The efficiency of using a heat pump in the construction of buildings on sites with a sunken roof of icy permafrost

Gorbunova Alina ORCID: 0000-0002-2939-7609 Postgraduate student of the Department of Geocryology; Faculty of Geology; Lomonosov Moscow State University 119991, Russia, Moscow, Leninskie gory str., 1 Gorbunova.alina2000@yandex.ru Gunar Alexey ORCID: 0000-0003-2878-0849 PhD in Technical Science Associate Professor of the Department of Geocryology; Faculty of Geology; Lomonosov Moscow State University 119991, Russia, Moscow, Leninskie gory str., 1 gunar_91@mail.ru Ozeritskiy Konstantin Vladimirovich ORCID: 0000-0003-1970-6735 PhD Student; Department of Civil Engineering; University of Calgary (Canada) AB T2N 1N4, Canada, Alberta, Calgary, 622 College Place Street ozeritskiy.k@gmail.com

Balihin Ermolai Postgraduate student of the Department of Geocryology; Faculty of Geology; Lomonosov Moscow State University 119991, Russia, Moscow, Leninskie gory str., 1 ermolaybalihin@mail.ru

Introduction

Climate warming has a significant impact on existing buildings and structures in the cryolithozone. So, for buildings in the north of the European part of the 60 – 80s. buildings according to the first principle of construction on MMG, the reduction in the bearing capacity of the pile foundation reaches 25% ^[1]. The districts of Salekhard, Nadym and Novy Urengoy are characterized by a decrease in soil bearing capacity by 15-25% ^[2]. For the territory of Vorkuta, with a stable increase in air temperature with a trend of 0.04 °C/year, the trend of change in the average temperature of MMG to a depth of 10 m is from 0.02 to 0.04 ° C / year from minus 1.5 ° C to minus 0.5 and plus 0.5 ° C, respectively, which will lead to a decrease in load-bearing capacity by more than 50% ^[3]. It is noted in ^[4] that a decrease in the bearing capacity of the soils of the foundations of buildings and structures in municipalities of the Arctic zone of the Russian Federation is projected in the range from 2 to 100% by 2050 and if current climatic trends continue to persist, the dangerous consequences of the degradation of permafrost soils will become inevitable.

Reducing the load-bearing capacity and increasing the amount of precipitation of buildings, due to thawing of MMG, requires the development and implementation of a system for monitoring the condition of foundations at almost all infrastructure facilities at MMG. However, monitoring and timely resolution of emerging problems is not the optimal way to operate facilities. Currently, we are faced with the task of developing and implementing solutions that will ensure the stable operation of facilities in the cryolithozone in conditions of global climate warming. One of such solutions is the use of heat pumps in construction on permafrost soils ^{[5, 6]}.

A heat pump (TN) is a device that allows you to accumulate heat from low—potential sources and transfer it to a heating system with a higher temperature using the work spent ^{[7, 8, 9]}. In the context of climate change, active soil cooling is becoming a promising solution for the foundations of buildings and structures. Ground-based heat pump technologies used in temperate regions as a source of renewable energy must also be adapted for cryolithozone conditions in order to maintain a consistently low ground temperature. The system, which operates on the basis of a heat pump, extracts heat from the ground using geothermal circuits and a coolant. The heat lost through the base of the building can be disposed of, and the ground temperature reduced to the required value.

Experience in the application of heat pumps to the cryolithozone

In practice, the concept of using TN in northern construction was first implemented by the outstanding Norwegian engineer Bjorn Instanes ^[10]. In 1986, in the settlements of Svergruv and Longyerbin, which are located on the island of Svalbard in the zone of continuous permafrost distribution, he equipped the bases of two warehouses (with an area of 900 m2 each) and one store with heat pump cooling units. With their help, the soils of the bases are maintained at a constant temperature of about minus 10 ° C throughout the year.

In 1988, experimental studies in the same direction were conducted in the Canadian settlements of Ross River and Old Crow in the Yukon ^[11]. The cooling systems of the two garages with an area of 350 m2 each consisted of polystyrene insulation, evaporator pipes and a gravelly cushion resting on frozen soils. An aqueous mixture of methyl hydrate with a boiling point of -10 ° C in the evaporator was used as a refrigerant. In both cases, slab foundations were used, which reliably ensured the stability of buildings during 2 years of observation. The compressor's electric drive used cheap energy from a local hydroelectric power plant, which provided significant savings.

In ^{[12, 13]}, a technique for year-round freezing of soils by supplying a coolant cooled by a refrigerating machine to thermoelements placed inside piles is proposed. Thermoelements keep the foundation soils in a frozen state due to thermal energy generated by technological sources located on the foundation (diesel generators, electric motors, etc.). The paper notes the possibility of placing thermoelements inside piles at different depths, which leads to more efficient cooling of the base soils.

A major project on the use of heat pumps was carried out in Longyearbyen, the largest settlement on Svalbard. The 3,400 m2 base is continuously cooled throughout the life of the building. A three-dimensional model simulating the temperature regime of the soil under the cooling plate has been created and verified. The estimated annual operating costs range from NOK 16,000 to NOK 54,000 (from 142.3 to 480.4 thousand rubles), depending on the efficiency of the heat pump and the cooling temperature. A well-functioning monitoring system is an integral component of this technology, allowing you to prevent unnoticed power outages that can lead to an increase in ground temperature by 4 °C within one year. The work notes the effectiveness of the use of heat pumps in areas with permafrost soils and the soil remains frozen even when the cooling system is turned off in winter for three months in a row. This means that the system can be turned off for a certain period of time during the dark season and powered by solar energy during the warm season. Thus, the results indicate the possibility of developing an autonomous cooling system that can be integrated with a renewable energy source such as solar energy to power the system ^[14].

In Russia, the use of heat pumps in construction on permafrost soils is not as widespread as abroad, however, work in this direction is underway and there are both theoretical and computational works ^[15] and field experiments ^[16]. A large number of inventions on designs and methods of using heat pumps in construction on frozen soils are still waiting in the wings for practical use. One of these inventions is a surface foundation with integrated heat pump circuits, the use and effectiveness of which is discussed in detail in this paper.

Relevance of the work performed

The most common method of erecting buildings on permafrost soils, developed and implemented in construction practice in the USSR, is the use of a ventilated underground under the structure. At the same time, the ventilated underground can be used both in areas with permafrost of a merging type and in areas with a sunken roof – in the first case, the underground cools the foundation soils, which increases their bearing capacity, and in the second regulates the amount of heat entering the array, preventing freezing and thawing of the soils under the building. Currently, methods of thermal stabilization using seasonal cooling units are widely used - they can be placed both under buildings and along the contour of the structure. To a lesser extent, geocryological parameter management technologies are used, such as clearing snow around a building or using thermal insulation to reduce seasonal thawing around buildings. However, all of the above construction methods have one common drawback – they are all climate-dependent. With an increase in the average annual air temperature, each of these methods loses its effectiveness. If the warming trend is not broken in the coming years, then after 2-3 decades, in the southern areas of permafrost, these methods will be ineffective and will require additional economic investments.

There is an opinion that the use of freezers to maintain the frozen state of the soil mass for 50-80 years of operation of the structure is initially an excessively expensive solution. However, the combination of a heat pump and modern thermal insulation materials can significantly reduce economic costs. Thermal insulation boards reduce the heat flow from the structure into the ground mass, and the use of a heat pump allows the pumped thermal energy to be converted into an additional heat source in heaters or water supply systems. A patent has been obtained in Russia and a low-depth slab foundation for light buildings is being developed, containing a cold and warm heat pump circuit ^[17]. The cold contour of the plate cools the soils of the base, and the heat withdrawn from the base goes to heat the floor of the building. Heat transfer from a warm circuit to a cold one is minimized by using a layer of thermal insulation (Fig. 1).

Figure 1 - Fragment of the surface foundation module: 1 – the upper part of the reinforced concrete slab; 2– the coil of the heating circuit of the heat pump; 3 – heat insulator; 4 – the lower part of the reinforced concrete slab; 5 – the coil of the cooling circuit of the heat pump ^[17]

In the construction of heavy buildings, the proposed technology will not allow for sufficient bearing capacity of the foundation soils. The solution to this problem is scaling – it is necessary to place the cooling circuit deep enough in the soil mass.

The use of a heat pump in the construction of buildings on sites with a sunken roof of icy permafrost rocks is considered on the example of one of the residential buildings in the city of Salekhard, Yamalo-Nenets Autonomous Okrug. Reinforced concrete piles with a cross section of 30 × 30 cm and a length of 11.0 m are used as the foundations of the structure. The loads transferred to one pile are up to 94.45 tons. The temperature inside the building is plus 20 °C. The duration of the heating period is from = 8 months = 5760 hours. The temperature of the underlying permafrost soils is t ₀ = -0.3 °C. The building has a foundation in the form of a reinforced concrete slab with integrated heating and cooling circuits of a heat pump (TN) in the form of a system of coils made of polyethylene pipes arranged in increments of s = 0.18 m. The outer diameter of the pipes is d _out = 0.06 m, the inner diameter is d _in=0.054 m.

A brief description of the research area

The district is located in the northwestern part of the West Siberian Lowland and is a relatively high-elevation plain of the Ob-Poluisky watershed.

The territory on which the environs of Salekhard and adjacent settlements are located belongs to the Subarctic climatic zone. The average air temperature during the year in Salekhard is -6.2 °C. The coldest month of the year is January with an average monthly air temperature of -24.2 °C, and the warmest is July, when the air warms up to an average of 15 °C. The average height of the snow cover reaches 57 cm (maximum in April).

Table 1 - Climatic characteristics of the research area

The construction site is located on the III lake-alluvial terrace. The geological section of the site is presented from top to bottom:

- up to a depth of 2.3 m - with bulk soils, dusty sands with construction debris;

- up to a depth of 3.7 m - lake-alluvial sands with dusty;

- to a depth of 9.9 m - lake-alluvial clays and loams;

- below 9.9 m - lake-alluvial sands, dusty and fine.

All soils are unsalted, do not contain coarse inclusions and organic substances.

The site is composed of permafrost soils (MMG) of merging and non-merging types with a depth of immersion of the roof of frozen soils from 3.4 to 13.1 m. The average annual soil temperature at a depth of 10 m ranges from plus 0.2 °C to minus 0.3 °C. The depth of seasonal freezing-thawing (SMS - STS) of soils is 1.5 - 2.2 m. Permafrost soils are represented by pulverized and fine icy sands of massive cryotexture, loams of layered cryotexture and clays of layered-mesh cryotexture. The thawed soils of the site are represented by sandy soils - dusty sands of medium density and medium degree of water saturation, soft-plastic loams and refractory clays. The physical and thermophysical characteristics of soils are presented in Table 2.

Table 2 - Physical and thermophysical characteristics of soils

Mathematical modeling of the temperature field

Setting the task

When assigning the calculation area, we were guided by the experience of setting such tasks, according to which the size of the area should exceed the size of the heat source by at least 2.5 times. Based on this, a three-dimensional soil massif with a depth of 70 m, with a plan size of 350.0×250.0 m, was adopted as the calculated area. The grid pitch is assumed to be adaptive from 0.1×0.1×0.1 to 1.0×1.0×1.0 m. The general view of the calculated area is shown in Figure 2. The upper boundary conditions are shown in Table 3.

Figure 2 - General view of the calculation area

Table 3 - Upper boundary conditions based on calibration results

Symbols: T _e – air temperature; α _e – heat transfer coefficient.

Accounting for changes in air temperature

The calculations were performed taking into account global climate change and temperature observations by comparing meteorological series before 1970 with series after 1970 and extrapolating them for the future. The methodology of such an analysis of the average annual air temperature was developed at the Department of Geocryology of Moscow State University in 2000 and named by the authors the methodology of author retrospective analysis ^[18]. The forecast formula (1) consists of the sum of periodic components modeling the natural course of air temperature, one linear component (trend) modeling anthropogenic changes in air temperature after the turn of the year. For the city of Salekhard, the forecast formula looks like this:

(1)

where t is the time in calendar years; T _cp (-6.21) is the average long–term temperature in the base range, ⁰ C; A _j is the amplitudes of harmonics, ⁰ C; ϕ _j is the phases of harmonics, rad; y _j is periods, years, N is the number of harmonics; g is the anthropogenic trend; t _p (1979) is a milestone year.

Calculation results

Based on the results of the work, the soil temperature was estimated at the depth of the pile foundation (11 m) after 50 years of operation of the building, depending on the distance of the cooling circuit from the heat-generating boundary and the operating time of the heat pump. The results of mathematical modeling are shown in Figure 3. The results of calculating the temperature field at the location of the cooling circuit 5 m from the heat-generating boundary at different operating times of the TN are shown in Figures 3-6. To preserve the soil of the base in a frozen state, the operating time of the TN must exceed 2 months, regardless of the distance of the cooling circuit from the heat-releasing boundary and more than 3 months at a distance from the heat-releasing boundary of less than 3.3 meters. To determine the optimal variant of the distance from the heat-generating boundary and the operating time of the TN, an assessment of the values of the bearing capacity on the ground was performed in accordance with SP 25.13330.2020 "SNiP 2.02.04-88 Foundations and foundations on permafrost soils" ^[19]. The calculation results showed that the bearing capacity on the ground is provided for the entire period of operation of the building with a working time of more than 3 months and 3 months at a distance of 4 meters or more from the heat-generating boundary.

Figure 3 - The results of calculating the ground temperature at a depth of 11 m, after 50 years of operation of the building, depending on the distance of the cooling circuit from the heat-generating boundary and the operating time of the TN

Table 4 - Results of calculation of the bearing capacity on the ground, kN

In addition to keeping the ground in a frozen state and ensuring stable operation of buildings and structures in the cryolithozone, heat pumps provide significant economic benefits and contribute to reducing emissions of carbon dioxide and other pollutants, which is important in the context of global climate change and the pursuit of sustainable development. In addition, by Government Decree No. 373 dated March 11, 2023, projects using electric heat pump technology can qualify for preferential financing through special bonds or loans, which can also significantly reduce the initial costs of installing and using TN.

Figure 3 - Temperature field after 50 years of operation when the cooling circuit is located 5 m from the heat-generating boundary and the operating time is 12 months

Figure 4 - Temperature field after 50 years of operation when the cooling circuit is located 5 m from the heat-generating boundary and the operating time is 4 summer months

Figure 5 - Temperature field after 50 years of operation when the cooling circuit is located 5 m from the heat-generating boundary and the operating time is 8 winter months

Figure 6 - Temperature field after 50 years of operation when the cooling circuit is located 5 m from the heat-generating boundary and the operating time is 2 months

Conclusion

The choice of a particular technical solution at the stage of project development should be based on a technical and economic comparison of options, however, dependence on the climate of standard design solutions creates some uncertainty in assessing the reliability of structures throughout their lifetime. The solution to this problem is to include methods of active thermal stabilization of soils in the project. For areas of high embankments, the cooling circuit allows you to create a buffer zone of cyclically freezing and thawing soils, which completely eliminates the heat flow from the building to the buried roof of permafrost. For areas with permafrost of the merging type, heat pumps can be considered as a backup device – if the climatic parameters laid down in the project go beyond the permissible limits, the inclusion of the cooling unit laid down at the project stage will exclude thawing of the base. The economic feasibility of the proposed solution is due to a reduction in operating costs and the almost complete elimination of problems associated with thawing of the frozen base.