Performances Analysis And Comparison Of Transcritical CO₂ Vapor-Compression/Ejection Refrigeration Cycles

Freon is widely used in refrigeration engineering, its mass production has caused serious damage to the environment. Carbon dioxide is friendly to the environment and has become the popular choice of Freon substitute, it has gained more attention and attracted the interest of researchers. Carbon dioxide has low critical temperature, therefore vapor compression refrigeration system are transcritical cycles in most conditions. It causes great throttling lose, applying ejectors in the cycles can recover throttling lose effectively. In order to research the performance of transcritical CO2 vapor compression/ ejection refrigeration cycle, three different cycles were analyzed and compared.The thermodynamic model of the transcritical CO2 single stage compression/ ejection refrigeration cycle (SEC) was established by fixed point iteration, the program was edit in MATLAB matching with REFPROP database. The COP of SEC is better than that of transcritical CO2 single stage compression basic refrigeration cycle (SBC) in all gas cooler pressure conditions. The maximum COP of SEC is 27.7% better than that of SBC. COP of SEC increases and optimum entrainment ratio slightly increases while evaporator temperature increases.The thermodynamic model of transcritical CO2 two stage compression/ ejection refrigeration cycle with two gas cooler (TEC+TG) and the calculation model considering two temperature factors change were established. Compared with transcritical CO2 two stage compression basic cycle (TBC+TG) and SBC, the performance of TEC+TG is the best. The maximum COP of TEC+TG is 21.69% better than that of TBC+TG, and 39.57% better than that of SBC. In the given calculation condition, the optimum intermediate pressure is 16.24% higher than the geometric mean of high stage gas cooler pressure and separator pressure.The designed transcritical CO2 two stage compression/ ejection refrigeration cycle (TEC+IC) is a novel two stage compression cycle. Temperature iteration of the intercooler exit was added in the thermodynamic model of TEC+IC. There were two fix point iterations nested and dichotomy application in the model. Compared with TEC+TG and SEC, the maximum COP of TEC+IC is the most improved, it is 6.27% better than TEC+TG and 16.15% better than SEC. The entrainment ratio of TEC+IC is value bigger than TEC+TG and SEC. When gas cooler pressure is higher than the optimum value, the COP of TEC+IC decreases faster than that of TEC+TG. The cooling COP of TEC+IC is less improved than TEC+TG while gas cooler pressure is higher than 10MPa. The COP of TEC+IC and that of TEC+TG show similar tendency while gas cooler exit temperature and evaporator temperature change simultaneously.By designing and calculating the experimental instruments of transcritical CO2 vapor compression/ ejection refrigeration cycle, the prototypes of compressor, ejector, separator and heat exchanger were made.