Earth temperature at depth 2. Earth heat. Geothermal heat pump systems of heat supply and efficiency of their application in the climatic conditions of Russia
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 is from +5 to + 7 ° C, and in summer from +10 to + 12 ° C. This warmth can prevent the house from freezing in winter, and in summer it can prevent it from overheating above 18 -20°C
by the most in a simple way The use of ground heat is the use of a soil heat exchanger (SHE). Under the ground, below the level of soil freezing, a system of air ducts is laid, which act as a heat exchanger between the ground and the air that passes through these air ducts. In winter, the incoming cold air that enters and passes through the pipes is heated, and in summer it is cooled. With the rational placement of air ducts, a significant amount of thermal energy can be taken from the soil with low energy costs.
A tube-in-pipe heat exchanger can be used. Internal stainless steel air ducts act here as recuperators.
Cooling in summer
In the warm season, the 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 supply and exhaust unit, in which a summer insert is installed instead of a heat exchanger for the summer period. Thanks to this solution, the temperature in the rooms decreases, the microclimate in the house improves, and the cost of electricity for air conditioning is reduced. Off-season work
When the difference between the temperature of the outdoor and indoor air is small, fresh air can be supplied through the supply grill located on the wall of the house in the above-ground part. In the period when the difference is significant, the fresh air supply can be carried out through the PHE, providing heating / cooling of the supply air. Savings in winter
In the cold season, outside air enters the PHE through the air intake, where it warms up and then enters the supply and exhaust unit for heating in the heat exchanger. Air preheating in the PHE reduces the possibility of icing on the heat exchanger of the air handling unit, increasing the effective use of the heat exchanger and minimizing the cost of additional air heating in the water / electric heater. How are heating and cooling costs calculated?
You can pre-calculate the cost of air heating in winter for a room where air enters at a standard of 300 m3 / hour. In winter, the average daily temperature for 80 days is -5 ° C - it needs to be heated to + 20 ° C. To heat this amount of air, 2.55 kW per hour is needed (in the absence of a heat recovery system). When using a geothermal system, the outside air is heated up to +5, and then it takes 1.02 kW to heat the incoming air to a comfortable level. The situation is even better when using recuperation - it is necessary to spend only 0.714 kW. Over a period of 80 days, 2448 kWh of thermal energy will be spent, respectively, and geothermal systems will reduce costs by 1175 or 685 kWh.
In the off-season for 180 days, the average daily temperature is + 5 ° C - it needs to be heated to + 20 ° C. The planned costs are 3305 kWh, and geothermal systems will reduce costs by 1322 or 1102 kWh.
During the summer period, for 60 days, the average daily temperature is around +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 evaluated using the coefficient - SPF (seasonal power factor), which is defined as the ratio of the amount of heat 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 ground per year, the ventilation unit consumes 635 kWh of electricity. SPF = 2634/635 = 4.14.
By materials.
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 very top earth's crust, opened 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 is poured onto the surface, having maximum temperature 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 Kola well located within the stable crystal 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 have fallen to Earth in different times and in different places meteorites. 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. Other elements periodic system Mendeleev on a global scale in terms of general distribution are of secondary importance. 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
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Voytkevich G.V. Fundamentals of the theory of the origin of the Earth. M., 1988.
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Zharkov V.N. Internal structure Earth and planets. M., 1978.
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Magnitsky V.A. Internal structure and physics of the Earth. M., 1965.
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Essays comparative planetology. M., 1981.
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Ringwood A.E. Composition and origin of the Earth. M., 1981.
Kirill Degtyarev, Researcher, Moscow State University them. M. V. Lomonosov.
In our country, rich in hydrocarbons, geothermal energy is a kind of exotic resource that, in the current state of affairs, is unlikely to compete with oil and gas. Nevertheless, this alternative form of energy can be used almost everywhere and quite efficiently.
Photo by Igor Konstantinov.
Change in soil temperature with depth.
Temperature increase of thermal waters and dry rocks containing them with depth.
Change in temperature with depth in different regions.
The eruption of the Icelandic volcano Eyjafjallajökull is an illustration of violent volcanic processes occurring in active tectonic and volcanic zones with a powerful heat flow from the earth's interior.
Installed capacities of geothermal power plants by countries of the world, MW.
Distribution of geothermal resources on the territory of Russia. The reserves of geothermal energy, according to experts, are several times higher than the energy reserves of organic fossil fuels. According to the Geothermal Energy Society Association.
Geothermal energy is the heat of the earth's interior. It is produced in the depths and comes to the surface of the Earth in different forms and with different intensity.
The temperature of the upper layers of the soil depends mainly on external (exogenous) factors - solar lighting and air temperature. In summer and during the day, the soil warms up to certain depths, and in winter and at night it cools down following the change in air temperature and with some delay, increasing with depth. The influence of daily fluctuations in air temperature ends at depths from a few to several tens of centimeters. Seasonal fluctuations capture deeper layers of soil - up to tens of meters.
At a certain depth - from tens to hundreds of meters - the temperature of the soil is kept constant, equal to the average annual air temperature near the Earth's surface. This is easy to verify by going down into a fairly deep cave.
When mean annual temperature air in the area is below zero, this manifests itself as permafrost (more precisely, permafrost). In Eastern Siberia, the thickness, that is, the thickness, of year-round frozen soils reaches 200-300 m in places.
From a certain depth (its own for each point on the map), the effect of the Sun and the atmosphere weakens so much that endogenous (internal) factors come first and the earth's interior is heated from the inside, so that the temperature begins to rise with depth.
The heating of the deep layers of the Earth is associated mainly with the decay of the radioactive elements located there, although other sources of heat are also named, for example, physicochemical, tectonic processes in the deep layers of the earth's crust and mantle. But whatever the cause, the temperature of rocks and associated liquid and gaseous substances increases with depth. Miners face this phenomenon - it is always hot in deep mines. At a depth of 1 km, thirty-degree heat is normal, and deeper the temperature is even higher.
The heat flow of the earth's interior, reaching the surface of the Earth, is small - on average, its power is 0.03-0.05 W / m 2,
or about 350 Wh/m 2 per year. Against the background of the heat flow from the Sun and the air heated by it, this is an imperceptible value: the Sun gives everyone square meter earth's surface about 4,000 kWh annually, that is, 10,000 times more (of course, this is an average, with a huge spread between polar and equatorial latitudes and depending on other climatic and weather factors).
The insignificance of the heat flow from the depths to the surface in most of the planet is associated with the low thermal conductivity of rocks and the peculiarities of the geological structure. But there are exceptions - places where the heat flow is high. These are, first of all, zones of tectonic faults, increased seismic activity and volcanism, where the energy of the earth's interior finds a way out. Such zones are characterized by thermal anomalies of the lithosphere, here the heat flow reaching the Earth's surface can be many times and even orders of magnitude more powerful than the "usual" one. A huge amount of heat is brought to the surface in these zones by volcanic eruptions and hot springs of water.
It is these areas that are most favorable for the development of geothermal energy. On the territory of Russia, these are, first of all, Kamchatka, the Kuril Islands and the Caucasus.
At the same time, the development of geothermal energy is possible almost everywhere, since the increase in temperature with depth is a ubiquitous phenomenon, and the task is to “extract” heat from the bowels, just as mineral raw materials are extracted from there.
On average, the temperature increases with depth by 2.5-3 o C for every 100 m. The ratio of the temperature difference between two points lying at different depths to the difference in depth between them is called the geothermal gradient.
The reciprocal is the geothermal step, or the depth interval at which the temperature rises by 1 o C.
The higher the gradient and, accordingly, the lower the step, the closer the heat of the Earth's depths approaches the surface and the more promising this area is for the development of geothermal energy.
In different areas, depending on the geological structure and other regional and local conditions, the rate of temperature increase with depth can vary dramatically. On the scale of the Earth, fluctuations in the values of geothermal gradients and steps reach 25 times. For example, in the state of Oregon (USA) the gradient is 150 o C per 1 km, and in South Africa - 6 o C per 1 km.
The question is, what is the temperature at great depths - 5, 10 km or more? If the trend continues, the temperature at a depth of 10 km should average about 250-300 ° C. This is more or less confirmed by direct observations in ultra-deep wells, although the picture is much more complicated than a linear increase in temperature.
For example, in the Kola superdeep well drilled in the Baltic crystalline shield, the temperature changes at a rate of 10 o C / 1 km to a depth of 3 km, and then the geothermal gradient becomes 2-2.5 times greater. At a depth of 7 km, a temperature of 120 o C was already recorded, at 10 km - 180 o C, and at 12 km - 220 o C.
Another example is a well laid in the Northern Caspian, where at a depth of 500 m a temperature of 42 o C was recorded, at 1.5 km - 70 o C, at 2 km - 80 o C, at 3 km - 108 o C.
It is assumed that the geothermal gradient decreases starting from a depth of 20-30 km: at a depth of 100 km, the estimated temperatures are about 1300-1500 o C, at a depth of 400 km - 1600 o C, in the Earth's core (depths of more than 6000 km) - 4000-5000 o WITH.
At depths up to 10-12 km, the temperature is measured through drilled wells; where they do not exist, it is determined by indirect signs in the same way as at greater depths. Such indirect signs may be the nature of the passage of seismic waves or the temperature of the erupting lava.
However, for the purposes of geothermal energy, data on temperatures at depths of more than 10 km are not yet of practical interest.
There is a lot of heat at depths of several kilometers, but how to raise it? Sometimes nature itself solves this problem for us with the help of a natural coolant - heated thermal waters that come to the surface or lie at a depth accessible to us. In some cases, the water in the depths is heated to the state of steam.
A strict definition of the concept " thermal waters" No. As a rule, they mean hot groundwater in a liquid state or in the form of steam, including those that come to the Earth's surface with a temperature above 20 ° C, that is, as a rule, higher than the air temperature.
The heat of groundwater, steam, steam-water mixtures is hydrothermal energy. Accordingly, energy based on its use is called hydrothermal.
The situation is more complicated with the production of heat directly from dry rocks - petrothermal energy, especially since sufficiently high temperatures, as a rule, begin from depths of several kilometers.
On the territory of Russia, the potential of petrothermal energy is a hundred times higher than that of hydrothermal energy - 3,500 and 35 trillion tons of standard fuel, respectively. This is quite natural - the warmth of the Earth's depths is everywhere, and thermal waters are found locally. However, due to obvious technical difficulties, most of the thermal waters are currently used to generate heat and electricity.
Waters with temperatures from 20-30 to 100 o C are suitable for heating, temperatures from 150 o C and above - and for generating electricity at geothermal power plants.
In general, geothermal resources on the territory of Russia, in terms of tons of reference fuel or any other unit of energy measurement, are approximately 10 times higher than fossil fuel reserves.
Theoretically, only geothermal energy could fully meet the energy needs of the country. Practically on this moment in most of its territory, this is not feasible for technical and economic reasons.
In the world, the use of geothermal energy is most often associated with Iceland - a country located at the northern end of the Mid-Atlantic Ridge, in an extremely active tectonic and volcanic zone. Probably everyone remembers the powerful eruption of the Eyjafjallajökull volcano in 2010.
It is thanks to this geological specificity that Iceland has huge reserves of geothermal energy, including hot springs that come to the surface of the Earth and even gushing in the form of geysers.
In Iceland, more than 60% of all energy consumed is currently taken from the Earth. Including due to geothermal sources, 90% of heating and 30% of electricity generation are provided. We add that the rest of the electricity in the country is produced by hydroelectric power plants, that is, also using a renewable energy source, thanks to which Iceland looks like a kind of global environmental standard.
The "taming" of geothermal energy in the 20th century helped Iceland significantly economically. Until the middle of the last century, it was a very poor country, now it ranks first in the world in terms of installed capacity and production of geothermal energy per capita, and is in the top ten in terms of absolute installed capacity of geothermal power plants. However, its population is only 300 thousand people, which simplifies the task of switching to environmentally friendly energy sources: the need for it is generally small.
In addition to Iceland, a high share of geothermal energy in the total balance of electricity production is provided in New Zealand and the island states South-East Asia(Philippines and Indonesia), countries Central America and East Africa, whose territory is also characterized by high seismic and volcanic activity. For these countries, at their current level of development and needs, geothermal energy makes a significant contribution to socio-economic development.
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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, underground system pipes, called an underground loop, and / or a pump for water circulation. 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, then 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. Internal equipment additionally 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, from where the water enters 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 in indoors and protected from change weather conditions thus, their service life can last for 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 NVC systems are increasingly becoming attributes of green homes, as part of the growing green building movement. Green projects made up 20 percent of all homes built in the U.S. 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 role for the environment, 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. Geothermal heat pumps NVCs 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 the moment comes necessary replacement 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 of miscalculated systems in the past 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 HC 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 air systems NEC 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
Description:
In contrast to the "direct" use of high-potential geothermal heat (hydrothermal resources), the use of the soil of the surface layers of the Earth as a source of low-grade thermal energy for geothermal heat pump heat supply systems (GHPS) is possible almost everywhere. It is currently one of the most dynamic emerging areas use of non-traditional renewable energy sources.
Geothermal heat pump systems of heat supply and efficiency of their application in climatic conditions Russia
G. P. Vasiliev, scientific adviser OJSC INSOLAR-INVEST
In contrast to the "direct" use of high-potential geothermal heat (hydrothermal resources), the use of the soil of the surface layers of the Earth as a source of low-grade thermal energy for geothermal heat pump heat supply systems (GHPS) is possible almost everywhere. At present, this is one of the most dynamically developing areas for the use of non-traditional renewable energy sources in the world.
The soil of the surface layers of the Earth is actually a heat accumulator of unlimited power. The thermal regime of the soil is formed under the influence of two main factors - the incident on the surface solar radiation and the flow of radiogenic heat from the earth's interior. Seasonal and daily changes in the intensity of solar radiation and outdoor temperature cause 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 thermal regime of soil layers located below this depth (“neutral zone”) is formed under the influence of thermal energy coming from the bowels of the Earth and practically does not depend on seasonal, and even more so daily changes in outdoor climate parameters (Fig. 1). With increasing depth, the ground temperature also increases in accordance with the geothermal gradient (approximately 3 °C for every 100 m). The magnitude of the flux of radiogenic heat coming from the bowels of the earth varies for different localities. As a rule, this value is 0.05–0.12 W / m 2.
Picture 1. |
During the operation of the gas turbine power plant, 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 external 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 located in general case both in liquid, and in solid and gaseous phases at the same time. At the same time, 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. If there is a temperature gradient in the thickness of the soil mass, water vapor molecules move to places with a lower temperature potential, but at the same time, under the action of gravitational forces, an oppositely directed flow of moisture in the liquid phase occurs. In addition, on temperature regime the upper layers of the soil is influenced by the moisture of precipitation, as well as groundwater.
To the characteristics thermal regime ground heat collection systems as a design object, one should also include the so-called “informative uncertainty” of mathematical models describing such processes, or, in other words, the lack of reliable information about the impacts on the environmental system (atmosphere and soil mass outside the zone of thermal influence of soil heat exchanger of the heat collection system) and the extreme complexity of their approximation. Indeed, if the approximation of the impacts on the outdoor climate system, although complicated, can still be implemented at certain costs of “computer time” and the use of existing models (for example, a “typical climatic year”), then the problem of taking into account the impact on the atmospheric system in the model influences (dew, fog, rain, snow, etc.), as well as the approximation of the thermal effect on the soil mass of the heat collection system of the underlying and surrounding soil layers, is practically unresolvable today and could be the subject of separate studies. So, for example, little knowledge of the processes of formation of groundwater filtration flows, their speed regime, as well as the impossibility of obtaining reliable information about the heat and moisture regime of soil layers located below the zone of thermal influence of a soil heat exchanger, greatly complicates the task of constructing a correct mathematical model of the thermal regime of a low-potential heat collection system. soil.
To overcome the described difficulties that arise when designing a gas turbine power plant, the developed and tested in practice method of mathematical modeling of the thermal regime of ground heat collection systems and the method of taking into account phase transitions of moisture in the pore space of the soil massif of heat collection systems when designing gas turbine power plants can be recommended.
The essence of the method is to consider, when constructing a mathematical model, the difference between two problems: the “basic” problem that describes the thermal regime of the soil in its natural state (without the influence of the soil heat exchanger of the heat collection system), and the problem to be solved that describes the thermal regime of the soil mass with heat sinks (sources). As a result, the method makes it possible to obtain a solution with respect to some new feature, which is a function of the influence of heat sinks on the natural thermal regime of the soil and is equal to the temperature difference between the soil mass in its natural state and the soil mass with drains (heat sources) - with the ground heat exchanger of the heat collection system. The use of this method in the construction of mathematical models of the thermal regime of systems for collecting low-potential ground heat made it possible not only to bypass the difficulties associated with approximating external influences on the heat collection system, but also to use in the models the information experimentally obtained by meteorological stations on the natural thermal regime of the soil. This makes it possible to partially take into account the whole complex of factors (such as the presence of groundwater, its speed and thermal regimes, the structure and location of soil layers, the "thermal" background of the Earth, precipitation, phase transformations of moisture in the pore space, and much more), which most significantly affect the formation of the thermal regime of the heat collection system and which are practically impossible to take into account in a strict formulation of the problem.
The method of taking into account phase transitions of moisture in the pore space of a soil mass when designing a gas turbine power plant is based on a new concept of “equivalent” thermal conductivity of soil, which is determined by replacing the problem of the thermal regime of a soil cylinder frozen around the pipes of a soil heat exchanger with an “equivalent” quasi-stationary problem with a close temperature field and identical boundary conditions, but with a different "equivalent" thermal conductivity.
The most important task to be solved in the design of geothermal heat supply systems for buildings is a detailed assessment of the energy capabilities of the climate of the construction area and, on this basis, drawing up a conclusion on the effectiveness and feasibility of using one or another circuit design of the GTTS. The calculated values of climatic parameters given in the current normative documents do not give complete characteristics outdoor climate, its variability by months, as well as in certain periods of the year - the heating season, the period of overheating, etc. Therefore, when deciding on the temperature potential of geothermal heat, assessing the possibility of its combination with other low-potential natural heat sources, assessing them (sources) temperature level in annual cycle it is necessary to involve more complete climatic data, cited, for example, in the USSR Climate Handbook (L.: Gidrometioizdat. Issue 1–34).
Among such climate information, in our case, we should highlight, first of all:
– data on average monthly soil temperature at different depths;
– data on the arrival of solar radiation on differently oriented surfaces.
In table. Tables 1–5 show data on average monthly ground temperatures at various depths for some Russian cities. In table. Table 1 shows the average monthly soil temperatures for 23 cities of the Russian Federation at a depth of 1.6 m, which seems to be the most rational in terms of the temperature potential of the soil and the possibilities of mechanizing the production of works on the laying of horizontal soil heat exchangers.
| Table 1 Average soil temperatures by months at a depth of 1.6 m for some Russian cities |
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| table 2 Soil temperature in Stavropol (soil - chernozem) |
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| Table 3 Ground temperatures in Yakutsk (silty-sandy soil with an admixture of humus, below - sand) |
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| Table 4 Soil temperatures in Pskov (bottom, loamy soil, clay subsoil) |
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| Table 5 Soil temperature in Vladivostok (soil brown stony, bulk) |
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The information presented in the tables on the natural course of soil temperatures at a depth of up to 3.2 m (i.e., in the “working” soil layer for a gas turbine power plant with a horizontal soil heat exchanger) clearly illustrates the possibilities of using soil as a low-potential heat source. The comparatively small interval of change in the temperature of the layers located at the same depth on the territory of Russia is obvious. So, for example, the minimum soil temperature at a depth of 3.2 m from the surface in the city of Stavropol is 7.4 °C, and in the city of Yakutsk - (-4.4 °C); accordingly, the range of soil temperature changes at a given depth is 11.8 degrees. This fact allows us to count on the creation of a sufficiently unified heat pump equipment suitable for operation practically throughout Russia.
As can be seen from the presented tables, a characteristic feature of the natural temperature regime of the soil is the delay in the minimum soil temperatures relative to the time of arrival of the minimum outdoor air temperatures. The minimum outdoor air temperatures are everywhere observed in January, the minimum temperatures in the soil at a depth of 1.6 m in Stavropol are observed in March, in Yakutsk - in March, in Sochi - in March, in Vladivostok - in April . Thus, it is obvious that by the time of the onset of minimum temperatures in the ground, the load on the heat pump heat supply system (building heat loss) is reduced. This point opens up quite serious opportunities for reducing the installed capacity of the GTTS (capital cost savings) and must be taken into account when designing.
To assess the effectiveness of the use of geothermal heat pump heat supply systems in the climatic conditions of Russia, the zoning of the territory of the Russian Federation was carried out according to the efficiency of using low-potential geothermal heat for heat supply purposes. The zoning was carried out on the basis of the results of numerical experiments on modeling the operating modes of the GTTS in the climatic conditions of various regions of the territory of the Russian Federation. Numerical experiments were carried out on the example of a hypothetical two-storey cottage with a heated area of 200 m 2 , equipped with a geothermal heat pump heat supply system. The external enclosing structures of the house under consideration have the following reduced heat transfer resistances:
- external walls - 3.2 m 2 h ° C / W;
- windows and doors - 0.6 m 2 h ° C / W;
- coatings and ceilings - 4.2 m 2 h ° C / W.
When carrying out numerical experiments, the following were considered:
– ground heat collection system with low density of geothermal energy consumption;
– horizontal heat collection system polyethylene pipes 0.05 m in diameter and 400 m long;
– ground heat collection system with a high density of geothermal energy consumption;
– vertical heat collection system from one thermal well with a diameter of 0.16 m and a length of 40 m.
The conducted studies have shown that the consumption of thermal energy from the soil mass by the end of the heating season causes a decrease in soil temperature near the register of pipes of the heat collection system, which, under the soil and climatic conditions of most of the territory of the Russian Federation, does not have time to be compensated in the summer period of the year, and by the beginning of the next heating season, the soil comes out with a reduced temperature potential. The consumption of thermal energy during the next heating season causes a further decrease in the temperature of the soil, and by the beginning of the third heating season, its temperature potential differs even more from the natural one. And so on... However, the envelopes of the thermal influence of long-term operation of the heat collection system on the natural temperature regime of the soil have a pronounced exponential character, and by the fifth year of operation, the soil enters a new regime close to periodic, i.e., starting from the fifth year operation, long-term consumption of thermal energy from the soil mass of the heat collection system is accompanied by periodic changes in its temperature. Thus, when zoning the territory of the Russian Federation, it was necessary to take into account the drop in temperatures of the soil massif, caused by the long-term operation of the heat collection system, and use the soil temperatures expected for the 5th year of operation of the GTTS as design parameters for the temperatures of the soil massif. Taking into account this circumstance, when zoning the territory of the Russian Federation according to the efficiency of the use of GTES, as a criterion for the effectiveness of the geothermal heat pump heat supply system, the average heat transformation coefficient for the 5th year of operation was chosen Kr tr, which is the ratio of the useful thermal energy generated by the GTST to the energy expended on its drive, and defined for the ideal thermodynamic Carnot cycle as follows:
K tr \u003d T o / (T o - T u), (1)
where T o is the temperature potential of heat removed to the heating or heat supply system, K;
T and - temperature potential of the heat source, K.
The coefficient of transformation of the heat pump heat supply system K tr is the ratio of the useful heat removed to the consumer's heat supply system to the energy expended on the operation of the GTST, and is numerically equal to the amount of useful heat obtained at temperatures T o and T and per unit of energy spent on the GTST drive . The real transformation ratio differs from the ideal one, described by formula (1), by the value of the coefficient h, which takes into account the degree of thermodynamic perfection of the GTST and irreversible energy losses during the implementation of the cycle.
Numerical experiments were carried out with the help of a program created at INSOLAR-INVEST OJSC, which ensures the determination of the optimal parameters of the heat collection system depending on the climatic conditions of the construction area, the heat-shielding qualities of the building, the performance characteristics of heat pump equipment, circulation pumps, heating devices of the heating system, as well as their modes. operation. The program is based on the previously described method for constructing mathematical models of the thermal regime of systems for collecting low-potential ground heat, which made it possible to bypass the difficulties associated with the informative uncertainty of the models and the approximation of external influences, due to the use in the program of experimentally obtained information on the natural thermal regime of the soil, which makes it possible to partially take into account the whole complex of factors (such as the presence of groundwater, their speed and thermal regimes, the structure and location of soil layers, the “thermal” background of the Earth, precipitation, phase transformations of moisture in the pore space, and much more) that most significantly affect the formation of the thermal regime of the system heat collection, and the joint accounting of which in a strict formulation of the problem is practically impossible today. As a solution to the “basic” problem, data from the USSR Climate Handbook (L.: Gidrometioizdat. Issue 1–34) were used.
The program actually allows solving the problem of multi-parameter optimization of the GTTS configuration for a specific building and construction area. At the same time, the target function of the optimization problem is the minimum annual energy costs for the operation of the gas turbine power plant, and the optimization criteria are the radius of the pipes of the soil heat exchanger, its (heat exchanger) length and depth.
The results of numerical experiments and the zoning of the territory of Russia in terms of the efficiency of using low-potential geothermal heat for the purpose of heat supply to buildings are presented graphically in Fig. 1. 2–9.
On fig. 2 shows the values and isolines of the transformation coefficient of geothermal heat pump heat supply systems with horizontal heat collection systems, and in fig. 3 - for GTST with vertical heat collection systems. As can be seen from the figures, the maximum values of Крр 4.24 for horizontal heat collection systems and 4.14 for vertical systems can be expected in the south of Russia, and the minimum values, respectively, 2.87 and 2.73 in the north, in Uelen. For central Russia, the values of Кр tr for horizontal heat collection systems are in the range of 3.4–3.6, and for vertical systems, in the range of 3.2–3.4. Relatively high values of Кр tr (3.2–3.5) are noteworthy for the regions of the Far East, regions with traditionally difficult fuel supply conditions. Apparently, the Far East is a region of priority implementation of GTST.
On fig. Figure 4 shows the values and isolines of the specific annual energy costs for the drive of "horizontal" GTST + PD (peak closer), including energy costs for heating, ventilation and hot water supply, reduced to 1 m 2 of the heated area, and in fig. 5 - for GTST with vertical heat collection systems. As can be seen from the figures, the annual specific energy consumption for the drive of horizontal gas turbine power plants, reduced to 1 m 2 of the heated area of the building, varies from 28.8 kWh / (year m 2) in the south of Russia to 241 kWh / (year m 2) in Moscow. Yakutsk, and for vertical gas turbine power stations, respectively, from 28.7 kWh / / (year m 2) in the south and up to 248 kWh / / (year m 2) in Yakutsk. If we multiply the value of the annual specific energy consumption for the drive of the gas turbine power plant shown in the figures for a particular area by the value for this area K p tr, reduced by 1, we will get the amount of energy saved by the gas turbine power plant from 1 m 2 of heated area per year. For example, for Moscow, for a vertical gas turbine power plant, this value will be 189.2 kWh per 1 m 2 per year. For comparison, we can cite the values of specific energy consumption established by the Moscow energy saving standards MGSN 2.01–99 for low-rise buildings at the level of 130, and for multi-storey buildings 95 kWh / (year m 2). At the same time, energy costs normalized by MGSN 2.01–99 include only energy costs for heating and ventilation, in our case, energy costs also include energy costs for hot water supply. The fact is that the approach to assessing the energy costs for the operation of a building, existing in the current standards, singles out the energy costs for heating and ventilation of the building and the energy costs for its hot water supply as separate items. At the same time, energy costs for hot water supply are not standardized. This approach does not seem correct, since the energy costs for hot water supply are often commensurate with the energy costs for heating and ventilation.
On fig. 6 shows the values and isolines of the rational ratio of the thermal power of the peak closer (PD) and the installed electric power of the horizontal GTST in fractions of a unit, and in fig. 7 - for GTST with vertical heat collection systems. The criterion for the rational ratio of the thermal power of the peak closer and the installed electric power of the GTST (excluding PD) was the minimum annual cost of electricity for the drive of the GTST + PD. As can be seen from the figures, the rational ratio of the capacities of thermal PD and electric GTPP (without PD) varies from 0 in the south of Russia, to 2.88 for horizontal GTPP and 2.92 for vertical systems in Yakutsk. In the central strip of the territory of the Russian Federation, the rational ratio of the thermal power of the door closer and the installed electric power of the GTST + PD is within 1.1–1.3 for both horizontal and vertical GTST. At this point it is necessary to dwell in more detail. The fact is that when replacing, for example, electric heating in Central Russia, we actually have the opportunity to reduce the power of electrical equipment installed in a heated building by 35-40% and, accordingly, reduce the electrical power requested from RAO UES, which today "costs » about 50 thousand rubles. per 1 kW of electrical power installed in the house. So, for example, for a cottage with calculated heat losses in the coldest five-day period equal to 15 kW, we will save 6 kW of installed electric power and, accordingly, about 300 thousand rubles. or ≈ 11.5 thousand US dollars. This figure is practically equal to the cost of a GTST of such heat capacity.
Thus, if we correctly take into account all the costs associated with connecting a building to a centralized power supply, it turns out that with the current tariffs for electricity and connection to centralized power supply networks in the Central Strip of the territory of the Russian Federation, even in terms of one-time costs, GTST turns out to be more profitable than electric heating, not to mention 60 % energy savings.
On fig. 8 shows values and isolines specific gravity thermal energy generated during the year by a peak closer (PD) in the total annual energy consumption of the horizontal GTST + PD system as a percentage, and in fig. 9 - for GTST with vertical heat collection systems. As can be seen from the figures, the share of thermal energy generated during the year by a peak closer (PD) in the total annual energy consumption of the horizontal GTST + PD system varies from 0% in the south of Russia to 38–40% in Yakutsk and Tura, and for vertical GTST+PD - respectively, from 0% in the south and up to 48.5% in Yakutsk. In the Central zone of Russia, these values are about 5–7% for both vertical and horizontal GTS. These are small energy costs, and in this regard, you need to be careful about choosing a peak closer. The most rational from the point of view of both specific capital investments in 1 kW of power and automation are peak electric drivers. Noteworthy is the use of pellet boilers.
In conclusion, I would like to dwell on a very important issue: the problem of choosing a rational level of thermal protection of buildings. This problem is today a very serious task, the solution of which requires a serious numerical analysis, which takes into account the specifics of our climate, and the features of the engineering equipment used, the infrastructure of centralized networks, as well as the ecological situation in cities, which is deteriorating literally before our eyes, and much more. It is obvious that today it is already incorrect to formulate any requirements for the building shell without taking into account its (the building) relationship with the climate and the energy supply system, utilities, etc. As a result, in the very near future, the solution to the problem of choosing a rational level of thermal protection will be possible only based on the consideration of the complex building + energy supply system + climate + environment as a single eco-energy system, and with this approach, the competitive advantages of the GTTS in the domestic market can hardly be overestimated.
Literature
1. Sanner B. Ground Heat Sources for Heat Pumps (classification, characteristics, advantages). Course on geothermal heat pumps, 2002.
2. Vasiliev G. P. Economically feasible level of thermal protection of buildings // Energy saving. - 2002. - No. 5.
3. Vasiliev G. P. Heat and cold supply of buildings and structures using low-potential thermal energy of the surface layers of the Earth: Monograph. Publishing house "Border". – M. : Krasnaya Zvezda, 2006.
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