Theoretical And Experimental Study On Transcritical CO₂Heat Pump Systems

The research on alternative refrigerants is now a hot spot in the area of refrigeration and air conditioning. The natural refrigerant CO2has great advantages in competition with other alternative refrigerants with its excellent thermal and environmental properties. Because of its large temperature change in the transcritical cycle, coupling with the temperature lifting process of water, CO2is very suitable as refrigerant for heat pump water heater. However, CO2has the shortcomings such as large throttling losses, low cycle efficiency. Due to this, methods had been proposed to improve the transcritical CO2cycle efficiency, such as introducing the internal heat exchanger and ejector. There are also a lot of work on the transcritical CO2system with internal heat exchanger and with ejector been done by the Institute of Refrigeration and Cryogenic of Zhejiang University. In the paper, further theoretical and experimental investigations were carried out on the performances of different transcritical CO2heat pump systems, on the basis of previous work, the different evaluation methods for system performance comparison are also discussed. The main work is as follows:Firstly, thermodynamic models are built up for the conventional transcritical CO2heat pump system (TCHS), transcritical CO2heat pump system with internal heat exchanger (TCHSI), transcritical CO2heat pump system with ejector (TCHSE), and transcritical CO2heat pump system with both internal exchanger and ejector (TCHSEI), individually. The impacts of superheating degree and gas cooler outlet temperature on the system performance are analyzed. The system performances of TCHS, TCHSI, TCHSE, and TCHSEI are compared.Secondly, by adjusting the water flow rate at the gas cooler, the water flow rate at the evaporator, and the opening of the throttling valve, the evaporation temperature was maintained constant at0℃, the superheating degree of CO2at the outlet of the evaporator was controlled at0℃,5℃,10℃respectively. The impacts of superheating degree at evaporator outlet on heating coefficient, heating capacity, power consumption of compressor, compression ratio, discharge temperature, mass flow rate are analyzed.Finally, by adjusting the flow rate of cooling water and the opening of.throttling valve, both the internal (including the evaporation pressure, the outlet temperature of gas cooler) and external parameters (including the flow rate of chilled water, the inlet temperature of chilled water, the inlet temperature of cooling water) are controlled. Under the conditions of the same evaporation temperature and superheating degree at the evaporator outlet, experiments were carried out on the4different transcritical CO2systems by obtaining the same hot water temperature. The trends of heating coefficient, heating capacity, power consumption of compressor, compression ratio, discharge temperature, mass flow rate as functions of hot water temperature are analyzed. System performances of the4systems are compared.