#
Evaluation of Various Ejector Profiles on CO_{2} Transcritical Refrigeration System Performance

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## Abstract

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_{2}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 CO

_{2}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.

## 1. Introduction

_{2}technology as a leading solution [2]. When R744 became known as a refrigerant, it was greeted by stiff skepticism and stern criticism among relevant scientific communities [3]. The reason for this was the characteristic heat sink operational pressure and low system efficiency, which call for another mechanism integration that fosters technical advancement and addresses unprecedented technological challenges. Recently, these challenges and the quest for system advancement have been addressed through better process design, which has shed more light on the merits of using CO

_{2}in cooling systems.

_{2}has superior thermal properties over other refrigerants. For instance, natural refrigerants represent the class with the highest thermal conductivity and specific heat capacity. They also possess the high latent heat of vaporization needed for a more efficient heat transfer within the evaporator. Moreover, CO

_{2}features a high volumetric refrigeration capacity, which is well known to significantly influence the heat transfer coefficient [4]. In addition, CO

_{2}has a low viscosity, which mainly reduces the initial investment cost due to the cost-effective geometrical properties of the valves, pipelines, and other ancillary components.

_{2}has a high saturation pressure of 4–12 times that of other refrigerants, which requires special technical considerations during the manufacturing process. Thus, due to the merits of the excellent properties of natural refrigerants, they are a favorable choice of working fluid [4]. The market has witnessed a surge in the applications using this refrigerant in cooling systems worldwide. This is encapsulated in the fact that ejectors improve the performance of transcritical refrigeration cycles when integrated with parallel compressors to attain the attendant pressure lift. The advantages of using the ejector have resulted in many ground-breaking types of research in terms of significant energy reduction, which is greatly reliant on the ejector geometries, refrigerant properties, and the core purpose of its applications.

_{2}multi-ejector outperformed other fluorinated working fluids in conventional-based solutions, especially in northern and central Europe [23]. The results showed 26.9% higher energy savings in average-sized supermarkets utilizing CO

_{2}as a refrigerant. The ejector has contributed to the air conditioning applications, and can reduce the total system power consumption by 8.3–8.6% in different system configurations.

_{2}transcritical refrigeration system. The performance of the ejectors and the overall system operational characteristics is emphasized as the main objective of the study.

## 2. System Configuration

## 3. System Performance Calculations

- the processes for all the analyses are steady-state;
- the pressure drop at the gas cooler, evaporator, and piping is not considered;
- the kinetic and the potential energies are neglected;
- the system is well isolated.

## 4. Results and Discussion

#### 4.1. Ejector Characteristic Functions

_{lift}= 2 bar, ER reaches 0.83, which is lower than when using VEJ1 alone. However, the multi-ejector allows entraining a 50% higher suction mass flow rate with the ejector combinations, but the motive mass flow rate also experiences a surge. When the pressure lift is increased, the ER drops gradually, which vanishes for VEJ1 from P

_{lift}= 8 bar where this profile is introduced as a normal expansion valve. In contrast, VEJ2 continues to produce a higher pressure lift to 12 bar with ER of 0.091. However, the ejector efficiency for VEJ1 + 2 acquired an optimum value of 25.4% reporting lower efficiency than using the VEJ1 profile alone but extended to cover a wide range of the operational condition. In other words, the combination of the ejector cartridges greatly influenced the system’s performance by improving the work rate recovered. The results demonstrated an increase in the recovered work rate to a maximum of 0.198 kW and recorded a rate that was 2.2-times higher overall than that of the single VEJ1 used under the same operating conditions based on Equation (3). Generally, when the systems are running in the transcritical state, the amount of flash gas increases, which increases the maximum work recovery potential, thereby reducing the ejector efficiency.

_{2}transcritical refrigeration systems are the expansion work rate recovery (Ẇ

_{r}) and the overall available work recovery potential (Ẇ

_{r,max}). These parameters indicate the power available to perform isentropic compression on the suction flow through the ejector to the separator and the maximum theoretical work recovery potential that depicts the total irreversibility of the ejector [22]. Figure 4 illustrates the work rate and maximum potential work recovery characteristics via different pressure lifts. The analysis was performed for the parallel compressor system layout as the baseline compared with varying configurations of ejectors for 10 kW cooling capacity. The results show the maximum work recovery rate of expansion in the high-pressure valve for the parallel system with expansion work ranging from 1 kW at P

_{lift}= 2 bar to 0.7 kW when the pressure lift increases to 12 bar. This indicates the significant throttling loss of CO

_{2}as a refrigerant compared with other low-pressure working fluids, especially at ambient temperatures that force the cycle to operate in transcritical mode. It can be seen that the smaller ejector cartridge VEJ1 could only recover up to 0.09 kW of the expansion work from the overall available work recovery potential of 0.3 kW. This ejector profile can only be used for a short range of liquid separator pressure with a pressure lift lower than 8 bar.

_{r,max}of the baseline system. Moreover, the multi-ejector block allows recovery of up to 0.2 kW of the expansion work, which represents 25.4% of the throttling losses according to the efficiency metrics and statistics. Therefore, this analysis is essential to map out each ejector’s performance and indicate the best range of operation conditions.

#### 4.2. Ejector System Performance Improvement

_{2}cycle was determined for the system operational dynamics, including the COP, as shown in Figure 5. The results were obtained for the parallel compressor system and compared with different ejector configurations and pressure lifts. The outcome reveals that the COP has a proportional relation with the pressure lift. Increasing the separator pressure for a higher pressure lift provides a higher system COP based on the compression ratio reduction, decreasing the required input power and improving the performance. When the VEJ1 is activated, the system COP witnesses an increase of up to 1.2% compared with the baseline layout. It should be noted that the operation range for this cartridge is relatively short, which cannot benefit the system when the pressure lift exceeds 8 bar. By comparison, COP degradation was recorded when both ejector profiles ran at a pressure lift of less than 3.1 bar, despite operating with both ejector cartridges.

_{lift}= 6 bar, representing a COP that is 4% higher than that of the booster system under the same working conditions. It can be noted that VEJ2 could support the system with a higher COP even for a pressure lift higher than 12 bar. It is also noteworthy that the system showed worse performance when operated at a low pressure lift compared to the booster baseline. The highest COP degradation was obtained at T

_{MN}= 35 °C, P

_{lift}= 2 bar for a value up to −2.9%. For this ejector configuration, the region of the COP improvement in the transcritical mode started at P

_{lift}higher than 3.15 bar. In general, the multi-ejector block supported with more than one ejector profile can enhance the performance of the cooling system and meet any capacity needed by switching the required ejector electric solenoid valve.

_{2}transcritical refrigeration cycles exhibit remarkable throttling loss, which is recovered during the expansion process due to the significant difference in pressure between the heat rejected and the evaporation temperature. The expansion process takes place at the high-pressure valve. The ejector proved to be a reliable solution that could be connected in parallel with the HPV to recover the amount of work in question and improve the system performance. Figure 7 illustrates the HPV exergy destruction rate for the baseline parallel system compared with different ejector cartridge combinations at the variant level of pressure lift. The results revealed massive exergy destruction for the baseline system exceeding 1 kW at the operation level with low pressure lift. Increasing the pressure lift in operation provides a lower amount of irreversibility in the expansion process due to the reduction in the parallel pressure ratio of the compressors, which decreases the input power needed. However, when the small ejector profile of VEJ1 runs, the expansion process losses decrease by 31% compared to all operation ranges without an ejector, bringing the maximum exergy destruction to 0.74 kW. When the second ejector cartridge of VEJ2 runs alone with the HPV, the exergy destruction recues by 53%. The result indicates a significant improvement in the exergy destruction by using both cartridges together with the HPV. In total, more than 84% of the exergy losses during the expansion can be reduced by both ejectors. These results provide crucial energy savings for the CO

_{2}refrigeration system operating at high ambient temperature and facing a high amount of flash gas in the second-generation layout of the transcritical systems.

## 5. Conclusions

- A total of 31% of available work was recovered by activating VEJ1, while the total efficiency acquired by both ejector combinations of VEJ1 + 2 registered an optimum value of 25.4%. However, the multi-ejector allows entraining a 50% higher suction mass flow rate with the ejector combinations, which greatly influences the system performance by improving the work rate recovered.
- CO
_{2}transcritical refrigeration cycles possess significant throttling loss, especially at lower pressure lift values. In contrast, the combination of both ejector cartridges represented 85% of the potential work that the ejector implementation can achieve compared with the conventional layout. - The multi-ejector concept was found to improve the overall system COP, which increased the refrigerating effect because a higher amount of liquid-phase refrigerant could be supplied to the evaporators. Moreover, the multi-ejector allowed pre-compression of the evaporator exit refrigerant prior to the intermediate pressure region and reduced the compressor input power needed to achieve this.
- In ejector technology, especially for those ejectors operating as supersonic ejectors in transcritical mode, the speed of sound and shock waves play a fundamental role and stand out as two crucial physical phenomena. They are responsible for choking flow and the increase in pressure inside the ejector. To consider the effects and dynamics of these parameters, an optimization CFD study should be performed to analyze these critical parameters.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

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**Figure 2.**The schematic and P-h diagram for ejector-boosted R744 transcritical refrigeration system.

**Figure 4.**The work rate and maximum potential work recovery characteristics based on different ejector configurations and pressure lifts.

**Figure 5.**System COP characteristics vs. pressure lift for the booster system layout at different ejector configurations.

**Figure 6.**The impact of the ejector system on the compressor power recovery as a function of different pressure lifts.

**Figure 7.**The HPV exergy destruction rate of the baseline parallel system and in the case of implementing different ejector profiles via different pressure lifts.

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**MDPI and ACS Style**

Elbarghthi, A.F.A.; Dvořák, V. Evaluation of Various Ejector Profiles on CO_{2} Transcritical Refrigeration System Performance. *Entropy* **2022**, *24*, 1173.
https://doi.org/10.3390/e24091173

**AMA Style**

Elbarghthi AFA, Dvořák V. Evaluation of Various Ejector Profiles on CO_{2} Transcritical Refrigeration System Performance. *Entropy*. 2022; 24(9):1173.
https://doi.org/10.3390/e24091173

**Chicago/Turabian Style**

Elbarghthi, Anas F. A., and Václav Dvořák. 2022. "Evaluation of Various Ejector Profiles on CO_{2} Transcritical Refrigeration System Performance" *Entropy* 24, no. 9: 1173.
https://doi.org/10.3390/e24091173