Ejector-boosted Transcritical CO2 Refrigeration System

M.Eng. Anas F. A. Elbarghthi

Doctoral thesis

Abstract:

Recently, the field of refrigeration and air conditioning has come under immense scrutiny as a result of their indirect contribution to global warming and climate change. This is due to the imminent danger posed on the globe by greenhouse gas emissions. This field is continually increasing, experiencing high-grade energy consumption, and calls for the developing of innovative alternative technologies for saving energy. The use of ejector allows overcoming the significant exergy destruction lays on the expansion processes of the cooling systems and led to spark improvement in the system performance by recovering some of the expansion work. The thesis focused on two things: investigate detailed experimental work on the ejector supplied R744 transcritical cooling system and the impact of the ejector profile on the system performance. The experiment was implemented on the commercial ejector cartridge type (032F7045 CTM ELP60 by Danfoss). The effect of different operating conditions determined by exit gas cooler pressure and temperature, evaporation temperature, and liquid separator pressure was examined. The ejector performance of the pressure lift, mass entrainment ratio, work rate recovery and efficiency were evaluated. In addition, exergy efficiency and the variation of exergy produced, consumed, and destructed related to the ejector profile were assessed based on the transiting exergy. The result revealed better overall performance when the ejector operated at transcritical conditions. The ejector was able to recover up to 36.9% of the available work rate and provided a maximum pressure lift of 9.51 bar. Moreover, it was found out that the overall available work recovery potential increased by raising the gas cooler pressure. Out of the findings, the ejector could deliver maximum exergy efficiency of 23% when working at higher motive nozzle flow temperatures along with providing lower exergy destruction. The experiment results show that the amount of the ejector exergy consumed and destructed were gradually increased with higher gas cooler pressure and, in contrast, decreasing with higher motive nozzle flow temperature. The ejector-supported system was theoretically compared with the parallel compression concept as the baseline system and carried out at different pressure lifts and exit gas cooler properties. The result indicated a COP and exergy efficiency improvement up to 2.05% and 1.92% for the set conditions, respectively, while the COP could be improved to a maximum of 11.22% when the system cooling load is minimized. Additionally, the ejector played a vital role in the system input power. Up to 3.46% of the energy consumption was reduced at subcritical heat rejection conditions. Operating the system with an ejector at a lower cooling capacity allows further overall power consumption reduction to 18%. In addition, the exergy analysis revealed a prominent lack of total system exergy destruction by employing the ejector in parallel with the high-pressure valve, which recovered 21% of the expansion work and saved 46% of the HPV exergy losses. Furthermore, the result exhibited a maximum system exergy loss of 7.8% that could be saved at the set condition and a maximum of 13.2% total system exergy destruction rate recovered by the ejector depending on the cooling load.

Abstract:

Recently, the field of refrigeration and air conditioning has come under immense scrutiny as a result of their indirect contribution to global warming and climate change. This is due to the imminent danger posed on the globe by greenhouse gas emissions. This field is continually increasing, experiencing high-grade energy consumption, and calls for the developing of innovative alternative technologies for saving energy. The use of ejector allows overcoming the significant exergy destruction lays on the expansion processes of the cooling systems and led to spark improvement in the system performance by recovering some of the expansion work. The thesis focused on two things: investigate detailed experimental work on the ejector supplied R744 transcritical cooling system and the impact of the ejector profile on the system performance. The experiment was implemented on the commercial ejector cartridge type (032F7045 CTM ELP60 by Danfoss). The effect of different operating conditions determined by exit gas cooler pressure and temperature, evaporation temperature, and liquid separator pressure was examined. The ejector performance of the pressure lift, mass entrainment ratio, work rate recovery and efficiency were evaluated. In addition, exergy efficiency and the variation of exergy produced, consumed, and destructed related to the ejector profile were assessed based on the transiting exergy. The result revealed better overall performance when the ejector operated at transcritical conditions. The ejector was able to recover up to 36.9% of the available work rate and provided a maximum pressure lift of 9.51 bar. Moreover, it was found out that the overall available work recovery potential increased by raising the gas cooler pressure. Out of the findings, the ejector could deliver maximum exergy efficiency of 23% when working at higher motive nozzle flow temperatures along with providing lower exergy destruction. The experiment results show that the amount of the ejector exergy consumed and destructed were gradually increased with higher gas cooler pressure and, in contrast, decreasing with higher motive nozzle flow temperature. The ejector-supported system was theoretically compared with the parallel compression concept as the baseline system and carried out at different pressure lifts and exit gas cooler properties. The result indicated a COP and exergy efficiency improvement up to 2.05% and 1.92% for the set conditions, respectively, while the COP could be improved to a maximum of 11.22% when the system cooling load is minimized. Additionally, the ejector played a vital role in the system input power. Up to 3.46% of the energy consumption was reduced at subcritical heat rejection conditions. Operating the system with an ejector at a lower cooling capacity allows further overall power consumption reduction to 18%. In addition, the exergy analysis revealed a prominent lack of total system exergy destruction by employing the ejector in parallel with the high-pressure valve, which recovered 21% of the expansion work and saved 46% of the HPV exergy losses. Furthermore, the result exhibited a maximum system exergy loss of 7.8% that could be saved at the set condition and a maximum of 13.2% total system exergy destruction rate recovered by the ejector depending on the cooling load.

Keywords

Natural fluid, CO2, Ejector, Transcritical system, Pressure lift, Efficiency, Work recovery, COP, Exergy destruction, Power consumption.

Language used: English

Date on which the thesis was submitted / produced: 2. 11. 2021

Thesis defence

• Supervisor: prof. Ing. Václav Dvořák, Ph.D.

Technical University of Liberec

Faculty of Mechanical Engineering

Doctoral programme / field:
Mechanical Engineering / Applied Mechanics