CO2 Refrigeration System Research Articles

Energy and advanced exergoeconomic analysis of a novel ejector-based CO2 refrigeration system and its optimization for supermarket application in warm climates

Energy and advanced exergoeconomic analysis of a novel ejector-based CO2 refrigeration system and its optimization for supermarket application in warm climates

Enhancing energy efficiency and sustainability in ejector expansion transcritical CO2 and lithium bromide water vapour absorption refrigeration systems

Enhancing energy efficiency and sustainability in ejector expansion transcritical CO2 and lithium bromide water vapour absorption refrigeration systems

Optimization analysis of Organic Rankine Cycle powered by waste heat of a supermarket transcritical CO2 multi-ejector refrigeration cycle

The exploitation of low-grade heat sources that are available from existing thermal systems processes consists a promising concept for the production of energy. In this research, the integration of an Organic Rankine Cycle (ORC) unit on a supermarket transcritical CO2 refrigeration system with ejectors is investigated, which is supplied with low-grade heat by the common discharge of the compressors. The optimization analysis proves that for different combinations of CO2 temperature and pressure in the ORC evaporator, as well as for different ORC condensation temperatures, there is a different optimum ORC evaporation temperature that leads to the highest energy production. Concerning the annual performance analysis conducted for the city of Athens, Greece, operation with isopentane is found to provide the greatest energy production with 10.36 MWh annually, which corresponds to a 3.69% reduction in the refrigeration system's energy consumption. For operation under the ambient temperature of 40 °C, the corresponding reduction of the refrigeration system consumption due to the presence of ORC is 7.54%. Finally, the financial analysis reveals that toluene is the most advantageous solution, leading to a payback period of 3.9 years, with a net present value by the end of the project's lifetime equal to 34.32 k€.

Review of Recent Advances in Transcritical CO2 Heat Pump and Refrigeration Cycles and Their Development in the Vehicle Field

Refrigerant substitution is an urgent need in the context of reducing carbon emissions and slowing global warming. CO2 is now being proposed as a promising solution based on its excellent properties and system performance, especially in low-temperature environments. This paper presents an overview of recent advances in system configuration and operation characteristics to improve the performance of transcritical CO2 heat pump and refrigeration systems. The paper first introduces the basic research background, system cycle, and thermodynamic characteristics. Secondly, CO2 cycle improvements with single modifications and modification combinations are reviewed. Then, some important operation characteristics and control methods are discussed. Additionally, the paper provides a detailed description of the development of transcritical CO2 heat pump and refrigeration systems in the vehicle field. At the end of this review, conclusions and opportunities for future work in this field are presented.

Estimation of evaporator valve sizes in supermarket refrigeration cabinets

In many applications, e.g. fault diagnostics and optimized control of supermarket refrigeration systems, it is important to determine the heat demand of the cabinets. This can easily be achieved by measuring the mass flow through each cabinet, however that is expensive and not feasible in large-scale deployments. Therefore it is important to be able to estimate the valve sizes from the monitoring data, which is typically measured. The valve size is measured by an area, which can be used to calculate mass flow through the valve — this estimated value is referred to as the valve constant. A novel method for estimating the cabinet evaporator valve constants is proposed in the present paper. It is demonstrated using monitoring data from a refrigeration system in a supermarket in Otterup (Denmark), consisting of data sampled at a one-minute sampling rate, however it is shown that a sampling time of around 10–20 min is adequate for the method. Through thermodynamic analysis of a two stage CO2 refrigeration system, a linear regression model for estimating valve constants is developed using time series data. The linear regression requires that transient dynamics are not present in the data, which depends on multiple factors e.g. the sampling time. If dynamics are not modelled it can be detected by a significant auto-correlation of the residuals. In order to include the dynamics in the model, an Auto-Regressive Moving Average model with eXogenous variables (ARMAX) is applied, and it is shown how it effectively eliminates the auto-correlation and provides less bias and higher accuracy of the estimates. Furthermore, it is shown that the sample time has a huge impact on the estimates. Thus, a method for selecting the optimal sampling time is introduced. It works individually for each of the evaporators, by exploring their respective frequency spectrum. That way, a reliable estimate of the actual valve constant can be achieved for each evaporator in the system and it is shown that sampling times above 10 min are optimal for the analysed systems.

Dynamic simulation study on regulation characteristics of transcritical CO2 multi-ejector refrigeration system

This paper presents a dynamic model of transcritical CO2 refrigeration system with multi-ejector which is developed based on the real CO2 properties and considering the internal transonic flow, phase change and choking, and its predictions are in good agreement with experimental data from the literature. The transient response of the system is studied by adjusting the speed of compressor, water flow of heat exchanger, opening of expansion valve and combination of ejector as disturbance conditions. The presented results show that the developed dynamic model has similar response rules with the same type of system. Changes of cooling water flow of gas cooler and chilled water flow of flooded evaporator can cause significant response of other pressures in the system except low-temperature evaporator, and can regulate the cooling and heating load to some extent. Opening of medium and low temperature expansion valve plays a decisive role in the system's medium and low temperature refrigeration capacity respectively, and the low-temperature expansion valve is the only regulation means of the low-temperature cooling capacity. Optimal operating combination of multi-ejector maximizes the system performance and ejector pressure lift ratio. Furthermore, the optimal ejector combination of the system under the refrigeration capacity of 75∼115 kW is studied and compared, a control strategy of adjusting ejector combination by "nozzle mass flow rate" is put forward to adapt to the high-performance operation of the system under different loads. Under the research conditions, the optimal ejector combination satisfies that the nozzle mass flow rate is stable in 0.066∼0.070 kg·s-1.

Design and performance simulation of a novel waste heat-driven liquid desiccant combined with transcritical CO2 refrigeration system for ocean-going vessels in typical shipping routes

Design and performance simulation of a novel waste heat-driven liquid desiccant combined with transcritical CO2 refrigeration system for ocean-going vessels in typical shipping routes

Thermodynamic and economic analysis of a supermarket transcritical CO2 refrigeration system coupled with solar-fed supercritical CO2 Brayton and organic Rankine cycles

Aiming to reduce the use of conventional energy sources, alternative methods should be applied in order to cover society’s needs. In this context, in the present study, the operation of a transcritical CO2 commercial refrigeration unit is investigated, coupled with two power production cycles. The first cycle is a supercritical CO2 Brayton system that is supplied with heat by a solar field including parabolic trough collectors, while the second one is an organic Rankine cycle, utilizing as a heat source the low enthalpy CO2 exiting the first cycle. The novelty of this system is attributed to the combination an advanced transcritical CO2 refrigeration system with power production cycles that operate under the optimal conditions, as well as to the reason that the operation of the refrigeration unit is independent from the solar energy, since it can provide refrigeration output regardless of the solar heat input. The results prove that the increased number of solar collectors leads to more advantageous power production results, although with higher capital costs. Furthermore, greater energy production is noted when the ambient temperature is lower and abundant solar power is available. For the case of installing five solar collectors and an organic Rankine cycle, it is found that the annual energy production of the total system is 173.72 MWh, while the annual energy consumption of the refrigeration unit is 466.66 MWh, meaning that 37.23 % of energy savings are achieved annually. For the same configuration, according to the life cycle cost analysis, the capital cost is found equal to 243.72 k€, leading to a payback period of 7.6 years. By the end of the project’s lifetime, the operation of the installed system returns 298.42 k€ to the investor.

Hybrid solar energy and waste heat driving absorption subcooling supermarket CO2 refrigeration system: Energetic, carbon emission and economic evaluation in China

A novel hybrid solar energy and waste heat driving absorption subcooled carbon dioxide (CO2/R744) booster refrigeration system (SE + WH) is proposed to improve the energy efficiency of the traditional CO2 refrigeration system used in supermarkets. The thermodynamic and energy conversion model of the system is established. Then, the optimization method of the two-dimensional golden section search algorithm is introduced to optimize the system energy efficiency taking the variation of ambient temperature and solar radiation intensity into account. Moreover, considering the maldistribution of solar energy and the influence of meteorological conditions in China, eight typical cities are selected to assess the annual performance factor and life cycle carbon emissions to achieve energy saving and carbon emission potentials. The performance of hybrid solar energy and waste heat driving absorption subcooled CO2 booster refrigeration system is the best with the COP enhanced by 3.05–42.30 % at the solar radiation intensity of 1200 W/m2 and the temperature of 0–35 °C in contrast to the baseline system. In addition, the discharge pressure of the new proposed hybrid system can be reduced by 7.86 % when the ambient temperature is 34 °C. The annual performance factor of the new proposed hybrid system in Haikou can be increased by 14.47 %, and the life cycle carbon emissions are effectively reduced by 39.54 %. The payback period is also reduced by 4.01 % and 3.37 % for Beijing and Shanghai. Consequently, the novel configuration of the hybrid absorption subcooling subsystem is recommended, particular in hot or warm climate areas with rich solar energy.

Evaluation of the ejector two-stage compression refrigeration cycle with work performance from energy, conventional exergy and advanced exergy perspectives

Low temperature cold chain is the top priority of logistics development at this stage. CO2 (carbon dioxide) refrigeration system is widely used. However, the low efficiency of CO2 refrigeration system is the biggest obstacle to its development. In this research work, EES (Engineering equation solver) is used to simulate the system, which is widely used in thermodynamic modeling and calculation, a novel trans-critical CO2 refrigeration cycle with ejector for low-temperature storage is proposed. The energy and conventional exergy model of the system are established, and the advanced exergy model is established based on the conventional exergy model, and the actual performance of the system is analyzed, and the optimization potential of the system components is analyzed through the commonness of the two exergy analysis methods. The results show that the exergy destruction of each component of the ejector is the largest in the system, and the optimization potential is the highest in both conventional and advanced exergy models. After studying the influence of main parameters in the system cycle on the system performance and performance, it is found that there is an optimum intermediate pressure and gas cooler pressure in the system to maximize the system performance and efficiency. According to the influence of the change of system parameters on the system, it is found that the outlet temperature of gas cooler has the greatest influence on the optimization of ejector.

Energy and exergy analysis of a novel dual-ejector booster transcritical CO2 refrigeration system for applications in warm climates

Energy and exergy analysis of a novel dual-ejector booster transcritical CO2 refrigeration system for applications in warm climates

Consistent control strategy for CO2 refrigeration systems based on refrigerant charge management

CO2 refrigeration systems operate in both subcritical and transcritical modes throughout a year. Different control strategies have been developed under different operation modes. However, this leads to complexity in implementation. In this work, we proposed a consistent control strategy (CCS) independent of operation modes for CO2 refrigeration systems to provide stable operation in a simple way. This strategy is grounded on the fact that discharge pressure control in transcritical mode is essentially regulating refrigerant charge in a running system. The active pressure control can be avoided with fine refrigerant charge management, thereby employing the same control strategy as in subcritical mode. Through numerical simulations, the characteristics of optimum charge in three typical CO2 refrigeration configurations were identified, which are single-stage system, two-stage system, and booster system. The optimum charges of three systems under different operating conditions all vary less than 22%, particularly the booster system fluctuates by 6% only. By adopting CCS, the average COP losses for three systems are all within -0.5% relative to their ideal optimum performance. Compared with re-optimized conventional multi-zone control, the maximum deviation is -0.3%. Therefore, the proposed new strategy can achieve comparable quasi-optimum performance, while significantly simplifying control scheme. It can be an attractive solution for small units which require simple and robust control.

Experimental and analytical investigation of CO2/R32 condensation heat transfer in a microchannel

Previous research has shown that CO2 mixtures can improve the thermal performance of CO2 refrigeration systems. Moreover, the use of a microchannel heat exchanger can reduce the refrigerant charge and improve the heat transfer coefficient. Therefore, in this experimental study, we investigated the condensation heat transfer characteristics of CO2/R32 in a porous flat microchannel with a hydraulic diameter of 0.715 mm. According to the results, the convective heat transfer coefficient decreased rapidly then increased slowly with an increase in the CO2 mass fraction. The minimum value was achieved when the CO2 mass fraction was approximately 55%. In addition, the convective heat transfer coefficient decreased with an increase in the condensation temperature. Higher vapour quality and mass flow rate conditions corresponded to higher convective heat transfer coefficients. The prediction correlations applicable to the in-tube condensation heat transfer exhibited low prediction results for CO2/R32, with a heat transfer coefficient prediction error for the newly established prediction model of 16.77%. This study provides theoretical support for analysis of the CO2/R32 condensation heat transfer mechanism in microchannels and the future design of microchannel condensers.

Performance Study and Multi-Objective Optimization of a Two-Temperature CO2 Refrigeration System with Economizer Based on Energetic, Exergetic and Economic Analysis

Performance Study and Multi-Objective Optimization of a Two-Temperature CO2 Refrigeration System with Economizer Based on Energetic, Exergetic and Economic Analysis

Evaluation of Various Ejector Profiles on CO2 Transcritical Refrigeration System Performance.

This study examines the potential impact of the different ejector profiles on the CO2 transcritical cooling system to highlight the contribution of the multi-ejector in the system performance improvement. The research compares the implementation of an ejector-boosted CO2 refrigeration system over the second-generation layout at a motive flow temperature of 35 °C and discharge pressure of 90 bar to account for the transcritical operation mode. The result revealed a significant energy saving by reducing the input power to the maximum of 8.77% when the ejector was activated. Furthermore, the multi-ejector block could recover up to 25.4% of the expansion work losses acquired by both ejector combinations VEJ1 + 2. In addition, the behavior of the multi-ejector geometries and operation conditions greatly influence the system exergy destruction. The analysis shows a remarkable lack of exergy destruction during the expansion process by deploying the ejector in parallel with the HPV.

Grey-box Fault Models and Applications for Low Carbon Emission CO2 Refrigeration System

CO2 refrigeration systems have attracted more attentions in recent years because of their zero-carbon emission advantages compared with traditional refrigerants. These systems are widely used in certain commercial buildings such as supermarkets. However, very limited energy studies have been done for such refrigeration systems under faults. This study investigated five common faults for supermarket transcritical CO2 refrigeration systems with display cases. The following major tasks were implemented for this study. (1) Five fault models (gray-box format) regarding the power consumption and supply air temperature were proposed for low- and high-temperature display cases, respectively. The models cover: ice accumulation on evaporator coil, evaporator fan partial failure, expansion valve failure, display door open, and condenser blockage. (2) These fault models were calibrated with field tests, and the modeling accuracies were in good agreement with measurement datasets. (3) The proposed fault models demonstrated more than 90% of fault detection for Fault Detection and Diagnostics (FDD) purposes for both field measurement and simulated data.

Visualization and Measurement of Swirling Flow of Dry Ice Particles in Cyclone Separator-Sublimator

The dry ice sublimation process of carbon dioxide (CO2) is a unique, environmentally friendly technology that can achieve a temperature of −56 °C or lower, which is a triple point of CO2 in CO2 refrigeration systems. In this study, a cyclone separator-evaporator was proposed to separate dry ice particles in an evaporator. As an initial step before introducing the cyclone separator-evaporator into an actual refrigeration system, a prototype cyclone separator-evaporator was constructed to visualize dry ice particles in a separation chamber. A high-speed camera was used to visualize the non-uniform flow of dry ice particles that repeatedly coalescence and collision in a swirl section. Consequently, the dry ice particle size and the circumferential and axial velocities of dry ice were measured. The results show that the equivalent diameter of the most abundant dry ice particles in the cyclone separation chamber is 2.0 mm. As the inner diameter of the separation section decreases, dry ice particles coalesce and grow from an equivalent diameter of 4 mm to a maximum of 40 mm. In addition, the comparison of the experimental and simulation results shows that the drag force due to CO2 gas flow is dominant in the circumferential velocity of dry ice particles.

Advanced exergy analysis of a CO2 two-phase ejector

Ejector is meaningful and valuable to improve the transcritical CO2 refrigeration system. To study in-depth on the exergy destruction is significant for the performance enhancement of CO2 ejector. This paper presents a comprehensive investigation on the exergetic performance of CO2 two-phase ejector using conventional and advanced exergy analysis method to explore the interactions among ejector components and their improvement potential. The effects of primary flow pressure, suction flow temperature and area ratio (motive nozzle throat area to mixing chamber area) on the ejector exergy destruction are discussed. The results indicate the largest exergy destruction in the ejector is caused by the mixing chamber, followed by the motive nozzle, diffuser and suction chamber. Based on the avoidable exergy destruction rate and exergy destruction proportion, the order of optimization potential for ejector components is motive nozzle, diffuser, mixing chamber and suction chamber. However, the sum of avoidable exergy destruction of mixing chamber is the largest, which has the highest priority for improvement of overall ejector performance. The exogenous exergy destructions of the mixing chamber and diffuser are largely caused by the motive nozzle, and the exergy destructions would be increased with the improvement of motive nozzle performance. There exists an optimal primary flow pressure 9.5 MPa to minimize the exergy destruction of the each component of the ejector. To reduce the suction flow temperature and the ejector area ratio can decrease the avoidable exergy destruction of the components. Moreover, the variation of operating parameter has no effect on the optimization sequence of the overall ejector and its components. This work would provide a helpful way to guide the optimization on the ejector efficiency.

The prediction for optimum combination of high pressure and intermediate pressure on a small refrigerated cabinet with CO2 transcritical two-stage cycle

It is universally acknowledged that the use of two-stage cycle can effectively improve the comprehensive performance of the CO2 refrigeration system. There exists an optimum combination of high pressure and intermediate pressure in a CO2 refrigeration system with transcritical two-stage cycle where the system yields the best coefficient of performance (COP). In this paper, a novel double-iteration method is used to predict the optimum combination of high pressure and intermediate pressure of a CO2 refrigerated cabinet system with transcritical two-stage cycle and the accuracy of prediction is experimentally verified. Then, the validated thermodynamic model is employed to identify the calculation accuracies of related existing correlations for calculating the optimum high pressure and the optimum intermediate pressure under various conditions. The results show that the thermodynamic model has enough accuracy to predict the optimum combination of high and intermediate pressures with an error less than 2.07%. Among the several major existing correlations, ′′Sarkar + Yari′′ (-10 °C≤Te≤10 °C, 30 °C≤Tgc,o≤50 °C) display the most accurate results to calculate the combination of high and intermediate pressures of a CO2 transcritical cycle. Meanwhile, the difference of thermodynamic performance predicted by the model and ′′Sarkar + Yari′′ correlations can be negligible (approximate to 0).

The Performance Analysis of an Integrated CO2 Refrigeration System with Multi-Ejectors Installed in a Supermarket

The field data from an integrated CO2 refrigeration system installed in a supermarket located north of the capital of Lisbon was analyzed. The goal was to demonstrate the effect and performance of multi-ejectors on a refrigeration system operating in a warm climate. The measurement results for the system with and without activating ejectors were compared. It was observed that the system with a multi-ejector operation had considerable performance superiority. The ejectors improved the cooling capacity of the medium temperature stage of the system by 17.4%. The system with active ejectors had 7.5% less total power consumption compared to the ejector off mode of the same system.