Performance evaluation of novel double internal auto-cascade two-stage compression system using refrigerant mixtures

Abstract

Heat pumps have been widely applied owing to their high energy efficiency. Two-stage and quasi-two-stage systems are effective in enhancing the performance of heat pumps under large pressure ratio. Meanwhile, refrigerant mixtures are commonly regarded as potential substitutes for heat pumps because they can provide more flexibility in searching for new alternatives. In contrast to previous conclusions regarding gas-injected heat pumps with pure or azeotropic refrigerants, the type of economizer has an important effect on the system performance. In this study, a novel double internal auto-cascade two-stage compression system suitable for refrigerant mixtures is proposed, which can effectively modulate the concentration of the refrigerant going to the evaporator and the injection port, drive the refrigerant rich in high-pressure composition to enter the evaporator, thereby improving the system performance. The results indicate that under the operational working conditions, the flashing pressure is the only independent parameter and the optimal value exists in this novel system. Compared with the flash tank system, the novel system exhibits a 9.6% higher heating capacity and a 6.1% COP higher. The largest increases in the heating capacity and COP relative to the intermediate heat exchanger system are 2.1% and 2.5%, respectively.