An optimized design of the ejector was presented

which was applied to a transcritical CO2 heat pump as an experimental water heater system. Experiments were carried out not only to investigate the effects of flow rate and outlet temperature of hot water and the high-side pressure on the system and the ejector performance

such as the heating coefficient of performance (COPh)

entrainment ratio

pressure lift and ejector efficiency

but also to reflect the ejector influence on system performance. The experimental results show that though the ejector efficiency was related to the entrainment ratio

their change with the hot water flow rate or the outlet temperature were different. In the other word

the entrainment ratio was decreased while the ejector efficiency was increased with the decrease of the cooling water flow rate or the increase of the outlet temperature. Under the experimental working conditions

pressure lift was kept constant and the COPh reached about 3.5. With this ejector

high side optimal pressure decreased dramatically

which results in safer operation for this system. However when the COPh reached the highest value

the ejector efficiency was not the best one. The experimental results also showed that there existed an optimal operation pressure for the transcritical CO2 heat pump with the ejector. What should be noted was under optimal hide side pressure

hot water outlet temperature was not the highest value but still more than 55℃. Therefore stable operation in the optimal high side pressure for the heat pump system is of great significance. It can not only greatly improve system performance

but also ensure attain higher water temperature of the hot water.