Refrigeration Cycle Research Articles

Thermodynamic analysis of a modified two-stage transcritical CO2 refrigeration cycle with an ejector and a subcooler

In the application scope of large-scale supermarkets, the practicality of transcritical CO2 refrigeration device has been confirmed to be quite satisfying. A modified two-stage transcritical CO2 refrigeration cycle with an ejector and a subcooler (MTC) is proposed in this paper. In the modified cycle, the ejector recovers expansion works and reduces irreversible losses in the throttling process. The subcooler provides subcooling degree for the CO2 entering the low-temperature (LT) evaporator, thus increasing the refrigeration capacity and improving the COP of the modified cycle. Thermodynamic analysis has shown that the exergy efficiency (ηex) and COP of MTC under given operating condition has been enhanced by 13.7 % and 14.2 % compared to BTC. The discharge temperature at the outlet of high-pressure compressor in MTC is decreased by 8.3℃. Under typical operating condition, the optimal discharge pressure of MTC is 9.07 MPa, which is lower than that of BTC. The correlation to calculate optimal discharge pressure for single-stage CO2 refrigeration cycle is also suitable for MTC under given operating conditions. The MTC has also shown better performance under variable operating conditions. For MTC, when the temperature at the outlet of gas cooler increases from 35 – 45℃, the COP and ηex are enhanced by 14.1 % - 16.1 % and 12.8 % - 14.2 % compared to BTC, respectively. When gas cooler outlet pressure decreases from 11.0 to 7.5 MPa, the COP and ηex are enhanced by 14.0 % - 22.8 % and 13.4 % - 18.7 %. As the evaporating temperature at the cold side of subcooler increases from -25 to -15℃, the COP and ηex increase from 1.82 to 1.98 and 27.4 % to 29.7 %. With the ratio of refrigeration capacity (Rec) between medium-temperature evaporator and low-temperature evaporator varies from 0.7 to 1.2, the COP and ηex are improved by 10.7 % - 16.3 % and 10.7 % - 15.4 % compared to BTC. There is the maximum exergy loss at gas cooler in the MTC, whereas that of BTC is located in the expansion valve before LT evaporator. The economic analysis shows the cost per unit of exergy of MTC is decreased by 11.5 % under typical operation condition. According to simulation results, the modified cycle has better performance in severe working conditions such as high gas cooler outlet temperature and low gas cooler outlet pressure in the given range of working conditions compared to BTC.

Economic Analyses of a New Power and Cooling System at Low Temperature Applications

Recent research suggests that the implementation of more efficient combined cooling and power systems, which enable the cogeneration of electricity and cooling, can enhance the efficiency of hybrid plants. The present investigation is motivated by finding that, in the literature review on combined power and cooling systems, there is very limited information on the coupling of the organic Rankine cycle (ORC) and the ejector refrigeration cycle (ERC) with low sink temperatures. A suggested approach to do this involves using hot exhaust gas and waste heat engines to power an ORC hybrid system. To enhance the ORC–ERC system's performance, three heater configurations use waste heat from the ORC turbine exhaust, ejector, and engine waste heat to heat the working fluid. Renewable energy sources are the primary focus of most current research initiatives. The present research focuses on unique ORC and ERC systems, considered as combined power and cooling systems, with the goal of improving exergy performance at low temperatures. The suggested ORC–ERC can generate energy destruction of 69.85 kW at a source temperature of 155 °C, with an exergetic efficiency of 76.9% at the turbine. Setting the entrainment ratio at 0.5 results in a total sum unit cost of products (SUCP) of 465 $/kW‐h for the ORC–ERC. Furthermore, the 37.83% exergy destruction ratio introduces heat exchanger 2 (HE2) as the primary cause of the suggested ORC–ERC's irreversibility. A detailed parametric study reveals that altering the hot source temperature and entrainment ratio improves the system's SUCP. The current examination at high sink temperatures may be expanded to an advanced exergoenvironmental investigation.

Design and economic analysis of an innovative multi-generation system in different Countries with diverse economic contexts

This paper presents an innovative trigeneration system designed to maximize energy utilization by simultaneously producing power, cooling, and freshwater. The system integrates multiple energy sources, including biomass, natural gas, and geothermal energy, and comprises a modified gas turbine cycle, a geothermal power plant, a triple-effect absorption refrigeration cycle, and a multi-effect desalination unit. Performance metrics are evaluated from both thermodynamic and economic perspectives across four case studies: Australia, the USA, China, and Russia. The economic feasibility analysis utilizes real-state economic data, incorporating natural gas prices, electricity prices for households, interest rates, and inflation rates specific to each country. The system achieved impressive outputs, with net power, cooling, and freshwater production rates of 6299 kW, 140.1 kW, and 13.41 kg/s, respectively. Exergy analysis revealed that the combustion chamber, heat exchanger, and multi-effect desalination units exhibit the highest irreversibility, with exergy destruction rates of 2961 kW, 1125 kW, and 995.3 kW, respectively. The fixed capital investment cost is estimated at $13.5 million, resulting in a payback period of 6.75 years and a net present value of $7.287 million. In Australia, the system demonstrated a particularly favorable economic performance, with a payback period of 3.98 years and a net present value of $36.19 million. Conversely, in China, the payback period exceeds the system's expected lifetime, highlighting the economic challenges in certain environments. These results underscore the system's potential for profitability and sustainability, particularly in regions with favorable economic conditions.

Transient analysis and thermal design of a solar-powered cooling system for an office building: Enhancements using phase change materials and zeotropic mixtures in ejector refrigeration cycle

This study explores an innovative integration of solar energy with thermal energy storage for power production and cooling system support, utilizing phase change materials (PCMs) to enhance solar system performance, particularly for the cooling demands of an office building. The cooling load of a target office building in Ardabil City, Iran, is calculated for a typical day. Optimization is conducted using Multi-Objective Particle Swarm Optimization (MOPSO) to select the optimal zeotropic mixture as the working fluid, aimed at maximizing efficiency and minimizing heat source requirements. The system is designed to meet the needs of a two-story office building. Analysis of photovoltaic modules integrated with PCM under various parameters led to the selection of 25 PVT-PCM units in a series. The total setup includes 730 modules covering 365 m2, achieving a mass flow rate of 2.92 kg/s and supporting a 40 kW cooling load in the Ejector Refrigeration Cycle (ERC). The development of this solar-powered cooling system presents a sustainable, economically viable solution for building cooling. It demonstrates significant advancements in thermal efficiency through the strategic use of PCMs and optimized zeotropic mixtures in ejector refrigeration cycles. The system's scalable design offers a model for adapting to diverse climatic conditions, marking a notable contribution to sustainable building technologies.

Assessment of a solar-powered trigeneration plant integrated with thermal energy storage using phase change materials

This study presents a comprehensive thermodynamic assessment of a trigeneration plant producing electricity, fresh water through multi-effect desalination (MED), and cooling through an absorption refrigeration cycle. The MED and absorption refrigeration systems utilize the rejected heat from the power cycle, driven by concentrated solar power (CSP). Situated in Qatar, the present system leverages the abundant solar irradiance to optimize the efficiency of electricity generation, water desalination, and cooling. The design features parabolic trough collectors with synthetic oil as the heat transfer fluid, direct thermal storage, and a Rankine steam turbine cycle with three turbine stages. The system also incorporates phase change materials (PCMs) based thermal energy storage (TES) to improve the system performance and offset the mismatch between demand and supply. The present system evaluation is based on energy and exergy analyses, while the Aspen Plus is used to simulate the power production and desalination operations, providing detailed insights into its efficiency and potential for large-scale implementation. The proposed system operates with an energy efficiency of 56.72 % and an exergy efficiency of 31.24 %, respectively.

Machine learning optimization and 4E analysis of a CCHP system integrated into a greenhouse system for carbon dioxide capturing

The world has seen an increase in the popularity of renewable-based energy systems. However, because of their erratic nature, fossil fuels cannot be entirely phased out. Utilizing carbon dioxide in greenhouses close to power plants and generating extra power electricity from dissipated thermal energy are appealing strategies that cause us to have a cleaner environment and affordable power electricity. To exploit the high enthalpy of expelled gases from a gas turbine, the gas turbine was coupled with an absorption refrigerant cycle and an organic Rankine cycle and analyzed. Additionally, a deep artificial neural network method was used to calculate all functions in a rapid optimization and accurate 4E analysis. The carbon dioxide emissions subsided from 0.9183 to 0.41 kg CO2/s, and the optimization time improved 257 times after using the trained machine learning model. An adsorption technique for CO2 separation is used for the proposed system because of its lower energy consumption. The maximum exergy and energy efficiencies were attained at 36.71 % and 49.2 %, respectively, and parametric studies are being assessed. Due to increases in harvests and efficiencies, the net annual interest has increased by 21.8 %, up to 22.5 million dollars.

Deep eutectic solvents and traditional refrigerants in absorption refrigeration cycles using molecular approaches

Absorption Refrigeration Cycles (ARC) represent a sustainable technology capable of effectively producing cold with modest energy consumption from waste heat or renewable sources. However, the right choice of working fluid pair absorbent–refrigerant is essential for the system to work properly. This contribution is devoted to analysing the performance of an ARC using a wide variety of refrigerants in combination with selected deep eutectic solvents (DES) as absorbents. Three choline chloride-based and one ionic liquid (IL)-based DESs as absorbers are combined with sixteen refrigerants. The PC-SAFT equation of state is employed to assess the thermophysical properties. Accurate modelling of the absorption cycle has revealed that the phase behaviour of the working mixture significantly constrains the temperature and pressure range within which the cycle can operate. Idealised ARCs are insufficient for accurately representing the capabilities and limitations of these systems. Of the sixty-four working pair mixtures, only eighteen are suitable for ARCs based on technical and thermodynamics criteria. The Coefficient of Performance (COP) has been updated with additional key performance indicators that consider environmental and safety factors. The maximum values of the COP for this mixture are around 0.92, while many options lie in ranges around 0.60. This comprehensive analysis helps establish a ranking of the best options. The results have shown that the IL-based DES using R227ea or R236ea is the most efficient system among all the systems studied, with performances of 0.92 and 0.74.

Comparison and analysis on comprehensive performance of CO2 multiphase refrigeration systems using liquefied natural gas cold energy

In order to fully apply liquefied natural gas (LNG) cold energy to the refrigeration system, four different types of CO2 multiphase refrigeration systems using LNG cold energy are designed. In this paper, (1) CO2 single-stage compressed solid and gas phase refrigeration cycle (SSCC1), (2) CO2 single-stage compressed solid and solid phase refrigeration cycle (SSCC2), (3) CO2 double-stage compressed solid and gas phase refrigeration cycle (DSCC1), and (4) CO2 double-stage compressed solid and solid phase refrigeration cycle (DSCC2) are combined with CO2 liquid phase secondary refrigerant cycle (RC), respectively, to effectively use LNG cold energy. The performance analysis, exergy analysis, economic analysis, and CO2 emission analysis of the proposed systems are carried out by establishing the mathematical models. The results show that the intermediate pressure of DSCC1-RC and DSCC2-RC reaches the best performance at 0.3 MPa, and the system performance decreases with the increase in intermediate temperature. The refrigerating capacity of the CO2 liquid phase secondary refrigerant cycle, the COP, and the exergy efficiency of four kinds of CO2 multiphase refrigeration systems decrease with the increase in the refrigerating capacity of the CO2 refrigeration cycle, while the power consumption of SSCC2-RC and DSCC2-RC decreases, SSCC1-RC and DSCC1-RC increased. The system with the shortest exergy loss is DSCC2-RC at 654.01 kW, while the system with the shortest payback period is SSCC2-RC at 0.88 years, and DSCC2-RC has the smallest CO2 emission. Four CO2 multiphase refrigeration systems and the ammonia combined refrigeration system with the same total refrigerating capacity are compared and analyzed, respectively; the results show that the performance, economy, and CO2 emission of CO2 multiphase refrigeration system are better than those of ammonia combined refrigeration system; and the exergy loss of CO2 multiphase refrigeration system is generally larger than that of ammonia combined refrigeration system because of the large temperature difference in heat transfer.

Optimization performance of quantum endoreversible Otto machines with dual-squeezed reservoirs

Abstract We consider a quantum endoreversible Otto engine cycle and its inverse operation-Otto refrigeration cycle, employing two-level systems as the working substance and operating in dual-squeezed reservoirs. We demonstrate that the efficiency of heat engines at maximum work output and the coefficient of performance for refrigerators at the maximum χ criterion will degenerate to η − = η C/(2 – η C) and ε − = ( 9 + 8 ε C − 3 ) / 2 when symmetric squeezing is satisfied, respectively. We also investigated the influences of squeezing degree on the performance optimization of quantum Otto heat engines at the maximum work output and refrigerators at the maximum χ criterion. These analytical results show that the efficiency of heat engines at maximum work output and the coefficient of performance for refrigerators at the maximum χ criterion can be improved, reduced or even inhibited in asymmetric squeezing. Furthermore, we also find that the efficiency of quantum Otto heat engines at maximum work output is lower than that obtained from the Otto heat engines based on a single harmonic oscillator system. However, the coefficient of performance of the corresponding refrigerator is higher.

Performance analysis of quantum harmonic Otto engine and refrigerator under a trade-off figure of merit

Abstract We investigate the optimal performance of the quantum Otto engine and refrigeration cycles of a time-dependent harmonic oscillator under a trade-off figure of merit for both adiabatic and nonadiabatic (sudden-switch) frequency modulations. For heat engines (refrigerators), the chosen trade-off figure of merit is an objective function defined by the product of efficiency (coefficient of performance) and work output (cooling load), thus representing a compromise between them. We obtain analytical expressions for the efficiency and coefficient of performance of the harmonic Otto cycle for the optimal performance of the thermal machine in various operational regimes. Particularly, in the sudden-switch regime, we discuss the implications of the nonadiabatic driving on the performance of the thermal machine under consideration and obtain analytic expressions for the maximum achievable efficiency and coefficient of performance of the harmonic Otto thermal machine. Particularly, we show that the quantum harmonic Otto cycle driven by sudden-switch protocol cannot work as a heat engine or refrigerator in the low-temperature limit. Finally, we show that in the high-temperature limit, the frictional effects give rise to a richer structure of the phase diagram of the harmonic Otto cycle. We identify the parametric regime for the operation of the Otto cycle as a heat engine, refrigerator, accelerator, and heater.

Thermodynamic analysis of new configurations of auto-cascade refrigeration cycles integrating the vortex tube

Auto-cascade refrigeration cycles have extensive application prospects for low-temperature refrigeration. To achieve low evaporating temperature, the cascade heat exchanger uses a significant portion of the cooling capacity, resulting in less cooling capacity entering the evaporator. A vortex tube with energy separation capability is integrated into the cycle. Then the refrigerant is further cooled, thereby reducing the cooling capacity of the cascade heat exchanger. Thus, more cooling capacity generated by the cycle is transferred to the evaporator. This paper proposes two novel auto-cascade refrigeration cycles integrating vortex tubes. The performance of novel configurations is evaluated through thermodynamic analysis. The results demonstrate that novel configurations outperform the baseline cycle in terms of energy and exergy efficiency. The coefficient of performance and exergy efficiency of cycles achieved maximum increases of 23.84 % and 11.62 %. The modified exergy analysis method distinguishes the destruction and indicates the direction of optimization. Furthermore, the economic and environmental analysis indicates that the novel cycles achieved a maximum reduction of 6.74 % in cost rate and 12.72 % in environmental impact under given conditions. Generally, the thermodynamic analysis reveal the potential of vortex tubes to enhance the performance of auto-cascade cycles.1

Energy and exergy analysis of a novel two-stage ejector refrigeration cycle using binary zeotropic mixtures

To improve the ejector refrigeration cycle performance, this paper presents a theoretical thermodynamic analysis of a novel two-stage ejector refrigeration cycle (TSERC) with a gas-liquid separator using R134a/R32 and R600a/R290 as refrigerant. By separating the relatively low-boiling-point and high-boiling-point refrigerants, the cycle compression ratio decreases, the cycle performance increases, and the energy utilization efficiency can be improved. Energy and exergy analysis were conducted for the traditional single-stage ejector refrigeration cycle (SSERC) and TSERC. The effect of the mixture mass fraction on cycle performance under a fixed external heat source operating condition was studied, and the cycle performance comparisons between TSERC and SSERC at different evaporation temperature, condensation temperature and generation temperature were conducted. Results show that for TSERC, the maximum COP values 0.126 and 0.11, the maximum exergy efficiency 4.51 % and 4 % are obtained as the low-boiling-point mass fraction equals 0.6 and 0.4 respectively for R134a/R32 and R600a/R290. It is found that exergy destruction mainly occurs in the low-pressure sub-cycle of TSERC, the top three largest exergy destruction components are ejector 1, condenser 1 and generator 1, while the smallest exergy destruction occurs in pump 2. In addition, cycle performance comparison between SSERC and TSERC shows that the maximum COP improvement increased by 41.6 % and 89.6 % for 134a/R32 and R600a/R290 for TSERC, while the maximum entrainment ratio improvement increased by 32.4 % and 87.6 %. Moreover, it is concluded that the cycle performance improvement of TSERC is more significant at lower evaporation temperature, higher condensation temperature, and lower generation temperature compared to SSERC.

Carbone-neutral oriented methanol-reforming HT-PEMFC cogeneration based on absorption power refrigeration cycle

Methanol is considered as a promising hydrogen carrier in accelerating the carbon-neutral applications of fuel cell cogeneration. However, the methanol reforming process emits CO2 whose efficient recycling is challenging due to the inevitable power and cooling consumption. To this end, this paper considers the methanol reforming based high-temperature proton exchange membrane fuel cells (HT-PEMFC) cogeneration system, equipped with ammonia absorption power-refrigeration cycle (APRC). The waste heat in the integrated system is utilized for the fuel preheating, evaporation and reforming, driving APRC, and water heating, achieving a cascaded utilization of thermal energy. By building the mathematical models for each subsystem, thermodynamic and economic assessments are investigated. The results confirm that the waste heat of HT-PEMFC is sufficient to drive both methanol reforming and APRC, with additional electricity output from APRC. At a current density of 0.53 A/cm2, the exergy efficiency of the integrated system under the design condition is 0.525 and reaches a maximum value of 0.565 at a reaction temperature of 180 °C and pressure of 1.2 bar. The techno-economic analysis reveals a dynamic payback period of 5.2 years and net present value of 1.14 M$ with a 20-year lifespan of the integrated system.

Optimizing compressed air energy storage with organic Rankine cycle and ejector refrigeration for sustainable power and cooling provision

In the pursuit of sustainable energy systems, integrating storage technologies is crucial. Compressed air energy storage (CAES) emerges as a significant option for ensuring reliable power supply during peak hours. This study focuses on a configuration combining a CAES unit with two integrated organic Rankine cycle and ejector refrigeration units (ORCERC). The primary objective is to ensure adequate power supply during peak hours in the discharge period, while also providing cooling capacity to enhance system flexibility and efficiency. The proposed system is evaluated from thermodynamic, exergoeconomic, and exergoenvironmental perspectives to determine optimal operating conditions for the CAES unit. A fluid selection process identified a suitable zeotropic mixture, R141b/Hexane, as the working fluid for both ORCERCs. The results demonstrate a net power production of 20,749.91 kW and a cooling load of 1448.23 kW, with cost and exergoenvironmental impact rates of $1193.59/h and 143.42 Pt/h, respectively. Additionally, the configuration achieves an exergy round-trip efficiency of 65.85 % and a payback period of 2.88 years. Increasing the HTES temperature from 1150 to 1250 K improved both the round-trip efficiency (RTE) and exergetic round-trip efficiency (ERTE) from 52.22 % to 53.39 % and 62.98 %–63.71 %, respectively, while reducing the product cost and exergoenvironmental impact rate from $1128 to $1117/h and 134.35 to 127.93 Pt/h, respectively. Three triple-objective optimization scenarios were considered, yielding diverse outcomes, each contributing unique insights into the system's performance and potential improvements.

The optimization of a low-temperature geothermal-driven combined cooling and power system: Thermoeconomic and advanced exergy assessments

Switching from fossil fuels to geothermal energy can provide a solution to environmental issues and the energy crisis caused by power and cooling generation systems. In this regard, this study examines an innovative cogeneration system that utilizes low-temperature geothermal energy to generate power and cool the built environment simultaneously. The proposed system incorporates a Kalina-34 cycle, complemented by an integrated absorption refrigeration cycle. System analysis encompasses thermodynamic and exergoeconomic assessments, as well as parametric investigations to gauge the effects of critical parameters on system performance. Following a parametric study, the TOPSIS multicriteria decision-making technique is applied to optimize turbine inlet pressure, vapor generator temperature difference, and ammonia concentration. Finally, the main source of irreversibility is compared by conventional and advanced exergy analyses under optimal conditions. Results show that under optimal conditions, the energy utilization factor (EUF), exergy efficiency, and unit co-generation cost (UCGC) are 11.15 % (30.5 % improvement), 45.54 % (20.1 % improvement), and 6.04 $/GJ (1.63 % reduction), respectively. Furthermore, while conventional exergy analysis prioritizes the improvement of the condenser-1, vapor generator, and turbine in that order, advanced exergy analysis suggests a different priority order, with condenser-1, turbine, and vapor generator being the main objects of improvement, respectively. Implementing this study will significantly improve and optimize the proposed system.

Experimental analysis of a vapor compression refrigeration cycle with heat pipe

Experimental analysis of a vapor compression refrigeration cycle with heat pipe

Experimental Investigation of R404A Indirect Refrigeration System Applied Internal Heat Exchanger: Part 1—Coefficient of Performance Characteristics

In this study, the performance characteristics of an R404A indirect refrigeration system (IRS) applied to an internal heat exchanger (IHX) is evaluated for supermarkets and hypermarkets. In a direct expansion system, R404A is the primary refrigerant and R744, a brine, is the secondary fluid. While there are abundant studies analyzing the theoretical performance of IRS, experimental research on IRS is lacking, and there are no papers that address the results of changes in the IHX in detail. In this study, the results achieved by modifying various parameters are experimentally evaluated to provide fundamental data for designing the optimal IRS. In the main results, looking at the trend of the increase in IHX efficiency, the change is very minimal when the efficiency is above 50%, so it is ideal to apply an IHX efficiency of about 50% considering economics and COP, etc. Applying the results in this study enables the operation and maintenance of IRSs as an eco-friendly system by achieving energy efficiency through optimizing the system coefficient of performance and securing economic feasibility by minimizing the R404A charging amount of the refrigeration cycle. To serve supermarkets and hypermarkets, R744 as a secondary fluid may help to realize an ecologically friendly, compact IRS system with a high heat transfer coefficient that can operate at low temperatures (−35 to 5 °C).

Energy-exergy-economic-environmental (4E) analysis and multi-objective optimization of a cascade refrigeration system

The present work focusses on 4E analysis of a 50 kW cooling capacity cascade refrigeration cycle covering the aspects of energy, exergy, economic and environment analysis. The numerical investigation and the multi-objective optimization is carried out for the system using the refrigerant pair R170-R600a and R41-R600a. The refrigerant pair is selected based on the environmental implications in terms of GWP and ODP. Multi-objective optimization of the objective functions is carried out using a heat transfer search optimization algorithm to evaluate the optimal performance of the system. The effect of evaporator temperature, condenser temperature, LTC condenser temperature and LTC condenser temperature difference on the exergy efficiency and total cost of the system is studied. A set of multiple optimal solutions is presented using the Pareto optimal curve and TOPSIS criteria is employed to select the optimal operating condition. Compared to the refrigerant pair R170-R600a, the system with R41-R600a operates at better exergy efficiency and lower total cost. At the TOPSIS selected optimal condition, exergy efficiency and the total cost of the CRS is 63.5 % and 65,228 $/year for the refrigerant pair R41-R600a and 62.6 % and 67,690 $/year for R170-R600a, respectively. The distribution of variables shows that the effect of the evaporation temperature, condensation temperature and the LTC condenser temperature is profound in obtaining the optimal solution.

Performance evaluation of different types of indirect evaporative coolers: A CFD-based comparative study

Indirect evaporative cooling is an environmentally friendly alternative to the vapor compression refrigeration cycle. Numerical modeling and comparison of different types of indirect evaporative coolers are presented in this study. The developed models are categorized according to the process and working channel arrangement. The categorization comprises Type 0, denoting conventional configuration, alongside Type 1 and Type 3, characterized by singular aperture air passages, and Types 2 and 4, characterized by multiple aperture air passages. The study employed CFD models to investigate the impact of different operational parameters such as air temperature, humidity, velocity and the ratio of working to intake air on various performance parameters. Dew-point effectiveness, cooling capacity per unit area, water consumption per cooling capacity and COP were determined as performance parameters. The study reveals that the performance characteristics of different cooler types vary under diverse conditions. In most examined cases, Type 0 demonstrates superior cooling capacity per unit area, whereas Type 3 exhibits higher dew-point effectiveness. Furthermore, Type 2, with its favorable water consumption per cooling capacity and COP, emerges as the optimal choice in many scenarios, also demonstrating superior dew-point effectiveness at high working air ratios.

Experimental and analytical study of a reverse-bootstrap air refrigerator for fresh air-conditioning

ABSTRACT The use of hydrofluorocarbons as refrigerants is reduced because of the increasing greenhouse effect. Air refrigeration system is environmentally friendly with air as working medium. In this study, a reverse-bootstrap air refrigeration cycle was proposed for fresh air-conditioning, which eliminated the hot heat exchanger to reduce volume and weight, and a full fresh air refrigerator was developed and tested in a psychrometric test room. Combining the characteristic curves of a motor-driven turboexpander–compressor (MTEC), a numerical model was established to analyze system off-design performance. The relative deviations between the predictions and experimental data were within 10%. The test and calculation were conducted at standard conditions (the outdoor and indoor dry and wet bulb temperature is 35/24°C and 27/19°C, respectively). The MTEC operating at design rotating speed 38 krpm could supply fresh air with a flow rate of 517.4 kg/h and temperature of 12.0°C, and a cooling capacity of 2.93 kW was obtained with a system Coefficient of performance (COP) of 0.315. The optimal COP and corresponding rotating speed will change with the variation of environmental parameters. As the outdoor temperature increases from 32 to 38°C, the optimal COP decreases by 29.6% and the corresponding rotating speed increases from 39.6 to 47.6 krpm. Moreover, when the outdoor relative humidity increases from 35% to 55% and the indoor temperature increases from 23 to 31°C, the COP decreases by 36.8% and increases by 31.9%, respectively. The advanced exergy analysis indicate the system avoidable exergy destructions account for 54.9%. Under the avoidable conditions, the efficiency of compressor, expander, and motor is 0.85, 0.85, and 0.95, respectively, the system COP can reach 0.946. The exergy destruction analysis of components indicates the importance to optimize the compressor.