The employment of natural refrigerant have drawn widespread attention

since natural refrigerant are completely environmentally friendly and they have high potential in refrigeration application. Among them

the NH3/CO2 cascade refrigeration system and NH3 compression refrigeration system with CO2 as secondary refrigerant have been viewed as the most promising refrigeration methods in freezer and cold storage especially. Nevertheless

there is limited agreement on the scope of application for these two types of system since no strictly related investigation has been conducted. In this paper

the refrigeration performance of the two systems above is analyzed

and conclusion is drawn on the basis of practical engineering applications. The range of application for these two types of system is marked off and explained in detail

aiming at providing reasonable suggestion for design and optimization of CO2 refrigeration systems in the future. It is shown that NH3/CO2 cascade refrigeration system is more competitive when the evaporating temperature is below -25 ℃

and the maximum suction pressure and and the maximum discharge pressure of CO2 compressor should not be lower than 1.97 MPa and 4.07 MPa respectively.