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Experimental comparison of cycle modifications and ejector control methods using variable geometry and CO2 pump in a multi-evaporator transcritical CO2 refrigeration system.

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Title: Experimental comparison of cycle modifications and ejector control methods using variable geometry and CO2 pump in a multi-evaporator transcritical CO2 refrigeration system.
Authors: Toffoletti, Gabriele1,2 (AUTHOR) gabriele.toffoletti@uniud.it, Barta, Riley B.1 (AUTHOR), Grajales, Steven M.1 (AUTHOR), Liu, Haotian1 (AUTHOR), Ziviani, Davide1 (AUTHOR), Groll, Eckhard A.1 (AUTHOR)
Source: International Journal of Refrigeration. Jan2025, Vol. 169, p226-240. 15p.
Subjects: Carbon dioxide, Heat exchangers, Refrigerants, Critical temperature, Global warming
Abstract: • Four CO 2 cycles were experimentally compared over four ambient conditions. • A variable-diameter nozzle and liquid CO 2 pump were assessed as ejector control methods. • The pump operated stably and was able to increase the ejector efficiency by 15 %. • Ejector and IHX cycles increased COP by 4.64 % and 9.47 %, respectively. To reduce the direct global warming impact of refrigerants in HVAC&R applications, low-global warming potential (GWP) refrigerants, including natural refrigerants, have been extensively investigated as alternatives to hydrofluorocarbon (HFC) refrigerants. Among the natural refrigerants, Carbon Dioxide (CO 2) offers several advantages, such as excellent transport and thermo-physical properties, being neither toxic nor flammable, and having a low price and high availability around the world. However, the high critical pressure and low critical temperature of CO 2 often lead to transcritical operation, resulting in lower efficiency due to the additional compressor power necessary to achieve transcritical operation relative to subcritical HFC cycles. Therefore, a number of cycle modifications are used to enhance the coefficient of performance (COP) of transcritical CO 2 cycles to meet or surpass those of HFC cycles. This paper provides a systematic experimental investigation of four such cycle architectures by employing the same multi-stage, two-evaporator CO 2 refrigeration cycle test stand, 3 of these configurations in transcritical and 1 in subcritical conditions. The four cycles architectures included intercooling, open economization, an internal heat exchanger and two different ejector control approaches. Specifically, a variable-diameter motive nozzle and a variable-speed liquid CO 2 pump located directly upstream of the ejector motive nozzle inlet were analyzed. Based on the experimental data, the maximum COP improvements are 4.64 % and 9.47 % when the ejector and the internal heat exchanger are used, respectively. The CO 2 pump, once successfully stabilized, can control the ejector, increase its efficiency by up to 15 % and increase the cooling capacity to a maximum of 6.2 %. Nevertheless, a reduction in COP is measured when the pump is in use; however, unlike the other three different configurations, it was only analyzed under subcritical conditions. [ABSTRACT FROM AUTHOR]
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Database: Engineering Source
Description
Abstract:• Four CO 2 cycles were experimentally compared over four ambient conditions. • A variable-diameter nozzle and liquid CO 2 pump were assessed as ejector control methods. • The pump operated stably and was able to increase the ejector efficiency by 15 %. • Ejector and IHX cycles increased COP by 4.64 % and 9.47 %, respectively. To reduce the direct global warming impact of refrigerants in HVAC&R applications, low-global warming potential (GWP) refrigerants, including natural refrigerants, have been extensively investigated as alternatives to hydrofluorocarbon (HFC) refrigerants. Among the natural refrigerants, Carbon Dioxide (CO 2) offers several advantages, such as excellent transport and thermo-physical properties, being neither toxic nor flammable, and having a low price and high availability around the world. However, the high critical pressure and low critical temperature of CO 2 often lead to transcritical operation, resulting in lower efficiency due to the additional compressor power necessary to achieve transcritical operation relative to subcritical HFC cycles. Therefore, a number of cycle modifications are used to enhance the coefficient of performance (COP) of transcritical CO 2 cycles to meet or surpass those of HFC cycles. This paper provides a systematic experimental investigation of four such cycle architectures by employing the same multi-stage, two-evaporator CO 2 refrigeration cycle test stand, 3 of these configurations in transcritical and 1 in subcritical conditions. The four cycles architectures included intercooling, open economization, an internal heat exchanger and two different ejector control approaches. Specifically, a variable-diameter motive nozzle and a variable-speed liquid CO 2 pump located directly upstream of the ejector motive nozzle inlet were analyzed. Based on the experimental data, the maximum COP improvements are 4.64 % and 9.47 % when the ejector and the internal heat exchanger are used, respectively. The CO 2 pump, once successfully stabilized, can control the ejector, increase its efficiency by up to 15 % and increase the cooling capacity to a maximum of 6.2 %. Nevertheless, a reduction in COP is measured when the pump is in use; however, unlike the other three different configurations, it was only analyzed under subcritical conditions. [ABSTRACT FROM AUTHOR]
ISSN:01407007
DOI:10.1016/j.ijrefrig.2024.10.001