RU2748086C1 - Method of thermal stabilization of permafrost soils - Google Patents

Abstract

FIELD: construction.

SUBSTANCE: invention relates to the field of construction, namely to devices for thermal stabilization by freezing soil using refrigeration machines. The method of thermal stabilization of permafrost soils includes shielding the soil from solar radiation and atmospheric precipitation by installing solar energy converters above the surface of a thermally stabilized soil section at a height not less than the local depth of snow cover (SEC. The creation of a layer that blocks the penetration of heat into the soil is carried out by cooling the near-surface layer of the soil using a refrigerating machine, which is fully or partially powered by the energy generated by the SEC. To remove heat by ground probes located in the soil layer no more than the depth of natural thawing, ground probes are placed or cooled gas or artificial snow is sprayed into the gap between the ground surface and the SEC.

EFFECT: prevention of thawing of permafrost soil without external power supply in a warming climate.

1 cl, 2 dwg

Description

Technology area

The invention relates to the field of construction, namely to devices for thermal stabilization by creating, using refrigeration machines, a cooled near-surface layer that blocks the penetration of heat into the soil, solar energy converters (photovoltaic and thermal) for powering these refrigerators and shielding the soil from exposure to solar radiation and liquid atmospheric precipitation (minimizing the supply of external heat).

State of the art

The methods of thermal stabilization of permafrost soils can be divided into the following main groups:

1) passive (screening of solar radiation and liquid atmospheric precipitation);

2) seasonal devices (SOU - thermosyphons, heat pipes, air ducts for heat removal);

3) active (refrigerating machines for heat removal).

A known analogue is the installation of screens on the slopes of the road embankment (patent CN 202850037 U, IPC E01F 7/02), where the proposed screens help to reduce the absorption of heat by the soil by reflecting and absorbing solar radiation and redirecting flows of liquid atmospheric precipitation away from the construction.

The disadvantage is that the heat flux remains uncompensated due to natural convection, which can exceed the shielded part. Such screens give the maximum effect exactly in the area (Tibet) for which they are proposed. For areas with permafrost soils in Russia, a less significant contribution of solar radiation to the thawing of soils is characteristic.

Thermosyphons are widely used for thermal stabilization of soils, the principle of which is based on freezing the soil in the cold season to such a state that it will not defrost in the warm season. A known analogue of such a device is a gravitational heat pipe (patent RU 2387937 C1, IPC F28D 15/02, published 04/27/2010). Thermosiphons are widely used for thermal stabilization of soils. The principle of operation is based on freezing the soil in the cold season to such a state that it will not defrost in the warm season.

The disadvantages of this method have now manifested themselves in connection with the warming of the climate in the Arctic and subarctic, when the actual operating conditions began to differ significantly from the calculated ones: during milder and shorter winters, there is no longer enough cold stored to prevent thawing of the soil during a longer and warmer summer. Since these devices are designed to use a natural source of cold, their use in combination with refrigeration machines, to which they have to resort in critical cases, is extremely ineffective, because the soil is frozen at a depth, and the soil on the surface remains thawed. In addition, thermosyphons create an uneven temperature field, which in modern conditions begins to lead to the emergence of thawed narrow sections between them, through which filtration processes are accelerated, ultimately contributing to the loss of bearing capacity and soil subsidence.

Known analogue - a device for cooling the soil, using the principle of forced convection (patent RU 2110647 C1, IPC E02D 3/115, E02D 19/14, published 10.05.1998). This is a SOU that has all the disadvantages of thermosyphons and, at the same time, requires an external power supply.

Known analogue - installation for freezing soil based on a vortex cooler (patent RU 2109878 C1, IPC E02D 3/115, published 04/27/1998). It differs from the previous analogue in the principle of obtaining cold air, therefore it can be used in the warm season. At the same time, the disadvantages associated with the need for an external energy source and excessive freezing of the soil layer remain.

The main analogue is known - Soil freezing system using a refrigerating machine (patent RU 2435904 C2, IPC E02D 3/115, published on August 27, 2009, Bul. No. 34). Refrigeration units can effectively prevent soil thawing, but their use is economically justified only in emergency cases, because require the supply of power plants and fuel for them. The disadvantages of this particular solution include the fact that the wells for soil probes are vertical, which is not always advisable. The vertical arrangement of the wells leads to the freezing of large volumes of soil, while, in fact, it is only necessary to remove heat from the surface, from where it enters the ground.

Thus, a common disadvantage of convective systems is the ability to work only in the cold season and freezing of the soil layer, when in fact, heat only needs to be removed from the surface layer, and active systems - an additional need for external power supply, while, as a rule, with insufficient energy efficiency when cooling excess soil volume.

The method proposed by the authors combines the shielding of the soil from the supply of heat from the environment, the use of solar energy to supply power to the refrigeration machine (CM), with the aim of greater energy efficiency - minimizing the need for cooling capacity by creating a near-surface blocking layer instead of freezing the soil layer. Unlike natural conditions, when an increase in the intensity of solar radiation (including on the slopes and slopes of the southern exposure) leads to more intensive thawing of the soil, when implementing the proposed method, more heat is removed from the soil, on the contrary. Elements of the proposed method can also be used in areas with existing thermal stabilization systems (most often, thermosyphons) - reducing the thermal load on the ground and feeding the HM, allowing the use of seasonal cooling devices in the warm season.

Disclosure of invention

The objective of the present invention is to eliminate the disadvantages of analogues, namely: insufficient reduction of the heat flow into the ground; the absence or the need to use external energy resources for heat removal in the warm season; heat removal from the depth of the ground, while the upper layers are thawed first. When using the proposed method, improvement is achieved through an integrated approach to solving the problem, namely a combination of: shielding solar radiation and precipitation by solar energy converters (PESI - photoelectric, thermoelectric, thermal); intensification of convective heat removal in the cold season due to a decrease in the height of the snow cover under the PESI; active heat sink using XM; fully or partially autonomous power supply of the XM with the energy generated by the PESI; heat removal not from a large volume of soil at a depth, but from a near-surface layer that blocks the penetration of heat (both from the environment and from artificial structures) into the soil, and this layer can be created both by soil probes and by spraying above the surface cooled gas soil or artificial snow instead of gas; in the presence of seasonal cooling devices in the cooled soil, XM can also be used to cool them in the warm season. Thus, the effect of using the proposed method is achieved by reducing unwanted heat input into the soil and intensifying heat removal from it.

In the absence of direct solar radiation, when the heat flow from the outside is minimal, the proposed method implements a passive mode, and in clear weather, an active mode with positive feedback is realized: with an increase in the intensity of solar radiation, the productivity of the HM increases. This connection also eliminates the problems for the slopes and slopes of the southern exposure, which in natural conditions increases the flux of solar radiation. The vertical surfaces of structures, when the PESI are located on them, in many cases will provide a higher peak power than from horizontal surfaces, since at latitudes north of the 60th parallel the sun rises above 45 degrees above the horizon for a very short time. The thermal inertia of the soil makes it possible to significantly smooth out the unevenness of the energy supply from the sun. However, in some cases, the accumulation of electrical, thermal energy, liquid refrigerant can also be used to ensure continuous and uniform operation of the chiller. The presence of a PESI screen near the soil surface reduces convective heat transfer in summer, and also minimizes snow accumulation under it, which contributes to better freezing of the near-surface soil layer in winter.

Since, first of all, it is required to prevent the penetration of heat into the soil, ground probes, as a rule, are horizontal, located at a depth of no more than the depth of the natural thawing of the ground. Ground probes already existing on the ground can also be used, even if they go below the natural thawing depth of the ground. In some cases, for example, if it is necessary to preserve surface ice massifs, instead of ground probes, a gas cooled to a temperature of no higher than 0 ° C can be used not heavier than air (including air) or artificial snow injected into the gap between the PESI and the protected surface. PESI can be located on an area larger than the heat-stabilized area to provide the required power.

List of figures

FIG. 1 shows a diagram of heat flows in natural conditions and during the implementation of the proposed method.

Figure 2 shows the calculated distribution of soil temperature in depth in natural conditions and when implementing the proposed method.

Implementation of the invention

In the heat flow diagram during the implementation of the proposed method in Fig. 1 (right), the solar radiation flux 1 affects the PESI 2, the convective heat flux 3 from the atmosphere (in some cases, the heat flux from the artificial structure is added to it) is converted into conductive heat flux 4 to the ground heat probes 5 and is carried out by the heat flux 6 into the cold circuit XM 7, driven by the energy generated from the PESI 8, providing the removal of the utilized heat flow 9, and the height of the snow cover 10 and the depth of natural thawing of the soil 11 form the boundaries of the location of the PESI 2 and soil probes 5. In the heat flow diagram without implementing the proposed method, FIG. ... 1 (left) shows the heat fluxes acting on the soil from solar radiation 1 and convection 3 (in some cases, the heat flux from an artificial structure is added to them), forming a conductive heat flux 4 in the soil, which, being uncompensated, leads to soil thawing and loss its bearing capacity.

PESI is installed above the surface of a heat-stabilized soil section at a height not less than the local depth of the snow cover. Variants are possible both with one- or two-axis orientation of the PESI in the sun (for example, according to patent RU 2716361 C1 (IPC F24S 30/00), published 2020-03-11), and without it. The installation option is selected from the condition of ensuring maximum shielding of the heat-stabilized area from direct solar radiation and conversion of its energy. It is advisable to organize the flow of atmospheric precipitation from the surface of the PESI outside the thermostabilized area. Thus, measures of passive protection of the thermally stabilized section from the heat input from the atmosphere are implemented. PESI can be as only photovoltaic (for example, according to patent RU 2287207 C1 (IPC H01L 31/048), published 2006-11-10), thermoelectric or combined thermo-photoelectric (for example, according to patent RU 128396 U1 (IPC H01L 27/142 ), published 2013-05-20), and in combination with thermal (vacuum solar collectors (for example, according to the patent RU 165800 U1 (IPC F24J 2/05; F24J 2/20; F24J 2/48), published 2016-11 -10), including concentrators (for example, according to the patent RU 179500 U1 (IPC F24J 2/05; F24J 2/06; F24J 2/32), published 2018-05-16), flat collectors (for example, according to the patent RU 195335 U1 (IPC F24S 10/70), published 2020-01-23)) - in the case of using an absorption CM (for example, according to patent RU 2224189 C2 (IPC F25B 15/04; F25B 15/00; F25B 15/10; F25B 33/00), published 2004-02-20).

The low coefficient of performance (COP) of absorption CMs relative to vapor compression (approximately 2.0 versus 4.0) is compensated for by the higher efficiency of thermal conversion of solar radiation energy than photovoltaic (0.6 versus 0.2). As a result, the product of KOP and efficiency is 1.5 times higher: 1.2 versus 0.8 (indicative values for industrial designs). It is also worth noting that with an increase in the ambient temperature, the efficiency of photovoltaic converters in most cases decreases, and that of thermal converters increases. However, some photovoltaic converters are good at converting diffuse solar radiation, which is not converted into thermal radiation, which must be taken into account when implementing the method in a specific area.

The cold contour of the chiller (regardless of the principle of its operation) is located within the thermostabilized section parallel to the soil surface (most often horizontally) at a depth determined by the calculation, based on local conditions and technical requirements, as a rule, in the range from 10 to 50 cm. the shallower the depth of the ground probes, the greater the heat flux they account for and the greater the cooling capacity of the system should be. To reduce the amount of earthwork, thermal insulation can be added on top of shallow soil probes. The greater the depth of the soil probes, the greater the depth of soil thawing. The location of soil probes below the depth of natural thawing is impractical. The use of shallow soil probes is especially important for rocky underlying soils. If the use of ground probes even of shallow occurrence is impossible, it is technically difficult, or it is necessary to preserve the surface ice massif, instead of ground probes, cooled gas no heavier than air or artificial snow can be used, injected into the gap between the PESI and the protected surface, creating a super-surface blocking layer. If measures for thermal stabilization of the soil have already been taken at the site for the implementation of the proposed method, in order to minimize costs, it is advisable to first consider the possibility of using soil probes already existing on the ground (for example, thermosiphons) instead of or in conjunction with the creation of a near-surface blocking layer.

Heat from the hot circuit of the XM is discharged in one or more of the following ways: into the atmosphere due to natural or forced convection; into the ground or into a body of water outside the thermostabilized area; utilized for the needs of heat supply; used in the second cascade of HM. The performance of the chiller is coordinated with the output power of the PESI. To ensure greater uniformity of the work of the chiller, in some cases, the accumulation of electrical and thermal energy is used; accumulation of refrigerant so that its condensation occurs during colder nights in order to increase the efficiency factor of the XM.

Implementation example

Soil probes with a diameter of 25 mm are placed parallel to the surface of the thawed soil at a depth of 20 cm in 20 cm increments using a cable layer. Cooling of the coolant based on a propylene glycol solution in water in the ground loop is carried out from minus 4 ° C to minus 10 ° C using a vapor compression heat pump (KOP 4.0) with heat dumping into the atmosphere. The calculated value of the heat transfer coefficient for soil probes is 45.7 W / (m ² K). The heat pump is powered from photovoltaic modules (solar panels) with an efficiency of 15%, located at a height of 50 cm above the ground surface level, after coordinating the electrical parameters (converting direct current into alternating current by an inverter). The calculation of the steady-state temperatures in the soil as a result of the application of the proposed method in the conditions of the city of Yakutsk is shown in Fig. 2: 12 - natural conditions, January; 13 - natural conditions, July; 14 - the proposed method, January; 15 - proposed method, July. The results of this calculation show that the use of the proposed method allows the soil to be kept in a frozen state throughout the year without external power supply, including a decrease in temperature at shallow depths in winter due to the minimization of snow cover under the PESI. Estimation of the cost of this implementation of the proposed method per unit area of the thermally stabilized surface shows that it is comparable to the use of thermosyphons.

If installation of ground probes is difficult (for example, rocky or coarse soil), impossible or impractical (for example, it is necessary to preserve surface ice), a blocking layer can be created above the ground surface. For example, for thermal stabilization of the glacier, unlike the previous example, ground probes are not used; at the outlet of the heat pump, air cooled to a temperature of minus 10 ° C is obtained, which is then blown into the gap between the solar panels and the ground surface through air ducts laid along the ground surface. For windy areas, on slopes and in other conditions associated with intensive air mass transfer, this method may be ineffective. Then, to create a more effective barrier layer, artificial snow generators are used, which spray it instead of cooled air in the gap between the ground surface and solar panels. Thus, in addition to more intense heat removal in comparison with cooled air when melting snow, a heat-insulating layer is also created, which reduces convective heat transfer into the ground. At the same time, snow is less sensitive to the movement of air masses than cooled air. To create artificial snow, both water can be taken from local reservoirs and streams, and precipitation collected from solar panels and other surfaces of structures. Collected precipitation usually requires less water treatment than natural water, but the amount of collected precipitation may not be sufficient to produce artificial snow in the amount of solar energy.

If there are already SOUs in the soil, the CM can be used to cool them in the warm season. For example, a coil heat exchanger is located on the condensing (aboveground) part of the thermosyphon, which allows it to be cooled in the warm season, thus extending the period of use of the LDS. At the same time, in the warm season, it is advisable to provide thermal insulation of the above-ground part of the SCU to reduce the loss of refrigeration capacity.

Claims (1)

A method of thermal stabilization of permafrost soils, including shielding the soil from solar radiation and atmospheric precipitation by installing solar radiation energy converters (PESI) above the surface of a thermally stabilized soil section at a height not less than the local depth of snow cover, creating a layer that blocks the penetration of heat into the soil by cooling the subsurface a refrigerating machine, which is fully or partially powered by the energy generated by the PESI, by removing heat by ground probes located in the soil layer no more than the depth of natural thawing, or by spraying chilled gas or artificial snow into the gap between the soil surface and the PESI.

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