Thermal fields at the Building-Ground boundary. Freezing depth. Influence of the snow cover of the earth. Earth vertical collectors Temperatures of different depths of the Earth
The surface layer of the Earth's soil is a natural heat accumulator. The main source of thermal energy entering the upper layers of the Earth is solar radiation. At a depth of about 3 m or more (below the freezing level), the soil temperature practically does not change during the year and is approximately equal to the average annual temperature of the outside air. At a depth of 1.5-3.2 m, in winter the temperature is from +5 to + 7 ° C, and in summer from +10 to + 12 ° C. This warmth can prevent the house from freezing in winter, and in summer it can prevent it from overheating above 18 -20°C
The simplest way to use the heat of the earth is to use a soil heat exchanger (SHE). Under the ground, below the level of soil freezing, a system of air ducts is laid, which act as a heat exchanger between the ground and the air that passes through these air ducts. In winter, the incoming cold air that enters and passes through the pipes is heated, and in summer it is cooled. With the rational placement of air ducts, a significant amount of thermal energy can be taken from the soil with low energy costs.
A tube-in-pipe heat exchanger can be used. Internal stainless steel air ducts act here as recuperators.
Cooling in summer
In the warm season, the ground heat exchanger provides cooling of the supply air. Outside air enters through the air intake device into the ground heat exchanger, where it is cooled by the ground. Then the cooled air is supplied by air ducts to the supply and exhaust unit, in which a summer insert is installed instead of a heat exchanger for the summer period. Thanks to this solution, the temperature in the rooms decreases, the microclimate in the house improves, and the cost of electricity for air conditioning is reduced. Off-season work
When the difference between the temperature of the outdoor and indoor air is small, fresh air can be supplied through the supply grill located on the wall of the house in the above-ground part. In the period when the difference is significant, the fresh air supply can be carried out through the PHE, providing heating / cooling of the supply air. Savings in winter
In the cold season, outside air enters the PHE through the air intake, where it warms up and then enters the supply and exhaust unit for heating in the heat exchanger. Air preheating in the PHE reduces the possibility of icing on the heat exchanger of the air handling unit, increasing the effective use of the heat exchanger and minimizing the cost of additional air heating in the water / electric heater. How are heating and cooling costs calculated?
You can pre-calculate the cost of air heating in winter for a room where air enters at a standard of 300 m3 / hour. In winter, the average daily temperature for 80 days is -5 ° C - it needs to be heated to + 20 ° C. To heat this amount of air, 2.55 kW per hour is needed (in the absence of a heat recovery system). When using a geothermal system, the outdoor air is heated up to +5 and then it takes 1.02 kW to heat the incoming air to a comfortable level. The situation is even better when using recuperation - it is necessary to spend only 0.714 kW. Over a period of 80 days, 2448 kWh of thermal energy will be spent, respectively, and geothermal systems will reduce costs by 1175 or 685 kWh.
In the off-season for 180 days, the average daily temperature is + 5 ° C - it needs to be heated to + 20 ° C. The planned costs are 3305 kWh, and geothermal systems will reduce costs by 1322 or 1102 kWh.
During the summer period, for 60 days, the average daily temperature is around +20°C, but for 8 hours it is within +26°C. Cooling costs will be 206 kWh, and the geothermal system will reduce costs by 137 kWh.
Throughout the year, the operation of such a geothermal system is evaluated using the coefficient - SPF (seasonal power factor), which is defined as the ratio of the amount of heat received to the amount of electricity consumed, taking into account seasonal changes in air / ground temperature.
To obtain 2634 kWh of thermal power from the ground per year, the ventilation unit consumes 635 kWh of electricity. SPF = 2634/635 = 4.14.
By materials.
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 a 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 internal 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.
To model temperature fields and for other calculations, it is necessary to know the soil temperature at a given depth.
The temperature of the soil at depth is measured using exhaust soil-deep thermometers. These are planned studies that are regularly carried out by meteorological stations. Research data serve as the basis for climate atlases and regulatory documentation.
To obtain the soil temperature at a given depth, you can try, for example, two simple methods. Both methods are based on the use of reference literature:
- For an approximate determination of temperature, you can use the document TsPI-22. "Railway crossings by pipelines". Here, within the framework of the methodology for the heat engineering calculation of pipelines, Table 1 is given, where for certain climatic regions, soil temperatures are given depending on the depth of measurement. I present this table below.
Table 1
- Table of soil temperatures at various depths from a source "to help a gas industry worker" from the times of the USSR
Normative freezing depths for some cities:
The depth of soil freezing depends on the type of soil:
I think the easiest option is to use the reference data above and then interpolate.
The most reliable option for accurate calculations using ground temperatures is to use data from the meteorological services. On the basis of meteorological services, some online directories work. For example, http://www.atlas-yakutia.ru/.
Here it is enough to select the settlement, the type of soil and you can get a temperature map of the soil or its data in tabular form. In principle, it is convenient, but it seems that this resource is paid.
If you know more ways to determine the soil temperature at a given depth, then please write comments.
You may be interested in the following material:
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 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.
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 can reduce 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, it was consistently warm in the kindergarten - 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, in our harsh Siberian conditions it has shown itself to be quite effective.”
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 facility 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, will 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, would 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 last year the Ambassador Extraordinary and Plenipotentiary of the Kingdom of Denmark came to the all-Russian meeting 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 budget 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.