THEORETICAL ANALYSIS ON A TWO-STAGE CO_2 REFRIGERATION CYCLE WITH EVAPORATING TEMPERATURE AT THE TRIPLE POINT TEMPERATURE

摘要

Owning to the advantages of environmental benefits, researches on natural refrigerant carbon dioxide cycles are revived. A CO_2 two-stage refrigeration system with evaporating temperature of 216.55K (triple point temperature) was proposed, in which the three-phase container allows stable refrigeration capacity. For different cooling / condensing temperature, the high pressure stage of the system can be operated at the subcritical cycle or the transcritical cycle. Thermodynamic model of this system was established and parameters were discussed. Influences of heat recovery in the low pressure stage were discussed. Simulation results show that the COP can reach 1.34 when the high pressure stage is the subcritical cycle and the discharging pressure of the low pressure compressor is 3.0MPa. With the increase of the cooling temperature, COP decreases for the subcritical cycle and there exists an optimal medium pressure, varying with the cooling temperature. In the transcritical cycle, the discharging pressure of the high pressure compressor is independent of the cooling temperature, which is different with that of the subcritical cycle.