Experimental Study On Transcritical CO₂Heat Pump Water Heaters

Along with the implementation of Montreal Protocol and Tokyo Protocol, the countries of the world begin to work together to fight against global warming and ozone depletion. The synthetic refrigerant HCFCs are being phased out, HFCs are subject to more and more restrictions. The environmental friendly CO2is one of the best alternative refrigerants due to its zero ODP, negligible GWP, non-toxic, non-flammable features and excellent heat transfer property. Studies show that ejector used in transcritical CO2heat pump system can bring about two advantages, on one hand, ejector entrains the evaporator outlet low-pressure CO2by using the gascooler outlet high-pressure CO2, which can reduce throttle loss in expansion process, the other hand, ejector lowers the inlet quality of CO2at evaporator, which can make the CO2specific cooling capacity increase. In this article, transcritical CO2heat pump water heaters are experimentally investigated. The main contents are as follows:Firstly, the thermodynamic models for transcritical CO2heat pump system (TRCS) and transcritical CO2heat pump system with ejector (TRCS-EJEC) are established to analyze the effects of evaporator temperature, gascooler outlet temperature and high-side pressure on system and ejector performances.Secondly, the experiments are investigated to assess the effects of primary flow pressure and entrained flow pressure on ejector flow rate, entrainment ratio, pressure lift and efficiency. Ejector performance on different operation conditions is analyzed and performance curve of ejector is acquired.Thirdly, experiments are carried out on a transcritical CO2water source heat pump water heater to compare the performance of TRCS with that of TRCS-EJEC under same evaporator temperature, gascooler outlet temperature and hot water temperature.Finally, the experiments are investigated on a prototype of transcritical CO2air source heat pump water heater. Through analysis of the trends in CO2flow rate, compressor compression ratio, heating capacity and COP, the effects of compressor isentropic efficiency and heat exchanger thermodynamical perfectness on heating COP are obtained.