An experimental study on the use of variable capacity two-stage compressors in transcritical carbon dioxide light commercial refrigerating systems.

• Fixed expansion device can provide good performance with a 2-stage rotary compressor. • High internal heat exchanger effectiveness increases minimum stable superheating. • Concentric tube and micro channel internal hex provide similar COP with stable superheating control. • Flash-gas bypass provides minor improvements in COP. • Flash-gas bypass increases cooling capacity due to lower evaporator inlet quality. This paper focuses on experimentally investigating different cycle architectures with two-stage variable capacity compressors for light commercial refrigeration CO 2 systems. A system with two-stage rotary compressor and intercooler was used as a baseline. Tests with an iHX showed the existence of a minimum stable/safe superheating. Proper superheating control (>3 K) with an iHX increases the COP by 12.9%, 16.0%, and 17.2% in the compressor frequencies of 45, 60 and 75 Hz, respectively. To prevent instability a capillary tube selected for the lowest frequency can be used, but it reduces performance by up to 15.5% at higher frequencies. A cycle with iHX, two-stage expansion and intermediate flash-gas bypass was also studied. This cycle provided an average increase in cooling capacity of 7.5%, and gains in COP of 15.5%, 18.4% and 18.1%, at 45, 60 and 75 Hz, respectively. Flash-gas bypass control showed little benefit to performance, but improved cooling capacity by up to 12.1% when compared to the cycle with an iHX. [ABSTRACT FROM AUTHOR]