With the increased awareness in the international community

the problems of ozone depletion and greenhouse effect have received extensive attention

more and more scholars are focusing on environmental-friendly refrigerants. CO2 demonstrated its value in terms of simplicity

availability

safety

reliability

and good heat transfer performance. This paper analyzes and summarizes the research progress of small-scale CO2 gas-liquid two-phase expanders. It is found that replacing the throttle valve with a CO2 expander in the cycle can effectively improve refrigeration efficiency. The cooling and heating efficiency of the cycle is 6.9% and 4.6% higher than that of the R134a cycle

and 25.76% and 16.17% higher than that of the cycle with a throttle valve

respectively. The higher the isentropic efficiency of the CO2 expander

the greater the system efficiency. As long as the isentropic efficiency of the expander is greater than 32%

the performance of the system with a CO2 expander is better than that with a regenerator. Different CO2 expanders have different efficiency

leakage

and friction loss due to their different structures. Piston type

rotor type

and other expanders have simple structures

while their leakage loss is relatively large. The efficiency of the sliding vane type

rotary vane type

and scroll type expander has been greatly improved

but their structure is more complicated

leading to more complicated manufacturing and increased production costs. The compression work is supplied to the compressor

increasing the flow rate during the expansion process

which requires designing a CO2 expander with an adjustable expansion ratio. At present

the research of CO2 expanders is basically concentrated in the laboratory

and this technology has not been commercialized yet. Therefore

the future development direction should combine experiments and theory to further study the efficiency improvement of CO2 expanders and the development of key components technology.