CO2 Air Conditioning Research Articles

Impacts of non-uniform airflow over gas cooler on the performance of transcritical CO2 air conditioning

The air flowrate and air inlet temperature over gas cooler in transcritical CO2 air conditioning for vehicles are easily unevenly distributed due to the influence of upstream obstacles. The unevenly distributed airflow affects heat exchange between CO2 and air, thus influencing the cooling capacity and power consumption of the air conditioning system. This study delves into the implications of such non-uniform airflow on the operational performance of transcritical CO2 air conditioning through simulation analysis. It elucidates the mechanisms by which the uneven distribution pattern, uneven distribution direction and non-uniformity affect the cooling capacity of transcritical CO2 air conditioning system. Moreover, the conditions of inhomogeneous airflow for enhanced heat transfer are explored by decoupling inhomogeneous flow rate and inhomogeneous temperature. Results indicate that non-uniform airflow of air temperature distributed as TB-HT (high in top and low in bottom) and air flowrate as TB-HB (high in bottom and low in top) increases the cooling capacity of 27.7 % at non-uniformity of 55 %. Conversely, uneven airflow of flowrate and temperature both distributed as LR-HL (high in left and low in right) at 40 % non-uniformity leads to a wastage of compressor power up to 1/2. These findings reveal the dual impacts of non-uniform airflow on the performance of transcritical CO2 air conditioning system, which emphasizes the importance of considering these impacts when designing automotive CO2 air conditioning systems.

Numerical simulation of heat transfer phenomena in a microchannel evaporator for a transcritical CO2 air conditioning system

This study presented a numerical simulation of heat transfer characteristics in a microchannel evaporator for a transcritical CO2 air conditioning system, and the simulated results have been validated by the experimental data. The evaporator consists of 6 passes with 29 flat tubes. Each flat tube has 10 microchannels having a 1.2 × 0.6 mm cross-section. The two-phase flow was simulated in the range of evaporation temperatures from 5 to 15 °C; the mass flow rates of 24.16, 30, and 34.96 g/s; and inlet vapor qualities of 0.5, 0.61, and 0.75, respectively. The results show that the two-phase flow with the evaporative temperature at the inlet of 5, 10, and 15 °C became superheated at the 3rd, the 4th, and the 5th pass, respectively. At the evaporation temperature of 15 °C, the heat transfer coefficient decreases from 8.1 to 3.6 kW/m2 K as the vapor quality increases from 0.75 to 1. With the mass flow rate of 30 g/s, in the tube length range from 0 to 1.1 m, the heat flux achieved around 1.55 kW/m2. The numerical cooling capacity gets the maximum value of 2.85 kW at the evaporative temperature of 10 °C. The numerical results are in good agreement with those obtained from the experimental results and the other published results, with a deviation of less than 10%.

Operational stability influenced by lubricant oil charge in the CO2 automotive air conditioning system

In a vapor compression refrigeration system, the oil enhances the efficiency of the compressor but impairs the performance of the heat exchanger. This paper focuses on the experimental study of the influence of oil quantity on the operational stability of CO2 air conditioning system. The operational stability of the system under different oil charge quantity and operating conditions is studied. The experimental results indicate that when the oil circulation rate increases to a certain value, the adjustment of electronic expansion valve (EXV) opening or the compressor speed will cause system fluctuation and the outlet of evaporator gets close to no superheat. The critical oil circulation rate of the experimental system is about 9.2 %. The analysis of the fluctuation shows that it becomes more severe with the increasing of the refrigerant mass flow rate. When the mass flow rate increases from 210 kg∙h − 1 to 250 kg∙h − 1 at 7000 r min−1 of compressor speed, the fluctuation frequency increases from 0.072 Hz to 0.095 Hz. The occurrence of unstable phenomena has remarkable impact on system performance, the maximum difference of this system in instantaneous cooling capacity and COP during the fluctuation period is respectively 0.5 kW and 0.13.

Experimental investigation on the cooling performance of direct and secondary loop CO2 air conditioning systems for electric vehicles

As a natural refrigerant, CO2 is considered one of next generation refrigerants of electric vehicle (EV) applications for excellent environmental and heating performance. The integration and safety performance of CO2 systems could be enhanced by secondary loop configuration, which is highly valuable for practical applications. This paper investigates the cooling performance characteristics of direct (DAC) and secondary loop (SLAC) transcritical CO2 air conditioning systems based on liquid cooled gas coolers (LCGC). A series of steady-state experiments were carried out for both systems to study the effects of compressor speed, EXV opening, outdoor air velocity, and indoor air flow rate on the cooling performances. Furthermore, the exergy destruction of each component and the exergy efficiency of both systems were analyzed based on the experimental results, showing that the evaporator needs to be better designed to balance heat transfer performance with pressure loss performance. The results showed that the cooling capacity and COP of SLAC system were reduced by up to 9.1% and 10.8% under the same compressor speed, compared with the DAC system. This study presents experimental references for further research and practical extension of the CO2 mobile air conditioner.

Energetic performance evaluation of an automotive CO2 air conditioning system with a dual-evaporator configuration

Inefficient performance of CO2 refrigerant in hot climates has become a major obstacle to its wide application in electric vehicles. In this paper, the vacant heating core in cooling mode was exploited to increase the heat transfer area of evaporator and improve system performance. With the aid of a validated evaporator model and an experimental test rig, performance improvements brought by this added area were evaluated. Results indicated that this area is not suggested to be directly added to cooling core. This necessitates a dual-evaporator (D-E) configuration. For case I (small CO2 pressure drop), the growth rate in heat transfer rate brought by D-E configuration is 19∼20% and less sensitive to parallel or series operation. Nevertheless, this growth rate for case II (large CO2 pressure drop) is greatly affected, 30.6∼32.6% for parallel operation and 7.5∼17.6% for series operation. Specifically, it is around 17.5% and 7.5∼13.6% for series cross-parallel and cross-counter arrangements, respectively. By comprehensive consideration, the cross-parallel arrangement is recommended to be adopted. Experimental results indicated that the cross-parallel D-E configuration greatly increases the evaporation temperature and exhibits a slight impact on optimum high pressure within the transcritical cycle. The enhancement in COP brought by this configuration is around 6∼12% without additional cost, and it rises with the decreased air flowrate through evaporator and air velocity through gas cooler and the increased ambient temperature. This indicates that this configuration is an effective and economical technology to alleviate the efficiency degradation issue of transcritical CO2 automotive air-conditioners.

Experimental Research on the Effect of Oil Charge Ratio on the Cooling Performance of CO2 Air Conditioning System for Electric Vehicles

From the perspective of the engineering application, the influence of the oil charge ratio on the cooling performance of the electric vehicle CO2 air conditioning system is experimentally studied in this paper. An online test device for oil circulation rate is set up on the built CO2 air conditioning system test bench. The experiment is conducted under different ambient temperatures and compressor speeds in the fresh air mode. As the oil charge ratio increases, the oil circulation rate increases as well as the oil mass flow rate, while the refrigerant mass flow rate decreases slightly. Under the refrigerant charge amount of 550 g and ambient temperature of 35 °C and 40 °C, this system reaches its best cooling performance when the oil charge ratio is in the range of 20% and 25%. And the maximum COPs are 2.46 and 2.36, respectively. The increase in the oil charge ratio is beneficial to the improvement of the isentropic efficiency and the volumetric efficiency of the compressor. The change in oil circulation rate has less effect on the pressure drop in the gas cooler than that in the evaporator.

The model predictive control strategy of the transcritical CO2 air conditioning system used in railway vehicles

• A transcritical CO 2 air conditioning system was developed for use in a railway vehicle. • The model predictive control strategy was used to optimize the system performance. • The proposed control method maximized the real-time COP without sacrificing comfort. • The simulation results showed that the proposed control strategy is effective. This paper presented a model predictive control strategy to optimize the operation of the transcritical CO 2 air conditioning system used in railway vehicles, which had to balance the needs of the passengers' comfort with energy-saving effects. In this work, the discharge pressure and evaporator air volume flow rates were optimized while maintaining comfort using a multi-variable control technique called model predictive control strategy. The objective function and the predictive model, which were proposed in this work combining data and physical laws, were the basis for the model predictive controller's ability to foresee future operation-conditions and calculate the optimal inputs. The validation results showed that the prediction error was less than 4.5%. The model predictive controller was adopted in GT-SUITE platform to realize the real-time maximization of the energy efficiency and maintain comfort requirement by adjusting discharge pressure, evaporator air volume flow rate and compressor speed. The simulation was conducted under fixed conditions and realistic conditions. As for the fixed conditions, the coefficient of performance achieved at 2.24, equaling to the maximum value of the map. Under the realistic conditions, the overall energy consumption of the control method using the MPC strategy was lower than that using the PID control strategy, and the average COP was increased by 7.4%. Further, there was a 1% error in coefficient of performance between the extreme value gained by the model predictive control method and the extreme value offered by the map method, which validated that the model predictive control strategy can be an effective control method for the optimal operation of the transcritical CO 2 air conditioning system.

The Effect of Fin Shape on the Heat Transfer and the Solution Time of a Microchannel Evaporator in a CO2 Air Conditioning System-A Numerical Investigation.

Numerical simulations on the fin shape of a microchannel evaporator in a CO2 air conditioning system were performed at the inlet evaporative temperature of 10 °C and the vapor quality of 0.61. Two types of fin shapes were dealt with: the straight fins and V-fins. The numerical results were verified by the experimental data. For the system under consideration and for the same heat transfer area and the heat transfer coefficient for the air side in the microchannel evaporator, the effect of the fin shape on the heat transfer was not different; however, the solution time and the physical memory for the straight fins were 1.3 and 1.45 times compared with the V-fins, respectively. Therefore, the V-fin shape should be used for numerical simulation to compare it with the straight fin shape. In this study, the evaporation of the refrigerant in the microchannel evaporator took place in four passes. The normal heat flux from the air through the fins and tubes was almost reached at 1550 W/m2 at the evaporative temperature of 10 °C. The results obtained from the experimental data were in good agreement with those obtained from the numerical results, with a deviation of less than 10%.

Experimental study on improved performance of an automotive CO2 air conditioning system with an evaporative gas cooler

A well-known challenge in the application of carbon dioxide (CO2) refrigerant for vehicles is the great energy efficiency attenuation in hot climates. In attempting to solve this problem, a novel design integrating evaporative cooling with automotive CO2 air conditioning system, where water was sprayed to the gas cooler, was introduced and experimentally investigated in present work. Results illustrate that with this design the CO2 temperature leaving gas cooler could be reduced to a lower level or even below the outdoor air temperature, thus inducing a significant performance improvement. A coefficient of performance (COP) enhancement percentage of 42.4% was achieved at 35°C outdoor condition. This percentage was increased up to 52.9% when outdoor air temperature increased to 45°C. With the decreased outdoor air velocity and the decreased outdoor air humidity, the merit of this design was more significant. These indicate that the evaporative gas cooler is a promising technology to alleviate the problem of performance deterioration in hot and dry weather conditions. A reduced spray water temperature could not cause a significant change in the COP enhancement. This may be attributed to that the sensible heat during droplet heat-up period was negligible compared to the latent heat of droplet evaporation. When the surface position of gas cooler chosen to be sprayed was moved from CO2 inlet to CO2 outlet, the COP enhancement increased, which suggests that water should be sprayed to the area where wall temperature is low.

Performance and economic evaluation of CO2 (R744) air conditioning system driven by compressed air engine

Traditional building air conditioning uses electricity or thermal energy to drive the refrigeration cycle. The energy released by the expansion of compressed air can also be used for driving the refrigeration cycle. Moreover, the expanded low-temperature air can be used in the air conditioning system too. In this paper, three forms of CO2 air conditioning systems driven by novel reciprocating piston compressed air engines were designed, namely EAGC (Exhaust Air for Gas Cooler), EAS (Exhaust Air for Sub-cooler), and EASC (Exhaust Air for Space Cooling), to analyze the performance and economics of CO2 air conditioning systems combined with compressed air energy storage technology. Twenty-seven typical cities were selected in different climate regions in China to evaluate the systems' performance and running cost under outdoor temperature changes and different electricity prices. The results show that compared with the electricity-driven CO2 air conditioning system, the three systems can greatly improve the system COP (Coefficient of Performance) but still cannot reach the performance level of the conventional freon refrigeration systems. However, in cities like Beijing and Shanghai with the large peak-to-valley electricity price difference, EASC system compared to the electricity-driven R410A air conditioning system running costs are lower, the average running cost per hour can be saved 11.96% and 16.35%, respectively. This means that the CO2 air conditioning system with compressed air energy storage technology has a certain economic application value and can be popularized on reasonable occasions. This innovative system design can help solve the contradiction between supply and demand of building energy, realize leisure energy consumption, and reduce the economic cost of energy consumption on the demand side.

The Model Predictive Control Strategy of the Transcritical Co2 Air Conditioning System Used in Railway Vehicles

The Model Predictive Control Strategy of the Transcritical Co2 Air Conditioning System Used in Railway Vehicles

Refrigerant distributions and dynamic migration characteristics of the transcritical CO2 air conditioning system

Research on refrigerant mass migration and distribution could not only provide deep understanding of the system performance enhancement, but also contribute to searching the high risk components caused by the Boiling Liquid Expanding Vapor Explosion (BLEVE) due to the high operation pressure of transcritical CO2 system. An experimental bench and a mathematical model of the transcritical CO2 automotive air-conditioning system were set up to explore the refrigerant migration characteristics at different operation conditions. Simulated results of the system were validated by experiments. The distribution characteristics of the refrigerant under different charge were studied and the refrigerant charge suggestions were given. Then, under the appropriate refrigerant charge, the migration characteristics of the refrigerant in the automobile air conditioning system were simulated in the process of startup and shutdown of the compressor and conversion of cooling mode. Furthermore, influence of the parameters, such as ambient temperature, air flow rate, compressor speed on the characteristics of the refrigerant distribution in the air-conditioning system were studied. The results could provide reference for the system design and system safety.

Cooling performance enhancement for the automobile transcritical CO2 air conditioning system with various internal heat exchanger effectiveness

• CO 2 system with internal heat exchanger (IHX) were investigated. • An experiment to change IHX under different conditions was designed. • Effects of IHX on system performance and parameters were studied. • Effect of various IHX under a wide working condition was analyzed. • Energy and exergy analyses with different IHX were conducted. Carbon dioxide (CO 2 ) is one of the promising refrigerant substitutes in mobile air conditioning system. To enhance the cooling performance, this paper presented an experimental investigation on the influence of the various internal heat exchanger (IHX) effectiveness on a transcritical CO 2 mobile air conditioning system. An experimental apparatus with regulatable IHX effectiveness was set up and tested. The effects of IHX effectiveness on the cooling capacities and coefficient of performance (COP) were studied under various outdoor air temperatures (30, 35 and 40 ℃) and compressor speeds (2000 to 7000 rev·min −1 ). Optimal discharge pressure and cycle state parameters with various IHX effectiveness were analyzed. Based on exergy perspective, the irreversibility of the main components and the cycle were discussed to give further reference for system optimization. Results showed that using IHX enhanced the COP of CO 2 system by the rates from 14.5% to 18.5% at various outdoor air temperatures. With increasing IHX effectiveness, the COP enhancement decreased, and the discharge temperature easily increased to its restriction value. The compressor and gas cooler occupied the main exergy losses of the system with IHX. The results could provide reference for the design and optimization of the CO 2 mobile air conditioning system.

Coupling Effect of Air Flow Rate and Operating Conditions on the Performance of Electric Vehicle R744 Air Conditioning System

The air flow rate on the gas cooler side is one of the key parameters affecting the performance and running safety of transcritical CO2 electric vehicle air conditioning systems. After experimentally analyzing the effects of the air volume flow rate in the gas cooler on the cycle parameters and system performance, a novel method to evaluate the optimal air flow rate was proposed. In addition, the effect of the gas cooler air volume flow rate on the key performance parameters of the system (e.g., optimal discharge pressure) was explored. Finally, the coupling effects of the compressor speed, ambient temperature and optimal air flow rate on the system performance was also exhaustively assessed. It was found that as the discharge temperature, the CO2 temperature at the gas cooler outlet and the discharge pressure did not vary more than ±2%, the corresponding gas cooler air volume flow rate was optimal. For the single-row and dual-process microchannel evaporator used in this work, the recommended value of the optimal gas cooler air volume flow rate was 2500 m3·h−1. The results could provide reference for the fan speed design of electric vehicle CO2 air conditioning systems, especially for the performance under idling model.

Energy and exergy analyses of a transcritical CO2 air conditioning system for an electric bus

Abstract The cooling performance of a transcritical CO2 air conditioning system for an electric bus was investigated using energy and exergy analyses. The effects of the working conditions, such as the compressor speed, electronic expansion valve (EEV) opening, inlet temperature and flow rate of indoor air and the inlet temperature of outdoor air on the energy performance were experimentally investigated. Exergy analysis of the system was performed based on the experimental data. Experimental results showed a coefficient of performance (COP) of 2.88 and a cooling capacity of 31.4 kW at a condition of 35 °C/27 °C (an outdoor air temperature of 35 °C and an indoor air temperature of 27 °C). As the outdoor air temperature increased to 45 °C, a COP of 1.87 was achieved with a cooling capacity of 22.8 kW. Therefore, the system possessed a good cooling performance in hot climates. Increasing the indoor air flow rate could improve the exergy efficiency of the system and increasing the indoor and outdoor air temperatures could reduce the exergy efficiency of the system. Among the exergy destructions of the components, the values obtained for the gas cooler and EEV accounted for significant proportions, offering great opportunities for improving the system performance. In extremely hot climates the exergy destruction of EEV comprised the greatest percentage and further research should be performed on the significant throttling loss of the expansion process.

Application and analysis of carbon dioxide refrigeration system with ejector in commercial super cold cabinet

In this paper, the transcritical CO2 refrigeration system with ejector is applied to a commercial ultra-cold cabinet. The influence of the key operating parameters of the ejector on the system performance is analyzed, and the measurement method of the experimental parameters is proposed. The results show that the ejector can eject the low pressure refrigerant into the reservoir by using the energy of the large pressure difference throttle, and the COP of the system can be increased by 9%-19%. This paper provides a theoretical basis for the design, development and optimization of CO2 refrigeration and air conditioning system with ejector.

Review of stationary and transport CO2 refrigeration and air conditioning technologies

Since its resurgence as a refrigerant in the 1990s, Carbon Dioxide (CO2) has grown in popularity and breadth of application. Its negligible global warming potential (GWP) eliminates the risk of being phased out due to legislation, and its non-toxicity, non-flammability, and low cost allows its use in many vapor compression cycle applications. However, the high critical pressure and low critical temperature require significantly more compressor power under moderate and high-ambient conditions and thus, necessitate the addition of cycle modifications to reach coefficient of performance (COP) values equal to or greater than those of other working fluids. This paper provides a review of research conducted on the use of CO2 vapor compression cycles in refrigeration and air conditioning (AC) cycles for both transportation and stationary refrigeration. A primary intention of writing this review is to offer evidence and justification for many common cycle modifications, then connect these modifications to a broad array of applications. Thus, advanced cycles in mobile and stationary applications are shown and the individual modifications present have their own independent justification to create a complete picture of the topic. The designs of complete systems, as well as specific components within the system, are included, as are the relations between the thermo-physical properties of CO2 and their benefits in these particular applications. Additionally, economic analyses of the feasibility of using CO2 in place of existing fluids are reviewed. Challenges facing the use of CO2 refrigeration cycles in stationary and transportation refrigeration are discussed, as are potential future advancements to overcome them.

Design Evaluation for a Finned-Tube CO2 Gas Cooler in Residential Applications

Towards the introduction of environmentally friendlier refrigerants, CO2 cycles have gained significant attention in cooling and air conditioning systems in recent years. In this context, a design procedure for an air finned-tube CO2 gas cooler is developed. The analysis aims to evaluate the gas cooler design incorporated into a CO2 air conditioning system for residential applications. Therefore, a simulation model of the gas cooler is developed and validated experimentally by comparing its overall heat transfer coefficient. Based on the model, the evaluation of different numbers of rows, lengths, and diameters of tubes, as well as different ambient temperatures, are conducted, identifying the most suitable design in terms of pressure losses and required heat exchange area for selected operational conditions. The comparison between the model and the experimental results showed a satisfactory convergence for fan frequencies from 50 to 80 Hz. The absolute average deviations of the overall heat transfer coefficient for fan frequencies from 60 to 80 Hz were approximately 10%. With respect to the gas cooler design, a compromise between the bundle area and the refrigerant pressure drop was necessary, resulting in a 2.11 m2 bundle area and 0.23 bar refrigerant pressure drop. In addition, the analysis of the gas cooler’s performance in different ambient temperatures showed that the defined heat exchanger operates properly, compared to other potential gas cooler designs.

Performance Enhancement Research of a CO2 Air Conditioning System with Propane Mechanical Subcooling for Electric Vehicle

Performance Enhancement Research of a CO2 Air Conditioning System with Propane Mechanical Subcooling for Electric Vehicle

Effects of various operating conditions on the performance of a CO2 air conditioning system for trains

This paper aims to investigate the effects of different operating parameters on the performance of a transcritical CO2 air conditioning system for trains. Similar designs of automotive CO2 air conditioning system are used in the train system such as internal heat exchanger (IHX) to improve the system efficiency. Experimental results are presented for the performance characteristics of the CO2 train air conditioning system with two independent circuits under different test conditions such as different refrigerant charge amount, compressor working frequency, indoor air volume flow rate and outdoor chamber temperature. Two different operating modes of the two circuits are also included in this research. For mode I while only one circuit is working, the test results show that the maximum cooling capacity and coefficient of performance (COP) is 24.3 kW and 2.7, during 55 Hz and 40 Hz compressor working frequency conditions respectively. Also, the cooling capacity and COP reach the minimum, 16.2 kW and 1.5, when the outdoor chamber temperature is 45 °C. For mode II while both two circuits are working, the results show that the cooling capacity and COP are 42.0 kW and 2.4, respectively. The system performance error between the test and numerical modeling which directs the design of the system is within ±5%. Furthermore, an optimum high pressure control algorithm for the transcritical CO2 cycle of the train AC system is also presented.