola well located within the stable crystalline shield of the platform. When laying this well, a geothermal gradient of 10 o per 1 km was expected and, therefore, at the design depth (15 km) a temperature of the order of 150 o C was expected. However, such a gradient was only up to a depth of 3 km, and then it began to increase by 1.5 -2.0 times. At a depth of 7 km the temperature was 120 o C, at 10 km -180 o C, at 12 km -220 o C. It is assumed that at the design depth the temperature will be close to 280 o C. Caspian region, in the area of ​​more active endogenous regime. In it, at a depth of 500 m, the temperature turned out to be 42.2 o C, at 1500 m - 69.9 o C, at 2000 m - 80.4 o C, at 3000 m - 108.3 o C.

What is the temperature in the deeper zones of the mantle and core of the Earth? More or less reliable data have been obtained on the temperature of the base of the B layer in the upper mantle (see Fig. 1.6). According to V. N. Zharkov, "detailed studies of the phase diagram of Mg 2 SiO 4 - Fe 2 Si0 4 made it possible to determine the reference temperature at a depth corresponding to the first zone of phase transitions (400 km)" (i.e., the transition of olivine to spinel). The temperature here as a result of these studies is about 1600 50 o C.

The question of the distribution of temperatures in the mantle below layer B and in the Earth's core has not yet been resolved, and therefore various views are expressed. It can only be assumed that the temperature increases with depth with a significant decrease in the geothermal gradient and an increase in the geothermal step. It is assumed that the temperature in the Earth's core is in the range of 4000-5000 o C.

The average chemical composition of the Earth. To judge the chemical composition of the Earth, data on meteorites are used, which are the most probable samples of protoplanetary material from which the terrestrial planets and asteroids were formed. To date, many meteorites that have fallen to Earth at different times and in different places have been well studied. According to the composition, three types of meteorites are distinguished: 1) iron, consisting mainly of nickel iron (90-91% Fe), with a small admixture of phosphorus and cobalt; 2) iron-stone(siderolites), consisting of iron and silicate minerals; 3) stone, or aerolites, consisting mainly of ferruginous-magnesian silicates and inclusions of nickel iron.

The most common are stone meteorites - about 92.7% of all finds, stony iron 1.3% and iron 5.6%. Stone meteorites are divided into two groups: a) chondrites with small rounded grains - chondrules (90%); b) achondrites that do not contain chondrules. The composition of stony meteorites is close to that of ultramafic igneous rocks. According to M. Bott, they contain about 12% iron-nickel phase.

Based on the analysis of the composition of various meteorites, as well as the obtained experimental geochemical and geophysical data, a number of researchers give a modern estimate of the gross elemental composition of the Earth, presented in Table. 1.3.

As can be seen from the data in the table, the increased distribution refers to the four most important elements - O, Fe, Si, Mg, constituting over 91%. The group of less common elements includes Ni, S, Ca, A1. The remaining elements of Mendeleev's periodic system on a global scale are of secondary importance in terms of their general distribution. If we compare the given data with the composition of the earth's crust, we can clearly see a significant difference consisting in a sharp decrease in O, Al, Si and a significant increase in Fe, Mg and the appearance of S and Ni in noticeable amounts.

The shape of the earth is called the geoid. The deep structure of the Earth is judged by longitudinal and transverse seismic waves, which, propagating inside the Earth, experience refraction, reflection and attenuation, which indicates the stratification of the Earth. There are three main areas:

Earth's crust;

mantle: upper to a depth of 900 km, lower to a depth of 2900 km;

the core of the Earth is outer to a depth of 5120 km, inner to a depth of 6371 km.

The internal heat of the Earth is associated with the decay of radioactive elements - uranium, thorium, potassium, rubidium, etc. The average value of the heat flux is 1.4-1.5 μkal / cm 2. s.

1. What is the shape and size of the Earth?

2. What are the methods for studying the internal structure of the Earth?

3. What is the internal structure of the Earth?

4. What seismic sections of the first order are clearly distinguished when analyzing the structure of the Earth?

5. What are the boundaries of the sections of Mohorovic and Gutenberg?

6. What is the average density of the Earth and how does it change at the boundary between the mantle and the core?

7. How does the heat flow change in different zones? How is the change in geothermal gradient and geothermal step understood?

8. What data is used to determine the average chemical composition of the Earth?

Literature

• Voytkevich G.V. Fundamentals of the theory of the origin of the Earth. M., 1988.

• Zharkov V.N. Internal structure of the Earth and planets. M., 1978.

• Magnitsky V.A. Internal structure and physics of the Earth. M., 1965.

• Essays comparative planetology. M., 1981.

• Ringwood A.E. Composition and origin of the Earth. M., 1981.

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

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

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

Using Earth's heat There are two types of thermal energy - high-potential and low-potential. The source of high-potential thermal energy is hydrothermal resources - thermal waters heated to a high temperature as a result of geological processes, which allows them to be used for 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. Insofar as heat pump efficiency increases with a decrease in the temperature difference between the evaporator and condenser, floor heating systems are often used for heating buildings, in which a coolant of a relatively low temperature (35–40 °C) circulates.

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

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

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

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

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

The main heat-exchange element of the system for collecting low-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 - the solar radiation incident on the surface and the flow of radiogenic heat from the earth's interior. Seasonal and daily changes in the intensity of solar radiation and outdoor temperature cause fluctuations in the temperature of the upper layers of the soil. The depth of penetration of daily fluctuations in the temperature of the outside air and the intensity of the incident solar radiation, depending on the specific soil and climatic conditions, ranges from several tens of centimeters to one and a half meters. The depth of penetration of seasonal fluctuations in the temperature of the outside air and the intensity of the incident solar radiation does not, as a rule, exceed 15–20 m.

The temperature regime of soil layers located below this depth (“neutral zone”) is formed under the influence of thermal energy coming from the bowels of the Earth and practically does not depend on seasonal, and even more so daily changes in 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. In this case, naturally, there is a change in the state of aggregation of moisture contained in the pores of the soil and, in the general case, both in liquid and in solid and gaseous phases simultaneously. In other words, the soil mass of the heat collection system, regardless of 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. Modeling the processes of heat and mass transfer 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 for their implementation: heat conduction in an individual particle, heat transfer from one particle to another upon their contact, molecular heat conduction in a medium filling 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 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 the temperature regime of the soil massif of systems for collecting low-potential soil heat is formed are shown in fig. 2.

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

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

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

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

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

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

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

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

• sufficient permeability of the soil, allowing replenishment of water reserves;
• good groundwater chemistry (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 large areas and do not depend on the intensity of solar radiation falling on the surface. Vertical ground heat exchangers work effectively in almost all types of geological environments, with the exception of soils with low thermal conductivity, such as dry sand or dry gravel. Systems with vertical ground heat exchangers are very widespread.

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

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

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

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

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

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

Systems with vertical ground heat exchangers can be used to heat and cool buildings of various sizes. For a small building, one heat exchanger is enough; 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 of building structures as soil heat exchangers, for example, foundation piles with embedded pipelines. The section of such a pile with three contours of a soil heat exchanger is shown in fig. 7.

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

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

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

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

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

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

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

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

Held in OJSC INSOLAR-INVEST studies have shown that the consumption of thermal energy from the soil mass by the end of the heating season causes a decrease in soil temperature near the register of pipes of the heat collection system, which, 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. Etc. 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 the heat of atmospheric air) and "from below" (due to the heat of the Earth); the value of heat gain "from above" and "from below" depends on the thickness and depth of the aquifer. Due to these heat transfers, the groundwater temperature remains constant throughout the season and changes little during operation.

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

German specialists measured the temperature of the soil massif, in which a vertical soil heat exchanger 50 m deep, 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 thermal load on the surrounding ground mass. The annual production of thermal energy is about 13 MWh

At a distance of 0.5 and 1 m from the main well, two additional wells were drilled, in which temperature sensors were installed at a depth of 1, 2, 5, 10, 20, 35, 50, 65, 85 and 105 m, after which the wells were filled clay-cement mixture. The temperature was measured every thirty minutes. In addition to the soil temperature, other parameters were also 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