Thermal Refrigeration Research Articles

Theoretical insights into Sb2Te3/Te van der Waals heterostructures for achieving very high figure of merit and conversion efficiency

Thermoelectric technology offers a promising solution for sustainable energy conversion, but maximizing efficiency and figure of merit (ZT) remains a significant challenge. This work explores the novel structural, electronic, and thermoelectric properties of Sb2Te3/Te van der Waals heterostructures (vdWHs) through first-principles computation and Boltzmann transport theory. Our study reveals that the Sb2Te3/Te vdWHs exhibit very low lattice thermal conductivity (0.28 Wm−1K−1) and high Seebeck coefficient (811 μVK−1), driven by energy-filtering effects across the interface and a band gap of 0.47 eV. Notably, the calculated ZT reaches a record-high value of 8.83 at 400 K, substantially surpassing previous benchmarks for similar materials. Unlike prior studies, we extend our investigation by tuning the dimensions to 2 × 2 × 1 and 3 × 3 × 1 supercells and exploring tri-layer Te/Sb2Te3/Te vdWHs. Our results show that the ZT of the 2 × 2 × 1 supercell reaches an even higher value of 9.14, further exceeding the performance of the unit cell. Additionally, the heterostructures demonstrate a remarkable thermoelectric conversion efficiency (η) of 32.25 % and thermionic refrigeration efficiency surpassing 51.9 % of Carnot efficiency at 400 K, highlighting their potential for high-performance cooling applications. These findings significantly advance the integration of high-efficiency heat-to-electricity conversion and cooling within a single material system, setting a new benchmark for thermoelectric performance and establishing Sb₂Te₃/Te vdWHs as a leading candidate for next-generation thermoelectric and electronic technologies.

Exergy-economic based multi-objective optimization and carbon footprint analysis of solar thermal refrigeration systems

The increasing carbon footprint associated with conventional cooling methods underscores the urgent need for sustainable alternatives. This study investigates the economic and environmental advantages of various solar-thermal cooling systems, with a focus on optimizing their performance across different climate conditions. Employing a multi-objective approach, the research emphasizes exergy-economic indices to optimize selected cycles. The analysis covers multiple refrigeration technologies, including liquid absorption, solid adsorption, and solid desiccant cycles. Results indicate that the liquid absorption cycle performs optimally in hot, arid climates, reducing the payback period to approximately 8 years when optimized. In hot and humid regions, the solid desiccant cycle proves most effective due to its superior humidity control, yielding a payback period of 5.3 years. For cold and mountainous areas, the solid adsorption cycle is preferred, with a payback period of 13.5 years, while moderate and humid climates benefit from the solid desiccant cycle for both cooling and humidity regulation. The exergy-economic factors for the solar refrigeration systems across semi-arid, hot and arid, hot and humid, cold and mountainous, and moderate and humid climates are 0.758, 0.602, 0.698, 0.74, and 0.575, respectively.

Dynamic simulation and exergy-economic assessment of solar thermal refrigeration systems in different climates

In this paper, the performance of solar-thermal cooling systems for air conditioning application is investigated in different climates of Iran. Diverse climate types are considered which makes the results applicable to other countries. Available commercial refrigeration cycles that are capable of integration with solar-thermal collector systems including the closed liquid absorption cycle, closed solid adsorption cycle, and desiccant cycle are studied. First, a solar and climatic map is proposed and the representative cities have been selected for each climate. Then, the climate parameters related to each region are extracted to create an hourly database. Also, the cooling load of a reference building is modeled based on the climate database, each refrigeration cycle is simulated using thermodynamic equations, and the dynamic model is generalized daily for the hot season in each city. In the last step, each cooling system is examined using an exergy-economic analysis, and technical and economic indicators are compared. The results show that the most suitable solar-thermal refrigeration cycle that can be installed in the central plateau and semi-arid areas of Iran is the closed-cycle liquid absorption system. The annual average exergy-economic factor and the annual solar fraction in this cycle are 0.7578 and 0.57, respectively. In the southern coastal areas, due to the high humidity and air temperature in summer, the solid desiccant refrigeration system with an exergy-economic factor of 0.6978 and a solar fraction of 0.2416 is preferred. The suitable solar-thermal refrigeration cycle in the cold and mountainous regions of Iran is a solid adsorption system with a silica gel absorber. The annual solar fraction and average annual exergy-economic factor in this cycle are 0.5158 and 0.7394. Also, in northern moderate regions, the solid adsorption refrigeration system with an average annual exergy-economic factor of 0.409 and average exergy efficiency of 0.12 is comparatively beneficial.

Thomson effect in thermionic refrigeration: Enhanced performance of graphene/2D-semiconductor/graphene heterostructure cooler

Two-dimensional (2D) materials and their heterostructures have been widely explored for high-performance energy conversion applications. The Thomson effect—a higher order transport process—plays an important role in thermoelectric devices, yet its effect on the performance of thermionic devices remains unknown thus far. Here, we investigate the performance of thermionic refrigeration in vertically stacked heterostructure (VHS) and laterally stitched heterointerface (LHS) composed of a graphene and a 2D semiconductor (i.e., MoS2 and WSe2) in the presence of the Thomson effect. Using a temperature-dependent Seebeck coefficient, we derived the analytical expressions of the cooling efficiency and the effective ZT. We shall show that the Thomson effect improves the coefficient of performance (COP) by up to 20%, particularly, in the case where the temperature difference between the cold and the hot electrodes is large. However, the Carnot efficiency decreases with the temperature difference. The overall COP is reduced by the Thomson effect. We calculate the COP in graphene/MoS2/graphene and graphene/WSe2/graphene VHS and LHS devices. We show that the LHS composed of WSe2 significantly outperforms the VHS and MoS2 counterpart. These findings provide an understanding of thermionic processes in the higher-order transport regime and shall offer insights into the design of novel 2D material heterostructure thermionic energy converters.

THE USE OF ALTERNATIVE ENERGY SOURCES FOR THE OPERATION OF ENGINEERING SYSTEMS OF DETACHED CONSUMERS

The operation of many of the engineering systems that are necessary for the functioning of a detached residential building depends on specific sources of alternative energy, the use of which can significantly reduce the consumption of traditional fossil energy resources. A review of scientific works devoted to the operation of engineering systems using alternative energy sources is given. Several schemes of power equipment have been developed to obtain thermal and electrical energy supplied to isolated consumers. When designing a fixed technical solar system using solar radiation for operation, the locations of all power equipment, the solar receiver pitch on the roof of the building and the values of its optimal angle of inclination to the horizon were determined. A scheme of a small transportable hydroelectric power plant has been developed, which generates electric energy using the pressure created by the flow of water. A thermal refrigeration unit is designed, which uses the heat of the air removed from the animal stall to heat the heated medium.

Performance assessment of a perovskite solar cell-driven thermionic refrigerator hybrid system

To develop a high-performance solar refrigeration technology, a novel refrigeration system composed of a perovskite solar cell (PSC) and a thermionic refrigerator (TIR) is constructed. Based on theories of photovoltaic effect, thermionic emission and non-equilibrium thermodynamics, the coefficient of performance (COP) and refrigerating capacity of the integrated system are mathematically expressed, in which the various irreversible losses are taken into consideration. The integration characteristics and coupling relationships between the PSC and TIR are investigated, and the working voltage regions for the combined operation of the subsystems are determined. Furthermore, the impacts of the area ratio, the work function, the environment temperature, the cold reservoir temperature and the heat transfer coefficient on systemic performance are evaluated in detail. Numerical calculation examples illustrate that the maximum COP and maximum refrigerating capacity of the integrated system are 0.81 and 81.2 W, respectively. Analysis results show that there exist optimal area ratio and work function to optimize the system performance. The present study may offer some fresh perspectives on the evolution of the solar refrigeration system.

Detailed CFD transient heat transfer modelling in a brake friction system

In a brake system the contact between the friction grooves of the discs and the steel elements leads to a consistent heat dissipation that is excised by the oil thermal vector in its flowing along the grooves channels. These are very complicated physical system were many variables may be taken in consideration, first of all the extensive integration of the different time scheduling actions that take place and that characterizes the translation of the interacting bodies and the correspondent deceleration, and also the heat dissipation derived from this contact. The simulations of similar physical system may result in too expensive computational efforts leading to very complicated simulations that need to take in account an extensive coupling between many physics, first of all the solid/liquid heat transfer and then the turbulence and the bi-phase models for the oil/air mixture that serves as thermal refrigerator vector in the braking system. Herein a simplified 3D model that focuses only on a discs / piston coupling is presented were the braking action is simulated in a simplified way modelling the velocity of rotation of the elements and the dissipated heat during the braking cycle. Focusing on the grooves channels the oil flows and thermal averaged flows are presented and detail contours are used to characterise a single brake event.

Thermodynamic analysis of operating strategies for waste heat recovery of combined heating and power systems

The multi-generation system often operates under off-design conditions due to the highly dynamic energy demand. Improving its off-design performance is crucial for energy conservation and emission reductions. To this end, this paper proposed a novel control strategy for the gas turbine cycle applied in combined heating and power (CHP) system, which effectively combines turbine inlet temperature control (TITC) and inlet air throttling control (IATC) methods, to improve system off-design performance and avoid discharging low-temperature gas. The hybrid control strategy can improve system efficiency by 3.70% on average over the traditional TITC method during the entire load range. Moreover, the maximum efficiency difference between these two methods reached 5.15% at the 30%-load factor conditions. To further showcase the effectiveness of the IAT-TITC method, the CHP system is integrated with an electric chiller and thermal refrigeration, respectively, forming four combined cooling, heating, and power (CCHP) system scenarios to supply the energy for a hotel building. Not only did the CCHP system scenarios with the IAT-TITC method show better performance, but also considering an electric chiller as an extra cooling supply machine can reduce primary energy consumption by 30.73%, 28.32%, and 11.56% in summer, transition season, and winter typical days, respectively, compared with separation system.

Steady state entanglement behavior between two quantum refrigerators

An entangled steady-state behavior between two quantum thermal refrigeration machines is evaluated. Each machine is made up of three qubits of two-level, where each qubit interacts with its proper reservoir. The coupling between the qubits and the reservoirs is investigated with weak interaction. In addition, thermodynamic quantities such as heat flux are examined as a function of entanglement. The effect of temperature on entanglement is also studied in the cases of resonance and non-resonance. The obtained results show that the steady-state entanglement is more robust in the case of resonance than in the case of non-resonance. However, the maintenance in this case corresponds to the higher cooling power of the machine.

Comprehensive Analysis of Electron Evaporative Cooling in Double-Barrier Semiconductor Heterostructures

Based on full quantum-transport simulations, we report a comprehensive study of the evaporative cooling process in a double-barrier semiconductor heterostructure thermionic refrigerator. Our model, which self-consistently solves the nonequilibrium Green's function framework and the heat equation, is capable of calculating the electron temperature and electrochemical potential inside the device. By investigating the dependence of those thermodynamic parameters as a function of the barrier thickness and height, we answer open questions on evaporative cooling in solid-state systems, and give a clear recipe to reach high electron refrigeration. In particular, simulation results demonstrate that the best cooling is obtained when (i) the device operates at the maximum resonant condition; (ii) the quantum well state is symmetrically coupled with the contacts. The present results then shed light on physical properties of evaporative cooling in semiconductor heterostructures and will allow the development of thermionic cooling devices towards unprecedented performances to be sped up.

Numerical investigation on the performance characteristics of photovoltaic thermal refrigerator integrating phase change material and water/nanofluid as fluid medium

To better manage the thermal energy of solar obtained during daytime and reduce the negative impacts caused by the global warming, Photovoltaic Thermal (PVT) refrigerator integrating phase change material (PCM) and with water/nanofluid (Al3O2) as fluids medium is among the best choices. In the present paper, a 3D model of water/nanofluid-based-PVT/PCM was numerically solved in ANSYS Fluent 18, to investigate the performance characteristics of the system under various parameters of weather, flow and PCM. By employing the methods of solidification/melting, conjugated heat transfer and second order discretization, the results in terms of top, out and bottom temperature, COP and energy stored in PCM were obtained and analyzed. The inlet of both mass flow rate and temperature were found to be the most influential parameters with highest COP and energy stored of greater than 5 and 2.9 kJ, respectively. Furthermore, the maximum temperature attained when PVT/PCM was used is 5.4 K lower than that of PVT, implying that PVT/PCM not only can act as a thermal energy management but also can reduce the negative impact of global warming. The system of nanofluid-based-PVT/PCM also achieved a good performance when operated under lowest values of weather parameters (long-wave intensity and ambient temperature).

Определение теплофизических характеристик тропических фруктов для их использования при производстве сухих молочных продуктов

Introduction. Canning requires thermophysical calculations for thermal or cold processing. These calculations are based on thermophysical characteristics of raw materials. The research objective was to analyze the thermophysical characteristics of tropical fruits.
 Study objects and methods. The study featured kiwi, papaya, avocado, and figs. Their thermal conductivity was analyzed with the help of stationary fiber plate method. The cryoscopic temperature was determined from the flat area of the thermogram obtained during freezing. The heat capacity and the amount of frozen moisture were determined by calculation based on the available data on the chemical composition of the fruits. The density was defined by using the hydrostatic weighing method, the sugar content – by the refractometric method, the moisture content – by drying to constant mass.
 Results and discussion. The research revealed the physicochemical parameters of kiwi, avocado, papaya, and figs. Papaya demonstrated the highest moisture content – 86.32 ± 0.02%, while kiwi appeared to have the highest density – 1,065 ± 1 kg/m3. Figs had the highest sugar content – 16.0 ± 0.1%. The thermal conductivity coefficient of fresh and frozen tropical fruits was determined experimentally and increased after freezing. Frozen avocado showed only a slight increase in thermal conductivity coefficient because of its low moisture content while frozen figs demonstrated a significant increase in the thermal conductivity coefficient – by 3.3 times. This product possessed the highest thermal conductivity: 0.63 ± 0.02 W/(m·K) for the fresh samples and 2.06 ± 0.02 W/(m·K) for the frozen samples. The thermal conductivity coefficient of kiwi and papaya increased by 2 and 4.2 times, respectively. The experiment also examined the effect of protective plastic wrap and ripeness on the thermal conductivity coefficient. The film proved to have a negative effect on the reliability of thermophysical analysis. The cryoscopic temperature was determined empirically. Such thermophysical properties as heat capacity, thermal diffusivity, and frozen moisture were based on the available chemical composition.
 Conclusion. The research revealed the physicochemical parameters of kiwi, avocado, papaya, and figs. It included a set of experiments on the thermal conductivity coefficient of fresh and frozen tropical fruits. The obtained values can be used to develop the optimal parameters of thermal processing, refrigeration, and thermal treatment of new products. They can also be useful for fortifying dairy and bakery products with exotic fruits.

Performance optimization of thermionic refrigerators based on van der Waals heterostructures

In this paper, an irreversible thermionic refrigerator model based on van der Waals heterostructure with various irreversibilities is established by utilizing combination of non-equilibrium thermodynamics and finite time thermodynamics. The basic performance characteristics of the refrigerator are obtained. The effects of key factors, such as bias voltages, Schottky barrier heights and heat leakages, on the performance are studied. Results show that cooling rates and coefficients of performances (COPs) can attain the double maximum with proper modulation of barrier heights and bias voltages. Increasing cross-plane thermal resistance as well as decreasing electrode-reservoir thermal resistance and reservoir-reservoir thermal resistance can enhance the performance of the device. The optimal performance region is the interval between the maximum cooling rate point and the maximum COP point. By modulating the bias voltage, the working state of the device can fall into the optimal performance region. The optimal performance of the refrigerator when using single layer graphene and a few layers graphene as electrode material is also compared.

Design and Performance of Solar PV Integrated Domestic Vapor Absorption Refrigeration System

The arrival of new technologies has increased the energy demand day by day and does not seem to slow down at any time soon. High energy demand is adding risk on energy depletion and cause of various environmental issues. Air conditioner, chiller, and refrigerator occupy a considerable amount of the world’s total energy usage and have also proved to be a massive contributor to various environmental impacts. This technology might sound like a luxury on the surface, but they are in high demand to achieve food security. They can also help lifesaving vaccines to reach even the isolated parts of the world. Even though solar thermal refrigeration is a popular field, this paper solely concentrates on PV integrated refrigeration. In this paper, a renewable integration technology where a solar photovoltaic system is used to supply the electrical energy required to drive an absorption cycle is studied and compared with the commercial AC absorption refrigeration system. The Coefficient of Performance (COP) of the AC and DC system was 0.18 and 0.14. The simple payback of the system is 10.2 years.

Analysis of the efficiency of application heat-used cooling systems of gas turbine drive air cycle

It was found that with an increase in temperature at the inlet of the compressor of a gas turbine engine, the efficiency of the installation decreases. The article presents a study of the influence of climatic conditions on the efficiency of the AI-336-1 / 2-10 gas turbine drive intended for the drive of gas pumping units and other industrial installations with a capacity of 10 MW. A method for increasing the efficiency of the installation based on air cooling at the compressor inlet is proposed. Comparison of various existing methods of air cooling at the compressor inlet is carried out. This air preconditioning avoids power loss during hot periods. It is shown that for the best air cooling and, as a consequence, maximizing the performance of a gas turbine unit, it is preferable to use refrigeration units. It seems expedient to cool the air with the help of refrigerating machines that utilize the heat of the exhaust gases. GTU of any circuit design has a high energy potential, what can be used for the production of cold using thermal refrigeration machines: absorption or steam jet. To cool the air at the inlet to the compressor of the AI-336-1 / 2-10 gas turbine drive, the possibility of using a heat-using ejector refrigeration machine was analyzed, which has such advantages as simplicity, small volume and the absence of movable wear parts and the possibility of using water vapor as a refrigerant. In order to determine the magnitude of the decrease in air temperature at the compressor inlet and, accordingly, the increase in the engine efficiency, calculations were performed. The analysis of the efficiency of the gas turbine unit depending on the ambient air temperature and the degree of air cooling in front of the compressor is carried out. The efficiency of the use of a steam jet refrigeration unit for reducing the air temperature at the compressor inlet has been substantiated. It is shown that air cooling at the inlet prevents a decrease in the power of the GTU and ensures the utilization of waste gas heat, leads to an increase in the economic and environmental performance of the installation, and ensures a reduction in fuel consumption. In connection with the foregoing, a detailed study of a technical solution based on the use of a steam-jet refrigeration machine is promising, what reduces the negative effect of high outside air temperatures on the energy and economic efficiency of the gas turbine plant.

A combined thermo-mechanical refrigeration system with isobaric expander-compressor unit powered by low grade heat – Design and analysis

Abstract Refrigeration and air conditioning systems consume about 17% of the world-wide electricity and their conventional refrigerants cause ozone depletion and global warming. In this study a novel thermo-mechanical refrigeration (TMR) system is developed and analyzed that is powered, instead of electricity, by thermal energy from waste heat or renewable sources in the ultra-low temperature range of 60–100 °C. A novel isobaric expander-compressor unit (ECU) is designed and combined with vapor compression refrigeration cycle to constitute the TMR system. The technological solutions (mainly towards simplification of the design) are crucial components of the study novelty. The suitable refrigerants for the system are systematically investigated, analyzed and selected from a list of 43 refrigerants. Nine fluids for the power loop (the isobaric expansion cycle) and nine fluids for the cooling loop (the thermal refrigeration cycle) were selected and compared based on their mode of operation (subcritical and supercritical), environmental effects and safety class. It is found that the HFO refrigerants such as R1234yf and R1234ze have acceptable performance with no ODP and very low GWP. Natural refrigerants R717 (ammonia) has the best performance in subcritical mode with toxicity as the main drawback. At heat source temperatures less than 85 °C, the system operation in subcritical mode is more efficient and more compact than in the supercritical mode. Thorough analysis and recommendations are made for the size of the ECU in terms of the diameters of the expander and the compressor.

Effect of quantum reflection over the barrier on thermionic refrigeration

We study the effect of quantum reflection over the barrier (ROB) in a thermionic cooling device. We find that the performance of refrigerators can be enhanced by the ROB effect if the bias voltage and the lattice thermal resistance of the semiconductor in the barrier region are both sufficiently high. Furthermore, the figure of merit ZT can be higher due to the ROB effect if the workfunction of the cathode is low and the lattice thermal resistance is high. The overall optimum ZT calculated with and without the ROB effect are 6.5 and 7.1, respectively. The origin of the ROB correction to ZT is that the quantum reflection becomes asymmetric for the carrier transport in opposite directions.

Numerical study of a Phase Change Material (PCM) embedded solar thermal energy operated cool store: A feasibility study

Solar energy has proved to be an ideal alternative for fossil fuels, however, due to its intermittency in availability, systems operated by solar energy face serious limitations. In this study, a simplified dynamic model is developed to investigate the behavior of a PCM-embedded cool store. The main application of this type of cool store is to be operated with a solar thermal refrigeration system to compensate for the intermittency in energy availability. 8640 kg of crops (potatoes) is considered to be stored inside the cool store along with PCM cold thermal energy storage units. 9 h of active and 15 h of inactive period are considered for 24 h operation of the refrigeration system based on the solar energy availability data. During the active period, PCM units charge, and during the inactive period, they discharge and regulate the temperature inside the cool store. It was revealed that by choosing the right properties, PCM can keep the temperature of the crops in the desired range during the inactive period of the refrigeration system. Also, it was observed that the amount of PCM used and the phase change temperature of the PCM has a significant impact on the performance of the system. The lowest temperature obtained for the crops during the inactive period of the refrigeration system is for the case with PCM to crops amount ratio of 0.026 with PCM phase change temperature of 9 °C in which the discharge process finishes around the end of the inactive period of the refrigeration.

Performance of ejector refrigeration cycle based on solar energy working with various refrigerants

The refrigerator unit based on electrically driven vapor compression technology consumes high energy, associated with high cost and loss of ecosystem. Recently, solar thermal energy emerges as a key research area and introducing it in refrigerator application has happened extremely fast. Solar thermal refrigeration systems based on ejector compression technology are thriving because of its utilization of low-grade energy sources, straight forward design, non-moving parts, lower maintenance costs, and durability. In this present work, the performance of a solar ejector refrigeration system (ERS) is calculated analytically using one-dimensional model. The performance parameters are calculated with the assumption that both the condensation and evaporation processes occur along the saturation line. Initially, the influence of the ejector area ratio, generator temperature, condenser temperature, and evaporator temperature on the performance of the ERS is analyzed in critical mode with water (R718) as the working fluid. The analysis shows that the coefficient of performance increases more than two times on increasing the ejector area ratio from 6.4 to 12.8. The results also show that a higher coefficient of performance (COP) is obtained at a higher ejector area ratio. The COP is found to be inversely related to the generator temperature, meanwhile the coefficient of performance increase more than 74% on increasing the refrigeration temperature from Tc = 8 °C to Tc = 15 °C. Then, the performance of ERS is obtained at the subcritical conditions at varying generator temperatures. At subcritical mode, COP is found to be directly proportional to the generator temperature, and the highest COP occurs at an optimum generator temperature. The performance of the ERS with various refrigerants as R717, R718, R245fa, R123, R141b, and R365fa is calculated at the critical mode. With refrigerant R717, the system shows higher performance as well as a cooling effect than other refrigerants. However, most environmental benign refrigerant R718 shows comparatively low performance, but it is found that R718 produces a significant cooling effect than other refrigerants excluding R717 refrigerant.

A Review on Exergy Analysis of Solar Refrigeration Technologies

Solar energy is becoming more and more useful in the modern day life in industrial, domestic and commercial sectors, because of his cleanliness from an environmental point of view and also contributes to the reduction of greenhouse effect gases such as CO2. Exergy analysis is a thermodynamic analysis technique based on the Second Law of Thermodynamics, which provides an alternative and illuminating means of assessing and comparing processes and systems rationally and meaningfully. Exergy analysis can assist in improving and optimizing designs. In this paper, the exergy analysis of solar thermal refrigeration cyles is reviewed. A review of the research state of art of the solar absorption and adsorption refrigeration technologies is also carried out. The cycles involved in these technologies are: open, closed, continuous and intermittent cycles. An overview of mesures of merit with regard to exergy (exergetic efficiency, exergy losses, exergy improvement and exergetic coefficient of performance) is presented. Besides, an historical and chronological view is done on the development scenario of exergy analysis in the world from 1824 until 2014. The main mathematical relations for the simulation of those cycles are presented.