What is the temperature of the earth at a depth of 2 meters. Thermal fields at the Building-Ground boundary. Freezing depth. Influence of the snow cover of the earth. Experience in the use of geothermal NVC systems
This might seem like fantasy if it weren't true. It turns out that in harsh Siberian conditions, you can get heat directly from the ground. The first objects with geothermal heating systems appeared in the Tomsk region last year, and although they allow reducing the cost of heat by about four times compared to traditional sources, there is still no mass circulation "under the ground". But the trend is noticeable and, most importantly, it is gaining momentum. In fact, this is the most affordable alternative energy source for Siberia, where solar panels or wind generators, for example, cannot always show their effectiveness. Geothermal energy, in fact, just lies under our feet.
“The depth of soil freezing is 2–2.5 meters. The ground temperature below this mark remains the same both in winter and in summer, ranging from plus one to plus five degrees Celsius. The work of the heat pump is built on this property, says the power engineer of the education department of the administration of the Tomsk region Roman Alekseenko. - Connecting pipes are buried in the earth contour to a depth of 2.5 meters, at a distance of about one and a half meters from each other. A coolant - ethylene glycol - circulates in the pipe system. The external horizontal earth circuit communicates with the refrigeration unit, in which the refrigerant - freon, a gas with a low boiling point, circulates. At plus three degrees Celsius, this gas begins to boil, and when the compressor sharply compresses the boiling gas, the temperature of the latter rises to plus 50 degrees Celsius. The heated gas is sent to a heat exchanger in which ordinary distilled water circulates. The liquid heats up and spreads heat throughout the heating system laid in the floor.
Pure physics and no miracles
A kindergarten equipped with a modern Danish geothermal heating system was opened in the village of Turuntaevo near Tomsk last summer. According to the director of the Tomsk company Ecoclimat George Granin, the energy-efficient system allowed several times to reduce the payment for heat supply. For eight years, this Tomsk enterprise has already equipped about two hundred objects in different regions of Russia with geothermal heating systems and continues to do so in the Tomsk region. So there is no doubt in the words of Granin. A year before the opening of the kindergarten in Turuntaevo, Ecoclimat equipped a geothermal heating system, which cost 13 million rubles, to another kindergarten, Sunny Bunny, in the Green Hills microdistrict of Tomsk. In fact, it was the first experience of its kind. And he was quite successful.
Back in 2012, during a visit to Denmark organized under the program of the Euro Info Correspondence Center (EICC-Tomsk region), the company managed to agree on cooperation with the Danish company Danfoss. And today, Danish equipment helps to extract heat from the Tomsk bowels, and, as experts say without too much modesty, it turns out quite efficiently. The main indicator of efficiency is economy. “The heating system for a 250-square-meter kindergarten building in Turuntayevo cost 1.9 million rubles,” says Granin. “And the heating fee is 20-25 thousand rubles a year.” This amount is incomparable with the one that the kindergarten would pay for heat using traditional sources.
The system worked without problems in the conditions of the Siberian winter. A calculation was made of the compliance of thermal equipment with SanPiN standards, according to which it must maintain a temperature of at least + 19 ° C in the kindergarten building at an outdoor air temperature of -40 ° C. In total, about four million rubles were spent on redevelopment, repair and re-equipment of the building. Together with the heat pump, the amount was just under six million. Thanks to heat pumps today, kindergarten heating is a completely isolated and independent system. There are no traditional batteries in the building now, and the space is heated using the “warm floor” system.
Turuntayevsky kindergarten is insulated, as they say, “from” and “to” - additional thermal insulation is equipped in the building: a 10-cm layer of insulation equivalent to two or three bricks is installed on top of the existing wall (three bricks thick). Behind the insulation is an air gap, followed by metal siding. The roof is insulated in the same way. The main attention of the builders was focused on the "warm floor" - the heating system of the building. It turned out several layers: a concrete floor, a layer of foam plastic 50 mm thick, a system of pipes in which hot water circulates and linoleum. Although the temperature of the water in the heat exchanger can reach +50°C, the maximum heating of the actual floor covering does not exceed +30°C. The actual temperature of each room can be adjusted manually - automatic sensors allow you to set the floor temperature in such a way that the kindergarten room warms up to the degrees required by sanitary standards.
The power of the pump in the Turuntayevsky garden is 40 kW of generated thermal energy, for the production of which the heat pump requires 10 kW of electrical power. Thus, out of 1 kW of electrical energy consumed, the heat pump produces 4 kW of heat. “We were a little afraid of winter - we did not know how heat pumps would behave. But even in severe frosts in the kindergarten it was consistently warm - from plus 18 to 23 degrees Celsius, - says the director of the Turuntaev secondary school Evgeny Belonogov. - Of course, here it is worth considering that the building itself was well insulated. The equipment is unpretentious in maintenance, and despite the fact that this is a Western development, it proved to be quite effective in our harsh Siberian conditions.”
A comprehensive project for the exchange of experience in the field of resource conservation was implemented by the EICC-Tomsk region of the Tomsk Chamber of Commerce and Industry. Its participants were small and medium-sized enterprises that develop and implement resource-saving technologies. In May last year, Danish experts visited Tomsk as part of a Russian-Danish project, and the result was, as they say, obvious.
Innovation comes to school
A new school in the village of Vershinino, Tomsk region, built by a farmer Mikhail Kolpakov, is the third object in the region that uses the heat of the earth as a source of heat for heating and hot water supply. The school is also unique because it has the highest energy efficiency category - "A". The heating system was designed and launched by the same Ecoclimat company.
“When we were deciding what kind of heating to install in the school, we had several options - a coal-fired boiler house and heat pumps,” says Mikhail Kolpakov. - We studied the experience of an energy-efficient kindergarten in Zeleny Gorki and calculated that heating in the old fashioned way, on coal, would cost us more than 1.2 million rubles over the winter, and we also need hot water. And with heat pumps, the cost will be about 170 thousand for the whole year, along with hot water.”
The system needs only electricity to produce heat. Consuming 1 kW of electricity, heat pumps in a school produce about 7 kW of thermal energy. In addition, unlike coal and gas, the heat of the earth is a self-renewable source of energy. The installation of a modern heating system for the school cost about 10 million rubles. For this, 28 wells were drilled on the school grounds.
“The arithmetic here is simple. We calculated that the maintenance of the coal boiler, taking into account the salary of the stoker and the cost of fuel, will cost more than a million rubles a year, - notes the head of the education department Sergey Efimov. - When using heat pumps, you will have to pay for all resources about fifteen thousand rubles a month. The undoubted advantages of using heat pumps are their efficiency and environmental friendliness. The heat supply system allows you to regulate the heat supply depending on the weather outside, which eliminates the so-called "underheating" or "overheating" of the room.
According to preliminary calculations, expensive Danish equipment will pay for itself in four to five years. The service life of Danfoss heat pumps, with which Ecoclimat LLC works, is 50 years. Receiving information about the air temperature outside, the computer determines when to heat the school, and when it is possible not to do so. Therefore, the question of the date of switching on and off the heating disappears altogether. Regardless of the weather, climate control will always work outside the windows inside the school for children.
“When the Ambassador Extraordinary and Plenipotentiary of the Kingdom of Denmark came to the all-Russian meeting last year and visited our kindergarten in Zelenye Gorki, he was pleasantly surprised that those technologies that are considered innovative even in Copenhagen are applied and work in the Tomsk region, - says the commercial director of Ecoclimat Alexander Granin.
In general, the use of local renewable energy sources in various sectors of the economy, in this case in the social sphere, which includes schools and kindergartens, is one of the main areas implemented in the region as part of the energy saving and energy efficiency program. The development of renewable energy is actively supported by the governor of the region Sergey Zhvachkin. And three budgetary institutions with a geothermal heating system are only the first steps towards the implementation of a large and promising project.
The kindergarten in Zelenye Gorki was recognized as the best energy-efficient facility in Russia at a competition in Skolkovo. Then came the Vershininskaya school with geothermal heating, also of the highest category of energy efficiency. The next object, no less significant for the Tomsk region, is a kindergarten in Turuntaevo. This year, the Gazhimstroyinvest and Stroygarant companies have already begun construction of kindergartens for 80 and 60 children in the villages of the Tomsk region, Kopylovo and Kandinka, respectively. Both new facilities will be heated by geothermal heating systems - from heat pumps. In total, this year the district administration intends to spend almost 205 million rubles on the construction of new kindergartens and the repair of existing ones. It is planned to reconstruct and re-equip the building for a kindergarten in the village of Takhtamyshevo. In this building, heating will also be implemented by means of heat pumps, since the system has proved itself well.
One of the best, rational methods in the construction of capital greenhouses is an underground thermos greenhouse.
The use of this fact of the constancy of the earth's temperature at a depth in the construction of a greenhouse gives tremendous savings in heating costs in the cold season, facilitates care, makes the microclimate more stable.
Such a greenhouse works in the most severe frosts, allows you to produce vegetables, grow flowers all year round.
A properly equipped buried greenhouse makes it possible to grow, among other things, heat-loving southern crops. There are practically no restrictions. Citrus fruits and even pineapples can feel great in a greenhouse.
But in order for everything to function properly in practice, it is imperative to follow the time-tested technologies by which underground greenhouses were built. After all, this idea is not new, even under the tsar in Russia, buried greenhouses yielded pineapple crops, which enterprising merchants exported to Europe for sale.
For some reason, the construction of such greenhouses has not found wide distribution in our country, by and large, it is simply forgotten, although the design is ideal just for our climate.
Probably, the need to dig a deep pit and pour the foundation played a role here. The construction of a buried greenhouse is quite expensive, it is far from a greenhouse covered with polyethylene, but the return on the greenhouse is much greater.
From deepening into the ground, the overall internal illumination is not lost, this may seem strange, but in some cases the light saturation is even higher than that of classic greenhouses.
It is impossible not to mention the strength and reliability of the structure, it is incomparably stronger than usual, it is easier to tolerate hurricane gusts of wind, it resists hail well, and blockages of snow will not become a hindrance.
1. Pit
The creation of a greenhouse begins with digging a foundation pit. To use the heat of the earth to heat the internal volume, the greenhouse must be sufficiently deepened. The deeper the earth gets warmer.
The temperature almost does not change during the year at a distance of 2-2.5 meters from the surface. At a depth of 1 m, the soil temperature fluctuates more, but in winter its value remains positive, usually in the middle zone the temperature is 4-10 C, depending on the season.
A buried greenhouse is built in one season. That is, in winter it will already be able to function and generate income. Construction is not cheap, but by using ingenuity, compromise materials, it is possible to save literally an order of magnitude by making a kind of economy option for a greenhouse, starting with a foundation pit.
For example, do without the involvement of construction equipment. Although the most time-consuming part of the work - digging a pit - is, of course, better to give to an excavator. Manually removing such a volume of land is difficult and time consuming.
The depth of the excavation pit should be at least two meters. At such a depth, the earth will begin to share its heat and work like a kind of thermos. If the depth is less, then in principle the idea will work, but noticeably less efficiently. Therefore, it is recommended that you spare no effort and money to deepen the future greenhouse.
Underground greenhouses can be any length, but it is better to keep the width within 5 meters, if the width is larger, then the quality characteristics for heating and light reflection deteriorate.
On the sides of the horizon, underground greenhouses need to be oriented, like ordinary greenhouses and greenhouses, from east to west, that is, so that one of the sides faces south. In this position, the plants will receive the maximum amount of solar energy.
2. Walls and roof
Along the perimeter of the pit, a foundation is poured or blocks are laid out. The foundation serves as the basis for the walls and frame of the structure. Walls are best made from materials with good thermal insulation characteristics, thermoblocks are an excellent option.
The roof frame is often made of wood, from bars impregnated with antiseptic agents. The roof structure is usually straight gable. A ridge beam is fixed in the center of the structure; for this, central supports are installed on the floor along the entire length of the greenhouse.
The ridge beam and walls are connected by a row of rafters. The frame can be made without high supports. They are replaced with small ones, which are placed on transverse beams connecting opposite sides of the greenhouse - this design makes the interior space freer.
As a roof covering, it is better to take cellular polycarbonate - a popular modern material. The distance between the rafters during construction is adjusted to the width of the polycarbonate sheets. It is convenient to work with the material. The coating is obtained with a small number of joints, since the sheets are produced in lengths of 12 m.
They are attached to the frame with self-tapping screws, it is better to choose them with a cap in the form of a washer. To avoid cracking the sheet, a hole of the appropriate diameter must be drilled under each self-tapping screw with a drill. With a screwdriver, or a conventional drill with a Phillips bit, glazing work moves very quickly. In order to avoid gaps, it is good to lay the rafters along the top with a sealant made of soft rubber or other suitable material in advance and only then screw the sheets. The peak of the roof along the ridge must be laid with soft insulation and pressed with some kind of corner: plastic, tin, or another suitable material.
For good thermal insulation, the roof is sometimes made with a double layer of polycarbonate. Although the transparency is reduced by about 10%, but this is covered by the excellent thermal insulation performance. It should be noted that the snow on such a roof does not melt. Therefore, the slope must be at a sufficient angle, at least 30 degrees, so that snow does not accumulate on the roof. Additionally, an electric vibrator is installed for shaking, it will save the roof in case snow still accumulates.
Double glazing is done in two ways:
A special profile is inserted between two sheets, the sheets are attached to the frame from above;
First, the bottom layer of glazing is attached to the frame from the inside, to the underside of the rafters. The roof is covered with the second layer, as usual, from above.
After completing the work, it is desirable to glue all the joints with tape. The finished roof looks very impressive: without unnecessary joints, smooth, without prominent parts.
3. Warming and heating
Wall insulation is carried out as follows. First you need to carefully coat all the joints and seams of the wall with a solution, here you can also use mounting foam. The inner side of the walls is covered with a thermal insulation film.
In cold parts of the country, it is good to use foil thick film, covering the wall with a double layer.
The temperature deep in the soil of the greenhouse is above zero, but colder than the air temperature required for plant growth. The top layer is heated by the sun's rays and the air of the greenhouse, but still the soil takes away heat, so often in underground greenhouses they use the technology of "warm floors": the heating element - an electric cable - is protected by a metal grill or poured with concrete.
In the second case, the soil for the beds is poured over concrete or greens are grown in pots and flowerpots.
The use of underfloor heating can be sufficient to heat the entire greenhouse if there is enough power. But it is more efficient and more comfortable for plants to use combined heating: underfloor heating + air heating. For good growth, they need an air temperature of 25-35 degrees at an earth temperature of about 25 C.
CONCLUSION
Of course, the construction of a buried greenhouse will cost more, and more effort will be required than with the construction of a similar greenhouse of a conventional design. But the funds invested in the greenhouse-thermos are justified over time.
First, it saves energy on heating. No matter how an ordinary ground-based greenhouse is heated in winter, it will always be more expensive and more difficult than a similar heating method in an underground greenhouse. Secondly, saving on lighting. Foil thermal insulation of the walls, reflecting light, doubles the illumination. The microclimate in an in-depth greenhouse in winter will be more favorable for plants, which will certainly affect the yield. Seedlings will easily take root, tender plants will feel great. Such a greenhouse guarantees a stable, high yield of any plants all year round.
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.
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 - sunlight 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 the average annual air temperature in a given 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 action 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 approximately 350 W h / 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 each square meter of the earth's surface about 4,000 kWh annually, that is, 10,000 times more (of course, this is on 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°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°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°C per 1 km, and in South Africa it is 6°C per 1 km.
The question is, what is the temperature at great depths - 5, 10 km or more? If the trend continues, temperatures at a depth of 10 km should average around 250–300°C. This is more or less confirmed by direct observations in ultradeep wells, although the picture is much more complicated than the 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°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°C has already been recorded, at 10 km - 180°C, and at 12 km - 220°C.
Another example is a well laid in the Northern Caspian, where at a depth of 500 m a temperature of 42°C was recorded, at 1.5 km - 70°C, at 2 km - 80°C, at 3 km - 108°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°C, at a depth of 400 km - 1600°C, in the Earth's core (depths of more than 6000 km) - 4000–5000° C.
At depths up to 10–12 km, 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.
There is no strict definition of the concept of "thermal waters". 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.
Water temperatures from 20-30 to 100°C are suitable for heating, temperatures from 150°C and above - and for the generation of electricity in geothermal power plants.
In general, geothermal resources in Russia, in terms of tons of standard 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. In practice, at the 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 volcano Eyyafyatlayokudl ( Eyjafjallajokull) in 2010 year.
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 of Southeast Asia (Philippines and Indonesia), the countries of 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.
The use of geothermal energy has a very long history. One of the first known examples is Italy, a place in the province of Tuscany, now called Larderello, where, at the beginning of the 19th century, local hot thermal waters, flowing naturally or extracted from shallow wells, were used for energy purposes.
Water from underground sources, rich in boron, was used here to obtain boric acid. Initially, this acid was obtained by evaporation in iron boilers, and ordinary firewood was taken as fuel from nearby forests, but in 1827 Francesco Larderel created a system that worked on the heat of the waters themselves. At the same time, the energy of natural water vapor began to be used for the operation of drilling rigs, and at the beginning of the 20th century, for heating local houses and greenhouses. In the same place, in Larderello, in 1904, thermal water vapor became an energy source for generating electricity.
The example of Italy at the end of the 19th and beginning of the 20th century was followed by some other countries. For example, in 1892, thermal waters were first used for local heating in the United States (Boise, Idaho), in 1919 - in Japan, in 1928 - in Iceland.
In the United States, the first hydrothermal power plant appeared in California in the early 1930s, in New Zealand - in 1958, in Mexico - in 1959, in Russia (the world's first binary GeoPP) - in 1965 .
An old principle at a new source
Electricity generation requires a higher water source temperature than heating, over 150°C. The principle of operation of a geothermal power plant (GeoES) is similar to the principle of operation of a conventional thermal power plant (TPP). In fact, a geothermal power plant is a type of thermal power plant.
At thermal power plants, as a rule, coal, gas or fuel oil act as the primary source of energy, and water vapor serves as the working fluid. The fuel, burning, heats the water to a state of steam, which rotates the steam turbine, and it generates electricity.
The difference between the GeoPP is that the primary source of energy here is the heat of the earth's interior and the working fluid in the form of steam enters the turbine blades of the electric generator in a "ready" form directly from the production well.
There are three main schemes of GeoPP operation: direct, using dry (geothermal) steam; indirect, based on hydrothermal water, and mixed, or binary.
The use of one or another scheme depends on the state of aggregation and the temperature of the energy carrier.
The simplest and therefore the first of the mastered schemes is the direct one, in which the steam coming from the well is passed directly through the turbine. The world's first GeoPP in Larderello in 1904 also operated on dry steam.
GeoPPs with an indirect scheme of operation are the most common in our time. They use hot underground water, which is pumped under high pressure into an evaporator, where part of it is evaporated, and the resulting steam rotates a turbine. In some cases, additional devices and circuits are required to purify geothermal water and steam from aggressive compounds.
The exhaust steam enters the injection well or is used for space heating - in this case, the principle is the same as during the operation of a CHP.
At binary GeoPPs, hot thermal water interacts with another liquid that acts as a working fluid with a lower boiling point. Both fluids are passed through a heat exchanger, where thermal water evaporates the working fluid, the vapors of which rotate the turbine.
This system is closed, which solves the problem of emissions into the atmosphere. In addition, working fluids with a relatively low boiling point make it possible to use not very hot thermal waters as a primary source of energy.
All three schemes use a hydrothermal source, but petrothermal energy can also be used to generate electricity.
The circuit diagram in this case is also quite simple. It is necessary to drill two interconnected wells - injection and production. Water is pumped into the injection well. At depth, it heats up, then heated water or steam formed as a result of strong heating is supplied to the surface through a production well. Further, it all depends on how the petrothermal energy is used - for heating or for the production of electricity. A closed cycle is possible with the pumping of exhaust steam and water back into the injection well or another method of disposal.
The disadvantage of such a system is obvious: in order to obtain a sufficiently high temperature of the working fluid, it is necessary to drill wells to a great depth. And this is a serious cost and the risk of significant heat loss when the fluid moves up. Therefore, petrothermal systems are still less common than hydrothermal ones, although the potential of petrothermal energy is orders of magnitude higher.
Currently, the leader in the creation of the so-called petrothermal circulating systems (PCS) is Australia. In addition, this direction of geothermal energy is actively developing in the USA, Switzerland, Great Britain, and Japan.
Gift from Lord Kelvin
The invention of the heat pump in 1852 by the physicist William Thompson (aka Lord Kelvin) provided mankind with a real opportunity to use the low-grade heat of the upper layers of the soil. The heat pump system, or heat multiplier as Thompson called it, is based on the physical process of transferring heat from the environment to the refrigerant. In fact, it uses the same principle as in petrothermal systems. The difference is in the source of heat, in connection with which a terminological question may arise: to what extent can a heat pump be considered a geothermal system? The fact is that in the upper layers, to depths of tens or hundreds of meters, the rocks and the fluids contained in them are heated not by the deep heat of the earth, but by the sun. Thus, it is the sun in this case that is the primary source of heat, although it is taken, as in geothermal systems, from the earth.
The operation of a heat pump is based on the delay in the heating and cooling of the soil compared to the atmosphere, resulting in a temperature gradient between the surface and deeper layers that retain heat even in winter, similar to what happens in reservoirs. The main purpose of heat pumps is space heating. In fact, it is a “refrigerator in reverse”. Both the heat pump and the refrigerator interact with three components: the internal environment (in the first case - a heated room, in the second - a cooled refrigerator chamber), the external environment - an energy source and a refrigerant (refrigerant), which is also a coolant that provides heat transfer or cold.
A substance with a low boiling point acts as a refrigerant, which allows it to take heat from a source that has even a relatively low temperature.
In the refrigerator, the liquid refrigerant enters the evaporator through a throttle (pressure regulator), where, due to a sharp decrease in pressure, the liquid evaporates. Evaporation is an endothermic process requiring heat to be absorbed from outside. As a result, heat is taken from the inner walls of the evaporator, which provides a cooling effect in the refrigerator chamber. Further from the evaporator, the refrigerant is sucked into the compressor, where it returns to the liquid state of aggregation. This is the reverse process, leading to the release of the taken heat into the external environment. As a rule, it is thrown into the room, and the back wall of the refrigerator is relatively warm.
The heat pump works in almost the same way, with the difference that heat is taken from the external environment and enters the internal environment through the evaporator - the room heating system.
In a real heat pump, water is heated, passing through an external circuit laid in the ground or a reservoir, then enters the evaporator.
In the evaporator, heat is transferred to an internal circuit filled with a refrigerant with a low boiling point, which, passing through the evaporator, changes from a liquid state to a gaseous state, taking heat.
Further, the gaseous refrigerant enters the compressor, where it is compressed to high pressure and temperature, and enters the condenser, where heat exchange takes place between the hot gas and the heat carrier from the heating system.
The compressor requires electricity to run, yet the transformation ratio (the ratio of energy consumed and produced) in modern systems is high enough to ensure their efficiency.
Currently, heat pumps are widely used for space heating, mainly in economically developed countries.
Eco-correct energy
Geothermal energy is considered environmentally friendly, which is generally true. First of all, it uses a renewable and practically inexhaustible resource. Geothermal energy does not require large areas, unlike large hydroelectric power plants or wind farms, and does not pollute the atmosphere, unlike hydrocarbon energy. On average, the GeoPP occupies 400 m 2 in terms of 1 GW of electricity generated. The same figure for a coal-fired thermal power plant, for example, is 3600 m 2. The environmental benefits of GeoPPs also include low water consumption - 20 liters of fresh water per 1 kW, while thermal power plants and nuclear power plants require about 1000 liters. Note that these are the environmental indicators of the "average" GeoPP.
But there are still negative side effects. Among them, noise, thermal pollution of the atmosphere and chemical pollution of water and soil, as well as the formation of solid waste are most often distinguished.
The main source of chemical pollution of the environment is the thermal water itself (with high temperature and mineralization), often containing large amounts of toxic compounds, and therefore there is a problem of waste water and hazardous substances disposal.
The negative effects of geothermal energy can be traced at several stages, starting with drilling wells. Here, the same dangers arise as when drilling any well: destruction of the soil and vegetation cover, pollution of the soil and groundwater.
At the stage of operation of the GeoPP, the problems of environmental pollution persist. Thermal fluids - water and steam - typically contain carbon dioxide (CO 2), sulfur sulfide (H 2 S), ammonia (NH 3), methane (CH 4), common salt (NaCl), boron (B), arsenic (As ), mercury (Hg). When released into the environment, they become sources of pollution. In addition, an aggressive chemical environment can cause corrosion damage to GeoTPP structures.
At the same time, pollutant emissions at GeoPPs are on average lower than at TPPs. For example, carbon dioxide emissions per kilowatt-hour of electricity generated are up to 380 g at GeoPPs, 1042 g at coal-fired thermal power plants, 906 g at fuel oil and 453 g at gas-fired thermal power plants.
The question arises: what to do with waste water? With low salinity, after cooling, it can be discharged into surface waters. The other way is to pump it back into the aquifer through an injection well, which is the preferred and predominant practice at present.
The extraction of thermal water from aquifers (as well as pumping out ordinary water) can cause subsidence and ground movements, other deformations of geological layers, and micro-earthquakes. The probability of such phenomena is usually low, although individual cases have been recorded (for example, at the GeoPP in Staufen im Breisgau in Germany).
It should be emphasized that most of the GeoPPs are located in relatively sparsely populated areas and in third world countries, where environmental requirements are less stringent than in developed countries. In addition, at the moment the number of GeoPPs and their capacities are relatively small. With a larger development of geothermal energy, environmental risks can increase and multiply.
How much is the energy of the Earth?
Investment costs for the construction of geothermal systems vary in a very wide range - from 200 to 5000 dollars per 1 kW of installed capacity, that is, the cheapest options are comparable to the cost of building a thermal power plant. They depend, first of all, on the conditions of occurrence of thermal waters, their composition, and the design of the system. Drilling to great depths, creating a closed system with two wells, the need for water treatment can multiply the cost.
For example, investments in the creation of a petrothermal circulation system (PTS) are estimated at 1.6–4 thousand dollars per 1 kW of installed capacity, which exceeds the costs of building a nuclear power plant and is comparable to the costs of building wind and solar power plants.
The obvious economic advantage of GeoTPP is a free energy carrier. For comparison, in the cost structure of an operating thermal power plant or nuclear power plant, fuel accounts for 50–80% or even more, depending on current energy prices. Hence, another advantage of the geothermal system: operating costs are more stable and predictable, since they do not depend on the external conjuncture of energy prices. In general, the operating costs of the GeoTPP are estimated at 2–10 cents (60 kopecks–3 rubles) per 1 kWh of generated capacity.
The second largest (and very significant) item of expenditure after the energy carrier is, as a rule, the salary of the station staff, which can vary dramatically by country and region.
On average, the cost of 1 kWh of geothermal energy is comparable to that for thermal power plants (in Russian conditions - about 1 ruble / 1 kWh) and ten times higher than the cost of electricity generation at hydroelectric power plants (5–10 kopecks / 1 kWh ).
Part of the reason for the high cost is that, unlike thermal and hydraulic power plants, GeoTPP has a relatively small capacity. In addition, it is necessary to compare systems located in the same region and in similar conditions. So, for example, in Kamchatka, according to experts, 1 kWh of geothermal electricity costs 2-3 times cheaper than electricity produced at local thermal power plants.
The indicators of economic efficiency of the geothermal system depend, for example, on whether it is necessary to dispose of the waste water and in what ways this is done, whether the combined use of the resource is possible. Thus, chemical elements and compounds extracted from thermal water can provide additional income. Recall the example of Larderello: it was chemical production that was primary there, and the use of geothermal energy was initially of an auxiliary nature.
Geothermal Energy Forwards
Geothermal energy is developing somewhat differently than wind and solar. At present, it largely depends on the nature of the resource itself, which differs sharply by region, and the highest concentrations are tied to narrow zones of geothermal anomalies, usually associated with areas of tectonic faults and volcanism.
In addition, geothermal energy is less technologically capacious compared to wind and even more so with solar energy: the systems of geothermal stations are quite simple.
In the overall structure of world electricity production, the geothermal component accounts for less than 1%, but in some regions and countries its share reaches 25–30%. Due to the linkage to geological conditions, a significant part of the geothermal energy capacity is concentrated in third world countries, where there are three clusters of the industry's greatest development - the islands of Southeast Asia, Central America and East Africa. The first two regions are part of the Pacific "Fire Belt of the Earth", the third is tied to the East African Rift. With the greatest probability, geothermal energy will continue to develop in these belts. A more distant prospect is the development of petrothermal energy, using the heat of the earth's layers lying at a depth of several kilometers. This is an almost ubiquitous resource, but its extraction requires high costs, so petrothermal energy is developing primarily in the most economically and technologically powerful countries.
In general, given the ubiquity of geothermal resources and an acceptable level of environmental safety, there is reason to believe that geothermal energy has good development prospects. Especially with the growing threat of a shortage of traditional energy carriers and rising prices for them.
From Kamchatka to the Caucasus
In Russia, the development of geothermal energy has a fairly long history, and in a number of positions we are among the world leaders, although the share of geothermal energy in the overall energy balance of a huge country is still negligible.
The pioneers and centers for the development of geothermal energy in Russia were two regions - Kamchatka and the North Caucasus, and if in the first case we are talking primarily about the electric power industry, then in the second - about the use of the thermal energy of thermal water.
In the North Caucasus - in the Krasnodar Territory, Chechnya, Dagestan - the heat of thermal waters was used for energy purposes even before the Great Patriotic War. In the 1980s–1990s, the development of geothermal energy in the region, for obvious reasons, stalled and has not yet recovered from the state of stagnation. Nevertheless, geothermal water supply in the North Caucasus provides heat for about 500 thousand people, and, for example, the city of Labinsk in the Krasnodar Territory with a population of 60 thousand people is completely heated by geothermal waters.
In Kamchatka, the history of geothermal energy is associated primarily with the construction of the GeoPP. The first of them, still operating Pauzhetskaya and Paratunskaya stations, were built back in 1965–1967, while the Paratunskaya GeoPP with a capacity of 600 kW became the first station in the world with a binary cycle. It was the development of Soviet scientists S. S. Kutateladze and A. M. Rosenfeld from the Institute of Thermal Physics of the Siberian Branch of the Russian Academy of Sciences, who received in 1965 a copyright certificate for extracting electricity from water with a temperature of 70 ° C. This technology subsequently became the prototype for more than 400 binary GeoPPs in the world.
The capacity of the Pauzhetskaya GeoPP, commissioned in 1966, was initially 5 MW and subsequently increased to 12 MW. Currently, the station is under construction of a binary block, which will increase its capacity by another 2.5 MW.
The development of geothermal energy in the USSR and Russia was hampered by the availability of traditional energy sources - oil, gas, coal, but never stopped. The largest geothermal energy facilities at the moment are the Verkhne-Mutnovskaya GeoPP with a total capacity of 12 MW power units, commissioned in 1999, and the Mutnovskaya GeoPP with a capacity of 50 MW (2002).
Mutnovskaya and Verkhne-Mutnovskaya GeoPP are unique objects not only for Russia, but also on a global scale. The stations are located at the foot of the Mutnovsky volcano, at an altitude of 800 meters above sea level, and operate in extreme climatic conditions, where it is winter for 9-10 months a year. The equipment of the Mutnovsky GeoPPs, currently one of the most modern in the world, was completely created at domestic enterprises of power engineering.
At present, the share of Mutnovskiye stations in the overall structure of energy consumption of the Central Kamchatka energy hub is 40%. An increase in capacity is planned in the coming years.
Separately, it should be said about Russian petrothermal developments. We do not yet have large PDSs, but there are advanced technologies for drilling to great depths (about 10 km), which also have no analogues in the world. Their further development will make it possible to drastically reduce the costs of creating petrothermal systems. The developers of these technologies and projects are N. A. Gnatus, M. D. Khutorskoy (Geological Institute of the Russian Academy of Sciences), A. S. Nekrasov (Institute of Economic Forecasting of the Russian Academy of Sciences) and specialists from the Kaluga Turbine Plant. Currently, the petrothermal circulation system project in Russia is at the pilot stage.
There are prospects for geothermal energy in Russia, although they are relatively distant: at the moment, the potential is quite large and the positions of traditional energy are strong. At the same time, in a number of remote regions of the country, the use of geothermal energy is economically profitable and in demand even now. These are territories with a high geoenergy potential (Chukotka, Kamchatka, the Kuriles - the Russian part of the Pacific "Fire Belt of the Earth", the mountains of Southern Siberia and the Caucasus) and at the same time remote and cut off from centralized energy supply.
It is likely that in the coming decades, geothermal energy in our country will develop precisely in such regions.
Kirill Degtyarev, Research Fellow, Lomonosov Moscow State University 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.
The increase in temperature of thermal waters and their host dry rocks with depth.
Change in temperature with depth in different regions.
The eruption of the Icelandic volcano Eyjafjallajokull 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 - sunlight 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 the average annual air temperature in a given 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 action 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 each square meter of the earth's surface about 4,000 kWh annually, that is, 10,000 times more (of course, this is on 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 super-deep well drilled in the Baltic crystalline shield, the temperature to a depth of 3 km changes at a rate of 10 ° C / 1 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 FROM.
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.
There is no strict definition of the concept of "thermal waters". As a rule, they mean hot groundwater in a liquid state or in the form of steam, including those that come to the surface of the Earth 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.
Water temperatures from 20-30 to 100 ° C are suitable for heating, temperatures from 150 ° C and above - and for generating electricity at geothermal power plants.
In general, geothermal resources in Russia, in terms of tons of standard 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. In practice, at the 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 of Southeast Asia (Philippines and Indonesia), the countries of 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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