Low Air Temperature Research Articles

The advantage of running a direct expansion CO2 heat pump with solar-and-air simultaneous heat sources: experimental and numerical investigation

Dual-source solar-air heat pumps represent a promising solution for overcoming the limitations associated with single-source utilization, thereby enhancing heat pump performance. However, running the heat pump by alternatively employing the more advantageous source requires the integration of a controller capable of continuously monitoring and predicting the heat pump's performance in response to dynamic environmental and operational variables. Even so, a selective alternate operation does not allow to get the maximum possible performance from the use of the two heat sources. A different approach to address this challenge is the simultaneous utilization of the two sources, by properly combining two evaporators in the CO2 circuit. This paper presents an experimental investigation of a dual-source heat pump using CO2 as refrigerant, which can operate in three different evaporation modes: air-mode (using a finned-coil evaporator), solar-mode (using a photovoltaic-thermal PV-T evaporator), and simultaneous-mode (using both the evaporators simultaneously). The novel solution presented here does not require to split the refrigerant flow rate between the two evaporators and at the same time it solves the problem of possible maldistribution at the inlet of the evaporators. Experimental data indicate that the heat pump operating in simultaneous-mode allows to increase the evaporation pressure and the coefficient of performance compared to operation in air-mode or solar-mode. The measurements have been employed for validating a model of the system, capable of predicting steady-state and dynamic performance under various environmental and operational conditions. Simulation results show that the simultaneous-mode operation can be outperformed by the solar-mode only at high irradiance and low air temperature, when the evaporation temperature gets higher than the air temperature. Finally, the impact of the number of PV-T collectors and solar irradiance on the heat pump performance has been simulated and discussed. On this regard, the simultaneous use of the two heat sources adds more flexibility to the system and its design, because even the availability of a small solar area can contribute enhancing the performance over the mere air source heat pump.

Examination of Operational Methods for a Low-Temperature Aquifer Thermal Storage Air Conditioning System Based on Operational Performance and Considerations of Thermal Storage and Pumping Volume Balance

Aquifer Thermal Energy Storage (ATES) systems are garnering attention as high-efficiency air conditioning technologies that contribute to the realization of a carbon-neutral society. This study focuses on an ATES system constructed in Japan, characterized by its complex geological conditions and thin aquifer layers. Detailed actual performance data measured over four years are presented, and performance analysis results show that a COP of 5 was achieved for the overall building cooling and heating system. In addition, the study provides a detailed analysis of the imbalance in heat quantity, remaining heat storage, and other factors based on actual data, identifying operational issues and summarizing specific improvement measures. Finally, as a proposal for a sustainable operational method to balance the cumulative heat storage and pumping volume of cold and hot water, specific operational procedures are summarized, including setting the return water temperature for the next season based on the dimensionless average pumping temperature of the previous season, and a flowchart is presented. There is no prior research that shows such specific operational procedures, and this can be considered an important achievement of this study.

The diffusion process of different moisture sources in the radiant cooling room and the condensation risk assessment

The condensation phenomenon severely limits the widespread application of radiant cooling systems. Although a series of anti-condensation methods have been proposed, it is more important to explore the diffusion process of moisture sources that cause condensation in the radiant cooling room, only in this way can targeted anti-condensation measures be carried out. In this regard, the numerical simulation method is used to explore the diffusion process of additional moisture caused by the increase in occupancy rate and window opening behavior, while monitoring the state of the air on the surface of the radiant terminal (ASRT) to determine whether condensation occurs and evaluate the condensation risk. The results indicate that the moisture emitted by the increased personnel will diffuse along the direction of their surface hot air flow when the occupancy rate increases, and the moisture in the outdoor air will diffuse along its flow direction after the window is opened. However, the change of moisture is accompanied by the change of heat and the condensation phenomenon will not occur first in the place with a large humidity ratio, but rather in areas with lower surface air temperatures in these two situations. In addition, condensation will not occur in all cases after the occupancy rate increases, and the minimum condensation time is still more than 20 min when the occupancy rate is maximum, which has sufficient time for human intervention to avoid condensation. Condensation occurs in all cases after the window is opened, with a minimum condensation time of 68 s. Window opening behavior carries a high condensation risk and should be avoided during the operation of the radiant cooling system. This study can provide a basis for adopting corresponding anti-condensation strategies.

Effect of vitamin and mineral complex enrichment of diet on physiological parameters and behavioral reactions of laboratory animals in experimental conditions of the Arctic Zone

Nutrition optimization by providing workers with specialized products enriched with vitamin and mineral components has great importance in solving problems in the health condition of workers in the Arctic zone. Objective: to evaluate the effect of vitamin-mineral complex (VMC) containing vitamins (А, D, В1, В2, В3, В6, В12 and C) and minerals (Fe, I, Se) on physiological indicators and indicators of orientative-research behavior of laboratory animals which characterize the adaptation process. Object of the study: white laboratory rats of «Wistar» type, organized into 4 groups: two experimental groups, kept in conditions of low air temperature (4–6 °C) and unusual photoperiodism, which received VMC with the diet, and two control groups, where animals did not receive VMC. One of them was maintained under «Northern Territories» conditions, the second — under comfortable conditions (usual photoperiodism and temperature +22…+24 °C). Indicators of body weight and temperature and functional activity of the nervous system were studied using the «Open Field» method. Statistical analysis was performed using standard application programs Statistica 10.0 with calculation of median (Me) and percentile interval. Acceptable level of reliability of the results was considered to be the probability of differences of 95 % and higher (p ≤0.05). The animals of the experimental groups showed constancy of temperature balance, statistically significantly lower anxiety level indicators, higher level of emotional activity compared to the animals of the control group. The gender peculiarities of anxiety indices were revealed, which were more pronounced in males, whose research activity was increased in the conditions of «polar night» and sharply decreased in the conditions of «polar day». The obtained stress-protective effect of intake of VMC containing daily norm of vitamins (C, Е, В1, В2, В3, В6, В9, В12, D3, carotene) and minerals (zinc, iron, iodine, selenium) indicates the safety and high significance of the use of VMC in the diet of shift workers in the Arctic zone.

A New Assessment of High Punctual Solar Trackers Effects on Poultry Welfare in Agrivoltaïc Open-Air Runs

Open-air poultry farming is currently developing with the increasing society demand for livestock farming better considering animal welfare. Outside animal comfort and open-air runs exploration could be enhanced by shelters such as trees or photovoltaic (PV) structure. The aim of this study is to confirm previous results to evaluate (i) the microclimates generated under high punctual PV trackers, (ii) the effect on laying hens comfort, (iii) the use of panels shadow area by hens. In three experimental sites, microclimates were studied and laying hens were counted in a control area, under such PV tracker and under a tree. Results showed that PV trackers, as big trees, lowered summer soil and air temperatures, radiation and lightness, decreased the occurrences of stress situations for hens, and that more hens were counted under trackers than in a control area. These results may help for optimizing such agrivoltaïc system and the hens welfare by improving the open-air run design with PV structures and vegetation.

Assessment of the impact of dam reservoirs on river ice cover – an example from the Carpathians (central Europe)

Abstract. This paper presents a method for determining the impact of dam reservoirs on the ice cover of rivers downstream of their locations based on a long measurement period (1950–2020) and synthetic-aperture radar (SAR) data. Two rivers and two sets of dam reservoirs located in the Carpathian Mountains (central Europe) were selected for this study. In order to estimate the influence of reservoirs, a logistic regression model was built to describe the relationship between the course of air temperature and the occurrence of ice cover (i.e., total ice cover and border ice) at water gauge cross sections upstream and downstream of the reservoirs. The influence of reservoirs was then defined as the differences between the number of days with ice cover predicted from air temperature and those observed at the water gauge cross sections. Additionally, the extent of the impact of the reservoirs was estimated based on SAR data (Sentinel-1) by identifying river sections downstream of the reservoirs on which total ice cover did not form despite the persistence of very low air temperatures. The study shows that dam reservoirs play an important role in decreasing the incidence of ice cover on rivers. The occurrence of ice cover as a result of reservoir operations could decrease by over 80 % in the sections immediately downstream of the reservoirs. The impact of the reservoir on river ice cover diminishes as the distance from the reservoir increases. Using SAR data, it was estimated that total ice cover did not form in sections 26–60 km downstream of the reservoirs, despite the presence of favorable thermal conditions. Based on the results of the study presented here, it can be assumed that in areas where many dam reservoirs are located, the ice regime of rivers is significantly transformed, which should be taken into account when studying river ice cover. This study also demonstrates that the logistic regression model and SAR data are useful tools for assessing the impact of dam reservoirs on river ice cover.

Physiological and hygienic assessment of a multilayer set of personal protective equipment against low temperatures made using membrane materials

Introduction. Advanced personal protective equipment (PPE) against low temperatures is a high–tech product created using the latest achievements of science, design designs, ergonomic, aesthetic solutions that determine its thermal protection properties. The use of membrane materials in PPE from low temperatures allows them to combine low weight and high thermal protection properties, low air permeability and sufficient vapor permeability necessary to ensure heat and moisture exchange between humans and the environment. The study aims to determine the thermal protection properties of various sets of multilayer workwear and indicators of heat and moisture exchange of volunteers when modeling professional activity at low air temperature. Materials and methods. The authors have conducted a study in a microclimatic chamber with the participation of six practically healthy male volunteers. The researchers have determined the thermal protection properties of multilayer workwear of various configurations in accordance with GOST R 12.4.185-99 and MUK 4.3.1894-04. As test samples, they have used pipes of three different configurations to protect against low temperatures, wind, water, general industrial pollution and mechanical influences, as well as short-term exposure to open fire. The assessment of heat and moisture exchange indicators of various PPE configurations was carried out by specialists when modeling work activities with measurements of the relative humidity level in the layers of the kits and the level of accumulated moisture in each element of the PPE. Results. The authors have determined the values of thermal insulation of the studied materials and the scope of their application. To assess heat and moisture exchange, two samples of PPE were selected, made using membrane and cotton fabrics with similar thermal insulation values. The analysis of the results of studies of moisture exchange indicators showed a slightly lower efficiency of moisture evaporation of the control sample. The indicator of the relative humidity of the air under the shelter space and between the layers of the package of materials and clothing has shown its informative significance for assessing the effectiveness of moisture transfer from the surface of the human body to the environment. Conclusion. The results of a physiological and hygienic assessment of a set of multilayer clothing made of membrane materials have shown that various combinations of its constituent elements make it possible to create kits that provide adequate protection for an employee in a wide range of air temperatures and wind speeds. The use of membrane materials in combination with structural elements (ventilation openings) provides a more efficient transfer of moisture from the skin surface to the external environment, as evidenced by the values of the relative humidity index of the air under the clothing space and between the layers of clothing. Ethics. The study was approved by the local Ethical committee of the Izmerov Research Institute of Occupational Health (Minutes of the meeting No. 6 dated 07/20/12)

Numerical simulation and geometric optimization of a desalination system using solar energy using two-phase mixing method with alumina nanoparticles

This research paper presents 2D numerical simulations of a single-slope solar desalination system from a nanomaterial science perspective, utilizing the two-phase mixture method. Alumina nanoparticles with a volume percentage of 0.1 % are introduced into the water storage tank for thermal analysis. The system features a trapezoid-shaped glass wall to harness solar energy. Key variables studied encompass ambient temperature, glass wall angle, front wall height, glass wall temperature, and air temperature inside the system. Temperature and velocity contours within the system are obtained through numerical simulations. To enrich the analysis, artificial intelligence techniques and response surface methodology are employed to examine the maximum and minimum temperatures. The results indicate that, at medium ambient temperatures, the lowest maximum air temperature inside the desalination system occurs at a front wall height of 0.2 m and a glass angle of 45°. Additionally, the lowest air temperature is observed at an ambient temperature of 10 °C and a front wall height of 0.05 m, corresponding to a glass angle of 27.5°. On the other hand, the highest maximum air temperature is recorded at a front wall height of 0.125 m, with a glass angle of 45° and an ambient temperature of 55 °C. Increasing the ambient temperature from 10 °C to 55 °C for a wall height of 0.2 m leads to a 26.81 °C rise in the air temperature inside the system, equivalent to a 9 % increase. The findings provide valuable insights for optimizing the design and performance of single-slope solar desalination systems integrated with alumina nanoparticles from the perspective of nanomaterial science.

Performance study of low temperature air heated rock bed thermal energy storage system

AbstractOne of the primary types of sensible heat storage systems in drying applications is the packed rock bed. However, to create large‐scale heat storage systems for industrial use, one must comprehend the hydrodynamic and effectiveness of the heat transport mechanism inside the bed. In this study, the thermal storage unit uses river rock as heat storage materials with equivalent particle diameters of 36 mm in bed 1 and 56 mm in bed 2. The rocks were stacked in a truncated cone‐shaped concrete wall section with an average diameter and depth of 1.1 m and 1.3 m, respectively and a volume of 2.32 m3. During the charging phase, two airflow configurations were used, one from the top with an air mass flow rate of 0.753 and 0.332 kg/m2‐s and the other from the bottom with an air mass flow rate of 0.955 kg/m2‐s. During the discharging phase, the entire flow configuration is from the bottom section. It was observed that the mass flow rate and particle equivalent diameter had an important effect on the thermal performance and behaviour of the rock bed during charging and discharging operations. Maximum efficiency was achieved with an airflow configuration provided from the bottom when charging at 0.955 kg/m2.s. Consequently, a sizable quantity of heat or energy (60 MJ) was retained. It was also observed that the relationship between air mass flow rate and particle size was significant, with smaller particles retaining more energy. When comparing bed 1 with bed 2 at this air mass flow rate, bed 1 stored 2.1 times more energy than bed 2. A wind tunnel experiment was used to measure the pressure drop in the packed rock bed. The pressure drop in the bed increases with an increase in particle Reynolds number and decreases with an increase in particle size. Rock bed heat transfer coefficient and Nusselt number were calculated using the correlation that has already been established in the literature smaller particles showed higher heat transfer coefficients and lower Nusselt numbers. This is due to the increase in particle‐to‐particle interaction and larger particle surface areas. For a given Reynolds number, the Nusselt number increases with the size of the rock particle.

Влияние температуры на терморегуляцию и активность соболя (Martes zibellina L. 1758)

The purpose of the study is to identify the dynamics of thermoregulation and activity of the sable in the process of changing environmental temperature. Objectives: assessment of the dynamics of changes in the temperature of sable fur at positive and negative air temperatures; studying the dynamics of physical thermoregulation of sable due to convection and heat radiation; the influence of low air temperatures on the activity of the sable and the body’s costs of supporting vital functions. During long-term observations of sable activity, data were collected on the length of the animals’ daily movement in the Turukhansk and Evenk Districts of the Krasnoyarsk Region under accompanying weather conditions. Data on the dynamics of sable body temperature depending on the ambient temperature were collected in the Krasnoyarsk Flora and Fauna Park “Roev Ruchey”. The temperature period of the surface layer of air is covered from –30 to 30 °C. The pyrometer Testo 830-T1 was used. These studies expand and clarify ideas about the influence of the temperature factor on aspects of sable activity at low air temperatures in natural habitats. Analysis of data on the dynamics of physical thermoregulation of sable due to convection and heat radiation revealed temperature ranges of optimum for sable. Temperature optimum – 0 °C. The range of favorable temperatures for moving sable is from –5 to –25 °C. The largest number of sable tracks was recorded at a tempe¬rature of –15 °C. The data obtained make it possible to estimate the costs of the animal’s body to support vital functions during the winter period. At an air temperature of –15 °C, a sable in a calm state loses 2.78 Kcal per hour. Under favorable conditions, the predator spends 3 to 4 hours searching for food, losing 33,400 calories at a time.

Enhancing control disorder and implementing V2X-Based suppression methods for electric vehicle CO2 thermal management systems

In recent years, the development of electric vehicles (EVs) thermal management systems has underscored the crucial role in ensuring driving safety and optimizing driving range has become increasingly prominent. However, the inherent dynamic complexity of EV operation coupled with automatic control systems, can sometimes lead to unstable behavior, resulting in performance degradation and safety risks for compressors and batteries. To effectively address this issue, an evaluation was conducted on the dynamic control characteristics of an EV thermal management system utilizing CO2 as the refrigerant in this study. Through mathematical modeling and experimental analysis, the erratic nature of the dynamic thermal process was first identified. The underlying reasons were elucidated, focusing on system control characteristics and intrinsic mechanisms. It was found that control disorder could induce abnormal actions in thermal management system components like compressors and expansion valve, leading to significant performance decline and issues such as liquid carryover in compressor suction. Furthermore, specific control disorder regions of CO2 heat pumps for EVs were delineated, providing a framework for assessing the likelihood of system control disorder. Notably, control disorder was more likely to occur under conditions of low indoor air flow rate, high ambient temperature, and low supply air temperature. Given the widespread nature of this issue and the lack of suitable solutions, two control disorder suppression schemes were developed using V2X technology and validated through simulation. Results showed that adoption of V2X communication technology prevented an average of 70.1 % COP degradation, ensuring stability and safety of compressors and batteries under various operating conditions. The research provides useful information for exploring the dynamic characteristics of CO2 thermal management systems, offering a novel approach to enhance the system stability and efficiency.

Hyporheic exchange flows in a mountainous river catchment identified by distributed temperature sensing

AbstractElevated stream temperatures under low‐flows, exacerbated by global warming, are a stressor that affects aquatic species directly or in combination with other stressors. Stream temperatures are influenced by energy fluxes across the air–water interface as well as by hydrological exchange processes occurring at the water–riverbed interface. Small‐scale stream temperature dynamics influenced by exchange flows are still underrepresented in stream temperature research. To investigate high‐resolution temperature dynamics and hydrological exchange processes at the sediment–water interface we applied fiber‐optic distributed temperature sensing (FO‐DTS) at two sites in the mountainous Kinzig catchment combined with mapping and measurement of additional environmental conditions. Two types of temperature anomalies could be observed at one site under conditions of low flow and high air temperature. Dampening effects coincided with riverine features such as pools, vegetation roots, fine sediment, and signs of streambank seepage which indicated hyporheic exchange flows. Increased heating of the substrate during the day was identified in shallow sections where sediment was exposed to the air and shading from riparian vegetation was patchy. At another site, at which the cable could not be buried because of the sediment composition, temperature anomalies in the overlying water indicated diffuse groundwater exfiltration. The results show that small‐scale processes in the hyporheic zone, low water tables, and riparian shading influence stream temperature in mountainous streams and can be identified with FO‐DTS under suitable conditions. The results improve our understanding of stream temperatures (in the hyporheic zone) and provide important information on how to improve hydrological modeling.

The assessment of chlorophyll-a retrieval algorithm and its spatial-temporal distribution using sentinel-2 MSI off the Banjir Kanal Timur River, Semarang, Indonesia

In aquatic ecology, Chlorophyll-a (Chl-a) plays a pivotal role in biogeochemical cycles, as well as a key indicator for the trophic state in the environment. Coastal area of Semarang is known as eutrophic waters. Thus continuous monitoring of Chl-a will be beneficial for the management of the impact of the trophic dynamic in the coastal waters of Semarang. Taking the benefit of the continuous monitoring of satellite measurement, the present study provides the assessment of Chl-a off the Banjir Kanal Timur (BKT) River, Semarang, using the proper remote sensing approach. In this study, algorithms were generated according to bands sensitive to Chl-a scattering properties, using multiple regression between in-situ Chl-a and Bottom of Atmospheric (BoA) reflectance from Sentinel-2 with 10 m resolution. A total of 43 water samples were collected to build the algorithm and 43 of the water samples were used for validation during the passing time of Sentinel-2 at the equator i.e., on 21 August 2021 at 9.00–11.00 local time. The results show that the performance of the algorithm generated in this study has high accuracy as denoted by the low RMSE, and MAPE, respectively. For the seasonal variation, the highest (lowest) concentration of Chl-a is observed in December (September), with values reaching 11.92 µg/L (7.06 µg/L). The seasonal fluctuation of Chl-a depends on air temperature and sunshine duration. High (low) air temperature and long (short) duration of solar exposure result in increased (decreased) Chl-a off the BKT River. Conversely, the elevated wind velocity only impacts the rise of Chl-a in the region far from the river estuary, which is associated with the mixing process. BKT waters are categorized as eutrophic conditions, with a concentration exceeding 6 µg/L. These algorithms can provide valuable insights into the health and productivity of coastal ecosystems, aiding in their monitoring and management.

New ways to improve the efficiency of railway transportation of viscous petroleum products

The paper proposes a new approach to solving the problem of increasing the economic efficiency of railway transportation of viscous petroleum fuels (fuel oils) at low air temperatures. The physical properties of fuel oils allow them to be obtained when poured into the tank of a tank wagon in a stratified state, when their density in the upper part of the tank is significantly less than in its lower part. This blocks the natural convection of hot fuel oil on the cold walls of the tank, and it cools only due to the molecular thermal conductivity which is very small. Upon cooling down, a relatively thin highly viscous layer forms on the inner walls of the tank of a tank wagon, which acts as a heat-insulating shell, and the bulk of it (more than 90%) retains high temperature and fluidity throughout the period of transportation. The thermal and hydrodynamic calculations were performed using modern computer technologies (the ANSYS 5.6 software package). The results obtained show that the need to heat up the fuel oil during unloading remains, but already requires significantly less time and heat energy. The proposed energy-saving technologies for the delivery of viscous petroleum products are especially relevant in Russia with its cold continental climate, long-haul transportation, and the current structure of the country’s wagon fleet. The value of the results obtained lies in the fact that the proposal can be implemented on tank wagons in circulation with minimal change to their design. The technology of operation of the drain equipment at unloading points will not change either.

Micro and macro urban heat islands in an industrial city: Bradford, UK

The urban heat island (UHI) phenomenon refers to the temperature difference(s) between urban/built environments and suburban/natural areas. It affects energy use, health, water quality, and in brief, the quality of life in cities. Understanding the magnitude of this issue could help urban planners and decision makers to better implement green interventions into their cities. This paper shows the results of field measurements carried out to study the UHIs in Bradford, as one of the most deprived cities in the UK with minimum green infrastructure. In the first phase of the study, air temperatures were measured in twenty locations (four areas) with different land covers in Bradford. These measurements were called micro UHI study, as the air temperatures were measured in areas within the boundaries of the city. Data were collected in late summer and early winter. The results showed that the UHI is more sensible in the colder day (with average 0.6 °C cooler air temperature in a green area compared to the city centre). Greener areas experienced higher wind and lower air temperatures compared to dense urban settings in December. In the second phase of the study (macro scale), diurnal air temperatures from a residential (urban) versus a rural site were compared during a heat wave episode (seven days). It was observed that the rural site was 0.8 °C cooler than the residential/urban site. The maximum temperature differences occurred during the nights/early mornings between the two sites (3.2 °C at 5:00am).

Zero-dimensional analysis of the effect of water vapor on reducing electrons in the plasma sheath

In this study, an air-water vapor ionization reaction model is developed within the quantum-kinetic (Q–K) model of the direct simulation Monte Carlo method to investigate the detailed mechanism of how water vapor reduces electrons. The zero-dimensional simulations of a typical non-equilibrium flow field downstream of a normal shock are designed, where the electron number density decreases by two orders of magnitude due to water vapor. We conclude that the introduction of water vapor reduces the mole fractions of oxygen atoms and nitrogen atoms through five pairs of reactions and enhances the reverse nitric oxide associative ionization reaction, leading to electron consumption. The phenomena and corresponding mechanisms under varying mole fractions of water vapor, air temperatures, and water vapor temperatures are investigated. Based on the mechanisms, we propose that the addition of hydrogen ions could improve the water's mitigation effect, which is then proven to be able to reduce the electron number density by another two orders of magnitude, not only at high air temperatures but also at lower air temperatures or lower mass injection rates.

Interannual Variations in Spring Snowmelt Timing of Alaskan Black Spruce Forests Using a Bulk‐Surface Energy Balance Approach

AbstractSpring snowmelt occurs for a short duration on an annual time scale, but their timings considerably affect the carbon and hydrological cycle in high‐latitude ecosystems. Here, we developed a simple snowmelt model, treating the ecosystem surface as a bulk‐surface layer. Energy fluxes across this bulk surface and the snow‐soil boundary determine snow temperature and the energy utilized for snowmelt. Parameterizing the bulk surface using decade‐long eddy covariance site data from two Alaskan open black spruce forests offered an opportunity to quantitatively evaluate meteorological drivers affecting snowmelt timings without the needs for detailed canopy information. The sensitivity analysis suggested that the total snowfall on the forest floor, ranging from 0.35 m in 2016 to about 1 m in 2018 and 2020, was the most crucial driver for snowmelt timing. This factor accounted for a 10‐day difference in the interannual variations in snow disappearance dates. The importance of the snowfall varied from year to year, and in 2013, the late snowmelt was characterized by low air temperatures, which increased sensible heat loss from the snowpack. The importance of atmospheric radiation was revealed in relatively warm years, such as 2016 and 2019. Our modeling approach necessitates adjusting one empirical parameter that reflects the heat conductivity from the bulk surface to the snow, based on observations. Nevertheless, despite this need for adjustment, the bulk‐surface approach helps identify important meteorological drivers underlying observed snowmelt within a simple theoretical framework.

Performance analysis of transcritical CO2 and common medium low-temperature air energy heat pump

To comply with the development of the “dual-carbon” goal, a new refrigerant for heat pump products is necessary. Firstly, mathematical models of enhanced vapor injection air energy heat pump and transcritical CO2 heat pump systems were established, and they were calculated using thermodynamic methods. The results show that when CO2 is used as the refrigerant, the discharge temperature is 30% and 22.7%, which is higher than that of R410A and R134a, and 4.9% lower than that of R32. At the compressor outlet, the volume flow rate of CO2 is 59.0%, 60.8%, and 64.2% less than R410A, R32, and R134a, respectively. In terms of power consumption, the difference between CO2 and R410 A is within 6%. The power consumption of the CO2 system is 0% ∼ 6.4% higher than that of the R32 system and 29.2% ∼ 46.5% lower than that of the R134a system. The COP of the CO2 system is within 6% of that of the R410A and R32 systems, which is 41% ∼ 87% higher than that of the R134a system. When the ratio of CO2 / R32 is 0 ∼ 0.2 and 0.9, the COP of the mixed refrigerant is better than that of the single refrigerant CO2.

A numerical study of heterogeneous nucleation of ice crystals and frost layer growth on horizontal cold surfaces

In engineering applications, the formation and growth of frost layers can significantly affect the normal operation of equipment, leading to undesirable influences. Therefore, it is imperative to understand the frost formation mechanisms as a basis for developing effective anti-frosting and defrosting methodologies. This study proposes a model for the growth of horizontal cold surface frost layers using the classical nucleation theory and the Eulerian multiphase flow model. Compared to numerous models that investigate the frost formation issues using the computational fluid dynamics (CFD) method, our proposed model takes into account the crystal growth period (the heterogeneous nucleation process of ice). The predicted results of the proposed model were found to be in good agreement with the data collected from five related experimental studies. In this regard, the maximum frost layer thickness error was 23.9 % with a mean error of 5.1 %, and the maximum density error was 22.8 % with a mean error of 8.1 %. Compared to existing models in the literature, the proposed model can reduce the maximum thickness and density uncertainties by 17.4 % and 10.2 %, respectively. Additionally, the effects of the surface contact angle and environmental parameters on the nucleation process were analyzed using the established model. It was found that a larger contact angle, higher cold surface temperature, and lower air temperature, humidity, and velocity were unfavorable conditions for supporting the formation and growth of ice crystals. Moreover, the nucleation completion time was shortened with the increase in the moist air supersaturation degree, varying as an exponential function. The findings of this study provide an important theoretical basis for the development of optimizing strategies for anti-frosting and defrosting.

Experimental study on exhaust internal circulation desiccant wheel evaporative cooling chiller

Evaporative cooling chillers reduce the water temperature by absorbing heat during the evaporation of water without the need for compressor assistance or refrigerants. It is an energy-saving and environmentally friendly cooling technology but it needs necessary improvements in the applicable environmental area and refrigeration performance due to ambient air wet-bulb temperature limitations. To obtain chilled water below the dew-point temperature of the ambient air, an exhaust internal circulation desiccant wheel evaporative cooling chiller is proposed and experimentally investigated. A combination of a heat pipe exchanger, a desiccant wheel, an air cooler and an air heater are applied to reduce the wet-bulb temperature of the ambient air before it passes through the direct evaporative cooler. At the same time, the low-temperature and low-humidity exhaust air from the direct evaporative cooler is used to circulate to improve chiller performance. The experimental results show that the average chilled water temperature of the proposed chiller is 10.2 ℃ lower than that of the direct evaporative cooling chiller under five typical environmental conditions. The wet-bulb efficiency of the proposed chiller is the highest at high temperatures and high humidities (36 ℃ and 80% RH), which is 6 times greater than that of direct evaporative cooling chillers. Therefore, the proposed chiller expands the application range of evaporative cooling technology, which is especially suitable for humid areas with low-grade heat utilization such as solar energy.