EntranceEarth temperature at depth 2. Ten myths about geothermal heating and cooling systems
Imagine a home that is always at a comfortable temperature, with no heating or cooling system in sight. This system works efficiently, but does not require complex maintenance or special knowledge from the owners.
Fresh air, you can hear the birds chirping and the wind lazily playing with the leaves on the trees. The house receives energy from the earth, like leaves, which receive energy from the roots. Great picture, isn't it?
Geothermal heating and cooling systems make this a reality. A geothermal HVAC (heating, ventilation and air conditioning) system uses the ground temperature to provide heating in winter and cooling in summer.
How geothermal heating and cooling works
The ambient temperature changes with the seasons, but the underground temperature does not change as much due to the insulating properties of the earth. At a depth of 1.5-2 meters, the temperature remains relatively constant all year round. A geothermal system typically consists of internal processing equipment, an underground pipe system called an underground loop, and/or a water circulation pump. The system uses the earth's constant temperature to provide "clean and free" energy.
(Do not confuse the concept of a geothermal NHC system with "geothermal energy" - a process in which electricity is generated directly from the heat in the earth. In the latter case, a different type of equipment and other processes are used, the purpose of which is usually to heat water to a boiling point.)
The pipes that make up the underground loop are usually made of polyethylene and can be laid horizontally or vertically underground, depending on the terrain. If an aquifer is available, 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. The indoor equipment further raises the temperature and distributes it throughout the building. It's like an air conditioner working in reverse. During the summer, a geothermal NWC system draws hot water from the building and carries it through an underground loop/pump to a re-injection well, 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 indoors and protected from changing weather conditions, so they can last many years, often decades. Routine periodic checks, timely filter replacement and annual coil cleaning are the only maintenance required.
Experience in the use of geothermal NVC systems
Geothermal NVC systems have been used for more than 60 years all over the world. They work with nature, not against it, and they don't emit greenhouse gases (as noted earlier, they use less electricity because they use the earth's constant temperature).
Geothermal NVC systems are increasingly becoming attributes of green homes, as part of the growing green building movement. Green projects accounted for 20 percent of all homes built in the US last year. An article in the Wall Street Journal says that by 2016 the green building budget will rise from $36 billion a year to $114 billion. This will amount to 30-40 percent of the entire real estate market.
But much of the information about geothermal heating and cooling is based on outdated data or unsubstantiated myths.
Destroying myths about geothermal NWC systems
1. Geothermal NVC systems are not a renewable technology because they use electricity.
Fact: Geothermal HVAC systems use only one unit of electricity to produce up to five units of cooling or heating.
2. Solar energy and wind energy are more favorable renewable technologies compared to geothermal NVC systems.
Fact: Geothermal NVC systems for one dollar process four times more kilowatts / hours than solar or wind energy generates for the same dollar. These technologies can, of course, play an important 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. HVK geothermal heat pumps are noisy.
Fact: The systems are very quiet and there is no equipment outside so as not to disturb the neighbors.
5. Geothermal systems eventually wear out.
Fact: Underground loops can last for generations. Heat exchange equipment typically lasts for decades as it is protected indoors. When it comes time to need to replace equipment, the cost of such a replacement is much less than a new geothermal system, since the underground loop and well are its most expensive parts. New technical solutions eliminate the problem of heat retention in the ground, so the system can exchange temperatures in unlimited quantities. There have been cases 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 HVAC air systems cost approximately $3,000 per tonne of heat or cold (homes typically use one to five tons). The price of geothermal NVC systems ranges from approximately $5,000 per ton to $8,000-9,000. However, new installation methods significantly reduce costs, down to the prices of conventional systems.
Cost savings can also be achieved through discounts on equipment for public or commercial use, or even large orders for the home (especially from big brands such as Bosch, Carrier and Trane). Open loops, using a pump and a re-injection well, are cheaper to install than closed systems.
Source: energyblog.nationalgeographic.com
Instead of a preface.
Clever and benevolent people pointed out to me not that this case should be evaluated only in a non-stationary setting, due to the huge thermal inertia of the earth and take into account the annual regime of temperature changes. The completed example was solved for a stationary thermal field, therefore it has obviously incorrect results, so it should be considered only as some kind of idealized model with a huge number of simplifications showing the temperature distribution in a stationary mode. So as they say, any coincidences are pure coincidence...
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As usual, I will not give a lot of specifics about the accepted thermal conductivities and thicknesses of materials, I will limit myself to describing only a few, we assume that other elements are as close as possible to real structures - the thermophysical characteristics are assigned correctly, and the thicknesses of materials are adequate to real cases of building practice. The purpose of the article is to get a framework idea of the temperature distribution at the Building-Ground boundary under various conditions.
A little about what needs to be said. The calculated schemes in this example contain 3 temperature limits, the 1st is the indoor air of the premises of the heated building +20 o C, the 2nd is the outside air -10 o C (-28 o C), and the 3rd is the temperature in the soil at a certain depth, at which it fluctuates around a certain constant value. In this example, the value of this depth is 8 m and the temperature is +10 ° C. Here, someone can argue with me regarding the accepted parameters of the 3rd boundary, but the dispute about the exact values \u200b\u200bis not the task of this article, just as the results obtained are not claim for special accuracy and the possibility of binding to a specific design case. I repeat, the task is to get a fundamental, framework idea of the temperature distribution, and to test some of the established ideas on this issue.
Now straight to the point. So the theses to be tested.
1. The ground under a heated building has a positive temperature.
2. Normative depth of soil freezing (this is more a question than a statement). Is the snow cover of the soil taken into account when reporting freezing data in geological reports, because, as a rule, the area around the house is cleared of snow, paths, sidewalks, blind areas, parking, etc. are cleaned?
Soil freezing is a process in time, so for the calculation we will take the outside temperature equal to the average temperature of the coldest month -10 o C. We will take the soil with the reduced lambda \u003d 1 for the entire depth.
Fig.1. Calculation scheme.
Fig.2. Temperature isolines. Scheme without snow cover.
In general, the ground temperature under the building is positive. Maximums are closer to the center of the building, minima to the outer walls. The isoline of zero temperatures horizontally only concerns the projection of the heated room on the horizontal plane.
Soil freezing far from the building (i.e. reaching negative temperatures) occurs at a depth of ~2.4 meters, which is more than the normative value for the conventionally selected region (1.4-1.6m).
Now let's add 400mm of medium dense snow with a lambda of 0.3.
Fig.3. Temperature isolines. Scheme with snow cover 400mm.
Isolines of positive temperatures displace negative temperatures outside, only positive temperatures under the building.
Ground freezing under snow cover ~1.2 meters (-0.4m of snow = 0.8m of ground freezing). Snow "blanket" significantly reduces the depth of freezing (almost 3 times).
Apparently, the presence of snow cover, its height and degree of compaction is not a constant value, therefore, the average freezing depth is in the range of the results of 2 schemes, (2.4 + 0.8) * 0.5 = 1.6 meters, which corresponds to the standard value.
Now let's see what happens if severe frosts hit (-28 o C) and stand long enough for the thermal field to stabilize, while there is no snow cover around the building.
Fig.4. Scheme at -28 about With no snow cover.
Negative temperatures crawl under the building, positive temperatures press against the floor of the heated room. In the area of the foundations, the soils freeze through. At a distance from the building, the soils freeze by ~4.7 meters.
See previous blog entries.
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 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 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 4000 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 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. 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, as early as 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 liquids are passed through a heat exchanger, where thermal water evaporates the working liquid, 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, as a result of which a temperature gradient is formed between the surface and deeper layers, which retain heat even in winter, similar to how it 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 electrical energy to operate, however, the transformation ratio (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, 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 thermal water itself (with high temperature and salinity), which often contains 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 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 hindered by the availability of traditional energy sources - oil, gas, coal, but never stopped. The largest geothermal power 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 Mutnovsky 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 PDS, however, 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 position of traditional energy is strong. At the same time, in a number of remote regions of the country, the use of geothermal energy is economically profitable and is in demand even now. These are territories with 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.
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 warmed up by the sun's rays and the air of the greenhouse, but still the soil takes heat away, therefore, the technology of "warm floors" is often used in underground greenhouses: 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.
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. At present, this is one of the most dynamically developing areas for the use of non-traditional renewable energy sources in the world.
Geothermal heat pump systems of heat supply and efficiency of their application in the climatic conditions of Russia
G. P. Vasiliev, scientific director of JSC "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 solar radiation incident on the surface and the flow of radiogenic heat from the earth's interior. Seasonal and daily changes in the intensity of solar radiation and outdoor temperature cause fluctuations in the temperature of the upper layers of the soil. The depth of penetration of daily fluctuations in the temperature of the outside air and the intensity of the incident solar radiation, depending on the specific soil and climatic conditions, ranges from several tens of centimeters to one and a half meters. The depth of penetration of seasonal fluctuations in the temperature of the outside air and the intensity of the incident solar radiation does not, as a rule, exceed 15–20 m.
The 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, in the general case, both in liquid and in solid and gaseous phases simultaneously. 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, the temperature regime of the upper layers of the soil is influenced by the moisture of atmospheric precipitation, as well as groundwater.
The characteristic features of the thermal regime of ground heat collection systems as a design object 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 effects on the environmental system (atmosphere and soil mass located outside the zone of thermal influence of the ground 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 for some new function, 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 sinks (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 velocity and thermal regimes, the structure and arrangement of soil layers, the “thermal” background of the Earth, atmospheric 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 the joint account of which in a strict formulation of the problem is practically impossible.
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 regulatory documents do not give a complete description of the 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 combinations with other low-potential natural heat sources, assessment of their (sources) temperature level in the annual cycle, it is necessary to involve more complete climatic data, given, 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, subsoil - clay) |
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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 made of polyethylene pipes with a diameter of 0.05 m and a length of 400 m;
– 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 the values and isolines of 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 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 that 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 environmental 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.