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To model temperature fields and for other calculations, it is necessary to know the temperature of the soil at a given depth.
Soil temperature at depth is measured using exhaust soil-depth thermometers. These are planned studies that are regularly carried out by meteorological stations. Research data serves 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 involve using reference books:
- For an approximate determination of temperature, you can use the document TsPI-22. "Transitions railways pipelines." Here, within the framework of the methodology for thermal engineering calculation of pipelines, Table 1 is given, where for certain climatic regions the values of soil temperatures are given depending on the measurement depth. I present this table here below.
Table 1
- Table of soil temperatures at various depths from the source “to help the worker gas industry» still from the times of the USSR
Standard 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 above reference data and then interpolate.
The most reliable option for accurate calculations using ground temperatures is to use data from meteorological services. Some online directories operate on the basis of meteorological services. For example, http://www.atlas-yakutia.ru/.
Here you just need to choose locality, soil type and can be obtained temperature map soil or its data in tabular form. In principle, it’s convenient, but it looks like this resource is paid.
If you know other ways to determine the soil temperature at a given depth, then please write comments.
You may be interested in the following material:
The surface layer of the Earth's soil is a natural heat accumulator. Main source thermal energy entering the upper layers of the Earth - solar radiation. At a depth of about 3 m or more (below the freezing level), the soil temperature practically does not change throughout the year and is approximately equal to average annual temperature outside air. At a depth of 1.5-3.2 m in winter the temperature ranges from +5 to + 7 ° C, and in summer from +10 to + 12 ° C. This heat can prevent the house from freezing in winter, and in summer prevent it from overheating above 18 -20°C
The most in a simple way Using the heat of the earth is the use of a soil heat exchanger (SHE). Under the ground, below the freezing level of the soil, a system of air ducts is laid, which act as a heat exchanger between the ground and the air that passes through these ducts. In winter, the incoming cold air that enters and passes through the pipes is heated, and in the summer it is cooled. With rational placement of air ducts, it is possible to extract a significant amount of thermal energy from the soil with little energy consumption.
You can use a pipe-in-pipe heat exchanger. Internal stainless steel air ducts act here as recuperators.
Cooling in summer
During the warm season, a ground heat exchanger provides cooling of the supply air. Outside air enters through an air intake device into a ground heat exchanger, where it is cooled by the ground. Then the cooled air is supplied through air ducts to the supply and exhaust unit, in which a summer insert is installed instead of a recuperator for the summer period. Thanks to this solution, the temperature in the rooms decreases, the microclimate in the house improves, and energy costs for air conditioning are reduced. Off-season work
When the difference between the outside and inside air temperatures is small, the supply fresh air can be carried out through a supply grille placed on the wall of the house in the above-ground part. During the period when the difference is significant, fresh air can be supplied through the PHE, providing heating / cooling of the supply air. Savings in winter
In the cold season, outside air enters through the air intake device into the PHE, where it is heated and then enters the supply and exhaust unit for heating in the recuperator. Preheating the air in the PHE reduces the likelihood of icing of the recuperator of the air handling unit, increasing effective time use of recovery and minimizes the cost of additional heating of air in a water / electric heater. How are the costs of heating and cooling air calculated?
You can preliminarily calculate the cost of heating air in winter for a room where air is supplied 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, you need to spend 2.55 kW per hour (in the absence of a heat recovery system). Using geothermal system The outside air is heated 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 - you only need to spend 0.714 kW. Over a period of 80 days, 2448 kWh of thermal energy will be consumed, 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 kW * h, and geothermal systems will reduce costs by 1322 or 1102 kW * h.
In the summer, for 60 days, the average daily temperature is about + 20 ° C, but within 8 hours it is within + 26 ° C. The costs for cooling will be 206 kWh, and the geothermal system will reduce costs by 137 kWh.
Throughout the year, the operation of such a geothermal system is assessed using the coefficient - SPF (seasonal power factor), which is defined as the ratio of the amount of thermal energy received to the amount of electrical energy consumed, taking into account seasonal changes air/ground temperatures.
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.
According to materials.
One of the best, most rational methods in the construction of permanent greenhouses is an underground thermos greenhouse.
Using this fact of the constancy of the earth's temperature at depth in the construction of a greenhouse provides enormous savings on heating costs in the cold season, makes maintenance easier, and makes the microclimate more stable..
Such a greenhouse works in the bitterest frosts, allows you to produce vegetables and grow flowers all year round.
A properly equipped in-ground greenhouse makes it possible to grow, including heat-loving plants. southern cultures. There are practically no restrictions. Citrus fruits and even pineapples can thrive in a greenhouse.
But in order for everything to function properly in practice, it is imperative to follow the time-tested technologies used to build underground greenhouses. After all, this idea is not new; even under the Tsar in Russia, sunken greenhouses produced pineapple harvests, which enterprising merchants exported for sale to Europe.
For some reason, the construction of such greenhouses was not found in our country widespread, by and large, it has simply been forgotten, although the design is ideal 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 being a greenhouse covered with polyethylene, but the return from the greenhouse is much greater.
The total internal illumination is not lost from being buried in the ground; 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 can more easily withstand hurricane gusts of wind, it resists hail well, and snow debris will not become a hindrance.
1. Pit
Creating a greenhouse begins with digging a pit. To use the heat of the earth to heat the interior, the greenhouse must be deep enough. The deeper you go, the warmer the earth becomes.
The temperature remains almost unchanged throughout 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 even in winter its value remains positive, usually at middle lane the temperature is 4-10 C, depending on the time of year.
A recessed greenhouse is built in one season. That is, in winter it will be fully able to function and generate income. Construction is not cheap, but by using ingenuity and compromise materials, it is possible to save literally an order of magnitude by making a kind of economical version of a greenhouse, starting from the foundation pit.
For example, do without the use of construction equipment. Although the most labor-intensive part of the work - digging a pit - is, of course, better to give it to an excavator. Manually removing such a volume of soil is difficult and time-consuming.
The depth of the excavation pit must 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 effectively. Therefore, it is recommended not to spare effort and money on deepening 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, the quality characteristics of heating and light reflection deteriorate.
On the sides of the horizon, underground greenhouses must 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
A foundation is poured or blocks are laid around the perimeter of the pit. The foundation serves as the basis for the walls and frame of the structure. It is better to make walls from materials with good thermal insulation characteristics; thermal blocks 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 the walls are connected by a series 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 greenhouses - 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 12 m long.
They are attached to the frame with self-tapping screws; it is better to choose them with a washer-shaped cap. To avoid cracking of the sheet, you need to drill a hole of the appropriate diameter for each self-tapping screw. Using a screwdriver or a regular drill with a Phillips bit, the glazing work moves very quickly. In order to ensure that there are no gaps left, it is good to lay a sealant made of soft rubber or other suitable material along the top of the rafters in advance and only then screw the sheets. The peak of the roof along the ridge needs to be laid with soft insulation and pressed with some kind of corner: plastic, tin, or other 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%, it is covered by excellent thermal insulation performance. It should be taken into account that 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 protect the roof if snow does accumulate.
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 second layer of the roof is covered, as usual, from above.
After completing the work, it is advisable to seal all joints with tape. The finished roof looks very impressive: without unnecessary joints, smooth, without protruding parts.
3. Insulation and heating
Wall insulation is carried out as follows. First you need to thoroughly coat all the joints and seams of the wall with the solution; here you can also use polyurethane foam. Inner side The walls are covered with thermal insulation film.
In cold parts of the country, it is good to use thick foil film, covering the wall with a double layer.
The temperature deep in the soil of the greenhouse is above freezing, but colder than the air temperature necessary for plant growth. Upper layer warmed up 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 with a metal grid or filled with concrete.
In the second case, soil for the beds is poured on top of 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 effective and more comfortable for plants to use combined heating: warm floor + air heating. For good growth, they need an air temperature of 25-35 degrees with a ground temperature of approximately 25 C.
CONCLUSION
Of course, building a recessed greenhouse will cost more and require more effort than building a similar greenhouse of a conventional design. But the money invested in a thermos greenhouse pays off over time.
Firstly, it saves energy on heating. No matter how you heat the winter time an ordinary above-ground greenhouse, 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 a deep greenhouse in winter will be more favorable for plants, which will certainly affect the yield. The seedlings will take root easily, and delicate plants will feel great. Such a greenhouse guarantees a stable, high yield of any plants all year round.
Instead of a foreword.
Smart and friendly people pointed out to me that this case should be assessed only in a non-stationary formulation, due to the enormous thermal inertia of the earth, and take into account annual regime 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 coincidence is 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 the materials are adequate real cases construction practice. The purpose of the article is to obtain a framework understanding of the temperature distribution at the Building-Ground boundary under various conditions.
A little about what needs to be said. Calculated schemes in in this example contain 3 temperature boundaries, the 1st is the internal air of the premises of a 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 some constant value. In this example, the value of this depth is assumed to be 8 m and the temperature is +10 o C. Here someone can argue with me regarding the accepted parameters of the 3rd boundary, but a dispute about the exact values is not the purpose of this article, just as the results obtained are not claim to be particularly accurate and can be linked to a specific design case. I repeat, the task is to obtain a fundamental, framework understanding of the temperature distribution, and to test some established ideas on this issue.
Now let's get straight to the point. So these are the points that need to be tested.
1. The soil under the heated building has a positive temperature.
2. Standard depth of soil freezing (this is more of a question than a statement). Is the snow cover of the ground taken into account when providing data on freezing 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 over time, so for calculation we will take the outside temperature equal to average temperature the coldest month is -10 o C. We take the soil with the reduced lambda = 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, minimums are towards the outer walls. The horizontal zero temperature isoline only touches the projection of the heated room onto the horizontal plane.
Freezing of the soil away from the building (i.e. reaching negative temperatures) occurs at a depth of ~2.4 meters, which is more normative value for a conditionally selected region (1.4-1.6m).
Now let's add 400mm of medium-density snow with lambda 0.3.
Fig.3. Temperature isolines. Scheme with 400mm snow cover.
Isolines of positive temperatures displace negative temperatures outward; under the building there are only positive temperatures.
Ground freezing under snow cover is ~1.2 meters (-0.4 m of snow = 0.8 m of ground freezing). The snow “blanket” significantly reduces the freezing depth (almost 3 times).
Apparently the presence of snow cover, its height and degree of compaction are not constant values, therefore average depth freezing is in the range of the results obtained from 2 schemes, (2.4 + 0.8) * 0.5 = 1.6 meters, which corresponds to the standard value.
Now let's see what happens if they hit very coldy(-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 O With no snow cover.
Negative temperatures crawl under the building, positive temperatures are pressed against the floor of the heated room. In the area of foundations, the soil freezes. At a distance from the building, the soil freezes to ~4.7 meters.
Cm. previous records blog.