WO2022075889A1

WO2022075889A1 - Method for thermally stabilizing permafrost soils

Info

Publication number: WO2022075889A1
Application number: PCT/RU2021/050325
Authority: WO
Inventors: Елизавета Сергеевна ШАРАБОРОВА; Егор Юрьевич ЛОКТИОНОВ; Таисия Васильевна ШЕПИТЬКО
Original assignee: Общество С Ограниченной Ответственностью "Пермафрост"
Priority date: 2020-10-07
Filing date: 2021-10-04
Publication date: 2022-04-14
Also published as: RU2748086C1; CN116635590A

Abstract

The invention relates to the field of building, and more particularly to devices for carrying out thermal stabilization by means of freezing the soil using refrigerating machines and also solar energy converters for powering said refrigerating machines, as well as by shielding the soil against the effect of solar radiation and atmospheric precipitation (minimizing the supply of external heat). In the claimed method, underground probes are arranged parallel to the surface of the soil at a depth less than the natural thaw depth of the soil, thus creating a near-surface barrier layer that prevents the deep penetration of heat from the environment. Unlike natural conditions, in which an increase in the intensity of solar radiation leads to more intensive thawing, the proposed method results in more heat being dissipated from the soil. The technical result of the invention is that of preventing the thawing of permafrost soil without the need for an external energy supply in the context of climate warming.

Description

Method for thermal stabilization of permafrost soils

Technical field

The invention relates to the field of construction, namely to devices for thermal stabilization by creating a cooled surface layer using refrigeration machines, blocking the penetration of heat deep into the soil, solar energy converters (photovoltaic and thermal) to power these refrigeration machines and screening the soil from the effects of solar radiation and liquid atmospheric precipitation (minimizing the supply of external heat).

State of the art

Methods for thermal stabilization of permafrost soils can be divided into the following main groups:

1) passive (shielding of solar radiation and liquid precipitation);

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

3) active (refrigerators for heat removal).

A known analogue is the installation of screens on the slopes of the road embankment (patent CN 202 850 037 U, IPC E01F 7/02), where the proposed screens help to reduce the absorption of heat by the soil due to the reflection and absorption of solar radiation and redirection away from the construction of streams of liquid atmospheric precipitation.

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

Thermosiphons are widely used for thermal stabilization of soils, the principle of operation of which is based on freezing the soil in the cold season to such a state that it will not thaw in the warm season. An analogue of such a device is known - a gravitational heat pipe (patent RU 2 387 937 C1, IPC F28D 15/02, published 27.04.2010). Thermosyphons 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 thaw in the warm season.

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SUBSTITUTE SHEETS (RULE 26) The disadvantages of this method have now become apparent 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, which they have to resort to in critical cases, is extremely inefficient, because. the soil at depth is frozen, and the soil at the surface remains thawed. In addition, thermosyphons create an uneven temperature field, which in modern conditions begins to lead to the appearance of thawed narrow sections between them, through which filtration processes are accelerated, ultimately contributing to the loss of bearing capacity and subsidence of the soil.

Known analogue - a device for cooling the soil, using the principle of forced convection (patent RU 2 110 647 C1, IPC E02D 3/115, E02D 19/14, published 10.05.1998). This is a SDA that has all the shortcomings of thermosiphons and at the same time requires an external power supply.

An analogue is known - a plant for freezing soil based on a vortex cooler (patent RU 2 109 878 C1, MIC E02D 3/115, published 27.04.1998). It differs from the previous analogue in the principle of obtaining cold air, therefore it can also 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 remain.

The main analogue is known - a system for freezing soils using a refrigeration machine (patent RU 2 435 904 C2, MIK E02D 3/115, published 27.08.2009, Bull. No. 34). Refrigeration units can effectively prevent soil defrosting, but their use is economically justified only in emergency cases, because. require the delivery of power plants and fuel for them. The disadvantages of this particular solution include the fact that wells for ground probes are vertical, which is not always advisable. The vertical location 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 soil.

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

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SUBSTITUTE SHEETS (RULE 26) The method proposed by the authors combines screening of the soil from the supply of heat from the environment, the use of solar energy to power the refrigeration machine (CM), with the aim of greater energy efficiency - minimizing the need for cooling capacity by creating a near-surface barrier layer instead of freezing the soil thickness. Unlike natural conditions, when an increase in the intensity of solar radiation (including on the slopes and slopes of southern exposure) leads to more intense thawing of the soil, when implementing the proposed method, more heat is removed from the soil. The elements of the proposed method can also be used in areas with already existing thermal stabilization systems (most often, thermosyphons) - reducing the thermal load on the soil and feeding the cold water, allowing the use of seasonal cooling devices in the warm season.

Disclosure of invention

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

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SUBSTITUTE SHEETS (RULE 26) 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 implemented: with an increase in the intensity of solar radiation, the productivity of the cold storage machine increases. Such a connection also eliminates problems for south-facing slopes and escarpments, on which solar radiation flux increases under natural conditions. Vertical surfaces of structures, when placed on them, will in many cases provide more 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 uneven 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 CM. The presence of a PESI screen near the ground 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 deep into the soil, soil probes, as a rule, are horizontal, located at a depth not exceeding the depth of natural thawing of the soil. Ground probes that already exist on the ground can also be used, even if they go below the depth of natural thawing of the ground. In some cases, for example, if it is necessary to preserve surface ice massifs, instead of ground probes, a gas not heavier than air (including air) cooled to a temperature not higher than 0°C or artificial snow can be used, 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

In 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 under natural conditions and when implementing the proposed method.

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SUBSTITUTE SHEETS (RULE 26) Implementation of the invention

In the diagram of heat flows during the implementation of the proposed method, Fig. 1 (on the right) solar radiation flux 1 affects PESI 2, convective heat flux 3 from the atmosphere (in some cases, the heat flux from an artificial structure is added to it) is converted into conductive heat flux 4 to ground thermal probes 5 and is carried out by heat flux 6 into the cold HM circuit 7, driven by the energy S generated from PESI, 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 placement of PESI 2 and soil probes 5. In the diagram of heat flows without implementing the proposed method, Fig. . 1 (left) shows the heat fluxes from solar radiation 1 and convection 3 affecting the soil (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 defrosting of the soil and loss of its bearing capacity.

PESI is installed above the surface of the thermally stabilized soil area at a height not less than the local depth of the snow cover. Variants are possible both with one- or two-axis orientation of PESY to the sun (for example, according to patent RU 2 716 361 C1 (MIC F24S 30/00), published on 2020-03-11), and without it. The installation option is chosen from the condition of ensuring maximum shielding of the heat-stabilized area from direct solar radiation and the conversion of its energy. It is expedient to organize the runoff of atmospheric precipitation from the surface of the PESI outside the thermally stabilized area. Thus, measures of passive protection of the thermally stabilized area from the influx of heat from the atmosphere are implemented. PESI can be either photovoltaic (for example, according to patent RU 2 287 207 C1 (MIC H01L 31/048), published 2006-11-10), thermoelectric or combined thermo-photovoltaic (for example, according to patent RU 128396 U1 (MIC H01L 27 /142), published 2013-05-20), and in combination with thermal (vacuum solar collectors (for example, according to patent RU 165800 U1 (MIC F24J 2/05; F24J 2/20; F24J 2/48), published 2016 -11-10), including with concentrators (for example, according to patent RU 179500 U1 (MIC F24J 2/05; F24J 2/06; F24J 2/32), published 2018-05-16), flat collectors (for example, according to patent RU 195335 U1 (MIC F24S 10/70), published 2020-01-23)) - in the case of using absorption HM (for example, according to patent RU 2 224 189 C2 (IPC F25B 15/04; F25B 15/00; F25B 15/10; F25B 33/00), published 2004-02-20).

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SUBSTITUTE SHEETS (RULE 26) The low coefficient of performance (COP) of absorption CMs relative to vapor compression ones (about 2.0 versus 4.0) is compensated by a higher efficiency of thermal conversion of solar radiation energy than photovoltaic (0.6 versus 0.2). As a result, the product of COP and efficiency is 1.5 times higher: 1.2 versus 0.8 (guide values for industrial designs). It is also worth noting that with an increase in the ambient temperature, the efficiency of photoelectric converters in most cases decreases, while thermal ones increase. 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 particular area.

The cold circuit of the HM (regardless of the principle of its operation) is located within the thermally stabilized area parallel to the soil surface (most often, horizontally) at a depth determined by calculation based on local conditions and technical requirements, as a rule, in the range from 10 to 50 cm. the lower the depth of the ground probes, the greater the heat flux they have, and the greater should be the cooling capacity of the system. To reduce the volume of earthworks, thermal insulation can be additionally laid on top of shallow ground probes. The greater the depth of the ground probes, the greater the depth of soil thawing. The location of ground probes below the depth of natural thawing is impractical. The use of shallow ground probes is especially important for rocky underlying soils. If the use of ground probes, even shallow ones, is impossible, technically difficult, or it is necessary to preserve the surface ice mass, instead of ground probes, cooled gas not heavier than air or artificial snow can be used, injected into the gap between the PESI and the surface to be protected, creating an above-surface barrier layer. If at the site for the implementation of the proposed method measures have already been taken to thermally stabilize the soil, 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 together with the creation of a near-surface barrier layer.

The heat from the HM hot loop is discharged by one or more of the following methods: to the atmosphere due to natural or forced convection; into the ground or into a reservoir outside the thermally stabilized area; disposed of for the needs of heat supply; used in the second cascade of XM. The performance of the HM will be matched with the output power of the PESI. In some cases, to ensure greater uniformity of the operation of the HM, the accumulation of electrical and thermal energy is used;

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SUBSTITUTE SHEETS (RULE 26) storage of refrigerant so that it condenses during colder nights to improve the performance factor of the XM.

Implementation example

Ground probes with a diameter of 25 mm are placed parallel to the surface of the thawed soil at a depth of 20 cm with a step of 20 cm using a cable layer. Cooling of the coolant based on a solution of propylene glycol in water in the ground circuit is carried out from minus 4°C to minus 10°C using a vapor compression heat pump (COP 4.0) with heat discharge into the atmosphere. The calculated value of the heat transfer coefficient for ground probes is 45.7 W/(m ² K). The heat pump is powered by photovoltaic modules (solar panels) with an efficiency of 15%, located at a height of 50 cm above the ground surface, after matching the electrical parameters (conversion of direct current to alternating current by an inverter). The calculation of steady-state temperatures in the soil as a result of applying 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 - the proposed method, July. The results of this calculation show that the use of the proposed method makes it possible to keep the soil in a frozen state throughout the year without external energy supply, including a decrease in temperature at shallow depths in winter due to minimization of snow cover under PESI. Estimation of the cost of this implementation of the proposed method per unit area of the heat-stabilized surface shows that it is comparable to the use of thermosyphons.

If the installation of ground probes is difficult (for example, rocky or coarse-grained soil), impossible or impractical (for example, it is necessary to preserve surface ice), a barrier layer can be created above the ground surface. For example, for thermal stabilization of a glacier, in contrast to the previous example, soil 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 soil surface through air ducts laid along the soil surface. For windy areas, on slopes and in other conditions associated with intensive air mass transfer, this method may not be effective. Then, to create a more effective barrier layer, artificial snow generators are used, spraying it instead of cooled air in the gap between the soil surface and solar panels. Thus, in addition to a more intense heat removal compared to cooled air during snow melting, it also creates

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SUBSTITUTE SHEETS (RULE 26) a heat-insulating layer that reduces convective heat transfer to 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 from local reservoirs and streams, and precipitation collected from solar panels and other surfaces of structures can be taken. Collected precipitation generally requires less water treatment than naturally occurring water, but the amount of collected precipitation may not be sufficient to produce artificial snow in the amount provided by solar energy.

If there are already SOAs in the ground, cold storage can be used to cool them during the warm season. For example, a serpentine heat exchanger is located on the condensation (above-ground) part of the thermosiphon, which allows it to be cooled in the warm season, thus extending the period of use of the SOU. At the same time, in the warm season, it is advisable to provide thermal insulation of the above-ground part of the SOU to reduce the loss of cooling capacity.

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SUBSTITUTE SHEETS (RULE 26)

Claims

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

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